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THE NAUTICAL INSTITUTE
TUG USE IN PORT
A Practical Guide
2nd edition
by
Captain Henk Hensen FNI
N.Cham. 387.166 H526 2.ed. 200';
Autor: Hensen, Henk,
Titulo: Tug use in port: a practical guide.
I\!IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII1III1111 ;;~;: 8
Ex.1
TUG USE IN PORT - 2nd edition
by Captain Henk Hensen FNI
1st edi tion published by The Nautical Institute
1997
2nd edition 2003
Published by The Nautical Institute
202 Lambeth Road, London, SEI 7LQ, England
Telephon e: +44 (0)20 7928 1351
Fax: +44 (0)20 7401 2817
Publications e-mail: pubs@nautinslorg
Worldwide web site: http:/ /www.nautinslotg
This edition Copyright © The Nautical Institute 2003
Sponsored by the Port of Rotterdam Authority
Cover picture The Hellespont Metropolis arriving in Rotterdam on her maiden voyage O ctober 2002 with
Fairplay tugs in attendance. Courtesy of Port of Rotterdam; Ben Wind Fotografie, the Netherlands
All rights reserved: No part of this publication may be reproduced, stored in a retrieval system, or transmitted in
any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written
permission of the publisher, except for the quotation of brief passages in reviews.
Although great care has been taken with the writing and production of this volume, neither The Nautical
Institute nor the author can accept any responsibility for errors, omissions or their consequences.
This book has been prepared to address the subject of tug use in port This should not, however, be taken to
mean that this document deals comprehensively with all of the concerns which will need to be addressed or even,
where a particular matter is addressed, that this document sets out the only definitive view for all situations.
The opinions expressed are those of the author only. Captain Henk Hensen was born in 1935, is a Master
Mariner and was a Port of Rotterdam pilot for 23 years. During his years as a pilot he was stationed at the Pilot
Office for five years. During that time he started simulator courses for harbour pilots and tug captains and partici-
pated in many port studies, including simulator research. He started a database for casualties in the Port of Rotterdam
and analysed them with the object of improving safety. Following his retirement he started his own consultancy,
Nautical Safety Consultancy, and works as marine consultant on the nautical aspects of port studies, tug advice and
simulator training.
All photographs and diagramS acknowledged
Typeset byJ A Hepworth
I Ropers Court, Lavenham, Suffolk, CO 10 9PU, England
Printed in England by
Modern Co lour Solutions
2 Bullsbridge Industrial Estate, Hayes Road ,
Southall, Middlesex, UB2 5NB, England
ISBN 1 870077 39 3
CONTENTS
Acknowledgements ii
Foreword iii
Author's Preface iv
Tug Use in Port - The Overview v
Glossary of Terms vi
List of figures ix
Chapter
1 Tug design factors 1
2 Types of harbour tug : 9
3 Assisting methods 33
4 Tug capabilities and limitations 43
5 Bollard pull required 68
6 Interactio n and tug safety 80
7 Towing equipment 94
8 Training and tug simulation 117
9 Escort tugs 134
10 Tug developments 163
References 174
Appendices
1 Port au thorities & towing companies which provided information 178
2 Safety of tugs while towing 180
3 Rules for escort vessels 182
Index 187
TUG USE IN PORT
ACKNOWLEDGMENTS
1st edition
The author would like to express his appreciation to the Rotterdam Municipal Port Management for their generou s
support, which made it possible to write this book.
Without the expertise and support of many individuals and companies this book could not have been completed
to the standard which has been achieved. The author is sincerely grateful for their contributions. Although it is hardly
possible to name them all, a small list of the persons and companies that have been so kind in providing information
or sharing their insights would include:
The Rotterdam towing companies, and in particular Smit Harbour Towage Company; Damen Shipyards, Gorinchem,
The Netherlan ds; Mr.Joh. deJo ng MSc, Marine Simulator Centre the Netherlands; Mr. David L. Potter, Marlow
Ropes, UK; The Glosten Associates, USA; Captain LarriJ ohn son, Marine Superintendent Foss Maritime, Seattle,
USA; US Coast Guard; and Thomas Reed Publications, UK.
Furthermore the author is greatly ind ebted to the following persons:-
Mr. W. Hoebee MSc, and his staff, and Captain W. Verbaan of the Rotterdam Port Authority, Mr. T.E. Tomasson
MSc, of MarineSafety International Rotterdam, for their generous and continuou s support.
Captain Evgeny Sarmanetov, former St. Petersburg pilot, for his excellent contribution regarding manoeuvring in
ice and Captain N. Golovenko, Rotterdam, for the Russian - English translation of this article.
Captain Victor ]. Schisler, Long Beach - pilot, U SA and Captain Nigel Allen, Southampton - pilot for their
professional contribution on escorting.
Those of all the port authorities and towing companies that compl eted the questionn aire and provid ed information
regarding tugs and tug assistance in their ports. The response to the questionnaires, which were sent by the Port
Authority of Rotterdam to a hundred ports around the world, was much high er than might be expected and the
information provided by those ports that completed the questionnaires was invaluable.The names of these persons
and the port authorities and towing companies are listed in Appendix I.
Finally, the author is sincerely grateful to Captain Herbert van Donselaar MSc, for sharing his keen professional
insight du ring the process of writing this book.
2nd edition
In 200 2 the bo ok was revised. Again many were helpful and contributed by providing information, sharing their
insights and always willing to answer questions. The author is grateful for the contributions of:
Mrs. Heike Hoppe of IMO, London, United Kingdom; Mr.JoopJansen and Erik Leend ers, Damen Shipyard, the
Netherlands; Mr. Randy S. Longerich, Puget Sound Rope, USA; Mr. Paul P. Smeets, DSM High Performance
Fibers, the Netherlands; Mr. David L Gray, Glosten Associates, USA, Mr. Robert Allan, Robert Allan Ltd , USA ;
Mr.J on M.Jakobsen, Statoil Mongstad, Norway; Mr. Erling Kvalvik, Norsk Hydro Produksjon a.s, Norway; Mr.
Jimmy Brantn er, Marine Towing of Tampa, USA; Mr. Richard Decker and Mr.John Collins, Seabulk Towing,
USA; Mr. Markus van der Laan , IMC Group, the Netherlands; Mr. Dave Foggie, The Maritime and Coast Guard
Agency, UK, while several others could be added.
Furthermore, the author is greatly indebted to the following persons:
Mr.Jaap C. Lems, Director Rotterdam Port Authority and Harbourmaster of the Port of Rotterdam, for his great
support; Captain Roger Ward, Tug Master and formerly Marine Man ager with Howard Smith Towage, Melbourne,
Australia,for the valuab le discussions and information exchange on practical aspects of harbour towage during
several years; Captain Gregory Brooks, Tug Master/Instructor, USA ; Captain Victor]. Schisler, Long Beach pilot,
USA; Capt Arthur Naismith, Voith Training Master; Captain Nigel Allan, Southampton pilot, UK; LT Keith Ropella,
Chief Vessel Traffic, MSO Valdez, Alaska, USA and Mr. Henrik Hammarberg, Det Norske Veritas, Norway, for
their professional contribution; on escorting, escort procedures, and / or regulations.
Finally, the Rotterdam Muni cipal Port Management generously supported also this revised edition of the book,
for which the author would like to express his sincere appreciation.
Without the help of all those mentioned it would have been impossible to revise the book in the way it has been done.
ii THE NAUTICAL INSTITUTE
FOREWORD
by
Executive Dir ector of the Port of Rotterdam
Mr. P. Struijs
Tug Use in Port, which includes escor t tugs, is a valuable addition
to nautical literature. Twenty
years ago few would have believed that it cou ld be possible to bu ild in so mu ch powe r and
manoeuvrability into the hull form of today's tugs.
With conventional designs it was impossible to achieve this capability, bu t now towage companies
which do not em brace thi s new techn ology are likely to find the compe tition overwhelming.
It is against this b ackground, and I trained as a naval architect, that I welcom e this book. It sets
out to demonstr ate the characteristics of the old and new and in doing so the reader can come to
appreciate how to transfer and adapt towing practices to optimise the use of all tugs in a mixed
fleet.
Whilst naval architects an d marine enginee rs have concentrated on fuel economy per ton mile
in deep sea vesse ls they remain unwieldy in confined waters. Similarly the car carrier and container
ship, although generally high er powered than the bulk carr iers, have special limitations imposed
by windage.
Happily whils t the deep sea vessel has become larger and relatively less man oeuvrable tugs have
grown in capability and so play an essential role in port economics. Indeed a port which can not
provide effective tug support becom es unviable an d it is important that the towing industry
recognises this.
So Captain Hensen an experienced pilot from my port has provided an essential service in
demonstrating how tugs can be used to best effect. The Port of Rotterdam is pleased to have
played its part as a major sponsor to this publication.
This book ex amines towage techniques an d the reader will be constantly re minded that
shiphandling with tugs is all about competent teamwork. On board the ship are the maste r, pilo t
and crew, on bo ard the tugs are the tug masters and crew and they have to work together. To be
effective all need a good knowledge of this professional area of activity particular ly as ships are
often attende d by a mixed variety of tugs. The foundation of how best to control operations is
laid out in this ve ry prac tica l guide .
The othe r chapters on tow ropes, training, bollard pull and escort work, all linked by a common
thr ead of safe working me thods makes this an ideal book for study. I believe it will favourably
influen ce the way tugs are designed and used . This is the hallmark by which this book will be
recognised and ' I have no h esitati on in recommending thi s well illustr ated text to towage
companies , ports, tug masters, pilots and sea staff alike. Everybody will ben efit from its practical
guidan ce.
TUG USE IN PORT iii
AUTHOR'S PREFACE
Wh en ships are assisted by tugs, experi~nce, teamwork , communication and above all insight into the capabilit ies
and limitations of ships and attending tugs are essential for safe and efficient shiphandling. This applies to th e tug
captain and his crew as well as th e ship master and pilot, particularly nowadays as older conven tional tugs are
increasingly being replaced by modern types with larger engine powers and increased capabilities. Reputable shipyards
build goo d tugs, and designers can predict how well their tugs will perform. However, they do not handle ships
themselves and have not experienced the tug assistance required: not in a river, channel or port approach nor in a
confined harbour basin, not during a storm or in strong currents nor in the midd le of a foggy night. Not even du ring
nice, calm weather. These are the situations and conditions in which pilots and tug captains have to hand le ships. So
it is essential that they know what can be expected from a tug in any specific circumstance. Only when these professionals
are fully aware of the capabilities and limitations of the various types of tugs in general and of an ind ividual tug,
including the effects on an assisted ship, are they able to utilise tugs in the safest and most effective way an d in
harmony with a ship's manoeuvring devices.
Good insight int o the operational performance of different types of tugs while assisting vessels is also of major
importance for tugboat companies. It allows them to determine what type of tug will pro vide optimum service for the
port, with respect to the local situation, environmental conditions and ships calling at the port.
The increasing use of simulation for research and training purposes requires an in-depth knowledge of tug capabilities
and limitations, in add ition to the data required for creat ing a tug simulator model. Only then can resu lts be achieved
that are safely applicable to daily practice and which form a contribution to safe shiphandling.
Th ere is a trend towards ev er more powerful tugs and more manoeuvrable modern vessels. Th is is leading to a
reduction in the number oftugs used to assist those ship s, so the role of harbour tugs becomes even m ore crucial than
before.
There are many reasons, therefore, why a book on tug assistance could be usefuL The aim of this book is to
improve the practical knowledge of harbour tugs and their different types, and to give a better insight into the
capabilities and limitations of these tugs while rendering assistance.
Not all aspects of shiphandling with tugs are addressed in detail within this book. This work sho uld be seen as a
basic guide to the reader, whilst at the same time encouraging further increase of knowledge. The references m entioned
at the end may prove usefuL '
The book is specifically written with th e needs of maritime professionals involved in the day-to-day pr actice and
training of shiphandling with tugs in mind, particularly pilots, tug captains and training instructors. It sho uld also be
of valu e to towing companies, shipmasters and mates of seagoing vessels and all other persons or orga nisations
involved, one way or ano ther, with tugs and shiphandling.
In th e second edition several subjects have been reviewed or extended, based on experience and kn owledge
gained during the last five years. Items that were found to be missing have been included, Ship's fittings for use with
tugs have been addressed more specifically, the escort chapter has been extended, new developments in the tug
world have been included, and several references used for th e book have been add ed for tho se who want to read
mo re about certain subjects.
The tug world is a fast changing world , although basic principles for tugs and tug operations do not change that much.
It is th e author's earnest hope that this book will contribute to improved knowledge of harb our tugs and lead to
increasing safety in tug and shiphandling operations in ports and port approaches around the world.
The author.
iv THE NAUTICAL INSTITUTE
TUG USE IN PORT
THE OVERVIEW
The contents of this book are outlined below.
A general review is presented first of factors which affect operational requirements for a harbour tug, such as the
different tasks for which they are used, the particulars of a port, the environmental conditions and ships calling at
the port.
• Various types of harbour tug are discussed in a general way, addressing the diversity of design, propulsion,
steering and manoeuvring capabilities.
After reviewing assisting methods in use worldwide, tug types are considered in more detail, including the
performance of different types of tug resulting from the location of propulsion devices, towing point and lateral
centre of pressure. Tug capabilities, limitations and effectiveness with respect to different assisting methods and
operating positions relative to a ship are discussed.
• The number of tugs required to handle a vessel safely is frequently a topic for discussion between pilots and
shipmasters. This iroportant subject is discussed taking into account the effects ofwind, current, shallow water and
confined waters. The number of tugs and total bollard pull used in several ports around the world is mentioned.
Much attention is given to dangerous operational
situations for tugs, such as interaction and girting, and to
environmental conditions such as fog.
Towing equipment is dealt with, particularly in relation to safe and efficient shiphandling.
Escorting and escort tugs, being a subject of specific interest nowadays, is dealt with separately.
Proper training for a tug captain and crew is essential in order that they handle the tug safely and efficiently. The
same applies to the pilot and/or master for shiphandling with tugs. Training is therefore an important subject in the
book, including siroulator training and research.
All subjects are, as far as possible, related to situations encountered in practice.
PIw,,,S~~Lbi., Cmwia
&verse-tractor tug> 'Seaspan Hawk' and 'Seaspan Falcon' (l.o.a. 25·9m, beam 9·lm, bp ahead 39 tons, bp astern 37·5 was) ready w mah fastat
thefrrward andport quarters with a bow line
TUG USEIN PORT v
Dead ship
Cross lines/gate lines
GLOSSARY OF TERMS
Assisting methods The term used to describe the way in which harbour tugs assist seagoing vessels.
Breasted/alongside towing : A tug securely lashed alongside a ship, usually with a minimum of three lines: head
line, spring line and stem line. Also called 'on the hip' or 'hipped up'.
Push-pull A tug made fast so that it can pull as well as push at a ship's side. Depending on the
type of tug, its location and the assistance required, it can be secured with one, two
or three lines.
Towing on a line A tug assisting a ship while towing on a line as is in common use in many European
ports.
Box keel An enclosed keel structure extending from the aft skeg (if fitted) to a point close to
the forefoot of a tug. A box keel is sometimes installed on ASD escort tugs to provide
a better course stability on astern and additional lift forces, resulting in higher towing
forces, when operating as stem tug in the indirect towing mode. In addition, a box
keel gives additional strength to the tug's hull and provides a better distribution of
dock forces when in dry-dock.
Course stability and directional stability:
Course stability is also called dynamic stability, stability of route or dynamic stability
of route (see References : Hydrodynamics in Ship Design, Vol. I. H.E. Saunders). It
is that property of a ship (which includes tugs) that, when disturbed, damps out
extraneous motions set up by the disturbance and to reduce them progressively to
zero . Course stability should not be confused with directional stability, which is,
strictly speaking, the ability of a ship to follow a certain direction, e.g. by means of
an automatic steering system. A ship closely following a selected heading has
directional stability but may be course unstable (see below), which results in frequent
rudder (or thruster) actions to hold the ship on its course .
Course stable ship With a constant position of the steering systems (rudders, thrusters, etc.), a ship is
defined to be course stable if, after experiencing a brief disturbance, it will resume
the original manoeuvre without any use of the means of steering. Course stability
on a straight course, with the rudder in the equilibrium position, is mostly only
considered. A turn initiated by a brief disturbance of a course stable ship will thus
not continue. However, after the disturbance has vanished, the actual course of the
ship will generally be altered. A course stable ship needs relatively large rudder
angles for course changing. A course stable ship has good yaw checking ability.
Course unstable ship A ship is called course unstable, if, after it is disturbed, it will immediately start to
turn. Course changing, with relatively high rates of turn, can be achieved with
relatively small rudder angles . A course unstable ship therefore generally has poor
yaw checking ability.
Separate lines from either side of the tow to the opposite quarter of the tug or the
opposite side of the tug's H-towing bitt.
A ship which cannot use her own propulsion.
Density of air as used
Density of sea water
as used
Escort tugs
Escorting tug
F(P}SO
Free sailing
Girting
Gob rope I gog line
1.28 kg/m'
1025 kg/m'
Tugs specifically built for escorting at high speeds.
Any type of tug escorting a ship underway.
Floating (Production) Storage and Offioading Unit,
A tug sailing independently.
Risk of capsizing, especially with conventional tugs, due to high athwartships tow
line forces. Also known as girding, girthing or tripping.
A rope or steel wire used on conventional tugs to shift the towing point
vi THE NAUTICAL INSTITUTE
H MPE
Hodde
IMO
Lbp
Loa
LWL
M BL
MG
Messenger
Norman pins
Nozzle
OCIMF
PlANe
Pendant/pennant
Propulsion:
Azimuth prop ellers
CPP
FPP
VS
PRT
Significant wave height
Snag re sistance
SPM
Sponson
Stemming
Stretcher
Towing point
Towlin e
Tripping
High-modulus polyethylene fibre under the trade names 'Spectra' and 'Dyneem a'
used for high performance ropes .
Kinking or twisting of a strand in a rop e which makes it unfit for use.
International M aritime Organization.
Length between perpendi culars.
Length overall.
Length at th e waterline .
Minimum Breaking Load (of arope},
Initial Metacentric Height.
A light rop e attached to the tow line in order to heave the tow line on board a ship.
Short iron bars fitted in th e gunwales of the transom to prevent the tow line from
slipping over the side gunwales. Sometimes called 'King Pins'.
A tube around the propeller to increase propeller performance. The nozzle can be
fixed or steerable.
Oil Companies International Marine Forum.
Permanent International Association of Navigation Congresses.
A separate part at the final part of a tow line which is most liable to wear on board
an assisted ship, at Ship's fairleads, etc. The pendant can be of different construction
to the tow line.
3600 steerab!e propellers, which can deliver thrust in any dire ction.
Also called: 'Z'pellers', 'Rexp ellers', 'Duckpellers' (azimuth propellers in nozzles).
Controllable pitch propeller{s}.
Fixed pitch propeller(s}.
Voith Schn eider propulsion : propulsion system with vertical propeller blades, also
called cydoidal propulsion system.
Prevention and Response Tug.
The approximate wave height as seen by an experienced observer when estimating
the height visually.
Resistance of a rope to single yarns being pulled out of the rope when it slides along
a surface, such as over a deck or through a fairlead. A snag is a loop of a yarn .
Single Point Moorings.
A strongly flared section in the side of a tug, commencing at or just below the
waterline, which results in substantial increase in deck area and reserve buoyancy
without increasing the beam at the waterline.
A tug coming under the bow of a ship at speed.
That part of a tow line, between the original tow line and pennant, which absorb s
the dynamic forces in th e tow line. Also called a spring and often made of nylon ,
polyester or a polyester/polypropylene combination .
Point ofapplication of the tow line force. It is the point from where the tow line goes
in a straight line towards th e ship.
A flexible hawser used for towing purposes.
A tug towing on a line swinging around and coming alongside a ship's hull due to
excessive speed by the ship in relation to a tug's capabilities and towing angl e. The
expression 'tripping' is also used for girting.
TUG USE IN PORT vii
Tug/ engine power :
BHP
SH P
BP
MCR
Ton
Tug simulation:
Interactive tug
Vector tugs
UHMW polyethylene
(UH MW PE)
VS-tug
Brake Horse Power : power delivered by the engine.
Shaft Horse Power: power delivered to the propeller shaft (approximately 97% of
BHP).
Bollard Pull, which in this book is expressed in the practical units of tons , equal to
1000 kgf (= 9·80665 kN).
Maxim um Continuous Rating (of
tug engine).
Th e practical unit used in this boo k for force, e.g. for bollard pull , equal to 1000 kg
force, and for 'weight', equal to 1000 kg.
A tug simulated on a bridge manoeuvring simulator, able to interact wi th other
bridge manoeuvring simulators, which are simulating other tugs and! or the assisted
ship .
Tugs simulated by just a force vector.
Ultra High Molecular Weight polyethylene.
Material used for dock fendering and for fenders of tug boats at places whe re a low
friction coefficien t is required.
A tug with VS pro pulsion.
viii THE NAUTICAL INSTITUTE
Figure
LIST OF FIGURES
Title Page
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
2.10
2.11
2.12
2.13
2.14
2.15
2.16
2.17
2.18
2.19
2.20
2.2 1
2.22
2.23
2.24
2.25
2.26
2.27
2.28
2.29
2.30
2.31
2.32
2.33
2.34
2.35
2.36
2.37
2.38
2.39
2.40
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
3.10
Port or Antwerp. Zandvlietsluizen. Tugs should be able to assist ships thr ough the locks and b ridges 1
Push-pull tugs ope rating in the Port of Osaka. Large manoeuvring area near the ber th _ 2
M.T. Capitol berthing atJetty 4 at Sullom Voe Oil Terminal 3
Tug assisting in open sea close to port entrance _ _ 3
In colder areas tugs should be able to operate in ice conditions 4
Car carrie r passing Calandbridge in the port of Rotterdam. Th e stem tug is an azimu th tractor tug 4
Azimuth tractor tugs (53 tons bo llard pull) of the KOTUG towing company towing an oil rig 5
Conventional twin screw tugs of 27 tons bollard pull towing on a line 6
H arb our tugs - factors influencing choice 7
Main types of harbour tug 8
Pusher tug Lam Tong .........................................................................................•..................................•........................................• 10
Plan of the navigation bridge deck and view of the whee lhouse of a modem Hong Kong pusher tug 11
Typi cal fender arrangement for a tug pushing under swell conditions and!or at flaring parts of a vessel 12
Bow fend ermade of reinforced truck tyres 12
Tyree used in addition to vertical bow fendering 13
Conventional twin screw tug - type Stan Tug 2909 13
Two generally used nozzle types 19A and 37 15
Steering nozzles, one with a moveable flap the oth er with a fixed fin 16
Construction of a steerab le nozzle with moveable flap 15
Fixed nozzle with a moveable flap rudder 15
Schilling rudder 16
Shutter rudder system with a fixed nozzle and two flanking rudders 16
'Iowmaster rudder system of tug Hamm 17
Twin screw tug moving sideways to starboard, also called flanking 18
Some assisting methods with conventional tugs 18
Combi-tug Petronella] , GoedJuJop of Wijsmuller Harbour Towage Amsterdam 19
~:::~~~:oe::~s=:cc::~~=:. ::: ::: : :: : : : : : :: : :::: : : :::: : : : :: :: : :: :: :: ::: ::: :::: :: ::: : : :: :::::: :: : : : : : : : : : : : : :: : : : :: : :: : : : : : : :: :: : : : : : : : : :: : : : : : : : : : : : :. : : : : : :::~~
Voith tractor tug 21
Prop eller blades of a VS tug 21
Prin ciple of Voith propulsion 22
Propeller control of VS tugs 22
A VS tug sailing ahead and astern 23
Some assisting methods with a tractor rug 23
Azimuth tractor tug Fairplay V•..................................................................................................•..................................................24
Integrated Schottel nozzles with open protective frames 24
J oystick for combined control of both thrusters 25
Thruster control unit for combined control of thrust and thrust directio n "" 25
Manoeuvring diagram for reverse-tractor tug 25
Reverse-tractor or pusher tug Lam 'Iimg 27
Thrusters of Cates ' reverse-tractor tugs 27
Assisting methods with a reverse-tractor tug 27
ASD-tug type 3110 28
Free sailing manoeuvring capabilities of an ASD-tug and reverse-tractor tug 29
Some assisting methods with an ASD ·tug 29
Relationship between brake horse power and bollard pull for different propulsion systems 30
Ranges in relationship between brake horsepower and b ollard pull for different tug types 30
Example of thrust vector diagrams 31
An assisting meth.od as used in some USA ports 32
Tugs alon gside at approach and push-pull while mooring/unmooring 34
Conventional USA tug secured with backing, spring and stern lines 35
Alongside towing (USA) 35
Forward tug secured alongside 35
Alongside towing in Cape Town for a 'dead ship' up to 100 metres in length 35
Rudder or steering tug 35
Conventional tug working stem to stem with a large passenger ship 36
Conventional twin screw tug EsperaTlQl 36
At approach, forward tug alongside and stern tug on a line; push-pull while berthing 36
Towing on a line at the approach and while mooring 37
TUG USE IN PORT ix
Figure Title Page
3.11 Ship is passing. narrow bridge and a conven tional tug forward is assisting with two crossed tow lines 37
3.12 Towing on a line at the approach nd push-pull while mooring 37
3.13 Combination of different assistiog methods 37
3.14 Ship approaches the be rth nearly parallel to the dock 39
3.15 Tug assistance in ice during approach to the berth an d while mooring 40
3.16 Tug sweeping ice aw.y from between ship an d dock 40
3.17 Mooring in ice wh en some 30 me tres free be rth is available in front of the bow position 40
3.18 Combination of tug and bow thruster while mooring 40
3.19 Good results when approaching the berth astern and m ooring star boar d side alo ngs ide 41
3.20 Tug assistance when mooring in ice with ships and powerful engines 41
3.21 Ship approaching the berth astern 41
3.22 Two tugs stem to stem clearing ice b etween ship and berth whil e othe r tugs keep the ship in position 41
3.23 Ship of m edium size departiog 42
3.24 Unmooring bow first 42
3.25 Channel through the ice prepared by ice breakers or strong tugs 42
4.1 Location of the pivot point for a ship at speed 43
4.2 Location of the pivot point in a ship with zero speed 44
4.3 Forces created on assistiog tug, moving ahead 45
4.4 Forces created on assisting tug, moving astern 46
4.5 Tug working on a gob rope 47
4.6 Swivel fairlead on the after end of a tug's deck for the gob rope 47
4.7 The large fairlead is the aft lying towing point on a VS tractor tug 47
4.8 Direct and indirect towing methods 48
4.9 VS tug operating in the indirect towing mode 49
4.10 Heeling forces working on a conventional tug when towing on a line 49
4.11 The effect of a radial hook 50
4.12 The effect of a radial hook , 50
4.13 Basic differ ence between tug types 52
4.14 Comparison between tractor type tugs and conventional tugs when towing on a line with a ship having headway 53
4.15 When port helm is applied and the tug pulls to starboard to counteract the port swing 54
4.16 Comparison of performance of tug types when pushing or pulling 55
4.17 Pushing force created by hydrodynamic force on a tug's hull 56
4.18 Effect of dynamic forces in the tow line 57
4.19 Performance and behaviour of a 40 metre conventional tug 58
4.20 Performance and behaviour of a 30 metre ASD-tug for pushing 58
4.21 Performance graphs for four and six koots speed 59
4.2 2 Performance graphs for eight koots speed 60
4.23 Different tug positions 62
4.2 4 Two conventional tugs assisting a tanker having headway in making a starboard tum 63
4.25 VS tug & dbridgeof Adsteam Towage, Southampton, UK 65
5.1 Bollard pull required to compensate for beam wind 70
5.2 Wmd height velocity ratio 70
5.3 Bollard pull required in a cross-eurrent 71
5.4 Effect of underkeel clearance on current force 72
5.5 Bollard pull required for beam waves 73
5.6 Open berth constru ction for bulk carriers 73
5.7 A tug's propeller wash hitting a ship's hull, reducing towing effectiveness 74
5.8 Different towing positions 75
5.9 'Coanda' effect 75
5.10 Total bollard pull in tons and average number of tugs for container and general cargo vessels 77
5.11 Total bollard pull in
tons and average number of tugs for tankers and bulk carriers (based on length overall) 77
5.12 Total bollard pull in tons and average number of tugs for tankers and bulk carriers (based on deadweight) 77
6.1 Effect of following water when passing through a channel with a deep loaded ship 81
6.2 Schematic flow - unsteady flowfield as felt by an observer in a stationary tug seeing a ship approaching 82
6:3 Pressure pattern and relative flow field around a bulk carrier 82
6.4 Interaction effects on a tug when proceeding along a ship 83
6.5 Effect of flow pattern around a ship on tug performance 85
6.6 A: Tug is waiting for the approaching ship to come closer to pass the tow line 87
6.7 Girtiog and tripping 88
6.8 Some specifi c manoeuvres by conventional tugs towing on a line including risk of girting or capsizing 89
x THE NAUTICAL INSTITUTE
Figure Title Page
6.9 D ue to excessive speed a tug at a ship's side may capsize if the stem line cannot be released 90
6.10 Due to low powered tugs and a strong beam wind, a container ship is drifting 91
6.11 ADS-tug 'Smit Marne 93
7.1 Radial towing hook with rail track 94
7.2 Radial towing hook of conventional twin screw tug Saona, Dominican Republic 94
7.3 After deck of a conventional tvvin screw tug with a to vving winch , i th radial system _ 95
7.4 Additional fairlead/towing point near the stern of combi-tug Hmdrik: P. Goedkoop 95
7.5 Two different gob rope systems 95
7.6 Conventional single screw tug Adelaar 96
7.7 After deck of ASD-tug Maasbank 96
7.8 Standard hook and a disc-hook with spring shock absorbers and different quick release systems 97
7.9 Single drum towing winch of azimuth tractor tug Iixelbank 97
7.10 Waterfall winch on board RTSpirit 98
7. 11 The friction drums of a traction winch 98
7.12 Split drum winch of the ASD-tug Melton 98
7.13 D ouble winch forward on the reverse tractor tug]ohn 99
7.14 Steel wire construction 102
7.15 Typical minimum breaking strengths 102
7.16 Fibre rope components and constructions 103
7.17 Table giving comparative weights and minimum breaking loads of eight strand ropes of different fibres 105
7.18 Table showing some characteristics of different fibre types 105
7.19 Tug reaction time and manoeuvring space required depending on towline length 108
7.20 The effect of different tow line lengths 108
7.21 Tug operating broadside 109
7.22 Static force in a a to, line 109
7.23 Two conventional twin screw tugs, Smit Ierland and Smit Denemarkm 109
7.24 VS tug Matchless II I
7.25 Reverse tractor tug Charles H Cates 1 _ 112
7.26 Quick release hook used on ferries of North Sea Ferriesfor securing a tow line when a tug is required 112
7.27 Automatic hook up system, Aarts Autohook 113
7.28 Typical emergency towing arrangement 114
7.29 One of the emergency towing systems in three phases of deployment 115
8.1 Simulator layout with five bridge manoeuvring simulato rs, a VTS simulator and instruction rooms 116
8.2 Desktop computer program Tug.Master, developed by The Glosten Associates, Seattle, USA 121
8.3 Bridge layout of a fullmission bridge simulator 124
8.4 Simulation track plot of a loaded tanker entering a port from the sea 125
8.5 Simulated ship and assisting tug passing a bridge 126
8.6 Schematic diagram of an interactive tug operations simulator 127
8.7 Field of view required for interactive tugs 128
8.8 Relationship between direction of view and control handles for an interactive tug with a 2250 out-of-window view 128
8.9 Heeling angle is an important factor in tug limitations. Twin screw tug Smit Siberiii 129
8.10 Model and model tank test for escort tugs to obtain hydrodynamic data, optimise tug design 132
8.11 Model and mod el tank test for escort tugs to evaluate performance 132
9.1 Major oil spills from tankers and their causes: No. of incidents & volume, World, 1976-89 134
9.2 Typical effect of frequency reducing measures 135
9.3 Direction of forces applied by assisting harb our tugs 137
9.4 Photographs taken during escort trials in Prince William Sound, Alaska, August/ September 1993 139
9.5 Terminology relating to direct and indirect towing methods 140
9.6 The reverse-tractor tug LynnMarie 140
9.7 Maximum direct bralcingforces azimuth drive 141
9.8 Approximation of steering forces of a 36 tons tractor tug 141
9.9 Definition sketch offorces on a tug and a ship 141
9.10 Importance of proper locations of centre of pressure and towing point.. 142
9.11 Aquamaster escort tug concept - The Towliner with towing arch l43
9.12 Steering forces required based on 15° rudder angle 145
9.13 Rudder forces in tons for different loaded tankers, speeds and rudde r angles 145
9.14 Tug Lindsey Foss applying steering forces in the indirect mode 146
9.15 Plots of a full scale trial with the loaded 125,000 dwt tanker Anofuraou and the pwpose built escort tug Lindsey Foss 147
9.16 VS escort tug Bess with modified tractor tug design 148
9.17 Specially designed tanker stern fittings on the former ARea tankers, now Polar tankers 149
TUG USE IN PORT xi
Figure Title Page
9.18 The Foss Transom Link 151
9.19 Ttvc escort tugs of towing company Foss Maritime keeping pace with a ship 153
9.20 Large VS escort tug Garth Foss .......................................................................................................................................•........... 154
9.21 A selection of escost(-ing) tugs at different ports. Situation 2002 155
9.22 VS escort tug Ajax 156
9.23 Powerful ASD escort tug Hawk 157
9.24 Can the escort tug prevent a grounding? 160
10.1 Novel new tractor tugdesign ·..vith sketch of the original shunters 164
10.2 Taiwanese reverse tractor tug No 3 Iao-Yu 164
10.3 The optimum harbour tug concept 164
10.4 ROTO R escort tug concept 164
10.5 The Rotor escort tug RT Magic 165
10.6 Modified ROTOR tug concept with aft thruster located more aft, behind the aft towing point 165
10.7 Typical assist modes with a ROTOR tug 166
10.8 SDM New River of Seabulk Towing (USA) 166
10.9 Side view of SDM Mark 11 167
10.10 Bow view ofSDM 167
10.11 Assistmodes SDMs 167
10.12 Characteristics of Design A and Design B of the carrousel tug 169
10.13 Combi·tug Mullratug 72 169
10.14 Modified combi-tug Multratug 72 during full scale trials 169
10.15 Towing forces based on model tests 169
10.16 Carrousel tug outer port design 169
10.17 Damen ASD tug 2477 with an open docking skeg, extending as a closed skeg forward 170
10.18 Compact tugs. Common assist modes 17l
10.19 Example of a compact tug - Cape Pasley 17l
xii THE NAUTICAL INSTITUTE
Photo; SmitIntmulJionnl
Three difJerrnJ tugtypes wwing ona line. The lugs portsilkforward andstarboardside aft are VS tugs of35 tons bollard puU. Ttutugstarboard side
forward isa twin screw conventional tugof37·5 tons bollardpuUandtheport tugaft isanASD-tugof62 tons bollasdpulL
When the tanker hastoberth starboardsilk a1mlgside the j etty, the ASD-tugandthe VS tugportsilkforward can, whm near thebmlt, easily dlange
to apushingpositian orpush-puU witlwut rekasing thetowline
TUG USE IN PORT xiii
Chapter ONE
TUG DESIGN FAcroRS
1.1 Differences in tug design and
assisting methods
METHODS OF ASSISTANCE PROViDED BY 111GS in ports and
port approaches around the world differ due to local
conditions and specific situations and have often grown
from long standing customs and traditions. These
differences in assistance methods and practices are often
reflected in the requirements for the tugs and hence in
the development of a range of tug types.
Over the past few years rapid development has been
observed amongst harbour tugs. New types have been
designed with high manoeuvrability and considerably
increased engine power. Modem steering devices, new
towing appliances and new materials for towlines , to
name a few, have been fitted. These developments affect
methods of tug assistance and the number of tugs used.
Following the Exxon Vald~disaster, the
requirement to
escort tankers in certain port approaches has resulted
in the development of specially built escort tugs.
As a result of the improved manoeuvring capabilities
of modern ships on the one hand and the improved
towing performance of modern tugs on the other hand,
the number of tugs required for assistance in port areas
is decreasing, Due to economic factors shipping
companies are facing, captains and pilots are often under
pressure to use the minimum number of tugs.
This reduction in the number of assisting tugs per
ship places the individual tug in a more crucial role . It
requires a high level of operational safety and reliability
from the tug and a high level of suitability for the job to
be carried out
In order to keep a port's tug services up to date and
to ensure safe, smooth shiphandling it is essential to keep
abreast of developments in harbour towage and
shipping, to have the most suitable tugs available and
to have well trained crews for the specific situation in
the port. This is all the more important when the
investment required for new tugs is so very high. Itmay
be necessary to reconsider the traditional approach.
It requires extensive research and knowledge of tugs
before an answer to the question "which type of tug or
which working method is the best for a certain port"
can be given. It requires a profound knowledge of the
different tug types, their capabilities and limitations, and
a good insight into the local situation.
The capabilities and limitations of different tug types
are dealt with in the following paragraphs.
The operational requirements that harbour tugs must
conform to with respect to ship assistance are mainly
determined by the following factors:-
The kind of port or harbour and approaches,
foreseeable future developments and the existing
geographical environmental conditions.
The type of ships calling at the port.
The services required in and around the port and, if
relevant, at offshore locations, e.g, SPMs, F(P)SOs
or oil rigs.
l'Iw"',/'brl of~"'""'P I G.w. O>olnu
FW'" 1.1 Prn1 ofAntwerp. ZondvlUtsluiQrz. Tugs s!wuld beobI. toassist ships tIorough the loda and bridges.
TUG USE IN PORT 1
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Highlight
l'r1rts underdevelopment
In many ports , developments take place such as new
berths or harbour basins and new ports are still being
designed. At an early stage it is desirable that tugboat
companies and pilots should participate in design studies
for these new ports, harbour basins, terminals, etc. In
this way tugboat companies and pilots can give advice
based on tbeir experience of shiphandling with the
available harbour tugs. Moreover, tug companies can
take account of these new developments when ordering
new tugs which are suitable for the new situation.
Regular consultation between port authorities, port
designers, tugboat companies and pilots will favourably
affect the accessibility of ports and harbours .
In container ports, especially where space is limited,
the requirement for large land space to stack containers
may not correspond with the minimum manoeuvring
area required for ships and tugs. Specific requirements
for tug assistance may be necessary, such as the type of
tug , engine power, towing equipment and assisting
method.
PADto: .4. 1II1uK-
Figure 1.4 Tug arsisting in open sea dos« toPOTt entraTla
As already mentioned, ports
close to the sea may be influenced
by waves and swell, leading to
additional requirements for tugs.
The same applies to tugs that have
to operate at offshore locations or
in ports in colder areas where ice
may be encountered. Limited
water depths in port areas where
harbour tugs have to operate will
give rise to special requirements
with regard to a tug's maximum
draft.
1.2.2 Environmental conditions
Port approaches
Port approaches are under the influence of the open
sea and can be wide or narrow, with sandy or rocky
banks, winding or straight entrances. Depending on the
local situation, tugs may be used in the port approaches
and should he capable of working in more open sea
conditions with waves and!or swell. Following the
Exxon Va/de'1: disaster there is a growing tendency to
require an escort for oil and gas tankers in port
approaches. Tugs used for escorting must comply with
very specific requirements.
Geographical environmental conditions are very
important from a tugboat company's point of view. The
majority of older ports are situated in river estuaries
and are particularly subject to the influence of tides or
seasonal effects. Fairways and rivers are constan tly
subject to changes. Differences in water depth, bridge
passages and lock entries may require the adoption of
time windows. The accessihility of these ports, therefore,
can be rather complicated. Tugs have to handle ships
,---- ------- ----------------- -.., safely and efficiently, Especially in
these ports, therefore, the
requirements to which a tug must
conform may change continuously
from the entrance or approach up
to the berth and the final mooring.
In some ports this problem is
solved by using different types of
tugs for the various parts of the
route .
Fwn' 1.3 M.T 'Capitol' berthing atJetty 4 at Sullom ", e OilTmninal
TUG USE IN PORT 3
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The usual course taken by vessels through a harbour or coastal waters
Phot.o:KOTUGIFotostUlh:oRijnmond Robert Nagtllurlu
Tug assistance always includes risks for the tug and her
crew. These risks can be minimised by good training
and by a well designed and equipped tug. The type of
tug also influences the level of safety. Depending on
the type of port, the environmental conditions, the ships
assisted, the assisting methods and the port regulations,
the safety requirements may differ by port. On the other
hand tug owners should require, regardless of the port
situation, the highest level of safety, which could dictate
a certain type of tug and tug equipment.
1.2.7 Safety requirements
The method of assistance used by tugs will depend on:
Port,jetty, terminal layout andloroffshore installation.
Types of ship .
Environmental conditions.
Navigational complexi ty of river, channels and port
approach.
Whether bridges and locks have to be passed.
Often on tradition.
Figure 1.7 Azimuth tractor tugs (53 tonnes hollardpull) of theKaruG towingcomparry towingan oil rig.
Dependingontheport, harhour tugs shouldalso be able tohandle offihare equipmen~ barges, fWaling cranes and soon.
These activities also demand a specific type and size port. They have built up their exp erience with these
of tug, as well as specific manoeuvrability, equipment tugs and with the tug's crews.They know the advantages
and towing methods, as is the case with tugs that have and the shortcomings of their tugs and are thus able to
to operate, for example, at SPMs, F(P)SOs or at oil rigs. anticipate. Changing over to a new system or to a new
type of tug may be associated with difficulties, will take
time and should be weighed carefully. Training and
instru ction will be needed, especially when the type of
tug and the way it operates is totally different from the
existing system . A well planned changeover to the new
system will be ne cessary. All this should be taken into
account when considering the introduction of a new
tug typ e or assisting method.
12..5 Assisting me th od In use
The type of tug used is largely dep endent on the
assisting method. Tugs have to meet, as far as possible,
the requirements related to the assisting method. The
assisting method may also depend on the availability of
mooring boats . When no mooring boats are available a
ship has to be brought up very close to the berth or
even alongside to be able to pass the mooring lines . In
these circumstances tugs should be able to push at the
ship's side.
1.2.6 Available experience 1.2.8 Summary
Pilots and tug captains are accustom
ed to the assisting
methods used in the port and to the types of tugs in the
No port is the same. Many factors influence the choice
of typ e of tug. such as local customs , environmental
TUG USE IN PORT 5
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CADE A PAGINA 6:
1.5 Conclusion
It is clear that no port is the same with respect to tug
requirements. Port layouts differ, as do the types of ship
frequentiog the port, the environmental conditions, local
tradi tions and consequently the types of tug and the
assisting methods.
Wh en a new tug is nee ded a simple answer to the
que stion "which typ e of tu g and/ or which towing
method is most suitable for the port" cannot easily be
given. Too many factor s play a role. It takes reliabl e
research, weighing all the advantages and disadvantages
against each other, in order to establish the requirements
for the most suitable tug for the port. Most important is
Factors influencing harbour tug choice
no t only wha t forces have to be considered but how,
when and under what conditions and circumstances,
such as ship's speed, co nfine ment, environm ental
conditions and underkeel clearance. This is the way
more and more modern ports and/or tug companies
work nowadays. Th e outcome may be a tractor type
with azimuth propellers or Voith-propulsion or even a
conve ntional type of tug. Escorting of tankers will set
additional requirements.
On the other hand tug owners want to operate as
few different types of tug as possible and prefer that th e
available types are put into action as frequently as
possible. Harb our tugs should, therefore, be as versatile
as possible.
Other
Passage! Environmental Types of Services Assistin g Existing AvaUable Safety Financial
Berth Conditions Ship required Methods Th", Experience ofThgs Aspects
Sea/Approach Swell General Offshore Towing Conventional Tug Tug Budget
cargo installation s on a single type type
River w aves ships line screw experienc e Tug
Barges Portl price
Channel Wind Container Push-pull Conventional Assisting State
vessels Floating twin methods regulations O perating
Waterdepth Current cranes Alongside screw experience costs
Car towing Classification
Locks/Bridges lee carriers Dockyards Tractor regulations
Escorting tugs VS
j etties in Fog Ro-rc Escorting Environmental
open sea ships Tractor conditions
Jetties in
tugs
Tanke rs! Azimuth
protected VLCCs
water ASD tugs
Gas
Harbour basins tankers Reverse
tractor
Terminals Bulk tugs
carriers
River berths SDM s
Ferries (Ship Docking
Mooring buoys Modul es)
Passenger
Mooring boats ships
Figur« 1.9 Harhour tugs - faCUITS influencingchoice
TUG USE IN PORT 7
TYFEOFTUG
I I
Propulsion forward J Propulsion aft J
Tractor tugs J Conventional '--type
ASD-type * f---
'-- Voith (Multi tug)
Schneider
Reverse-tractor f---
L-- Azimuth typ e
propellers
Combi * f---
type
• Tugs that can operate as a conventional tug and as a reverse (tractor-) tug
Note :
The ROTOR tug discussed in par. 10.1.1 is in fact a tractur tug with a dynamic skeg, being a third thruster.
The SDM (Ship Docking Module) discussed in the same paragraph does not belong to any of the categories
mentioned above.
Figure 2.1 Main types ofharbour tug
8 THE NAUTICAL INSTITUTE
Chapter TWO
TYPES OF HARBOUR TUG
2.1 Classification of harbour tug types
T UG TYPE S ARE NMIED AITER THEIR MAIN CHARACTERISTICS,
i.e. the type of propulsion, propulsion manufacturer,
location of propulsion or steering system. Names include
conv entional tugs, Voith-Schneider tugs, Z-peller tugs,
Kort nozzle tugs and tractor tugs, amongst others. There
is no uniform naming system in use and this can be
confusing. For example, when talking about a Z-peller
tug, what is meant? Is this a tug with azimuth propellers
forward or with azimuth pro pellers aft? The difference
does not see m so great, but considering tug performance
while rendering assistance, it is. Afte r all, that is what
tugs are used for - to render assistance. As will be seen
later, it is better to classify tugs according to their location
of propulsion and towing point It makes things easier
to understand.
Naming tugs this way there are only two main
classifications , which can be groupe d as follows:
a) Tugs with their propulsion aft and towing point
ncar midships. These are basically conventional
types of tug .
This category includes all normal conventional types
such as single scr ew and twin screw tugs.
b) Tugs with their towing point aft and propulsion
forward of midships. These are tractor tugs.
In this catego ry are:
Tractor tugs with Voith propulsion .
Tractor tugs with azimu th propellers.
T here are intermediate types of tug that can be
classified either as conventional or tractor tugs,
depending on the way they operate. These are:
Reverse-tractor or pusher tugs - tugs with azimuth
propellers aft and towing point forward, built to
operate mainly over the tug's bow, as can be see n
for example in J apan, Hong Kong and Taiwan.
Tractor tugs normally work with their towing point
- the tug's stern - towards the ship and their
propellers - near the tug's bow - away from the ship.
Reverse-tractor tugs operate in -the sam e way
regarding the towing point and th e pro pellers,
consequently the tug itself lies in the reverse direction.
Azimuth Stem Drive (ASD) tugs. These are multi-
purpose tugs with azimuth propellers aft which are
built to operate over the tug'sbow as a reverse-tractor
tug as well as over the tug's stern like a conventional
tug. Most ASD -tugs have a towing winch forward
and one on the after deck while some have simply a
towing hook instead of a towing winch aft. Because
an ASD-tug can operate as a'reverse-tractor tug, it is
often m entioned together with reverse-tractor tugs.
Although the term ASD-tug is frequ ently used, it is
not such a good name, b ecause rever se-tractor tugs
also have azimuth propulsion under the stern . Multi-
tug is a b etter name.
Modified older tugswith a360° steerable bow thruster
(combi-tugs) and equipped ....,th an additional towing
point at the after end of the tug. These tugs can
operate as a n ormal conven tional tug or like a tractor
tug when using their aftermo st towing point.
So the following types of tug can be seen, all
belonging to one or both of the above groups:
Conventional tugs.
Trac tor tugs with azimuth propellers or Voith
prop ulsion.
ASD-tugs.
Reverse-tractor tugs.
Combi-tugs.
The table in figure 2.1 gives an overview of the
classification of harbour tugs.
The re are, of course, m any d iffer ences in
constru ction , hull de sign, propulsion and rudder
configuration and so on with in each tug type. The
different types of tug are therefore discussed in more
detail star ting with some general aspects regar ding tug
performance and safety of operations.
2.2 Important general requirements for
good tug performance
For good harbour tug safety and performance, the
following factors are important:
2.2.1 Thg performance and safety
Response time
Harb our tugs should have a short respo nse time and
their manoeuvrability should be such that the tug can
react in a minimum of time. It is therefore im por tant
that measures are taken to increase the manoeuvrability
of harbour tugs and shorten their response time .
Not only is a short response time required when
assisting a vessel, but also for making fast. Due to ever
d ecr easin g number s in a ship's crew, th e time taken to
make tugs fast is increasing. Thus the requirement for
tugs regarding fast an d easy han dli ng o f towing
equipment becomes an e lement of in creasing
importance in orde r to im prove their response time.
TUG USE IN PORT 9
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near stern of a ship 
Effectiveness and safety ofoperations
It is not only manoeuvrability,
but also bollard pull
and underwater shape that make a tug effective and
th erefore suitable .Ior th e j ob . For example, lar ge
containe r vessels with containers stacked six high on
deck need powe rful tugs in case of strong wind s. Wh en
a ship is underway at spe ed, loss of tug's effectiveness
due to th e ship's speed and/or towing direction should
be as small as possible. The effectiveness and safety of
a tug is also related to factors such as the tug's stability
and suitability of towing equipment.
Required manoeuvring space
Th e manoeuvring space required by assisting tugs
sho uld, depending on the situation, be as small as
possible . This ca n be achieved b y good tu g
manoeuvrability, limited tug dimensions and proper
towing equipment.
Other practical aspects of importance for good tug
performance and safety of operations are as follows:
2.2.2 Wheelhouse construction and layout
Visibility
A tug'swheelhouse should be placed and constructed
such that, at his manoeuvring station, the tug captain
has a good view of the tug's fore and aft ends and tug
sides . He must also have a good view of:
The towline and towing equipment.
The working deck.
Contact areas between tug and ship.
The assisted ship.
Other assisting tug boats.
The direction of ope ration.
This requires a field of view at the manoeuvring
station(s) as unobstructed as possible, with an angle of
view as close as possible to 360°.
TIu HQrIg KongSalvagt & 10wage Co . Ltd.
Figure 2.2 Pusher tug 'Lam Tong' (l.o.a 26·7m, beam 805m, bp
4JT) with a cockpit uhedhmue. S'" has vertical andhorizontal
heauy dutyfentkring withwater lubrication at thebowplur vertical
sum andhminmtal side fender systems
10 THE NAUTICAL INSTITUTE
In addition to the all round view, well designed
whee lhouses also have small windows that face upwards,
which is important when making fast to vessels with a
high forecastle, stern or freeboard. On some modern
tugs very small wheelhouses are constructed wi th large
window s and a nearly 3600 view.
Manoeuvring stations
When making fast to a vessel and while assisting, a
tug captain should be able, in one glance from his
man oeuvri ng statio n, to see the mo st essentia l
information available from outside, without jumping
from side to side in his whee lho use and without getting
painful legs, neck or back. T he ess en tial outside
information co mes from:
a) The towline(s) - their direction and tension.
b) The assisted ship: such as relative heading and speed,
distance off and the way the assisted ship reacts to
th e applied tug forc es. When pu shing, essential
information also comes from the co ntact area
between tug and ship.
c) The combined ship/ tug direction of movement with
regard to channel or fairway boundaries, other traffic
and nearby berths and banks.
Depending on the typ e of tug and th e assisting
method in use, this essential information may come from
totally different or eve n opposite directions. The
directions may change during one and the same trip
and are dependent, in any case, on the assisting method .
In a reverse-tractor tug, which is assisting from over the
tug's bow, nearly all the essential outside inform ation
com es from forward and should be available in one
outside look from the manoeuvring station. T his can be
achieve d with one forward facing statio n . If the
manoeuvring station is well planne d, the tug captain
may have an unobstruct ed view in th e working
direction , even from a seated po sition, of the winch ,
working deck, bow and side fend ers and th e assisted
ship.
For all other typ es of tug and/or other assisting
methods th e visibility re quirements m ay be totally
different. For instan ce, a tractor tug used for push-pull
operations works over th e stern. The n an aft facing
manoeuvring panel is needed. When the same tug is
free sailing a forward facing man oeuvring pan el is
required , Depending on the wheelhouse construction,
a central manoeuvring panel for this type of tug could
be useful, capable of being operated in both directions,
forward and aft. On other tugs more manoeuvring
panel s may be required, of course, depending on th e
wheelhouse size and construction . Som e harbour tugs
even have three manoeuvring panels facing forward and
one facing aft. Care should be taken in order that reliable
change-over between manoeuvring panels is possible
without the risk of failures or mistakes.
Co n trols at the man oeuvring panels should be
arrange d such that they can be operated in a logical
way in relation to the tug's direction of movement.
Pushing a lever down and away in the direction the tug
cap tain is facing should result in an incr ease of
movem ent in that direction. Turning a wheel or moving
a joystick to the left should turn the tug in that direction,
rega rdless ot"whether the direction of movement is
ahead or as te rn. Any illogical way of control or
complexity in control easily lead s to human control
failures, particularly when under tension .
It is clear that the wheelhouse layout and the numb er,
location and orientation of manoeuvring pan els depend
largely on the type of tug and the usual assisting method
and should be carefully con sidered, also taking into
accoun t the op tim u m vi ew needed fr om the
manoeuvring station when coming alongside a ship or
berth. Modern tugs so me times have one cent ral
manoeuv ri ng panel in an optimal designed small
wheelhouse, like a kind of cockpit.
At th e manoeuvring stations the captain should also
have a good view of his instruments, including the radar.
Communication and quick release systems, which will
be discussed later on, should be within hand reach at
all manoeuvring pan els. Towing winch control from the
wheelhouse is also recommend ed for harb our tugs. The
Photo:Author
Figure 2.3 Plan of tlu navigationbrit!t,e deck and viewof the
wheelhouse of a modern HongKongpusher tug. Thecaptainis
handlingthepropellercontrols andthe mate thetowing wind!
towline length ca n th en alway s be adjusted wh en
required without calling a man to the towing winch .
The number of crew members on modern harbour tugs
is very limited nowadays.
Communicatio n
Good co-operation between the pilot and tug captain
is a basic requireme nt for safe and efficient shiphandling
with tugs. Such co-operation is only possible with good
procedures and efficiently wo rking communication
systems. Radio communication systems on board tugs
sho uld the refore be reli able. A double VHF set is
recommended.
2.2.3 'lUg superstructure and underwater d esign
Tugs regularly have to work near a ship's bow or
stern, whe re the flare and overhang are often fairly
pron ounced. It is necessary, therefore, that the tug's
superstructure is located well inboard of the deck edge ,
so that risk of tug damage can be avoided as much as
possible when working near the ship's bow or stem or
when the vessel or tug is rolling when alongside a ship.
Underwater design of the tug should be such that
the propulsion units will not hit the ship's hull when
th e tug is rolling alongside. In this regard harbour tugs
have to assist all kind of vessels, including submarines
in some ports. Tug propellers may hit the submarine
hull whe n a tug is required to come alongs ide for
assistance or for bringing the pilot on board. In that
case Single screw harbour tugs were usually best.
2.2.4 Fendering
Tugs should be equippe d with good fendering.
Appropriate fendering protects both the assisted ship
and tug from dam age and decreases the tendency to
slide along the ship's hull when the tug is pushing at an
angle to th e ship' s hu ll. Fenders are construc ted of
rubber or synthetic rubber products. Bey ond th e
m echanical requirements ofload versus deflection and
energy absorption, which is given in curves, attachment
methods and structural limits, con sideration
should also
be given to the material used in the fender. The material
used should have good resistance to pollut ed water,
ozone, UV radiation and high and low temperatures.
The following factors are of importance in the choice
of a tug's bow and/or stem fendering:
The way the tug is assisting vessels, for instance
towing on a line or push-pull, and whether the tug
will push by the stern andlor by the bow.
The size and engine power of the tug which are
important factors for the horizontal load an d kinetic
energy transmitted durin g contact and pushing.
Size of contact area.
The type and size of vessels to be handled e.g. ships
with large bow flare and/or overhangin g stem. Tugs
TUG USE IN PORT 11
pushing near the bow or stem of these ships may
need extra fendering on top of the bow to prevent
damage to tug or ship.
The environmental conditions such as waves and
swell. Th ese conditions will give rise to addi tional
forces in the fendering, for which it must be able to
compensate .
The tug's bow and stem construction.
Tug fendering varies enormo usly. One frequently
used fen der sys tem is the extru de d profil e typ e.
Extruded fend ers are produced in different lengths and
in a wide variety of profile s and sizes. They can have a
hollow D-shape profil e, can be rectangular, cylindrical
or solid, can be precurved to fit the tug bow or stern, be
chamfered or drilled. Extruded fenders are very flexible
from the point of vi ew of d esign . Extrusion is a
manufacturing method whe re by un cur ed rubber is
forced through a die to produce th e required profil e
and then the lengths offormed rubber are vulcanised.
Moulded m odular or block fend er systems offer
many of the advantages of extruded fenders and, in
addition , allow for secure attachment and ease of repair
since with this type individual blocks can be replaced.
Photo: Schuyler Rubber Co. lnc., USA
Figure 2.5 Bowfender made ofreinforced truck tyres
specific size and compressed onto steel supporting rods.
This fender type, made in the USA, is suitable for bow
fend ers, stern fenders and side fend ers. There is one
specific type which has a large absorption ability an d is
very soft, thus having a large contact area and 'sticking
ability' when under load.
The following is an indication of some permissible
hull pressures, which vary by ship' s type and size:
Tugs may also be fitted with foam-filled or pneum atic
fenders, especially wh en working in exposed ar eas.
Sometimes 'non-marking' fenders are required, for
instance when ships with white or grey hulls have to be
handled , such as cruise or navy vessels . In that case
manilla rop e fend ers, in addition to the standard tug
fendering, may still be used or the tugs may b e equipped
with grey rubber fendering.
Bow fend ers should have a large contact area and
radius to reduce th e pressure on the ship's hull . The
same applies to the stern fend ers of tractor-tugs since
these tugs are pushing with the ir stern . Tyres are often
used in additi on to bow and stern fenders to protect the
fenders and enlarge the contact area and are often used
along tug sides since they can easily be replaced when
damaged.
<200 kNlm'
< 350 kN/m'
<300 kNlm'
< 250 kNlm'
150-200 kNlm'
400-700 kNlm'
Oil tankers of m ore than
60,000 dwt
Container ships:
3rd generation
4th gen eration
5th and 6th generation
(Superpos t Panamax)
VLCCs
General cargo ships of
20,000 dwt and less
RoyalBakkerRuM", TheNetherlands
Figure 2.4 1jpicalfi nder arrangement f or a tugpushing
underswellconditionsand/or atflaringparts of a vessel, consistingof
vertically instal/ed moulded blocks and horizontal hollowcylinders of
. . - extrudedrubber
A tug's bow andlor stern can be equipped with
horizontal fendering, for instance extruded fenders of
cylindrical profile, or with vertical block fendering. A
combination of these types is often used. Block fenders
can easily be replaced when damaged, and for fend ers
on bow and stern which are intensively used, basic
vertical block fendering is very suitable.
Weldable fenders with steel backings are yet another
fender type, used when very secure attachment is
required.
Other types of fendering include tho se made of
reinforced tru ck or aircraft tyres which are cut to a
12 THE NAUTICAL INSTITUTE
Photo:Author
Figure 2.6 Trw usedinaddition tot mtical bowfendmng
Fender mater ial should have a large coefficient of
friction in order to keep the bow or stern in position
when the tug is pushing under an angle to the ship 's
hull. Sliding along the ship's hull , tug berth or alongside
other tugs, and rolling and pitching along the ship's side
due to waves will easily damage tug fend ers. To avoid
early damage of the fend ering, as for instance the side
fend erin g, or where no grip is required , fenders can be
used with a low friction coefficient or can have a top
layer of UHMW polyethylene, which has an extremely
Further relevant inform ation for harb our tug design
in general and for ASD-tug design in particular can be
found in 'Designers' Che cklist No 1. Azimuth Stern
Drive Tugs (ASD)' (see References).
Harbour tugs assisting submarines may also hav e
underwater fendering to avoid contact damage to th e
submarine's hull. In addition, an ASD or reverse-tractor
tug's hull may be expanded with fendered steel sponsons
on the quarters to ensure that th e nozzles of the tug's
azimuth propellers never come in contact with the
submarine being assisted, the so-called 'propulsion uni t
protective sponsons'.
The height of a tug's fendering above water level is
a factor to be considered. When pushing under an angle
at a ship's side while the ship has headway or sternway,
the hydrodynamic forces on the tug create a list. It is
evident that the higher the bow fender above the water
the larger the resulting heeling mom ent will be.
Specific types of fenders can be pro vided with water
lubricati on to reduce the friction between tug and ship
and so prevent damage and wear, especially when
pushing against a slab-sided ship in swell conditions.
This type of fendering can, for instance, be found on
tugs in the port of Hong Kong (see photo of the reverse-
tractor tug Lam Tong - figure 2.2).
< 200 kN/m'
Gas carriers (LNG/LP G)
and Bulk ca rrie rs
low friction coefficient. The coefficien t of friction of
rubber to steel is approximately 0·8. The friction force
F = c x P, where P is the impact force of the tug and c
the friction coefficient. UHMW polyethylene has a
friction coefficient of 0·15.
Suitable and reliable towing equi pm ent is also
important for good harb our tug performance and safe
working. This is dealt with in Chapter 7.
o
.,
Damm Shipyards. 17u NttJurlmuis
Frgure 2.7 Consentionaltwin screw tug - type Stan Tng 2909. L».a. 29-6m, beam 9-3m, bp depending on installed engint power:3D-60tons
TUG USE IN PORT 13
2.3 Conventional types of tug
2.3.1 General
The largest number of tugs still belong to this type.They
can be seen all over the world and are still built in large
numbers. Conventional tugs are used for push-pull
assis tance, alongside towing and in part icular, in
European ports, for towing on a line.
There is a large variety of conventional tugs. The
most simple one is a single screw tug with a single plate
rudder. Mainly due to the location of the towing point,
the tugs have limitations regarding performance and
safety. When towing on a line the main risk is of girting.
A towing winch with a quick release m echanism lowers
this risk. The same applies to a quick release towing
hook, if it works under the extreme condition of girting,
which is not always the case . The astern power of
conventional tugs is generally low. Wh en making fast
near the bow of a ship , interaction forces between the
ship and the tug should be allowed for, which can better
be done with tugs th at can produce good side thrust,
such as tractor-tugs. Girting and interaction are dealt
with in Chapters 4 and 6.
The towing point of these tugs generally lies about
0·45 x LWL from aft, although shorter distances may
b e found . The aft tow ing point on American
conventional tugs lies further aft, which allows the
opportunity to extend th e deckhouse further aft. A more
aft placed towing point limits the tug's effectiveness
when towing on a line at spee d but this way of towing is
not normal pr actice in the USA.
In USA ports whe re tugs are used for towing on a
lin e, conventional tugs can be found with a more
forward lying towing point.
Experience is an important factor in handling
conventional tu gs safely while assisting ships under
.speed and with a well qualified captain these tugs can
be very effective while rendering assistance. To increase
the tug' s effectiveness and/or manoeuvring capabilities
there are several possibilities, as explained below.
2.3.2 Propulsion and rudders
Propulsion and propeller control
Nearly all tugs are equipped with diesel engines
th ough an occasional old harbour tug with a steam
engin e may still be found somewhere outside a maritime
museum. Diesel engines on harbour tugs are high or
medium speed engines. The high engine revolutions
have to be brought down by reduction gearing to the
required propeller revolutions. To reverse the propeller
thrust, different systems are in use. .
The direct-reversing system is the oldest and can still
be found on som e conventional tugs. The engine ha s to
14 THE NAUTICAL INSTITUTE
be started on ahead and on astern . On some tugs engines
can be controlled from the whee lho use, while on others
it still has to be done by an engineer. The number of
manoeuvres is limited by the volume of starting air
. available . The response time from ahead to astern and
back differs by tug and by the direct-reversing handling
system fitted.
Diesel-electric propulsion systems can still be found
in some harbour tugs. The diesel engin e(s) drives electric
generators \v·hich in turn drive electric motors . These
electric motors drive the propeller. This system is easily
contro llable from the wheelhouse . It has th e large
advantage that it can deliver any propeller shaft speed
ahead and astern without delay. The system is exp ensive,
though . It has high initial costs and higher maintenance
expenses compared to oth er systems .
Most commo n now adays on harbour tugs are high
and medium speed diesel engines with reduction gears
and pneumatic-hydraulic couplings. (See References for
'Operational benefi ts of hi gh-speed electronic diesel
engines'). Other type s of couplings can be used. On
tugs with fixed propellers the propeller thrust is reversed
by m eans of a reverse-reduction gear, while on tugs with
controllable pitch propellers (cpp) thrus t is re versed by
changing the prop eller pitch .Torqu e pro blems may arise
when a fixed pitch pro peller is reve rse d at high tug
speeds. These problems can be re duce d or overcome
by proper design (= the correc t combination of engi ne,
prop eller and gear) and tuning ofthe whole propulsion
system. Shaft brakes are used, depending on engine and
propeller type.
Engine revolutions and propeller pitch are remotely
cont rolled fr om the whee lhouse . Manoeuvring,
espec ially with a cpp, is very smooth. When th e cpp
control system is equipped with a combinator control,
prop eller revolutions are regulated in accordance with
propeller pitch . The pitch of a cpp js regulated by a
hydraulic system. Cpp control systems, including remote
control systems, th e hyd rauli c system and emergency
stop require regular checkup s and goo d maintenance .
Failure in the hydrauli c or remote control system can
cause serious damage to tug, ships or berths. Modem
cpp systems have reliable backup systems.
Propeller effidency and manoeuvrability
The propellers of conve ntio nal tugs can be fitted in
open frames or fitted in nozzles. Going full astern, an
open fixed pitch propeller will - in general - deve lop
about 60% of its maximum ahead thrust. An open cpp
going astern develops some 40 to 45% of maximum
ahead thrust. The lesser efficiency astern of a cpp has
to do with the specific design and working of a cPl' .
Propellers are designed for maximum efficiency going
ah ead . A fixed pitch propeller will turn, when astern
thrust is requ ired, with the same pitch in the reverse
direction as on ahead. The propeller blades of a standard
cpp have a sm aller width near the hub and the refore,
when the blades are set for ahead, a larger forward pitch
angle than near the tip of the propeller. When the blades
are turned for astern thrust, the lower part of the blades
will consequently have a smaller pitch than the top of
the blades , which results in less efficiency going astern
compared to a fixed pitch propeller.
Nozzles increase thrust and consequently bollard pull
significantly. Mr. Ludwig Kort, an aerodynamicist,
designed the first nozzles as far back as 1927. The first
one was introduced into service in 1932 and was
originally designed to protect canal banks from propeller
wash. The effect of a nozzle is most pronounced with
high propeller loads at low speeds. Harbour tugs have
to perform in that way. Nozzles increase thrust by 15-
25% in towing and pushing conditions.
".
"
propellers and the same type of nozzles. With a specific
propeller design a much higher value can be reached
for astern performance of controllable pitch propellers,
but then ahead efficiency will be lower.
Note:
The 19A nozzle, and several variations more or less
on this design , are used for azimuth thrusters, either
with fixed or with controllable pitch propellers, because
astern thrust is achieved by turning the nozzle.
A nozzle seen on several tugs with azimuth
propulsion is the Nautican nozzle, which is the same as
the Lips HR (= high efficiency) nozzle. Ahead efficiency
of this nozzle is higher than of nozzle type 19A and 37,
approximately 8% in bollard pull conditions, while
astern performance of the Nautican nozzle is better than
of nozzle type 19A, but not better than of nozzle type
37. As said, astern performance is not relevant for tugs
with azimuth thrusters.
Conventional tugs with controllable pitch propellers
.n nozzles (nozzle type 37) achieve, whe n pulling astern,
tbout 45% of maximum ahead ballard pull, while this
gure is about 65% for tugs equipped with fixed pitch
Various types of nozzles (figure 2.8) have been
developed while research is still going on. Nozzle type
19A is very common because of its cost-effective design
and is typical for ahead thrust requirements. Nozzle type
37, a 'backing nozzle', has been developed to give better
efficiency going astern, which results in only a little less
efficiency going ahead. The same applies to the Hannan
Ring Nozzle, which is a normal type 19A nozzle with
slots cut in at the after end giving good astern thrust -
abou t 70% of the ahead value with-fixed pitch propellers
and special blades and 60-65% with ordinary blades.
Nozzle type 37 is a type of nozzle often used for
conventional harbour tugs.
Some twin screw tug s have two indepe ndently
controlled steerable no zzles, so increasing the tug's
manoeuvrahility furth er.
Nozzles can be also be steerable. Their manoeuvring
performance is superior to norm al rud der arrangements.
Rudder angles of no more than 25° - 30° are used due
to the greater side thrust. A tug's manoeuvrability when
going astern with a nozzle rudder system is very good.
When going astern the tug will swing to port or starboard
depending on the direction of the steering nozzle. A
vertical fin or a movable flap may be fitted at the end of
the steering nozzle. (see figures 2.g and 2.10).
Willi&&n. Iogrn;"";;",. G<rnum)
Figure 2.10 Construction
ofa suemble nozzk with moteableflap
Nozzles increase the efficiency of the propeller but
decrease steering capabilities. The fItting of a nozzle is
equivalent to increasing the lateral area of skegs. Special
rudder systems are therefore often used.
Figure 2.9
Steering
noales, one
with a
moteableflap
1M other with
aftxedfin
Figure 2.8
Twogenerally
usednoz:;:k typtJ
79A and 37
o . p
.-
+-j+-+- ~
TUG USE IN PORT 15
FlgUTe 2.72
Schilling rudder
Figure 2.73 Slwturrudder
lJsfmlwithafixed Male
andtwojlmJeingrudders
Flanking rudders
Flanking rudders are installed in front of the tug's
propeller and both single screw and twin screw tugs
may be so fitted. Flanking rudders are often installed in
conjunction with other rudder systems, such as a single
rudder behind the propeller or a Towmaster rudder
system and are especially used in conjunction with fixed
nozzles. In general there
are two flanking rudders
situated before the
propeller nozzle. The
flanking rudders are
operated by separate
controls and enhance
steering performance
when moving astern or
when towing astern on a
towline from the tug's bow.
When going ahead they
are kept amidships.
new tug Sayya/at Abu Dhabi. Schilling Monovecrudders
have no movable parts. Horizontal slipstreamguideplates
are fitted at the top and bottom of the rudder. The rudder
itselfhas a high liftblade profile with a wedgeprofile, so-
called 'fishtail', at the end of the rudder blade.The rudder
develops 30-40% more lift compared to a conventional
rudder and maximum lift is obtained at a rudder angle of
approximately 40°. The
rudder canbeusedup to 70°
angle and at this angle the
propeller slipstream is thus
deflected 90° and works
more like a side thruster.
When moving astern the
rudder is more effective
than normal rudders. With
a Schilling Monovec
rudder, turning on the spot
is almost possible while
speed is dropping very fast.
1bwmaste1' system
The Towmaster rudder system is a shutter rudder
type used in conjunction with fixed nozzles. It consists
of several rudders mounted behind and sometimes also
ahead (flanking rudders) of each nozzle. Behind the
nozzle are normally three and ahead of the nozzle two
Two Schilling rudders, called SchillingVecTwin, can
be used behind a propeller and make the vessel very
manoeuvrable. Eachrudder has a separate steeringgear.
The rudders can be turned by joystick a maximum of
105° outboard and 40° inboard. A maximum side thrust
of 70% of ahead thrust can be achieved. Depending on
the two rudder angles, it allows the degree of thrust from
a conventionally mounted propeller to be controlled
and the thrust direction vectored through 360°. Thus
the need to reverse the shaft direction or propeller pitch
is eliminated.
Conventional tugs can be single screw, twin screw
and even triple screw, e.g, the USA harbour tug Scott T.
Slatten. Manoeuvrability of twin and triple screw tugs
will, in general, be better than of single screw tugs.
Movableflap-rndthTs
There are several types of
movable flap rudders, such as
Becker, Barke, U1stein,]astram
and Promac Stuwa. At the end
of the rudder blade is a
movable flap, controlled by
linkage, comprising about 20-
30% of the total rudder area.
Maximum helm angle differs
by type and is about 40-50°.
Each type of flap rudder has its
own specific characteristics.
The flap angle is a function of
the helm angle and with a
Becker rudder, for instance, it
will be about three times the
l'/w1D..H",,<H..... G<muml main rudder angle for the
Figure 2./1 lower range and decreasing to
Fixed nome with a factor of two for the upper
a moveable flap rudder range rudder angles.
Maximum lift, which is achieved at a rudder angle of
approximately 30°, is increased by 60-70% compared
with a conventional rudder of the same shape, size and
area. Sideways thrust ranges up to 50% of ahead thrust.
At maximum rudder angle the propeller stream will,
depending on rudder size and balance, be diverted
approximately 90°. At speed the vessel can turn very
quickly and speed willdrop fast. When dead in the water
the vessel can nearly turn on the spot. Performance of
the rudder when the tug has speed astern is about the
same as that of an unflapped rudder. Tugs may have
more than one movable flap rudder behind a nozzle .
The manoeuvrability of conventional tugs can be
increased by the use of specific rudder types or rudder
systems. Several different rudder systems are in use,
often in combination with nozzles, such as:
Schilling rudders
Schilling rudders can also be found on tugs e.g. the
In general tugs are equipped with balanced, semi-
balanced or spade rudders. By far most tugs have
balanced rudders. Single plate rudders are also stillused.
With the spade, balanced or semi-balanced rudder the
leading edge of the rudder extends forward of the rudder
shaft. This , together with the shape of the rudder, results
in higher propeller efficiency and a lower steering
couple, so a smaller steering gear can be used. Spade
rudders are hanging free, are not attached to a heel,
and are consequently more stoutly constructed than a
balanced rudder. Single plate rudders decrease propeller
efficiency, need a higher steer ing couple and
consequently a larger steering gear.
16 THE NAUTICAL INSTITUTE
rudders. Rudder angles are possible up to 60°. The
Towmaster system provides good thrust and steering
characteristics ahead and astern at the expense of
increased complexity. Astern thrust can be more than
70% of ahead thru st. Even recently built tugs are still
equipped with this system , such as tugs of the Kuwait
O il Company, the tug AI-Hawtah of the Saudi Arab ian
Oil Co., tug Pegasus of the Mobil Refinery, Port Stanvac,
Australia and the tug Neeltje P and her sister tugs of
Terminales Maracaibo , Venezuela. The Michigan Vane
Wh eel used on some tugs in the USA is a comparable
system, with several high aspect ratio rudders, e.g. three,
behind a fixed nozzle; the same applies to the Nautican
High Aspect Ratio Triple Rudder system.
Plwt4: DammShJ.jJyo.uu, VIt Nttlterlands
Figure 2.14 Towmaster rudder sysUm oftug'Hamm'.L.o.a.38m,
beam 11m,Bp 70 tonsahead and50 tans astern
Other systems
Besides the rudder systems mentione d above, many
other systems exist, such as different types of fishtail
rudders and the pr eviously mentioned triple screw tug
Scott T.Allenwith her thr ee ru dders, of which the cen tre
rudder can be ope rated in dependently from th e
outboard rudders.
Bow thruster
Conventional harbour tugs are sometimes equipped
with a tunnel bow thruster. The effectiveness of a tunnel
thruster is not high when the tug has speed ahead. With
only two knots speed the effectivene ss of th e bow
thruster may already be reduced by 50%. Seagoing
harbour tugs operating in port areas as well as at sea for
offshore work often have a bow thruste r, which enables
them to keep position better near oil platforms.
Conv entional tu gs may be equipped wit h a
(retrac table) 360° steerable bow thru ster. Th ese bow
thrusters are much m ore effecti ve and can op erate in
any dir ection . Tugs with this kind of bow thruster are
the previously mentioned combi-tugs.
2.3.3 Manoe uvri ng conve ntional tu gs
Single screw tugs
Three aspects are im portant in manoeuvring a
norma l single screw conventional tug:
The aft location of the rudder and propul sion.
The transverse effect of the propeller when turning
for astern.
The low astern po\\'er.
When ahead thrust is appli ed with port or starboard
helm, the tug's stern moves in a direction opposite to
the intended direction of turn due to the aft location of
the propeller and rudder.This contrasts with tractor tugs
where the steering forces are applied in the direction of
turn . Thi s is a subject further dealt with in Chapter 6
when discussing interaction effects between
tug and ship.
Turning on the spot, or nearly on the spot, is only
possible with the previously mentioned high lift rudders.
No sideways movement ofa single screw tug is pos sible,
not eve n with h igh lift rudders, though sideways
m ovement is po ssible with high lift rudders in
co njunction with a bow thruster.
Th e transver se effect or 'paddle whee l effect' is
caused by the propeller wash hitting the stem at right
angles when the propeller is turning for astern. Nearly
all single screw tugs have a right handed propeller, which
means a clockwise turning propeller going ahead in case
of a fixed pitch propeller and an anti-clockwise turning
propeller in case of a controllable pitch propeller. When
the prop eller is set for astern, propeller wash hits the
tug's stern on the starboard side and the stem moves to
port - consequentl y the bow turn s to starboard. The
more sternway the tug has the more effective the rudder
is and it may even be possible to bring the tug onto a
straight course by applying rudder. The paddle whee l
effect together with the low astern power results in poor
performance going astern in single screw tugs.
When moving astern a tug's stern can be controlled
when the tug is equipped with a steering nozzle or with
Towmas ter or flanking rudders . Steering nozzles or
flanking rudders can be set for the direction the stern
has to move.
Twin screw tugs
Twin (or triple) sc re w tugs are much more
manoeuvrable than single screw tugs. They can turn on
the spo t without m aking headway and can easily
manoeuvre straight astern. Turning can be done by
reversing one propeller and setting the other for ahead
while applying helm in the intended direction.
Prop ellers of twin screw tugs, whether controllable
or fixed pitch, are often inward turning except on tugs
designed to operate in ice conditions. The advantage of
in-turning prop ellers is higher propeller efficiency. A
disadvantage with fixed pitch propellers is the larger
TUG USE IN PORT 17
turning diameter , because the starboard propeller is left
handed and the port.one is right handed. Wh en using
the propellers as a couple, the transverse effect of the
screws opposes the turn .
t
I
Figure 2.15 Twin screw tugmoving sidewaystostarboard,
also calledflanking, by sating 1M port engineonahead andstarboard
engine onas/ern while applyingport htlm. In tIucase of in-turning
fixedpit<h propellers the transverse thrust ofthe innerpropeller will
enlarge the side thrust tostarboard
With inward turning fixed pitch propellers a tug can
move sideways (see figure 2.15), so-called 'flanking ' .
When the tug has to move sideways to starboard, one
wou ld thin k of sett ing the starboard propeller to ahead
and the port propeller to astern . Th is works only when
the tug is equipped with a bow thruster. However,
witho ut a bow thruster this pro peller setting does not
move the whole tug sideways, bu t only the stern to
starboard. By setting the propellers in the opposite way,
with the starboard propeller astern, the port propeller
ahead and the rudder to port, the tug w:ill move sideways
to starboard witho ut gathering headway, depending on
trim, wind and current influence . The transverse effect
of the inner propeller will enhance the side thrust.
2.3.4 Conventional tugs in shiphandling
Conventional tugs are used for all methods of tug
assistance but are not equally suitable for all methods.
When assisting a vessel under speed a conventional tug
is effective when towing on a line but as a stern tug,
owing to the location of the towing poin t, it has severe
limitations. When the ship has more than approximately
three knots headway the after tug can only assist at one
side ofthe ship and cannot shift to the other side nor is
it able to control the speed of the assisted ship. The
towing point being near midships implies a risk of girting.
Wh en towing on a line, conventional tugs are not
suitable to changi ng over, while th e towline is still
fastened, to pushing at the ship's side. Thi s might be
desirable, for instance, on arrival at a berth. For a quick
change-over from pulling to pushing and vice versa while
the towline is still fastened the conventional tug would
18 THE NAUTICAL INSTITUTE
have to push with the stern . The manoeuvre itself is
already difficult unless the tug is equipped with a bow
thruster or if it is a twin screw tug. However, when
pushing with the stern the tug' s propellers are so close
to the ship's hull that the interrupted water flow towards
the prop ellers will result in low prop eller efficiency. In
addition, the stern fend ering of conventional tugs are
normally not designed for pushing with the stern. In such
a situation it is better to release the tug from the bow or
stern in order to be able to push at the ship's side.
For tug operations at th e ship's side a normal
conventional tug can push but it is not the most efficient
one for pulling on a tug's bow line, due to the limi ted
astern power. Specific rudder configurations, such as
the Towmaster system for example, will increase astern
thru st. Normal single screw conventional tugs can
neither pull at right angles because of the transver se
effect of the propeller, no r can a single screw tug pull at
right angles with a cross current or strong cross winds.
The same kind of pro blem arises when the assisted ship
is moving ahead or astern while the tugs are pulling. It
will then be imp ossible to stay pulling at right angles.
Additional measures should then be taken, such as a
line from the stern of the tug to the ship to keep the tug
in the best pulling dir ection. A bow thruster does not .
improve the situation as the conve ntional tug operates
while pu lling with th e tug's bow headed towards th e
ship's hull. Steering nozzles, Towmaster and nanking
rudders mak e it easier to keep the tug at righ t angles
when pulling. Twin screw conve ntional tugs can make
use of their propellers to keep the tug at right angles,
alt hough this will be at the expense of loss of
effectiveness.
Figure 2.16 Some assisting methods with conventional tugs
The capabilities and limitations of conventional tugs
in relation to oth er tug types are discussed in Chapter 4.
Som e assisting m ethods with conventional tugs are
shown in figure 2.16.
with a special rudder and/ or propell er arrangement
which increases propeller efficiency.
Figure2.17 Camhi-tug 'Petronella], Goedkoop' ofWijsmulhr
Harbour TowageAmsterdam. Lio.a. 28·5m, beam 6·9m. Main engine
900 bhp. Om epp infixed nowe andtwin rudders. Retractable 361J'
steeroblebowthruster of 420bhp, typeAquamaster UL 316/2600.
Bollardpull ofmain engine 15t. Bollardpull main engim + bow
thruster2Ot. Maximum speed ahead 11·9 knots, astern 102 knots
whenusingboth main engineand bow thruster: The tug is equipped
witha specialfairleadat the stern anda towing winch. Line '1'
shows the tow linein its (normal'posit£on and '2' the tow line passing
through thefairlead .
- - .../'
,; --------
----- -------
- - - - - -.::~
As an example, an azimuth bow thrus ter of 400 hp
can increase the top speed of a tug of 27 metres length
and engine power of 1500 bhp by half a knot. With just
the bow thruster working a speed of about five knots
can be achieved. The towing force of the tug is increased
by five tonnes if the main propulsion and the bow
thruster work in the same direction. This is all in addition
to better manoeuvrability.
By installing a conventional single screw tug with a
3600 steerable bow thruster, also called azimuth bow
thruster, these disadvantages can be overcome . Tugs
equippe d with such a bow thruster are the so-called
combi-tugs. The first combi-tugs app eared in the early
1960s. A tug equipped with this type of bow thruster
can, with the aid of the main propulsion and the bow
thruster, turn on the spo t, sail straight astern at a
fair
speed and mov e sideways as well (see figure 2.(8).
Setting this type of bow thruster in the same direction
as the propulsion also gives additional bollard pull ahead
and astern and increases maximum speed. In most cases
this type of bow thru ster is equipped with a nozzle and
can be of retractabl e or fixed type. An azimuth bow
thruster with a nozzle pr opeller below th e keel , in
contrast to a tu nnel bow thruster, achieves high
efficiency in any direction even when th e tug is moving
quickly. This provides an additional increase in the tug's
man oeuvrability,
I_ '~ • • ..
-- -,-- -I,-
.«(-----1- ---
For older tugs this is a satisfactory and inexp ensive
way of improving manoeuvrability and bollard pull. As
examples of converted tug s, at Am sterdam, The
Netherlands, some older tugs have been converted to
combi-tugs and at San Pedro, California, USA , the tug
SanPedro (now Pacific Combl) has been converted into a
combi-tug with a similar conversion to the tug Point
Gilbert and Flying Phantom of Cory Towag e (now
Wijsmuller Marine) in the UK. The San Pedrohas been
equipped with a 600 bhp bow thruster, which has
increased the tug's bollard pull by 400/0, from 25 to 35
tons and has improved the manoeuvring capabilities.
Moran Towing Company, USA, revitalised its fleet of
single screw tugs by installing retractable azimuth bow
thrusters and a larg e fairlead aft. New tugs are also
equipped with azimuth bow thrusters, all of them of the
retractable type.
Going astern
2.4.1 Designing and manoeuvring combi-tugs
Figure 2.18 Free sailing manoeuvres witha ccmbi-tug
Side stepping
Turningonthe spot
2.4 Combi-Tugs
As discussed above, the manoeuvrability of single
Screw conventional tugs can be improved by the use of
high lift rudders. However, th e disadvantage of many
single screw tugs without steerable nozzles, Towmaster
system and/or flanking rudders, is that moving straight
astern is hardly possible and no single screw tug can
move sideways unless fitted with a tunnel bow thruster
in combination with high lift rudders. The astern power
of single screw tugs is also low, unless the tug is equipped
If the azimuth bow thru ster is not in use it causes
extra resistanc e. This is one of the reasons for making
the bow thruster retractable . In shallow waters a
retractable type is necessary_ Care is required in using
the azimuth bow thruster when underkeel clearance is
small and it should be retracted in good time . A good
working alarm system whe n the water depth is not
sufficient for safe working of the bow thruster is strongly
recommended.
TUG USE IN PORT 19
2.4.2 Combi-tugs in shiphandling
Combi-tugs can tow on a line forward as well as aft.
As a forward tug th e combi-tug operates like a
conventional tug, but has the advantage of increased
maximum speed , manoeuvrability and ballard pull.
Also, the risk of girting is reduced and response time is
less due to the higher manoeuvrability.
As a ste rn tug combi-tugs can"ope ra te as a
conve ntional tug at low speeds and can easily work over
the tug's stern at higher speeds because of the azimuth
bow thruster. However, since conventional tugs have
their towing point approximately 0·45 x LWL from aft,
working over the tug's stern nee ds an additional towing
poin t near the stern to prevent girting, especial ly when
the assisted ship has a higher spee d. On conve ntional
tugs the towing point can be moved aft by a gob rope,
and on some tugs by a gob rope from a gob rope winch.
The gob rope is then led from the winch through an
eyelet or swivel fairlead at the tug's stern. At the free
end of the wire is a large shackle which can be pu t
around the towline. By heaving on the gob rope winch
the towing point can thus be brought as far as
possible aft.
This system is furth er explained in paragraph 7.2. A
gob rope arrangement normal ly needs two persons on
deck. With the redu ced numbers in tug's crews a handier
A combl·tug asslamtug
Combl·tug pushing
Figure2.19 Some assisting methods witha camhi-tug
20 THE NAUTICAL INSTITUTE
and safer sys te m wa s develo ped b y the former
Goedkoop H arb our Towage Company of The
Netherlands (n ow Wij smuller H arb ou r Towag e
Amsterdam). A strong fairlead has been attached to the
deck close to the tug' s stern.This fairlead can be opened
at one side so that the towline can easily be put in or
taken out. \Vith this additional towing point at the tug's
after end the combi-tug can operate similarly to a tractor-
tug, that is with the stern towards the assisted ship .
To show the capabilities of a combi-tug consider an
arriving sh ip. The combi-tug makes fast aft an d
approa ches stern first to the stern of the ship to pass the
towline (see figure 2.19 position 1). The ship to be
assisted may still have rathe r a high speed, e.g. about
seven to eight knots. As soon as the towline has been
secured and the aft towing point is in use by means of a
gob rope or fairlead, the combi-tug can control the
vessel's speed (position 5) or assist in steering (positions
2 and 3). To reduce ship's speed, the tug's propulsion
and the bow thruster will be set in the same direction to
increase the tug's pulling force. Assisting steering is
achieved by th e tug shee ring out to port or starboard
with the main propulsion going astern and the bow
thruster working sideways. In positions 2 and 3 th e
incoming water flow creates lift forces on the tug and
consequently a force in the towline. When th e ship's
speed reduces, the effect of the tug in position 2 and 3
will becom e less due to the reduced lift forces. The gob
rope is then released or the towline take n out of the
fairlead . The original towing point is then in use again
and the tug can operate again as a normal conventional
tug (position 4).
In circumstances where there are strong cross winds
and/or currents, and much effort is requi red from the
tug to compensate for those forces, the tug is m ore
effective when it proceeds with the assisted ship as a
normal conventional tug (position 4) and thus can use
its full ahe ad power. When required, th e bow thruster
can be used to incr ease bollard pull. The lift forces on
the tug caused by the water flow increase the force in
the towline.
If so required thetug can , eve n when the assisted
ship has forward speed, shift to a position behind the
ship's stern by using th e gob rope or fairlead, bow
thruster and main propulsion (position 4 --7 5). This
can be done faster compared to a normal conventional
tug. Conversely, moving from a position abaft the stern
to a po sition m oving with the assisted ship is, because
of the bow thrus ter, possible at a somewhat higher speed
than with a normal conventional tug.
It has been mad e clear that th e advantages of a
combi-tug are greatest when the tug operates as a stern
tug on a line. For that reason this type of tug often assists
during quite long passages as a stern tug for speed and
steering control. The cornbi-tug can also be used at the
ship's side, such as for push-pull operations.
The towing winch (6) is located aft of
midships. It may also be just a towing hook.
The towing point, a large fairlead or towing
staple (7), through which the towing line
The large skeg is typical for tractor tugs
an d in particular for VS tractor tugs. It gives
course stability and brings- the centre of
hydrodynamic pressu re further aft, which
is advantageous to both safety and towing
performance when towing on a line,
especially towing perform ance whe n oper-
ating as an after tug at higher speeds.
University of Washington in Seattle , USA. Tugs with
Voith Schneider propulsion system appeared in the
1920s and 1930s. In the 1950s Wolfgang Bear of the
Voith Company designed a shiphandling tug with the
cycloidal propulsion under the tug's forebody and the
towing gear on the aft deck. Many limitations
of
conventional tugs were overcome by the introduction
of this totally new concept, which was called a Voith
Water Tractor.
Figure 2.21
Propeller
blades
ofa
VStug
Phow:
J M. VO>lh GmbH.
Grml4l1J
The cycloi dal propulsion system is, in fact, a kind of
controllab le pitch propeller (see the side-view of Voith
tractor tug, figure 2.20). The engine works at constant
rpm and magnitude of thrust and the thru st
direction is regulated from the wheelhouse.
Different engine rpm settings can be
selected. Full engine rpm is required when
full towing or push ing power is required or
a t high free sailing sp eed s. In other
situations lower rpm settings can be used.
The VS propulsion system for tugs consists
of two units with vertical propeller blades
whose pitch and thrust direction can be
regulated uniformly through 360· without
delay. The protection plate (2) protects the
propeller blades and works like a nozzle,
thus increasing propeller efficiency. During
docking,the tug stands on these protection
plates an d on the skeg (4).
8 Second towing
position
4Sug
5 Ferukr
6 10wing unndi
3 J'oith turbo
coupling
Figurt 2.20
VOith traaor tug
7 Towing staple
9 WheelJwwe
2.5.1 Design
2.5 Tractor-tugs 'with cycloidal propellers
Note: A second
towingpoint is
only fitted in a
smallnumberof
VS tugs, andis
discussedfurther in
Chapters 4 and9
1 Voith·Schneidn
praptlhr
When operating at the ship 's side, a combi-tug has
many of the disad vantages of a normal conventional
tug. The combi-tug can either push with the bow or
with the stern . When pushing with the bow while the
ship has some speed , the how thruster can be help ful to
keep the tug's bow in po sition and prevent sliding along
the ship's hull. The bow thruster will also give an
additional transverse pushing force [see figure 2.19).
When pushing with the stern, the effectiveness of the
tug is reduced due to the restricted wate r flow towards
the propeller an d it is mo re difficult to bring or hold the
tug at right angles to the ship's hull when th e ship has
some speed, due to the low power of the bow thruster.
In particular, when working over the tug' s bow, pulling
effectiveness at speed is low.
2 Proptlur guard
platt
Tractor tugs have their propulsion under the
forebody. Those with a vertical blade system, or
cycloidal propulsion system, are the so-called Veith-
Schneider or Voith tugs (VS tugs). The first vertical axis
propeller, similar to the Voith Schn eider propulsion
system, was developed in the early 1920s by Professor
Kirsten of the Aeronautical Engineering Faculty at the
j. M. Thith. GmbH, Gn many
TUG USE IN PORT 21
•
•
•
~ .
•
•
>0 "·" "" 10
"It"""'""",, '
..
,o . , . r. , ... ..
"' ,,' '' ' ,,'''' ,,, ,,,
..
""
..
"
..
pllch..-. f\AI'tl..e ("11m)
....h.... appro",. 5 por'l (Ila/board)
enu.. aheld (lttllfTl !
apptox. 80 %
lI1ln........ " 1U'1.pplOX. 50 %
pI1cto ...Uw-cl (IsIem)
........ 11 porl (NrboIrd)
1hr'......ad (litem) ••
nr-fIII ..... 1011 ...
pItCh ...... u ar-.s or~
_...-
~ -.s lhWnl- 11)0'"
"-...s. l/YUIt • 0
j. M. VoilAGmhH, GmMny
Figure 2.23 Propea.r control ofVS tug'
..
..
..
..
..
2.5.2 Propeller control
Th e direction and magnitude of propeller thrust is
remotely controlled from the wheelhouse. The remote
control may be mechanically operated bya push-pull
rod gear. This is a very reliable system for tugs and best
when the distance between wheelhouse and propeller
is short. With long distances between the bridge/
wheelhouse and propeller and whe n several
manoeuvring stands are installed other remote control
systems are recommended. Hydraulic, pneumatic,
electric and even computerised rem ote controls, even
with jo ystick control, are alternative solutions. How
propeller thrust is regulated can be seen in figure 2.23.
Transverse thrus t is contro lled by the wheel and
longitudinal thrust is controlled by pitch levers. So thrust
setting is a combination of transverse and longitudinal
thrust. Transverse direction has pri ority. When full
transverse thrust is used (wheel hard to port or starboard)
no longitudinal thrust will be available, even when the
pitch levers are set in pitch position. It can be seen that
the full 100% thrust cannot be applied in any direction.
The two units of a VS tug can be con troll ed
ind ependendy or together for longitudinal thrust but
only controlled togeth er for transverse thrust.
forward. The towing point lies 0· ' - 0·2 x LWL from
aft, although this may differ by tug depending on
operational requirements.
"..
Th e maximum draft,
including the propulsion
un its, of a VS tug is
relatively larger than that
of conventional tugs, due
to the weight of the
propulsion units, the
propeller location and
dimensions. The location
of the propulsion units is
approximatel y 0·25 -
0·30 x LWL from
J M. Vilith GmbH, GmruJ7I]
Figure 2.22 Principk of
Voith propulsion
Most modem tugs have small wheelhouses
with optimal visibility. The same applies to
modem VS-tugs, like the one shown in figure
2.20. The small and optimum wheelhouse (9)
often has one central manoeuvring panel for
propeller control.
passes, lies far aft and usually exactly above the
middle of the skeg. The hull is relatively wide
and flat to provide sufficient space for the two
propulsion units. VS tugs have heavy duty
fendering (5), especially at the stem, because
when pushing, the tugs push with the stem.
Th e principle of a cycloidal VS propeller is
shown in figure 2.22. Links leading to the
steering centre N are fitted to th e vertical
propeller blades. The steering centre N can be
moved out of the centre a by two hydraulic
cylinders. One hydraulic cylinder works in the
longitudinal direction and the other one in the
transverse direction. The propeller blades create
a thrustin a direction depending on the location
of the steering centre N . In sketch I there is no
thrust; the propellers are 'idling' . In sketch 2
the steering centre is moved by one hydraulic
cylinder to port. Th is offset location of the
steering centre N results in forward thrust. In
sketch 3 the steering point
N is moved by the two
h yd raulic cylinders to
port and forward, which
gi ves th rus t in the
indica ted direction S. So,
the thrust can be
regulated for any
direction by moving N.
The nominal direction of
thrust is perpendicular to
the line O -N and the
magnitude of thrust is
proportional to the
distance O-N. In tugs,
there are always two VS
propeller units , which are
installed side by side.
22 THE NAUTICAL INSTITUTE
2.5.3 Manoeuvring
VS tractor-tugs are highly manoeuvrable, can turn
on the spot, deliver a high amount of thrust in any
dir ection and sail straight astern at high speed. Astern
thrust is nearly equal to ahead thrust. Many of the
disadvantages of conventional - especially single screw
- tugs, such as low astern power, no or low side thrust
and in so me situations transv erse effect of the propeller,
do not apply to VS tugs. Because it is possible to apply
side thrust tractor tugs are also safer when making fast
near the ship's bow and interaction forces can be better
compensated.
Sailing ahead as well as astern is easily achieved by
use of the wheel, as shown in figure 2.24. Turning on
the spot can be done by setting the wheel hard to port
j. M. JIOithGmbH, Gmno.ny
Figure 2.24 A VS tug sailiug ahead andastern
or starboard. A VS tug can be moved sideways e.g. to
port. The po rt pitch lever is set for ahead and the
starboard for astern, while turning the wheel to port.
The turning moment of the propellers is eliminated by
the action of the wheel and the tug moves sideways .
Propeller effectiveness is less on astern therefore ahead
pitch should be set somewhat lower than astern pitch.
VS tug propulsion produces
little wash, which is
invaluable when skimming oil and, for example, when
working with full thrust close to deep loaded lighters as
can be the case in narrow harbour basins.
The full bow of tractor-tugs and the flat and wide
hull bottoms which are nece ssary to create sufficient
room for the propulsion units adversely affect their sea
keeping behaviour. According to the experience of some
VS tug captains, so do the bottom plates of the VS
propulsion units in rough sea conditions.
A nu mbe r of VS-tugs, parti cular ly those used for
escorting, are designed such that they better meet the
demands of operating 'skeg-first'. This, however, does
not alter the basic principles of the tractor tug.
2.5.4 VS tugs in shi phandling
VS tugs are used for towing on a line and for
operations like push-pull (see figure 2.25). For towing
and pushing operations the maximum longitudinal pitch
is limited (to approximately pitch 8 for towing/pulling
and pitch 9 for pushing) to avoid overloading the engine .
In push -pull ope ratio ns the disadvan tages of
conve ntional tugs of having low astern power and/or
not being able to pull at right angles to the ship do not
,apply to VS tugs. As already mentioned, VS tugs have
nearly equal power astern and ahead and can apply
thrust in any direction.
While towing on a line a VS tug forward or aft can
change to pushing without releasing the towline, which
is very handy while approaching the berth (see figure
2.25, situation 3). The forward tug can change to a
====1~.
,
Figure 2.25 SOTM assistiugmethods witha traaortug
pushing position at a ship's speed up to approximately
two knots. A towing winch is always useful with this
kind of operation in order to con trol the length of the
towline and to enhance safety.
VS tugs can also make fast directly to a ship 's side as
push-pull tugs (see figure 2.25, situation 4) approaching
the ship either stern or bow first. Ship's speed should
then not be more than about five knots. Although VS
tugs are not the most effective type of tug as.a forward
tug towing on a line for a ship under spee d, due to
performance restrictions imposed by the location of the
towing point, they are very suitable as after tug for course
and spee d control. Course control can then be carried
out with ships having headway and, contrary to what is
possible with conventional tugs, to starboard as well as
to port.
TUG USE IN PORT 23
Extremely usefull
Intensidade
Course control is carried out at higher speeds by the
indirect me thod (see figure 2.25, situation 2), making
use of the hydrodynamic forces on the tug's hull, or at
lower speeds by the direct method (see figur e 2.25,
situation I). Forces in the indirect method can be far
highe r than the tug' s ba llard pull . When braking forces
are required, pitch levers should be adjusted to ship's
speed to avoid overloading the engine and a minimum
of wheel should be used. Th e different manoeuvres that
can be carried out with a VS tug are shown in the
manoeuvring manual ofJ.M. Voith GmbH.
2.6 Tractor tugs with azimuth propellers
2.6.1 Design
Tractor tugs with azimuth propellers have two 3600
steerable thrusters under the forebody. Th ere are several
manufactur er s of az imuth th ruste rs, in cluding
Aquamaster, Schottel, KaMe Wa, Niigata , Kawasaki,
Vlstein and Brunvoll. So me of the Eu ropean
manufacturers mentioned have merged. Differ ent
names are used for azimuth thrusters, such as Z-pellers,
Rexpellers and Duckpellers, amongst others. Although
th e thruster systems are gene ra lly similar, each
manufacturing comp any has its own specific design.
The first azimuth propellers were introduced into
service in the \960s. The first tug fitted with azimuth
propellers was the German harbour tug]anus (1967).
Azimuth propellers can be fixed pitch , e.g. mostly with
Niigata, or controllable pitch . Fixed pitch propeller
revolutions can be regu lated by a speed modulating
Schout/·wtlft, Gnmany
Figure 2.27 Integrated Sdiouel ;'o<.des with open protectioe frames,
decreasinga tug's maximum draft byapproXimately 0-5m
without affecting the tug's performame
clutch, which enables the pro peller spe ed to be
controlle d in a stepless manner from ze ro up to
maximum. This more or less eliminates the need for
contro llable pitch propellers and is much less expensive.
Azimuth propellers are fitted in nozzles to increase
propeller efficiency, (for nozzle types, see par. 2.3.2) In
the event of grounding, propeller protection is provided
either by protection or docking plates. Docking plates
are fitted underneath or in fron t of the propeller and
give on ly limited prot ection for the propellers.
Protection plates serve also when docking.
-
Schotte~ 1MNetMrlands
Figure 2.26 Azimuth tractortug 'Fairplay V'. L.o.a. 26·7m,beam 8·8m, bp2!!t. Infront
of the thrusters is thedoclring plate
24 THE NAUTICAL INSTITUTE
The basic design of the tug itself does not
differ much from VS tr actor tu gs. The
displacement of a VS tug is more than that
of a comparable azimuth tractor tug of the
same engine power, du e to the high er
weight of the VS pro pulsion systems and
to the requirements for more stiffening due
to the wider hull openings for the VS units.
.An azimuth tractor tug of the sam e
dimensions and engine power will
therefore have less hull draft.
Towing point location is generally similar
to that in VS tugs. The skeg is sometimes
smaller and the location of the towing point
is often less strictly related to the location of
the skeg as with VS tractor tugs. Th e towing
point lies approximately 0·1 x LWL from aft
and the propellers are fitted at 0·30 - 0·35 x
LWL from forward. A smaller distance is
found, 0·25 x LWL for instance, on some
Italian tractor-tugs at Genoa, Italy. Thrusters
Photo:Author
Figure 2.28 J oy,tUkfor combined control ofboth thrusters. The
direction oflug's motemmt is indicated around thej oy'tidc. Speed
control is carried out by separate loxn
Plwto: StorJ:·KwaJlJ, T1u Nethtrl.zruls
Figure 2.29 Thruster control unitfor combined control of thrust and
thrust direction. The unitsare asailableforfixed pitrh and
conlrollablepitrh propellers
o Clutchoff
• and! Of!
_ Oi..CIlonof
Aa..... b.~ irId
Niigata Engineering Co. UrI.Japan
Figure 2.:jO Manocuvringdiagram for reuerse-tracior tug. When the tug has a Uni-leter type manoouaingpansl; theUni-lever isusedin
combination with the dual speed control handles. When the tug has the standard typc manoeuvringpane~
manoeuvring is t/Q1Jl by thesteering whee~ the dualahead-astern handles anddualspeed control handles.
A comparable system, :4quaduo' ofAquamasterlKilMeJ#z is installed inASD-Iug'of Adsteam Towage, UK
TUG USE IN PORT 25
placed further forward increase a tug's effec tiveness while
assisting. The thrustersdeliver practically the same amount
of thrust in any direction, though astern thrust might be
about 5% less. \'!hen the thrusters int eract, as when
producing side thrust, total thrust efficiency will be less.
Thrusters should then be set at a small angle to each other.
2.6.2 Propell er control
T hrusters can be controlled by a single device for
each thruster separately in respect of the amount of
thrust (propeller pitch for cpp or prop eller revolutions
for fpp) and thrust direction or controlled together by a
joystick. Altern atively, by a control system consisting
of two steering levers (ahead-astern handles), a steering
wheel to give angle adjustment to both thrusters and
two speed control levers. For the latter two methods
sec the manoeuvring diagram (figure 2.30) of Niigata
for joystick, steering wheel and control handle positions
and the resulting tug movements for a tug with azimuth
thrusters at the stern.
When combined thruster control is by joystick (also
called a Uni-lever, Combi-lever, master pilot, or similar
names), th e thru
sters are automatically set for the most
suitabl e direction in order to manoeuvre the tug as
indicated at the joystick control. Some azimuth thruster
types have joystick control for the direction of tug's
movement while the amount of thrust has to be
regulated separately. Others have combined control of
thrust force and direction.
Tugs with combined joystick control can also control
each thruster separately, but on some tugs this may be
too complicated due to the number of handles to be
operated. Combined joystick control of both units is
limited to pre-programmed tug manoeuvres, so separate
control of the thru sters has some advantages owing to
the large number of possibilities, especially when ship
handling man oeuvres arc complicated. It should th en
be possible that thru st and direction for each thruster
can be regulated in a simple and logical way.
Azimuth thrusters with controllable pitch propellers
hav e the advantage that pitch can quickly be reversed
for astern thrust However, wh en full power astern is
required thru sters should be turned for astern.
2.6.3 Manoeuvring
The manoeuvring characteristics of azimuth tractor-
tugs are more or less comparable to those ofVS tractor-
tugs. They are also safe working tugs and highly
manoeuvrable, canturn on the spot, move sideways
and have nearly the same ballard pull ahead as astern.
Because of the relatively shallower draft, sometimes
another skeg design and almost 100% thrust in any
direction, the manoeuvring characteristics of thes e tugs
may be somewhat different compared to VS tugs.
26 THE NAUTICAL INSTITUTE
2.6.4 Azimuth tractor tugs in ship handling
The assisting capabilities of azimuth tractor tugs are
comparable to those ofVS tractor tugs. They are suitable
either for operating at the ship' s side or for towing on a
line (see figure 2.25). Azimuth tracto r tugs fitted with a
smaller skeg and!or a towing point not located at the
correct position are less effective as a stern tug compared
to the VS tractor tugs, when ope rating in the indirect
towing method at highe r speeds. On the other hand,
because of the ir 100"ver underwater resistance - mainly
due to the relatively shallowe r draft - and the ability to
provide nearly 100% thrust in any direction, azimuth
tractor tugs will be more effective at spee d when direct
towing as a stern tug and as a forward tug when towing
on a line, again dep ending on a proper location ofthe
towing point. The influence of the location of the towing
point on the performan ce of tr actor tugs is furt her
discussed in Chapter 4.
2.7 Reverse-tractor tugs
2.7.1 Design
Reverse-tractor tugs, also called pusher tugs, are tugs
with two azimuth propellers under the stern . They are
more or less specifi cally de signed for th e assisting
method used, for instance, in a large number of ''\Test
Pacific ports - assisting over the tug's bow. These tugs
have a large towing winch forward and only sm aller
towing equipm ent aft e.g. a towing hook. The towing
point aft often lies too far aft to be effective if these tugs
were to tow on a line at speed like a conve ntional tug.
Sometimes the towing point lies nearl y above the
thru sters aft.
Azimuth propeller systems in use are J ap an ese or
European made and can b e fi tte d with fixe d or
controllable pitch propellers in nozzles. In th e case of
fixed pitch propellers, revolutions can be regu late d by
a speed modulating clutch , which controls the propeller
spe ed in a stepless manner from zero. Because the
thrusters are fitted under the stem the maximum dra ft
of reverse-tractor tugs is less than that of comparable
real tractor tugs. Hull draft is less th an the hull dr aft of
a similar VS tractor tug, for reasons already explained
when discussing azimuth tractor tugs.
The propulsion units are located approximately 0·[
x LWL from aft. The pushing point and forward towing
point is at the forward part of the bow. Wheelhouse
construction is completely adju sted to th e assisting
method. The manoeuvring station is designed in such a
way that the tug captain ha s an un obstructed view of
the forepart of th e tug, the towline and th e assisted ship
while seated behind the manoeuvring panel and the
assorted instrumentation and control handles around
him.
tractor tugs do the same but are then heading in the
reverse direction. That's why th ese tugs are call ed
reverse-tractor tugs.
PROFILE
What has been mentioned about azimuth tractor tugs
with respect to manoeuvring also appli es to a large
extent to reverse-tractor tugs. They can be used for
towing on a line or for assisting at the ship's side as
shown in figure 2.33. They can easily change, when
towing over the tug's bow, to a pushing position at the
o 0
....... ......
r
.~~
'.
UPPER DECK PLAN
1M HongKongSalvagt & Towage Co. Ltd
Figure 2.31 Reverse-tractor orpushertug 'Lam Tong', l.o.a.
26·1m, beam 8·5m, hp 431
C. H Caus & Sf11/.1 Limi.ttd, Qll'u.4a
Frgure 2.32 Thrusters ofCates' reome-traaor tugs
2.7.2 Propeller control, manoeuvring capabilities
and shiphandling
Prop eller control with reverse tractor tugs is the same
as with azimuth tractor tugs. Because of the two azimuth
thru sters and the forward lying towing point reverse-
tractor tugs are highly manoeuvrable and safe working
tugs. They can tum on the 'spot and mov e sideways,
(see fig. 2.35) The astern power of these tugs is generally
about 10% less than ahead power, due to the shape of
the after hull. The name reverse-tractor tug implies that
the tugs operate similarly to tractor tugs but in the
opposite way. Tractor tugs always operate with the
towing point towards the assi sted ship and th e
propulsion units away from the assisted ship. Reverse-
Figure 2.33 Assisting methods with a reverse-tractor tug
ship's side or for push-pull while berthing. A towing
winch is useful to enable the towing line always to be a
suitable length or to pick up any slack in the line.
When operating at the ships side these tugs are very
effective at speed.
Although this type of tug is also used for towing on
a line , as a forward tug it will not be effective in steering
ships having headway. The tug has to move astern and
its towing point lies at the forwardmost end of the tug,
giving a similar decrease in steering efficiency when
speed increases as with a tractor tug.
As a stern tug, reverse-tractor tugs are very suitable
for steering and speed control for ships at speed, whether
making use of the indirect or direct method. In th e
indirect m ethod reverse-tr actor tugs are in ge neral
somewhat less effective in steering compared to a similar
VS tug in the same situation, but in the direct method
rever se-tractor tug s might be some more effective
because of the lesser draft The effectiveness of tugs is
dealt with in more detail in Chapter 4.
2.8 Azimuth Stern Drive (ASD) tugs
2.8.1 Design
Conventional tugs hav e certain advantages and so
do reverse-tractor tugs. ASD-tugs are nearly the same
as reverse-tractor tugs but are desigrled in such a way
that they can operate like a reverse-tractor tug as well
as a conventional tug, thus combining the advantages
TUG USE IN PORT 27
Mas o ASD tem essa capacidade!
-,9.
ShipyardDamen; 'I1le Netherltmds
Figure 2.34 ASD-tug type 3110. L.o.a. 30·7m, beam 70·6m, bpdepending oninstalled enginepower 37-57tons (ahead)
Note:: Underusuer body design ofthis ASD-tug type has been optimised during recent years, which includes a large skeg extendingfrom
approximately 0.3 x water length from aft till theftrefoot, with the deepest partaft.
of both types. ASD-tugs have a towing winch forward
and a towing winch or towing hook aft. The aft towing
point is at a suitable location for towing on a line, viz.
0·35 - 0·4 x LWL from the stern. Like reverse-tractor
tugs, they have two azimuth propellers fitted under the
stern at roughly
the same location, about 0·1 x LWL
from the stern.
The azimuth thrusters of ASD-tugs are made by the
same manufacturers as the azimuth thrusters of tractor
tugs. In addition, Holland Roer Propeller (H RP) can
be mentioned. Their maximum draft is less than that of
comparable tractor tugs, as mentioned when discussing
reverse-tractor tugs. They may be equipped with a
tunnel bow thruster, especially when used for offshore
operations. Tunnel bow thrusters are not very effective
when a tug has speed ahead, but are very useful for
position keeping. Interest in this typ e of tug is still
growing because of their manoeuvrability and multi-
purpose capabilities.The latest development is installing
an azimuth bow thruster. This has been the case with
the 4000 hp ASD-tug Z-Two of towing company Tugz
International LLC (USA). A retractable azimuth bow
thruster of approximately 1000 hp was installed, so
28 THE NAUTICAL INSTITUTE
increasing the tug's manoeuvrability, its position keeping
abilities, maximum ballard pull ahead and astern and
maximum achievable sideways thrust.
2.8.2 Propeller control, manoeuvring capabilities
and ship handling .
Propeller control is the same as with azimuth tractor
tugs. The manoeuvring capabilities of free sailing ASD-
tugs and reverse-tractor tugs are shown in figure 2.35.
These tugs can deliver thrust in any direction, though
maximum stem thrust is some 5 to 10% lessthan on ahead.
Conventional tugs are effective as forward tugs
towing on a line, while reverse-tractor tugs are effective
aft and are also very suitable for push-pull operations.
ASD -tugs are very effective and suitable for all kinds of
shiphandling, owing to their ability to assist like both a
reverse-tractor tug and a conventional tug. When towing
forward on a line like a conventional tug (see figure
2.36, 1) the ASD-tug is very effective, although the risk
of girting exists. The risk is minimised when the tug is
equipped with a reliable quick release system.
As a stem tug on a line an ASD -tug works over the
bow (situation I and 2). This is effective for speed control
and course control to both sides. Effectiveness when
assisting in indir ect mode (situation 2) is generally
somewhat less when compared to VS tractor tugs, but
ASD-tu gs may be somewhat more effective when direct
pulling (situation I), because of their relatively shallower
draft.
~ ]2])
~ IID)-
Figure 2.35
-~ W FTU sailingmanoeuvringcapabilities
of an
{~ W ASD-Iugandreuerse-tractor
tug
~ :tID!
~ »ill ~
t
c: lID)
/
1 .
\
~I••~~
C§2 · ·: .~
'lJ . lJ
<1Q.! .~
4
Figure 2.36 Some assisting methods withanASD-tug
Like reverse-tractor tugs, ASD -tugs can also easily
change from towing on a line to push-pull without
releasing or changing the towline position (situation 3).
The forward ASD-tug should th en assist like a reverse-
tr actor tug (situation 2). A bow thruster is, as for a
reverse-tractor tug, useful for bringing and keeping the
tug's bow in position at the ship's side. For this kind of
ope ration a towing winch is very useful in order to
control the length of the towline and to pick up the slack
when necessary. ASD-tugs are also very suitable for
assisting at the ship's side, because of their high reversing
power and their 3600 steerable thru sters.
If an ASD-tug is equipped with an azimuth bow
thruste r as mentioned in par. 2.8.1,then the manoeuvres
discussed can be execute d faster and more effective .
2.9 Tug performance
With respect to tug performance it is goo d to
understand some basic principles. The first item deals
with performance at speed, which is discussed in detail
in Chapter 4, and the second mainly with ballard
pull conditions.
I) Wh en the tug's propeller wash is more or less with
the direction of the water flow, the propeller is said
to be operating in positive flow cond itions. This is,
for instance, when a bow tug is pulling a ship having
headway. Wh en the tug' s propeller wash is more or
less against the direction of the water flow, it is said
to be operating in negative flow conditions. This is,
for instance, when a stem tug is braking a ship's speed .
Although greater thrust is produced when operating
in a negative flow, torque loadings on the propeller
and engine increase considerably, particularly with
increasing speed of the water flow. As the negative
flow may also result in an unstable flow trough the
propeller, it may produce fluctuating loads and
vibrations.
2) The line pull is essentially dependent on the square
of the propeller revolutions , and the engine power is
dependent on the cube of the revolutions.This means
that if propeller revo lutions are doubled, the force
will increase with a factor of four , while the required
engine power increases by a factor of eight. This
re la tionship not on ly applies to ballard pull
conditions, but approximately to most tug operations
in port.
The efficiency of an open propeller - as already
mentioned - can be increased by fitting a nozzle. Tugs
with the same BHP may have a different bollard pull
depending on whether the propellers are fitted in a
nozzle or no t. Also, th e type of pro pe ller fitted is
impor tant. To de termine the towing force of a tug,
bo llard pull tests are carried out at different engine
ratings, particularly at the manufacturer's recommended
TUG USE IN PORT 29
continuous rating (MCR). Tests can also be carr ied out
at engine overload conditions , for instance with a
maximum rating that can be m aintained for a minimum
of one hour , and also with just one propeller working.
Figure2.38 !IJlnges in relationshipbetween brakehorsepower
andbollardpullfOr different tug types
Figure2.37
Relationshipbetween brake horse powerandbollardpull
fOr differentpropulsionsystems (see'text}
Brake HorsePower (BHP) is measured at theflywheel
ShaftHorsePower (SHP) is measured at thepropellershaft
SHP~± O·97 x BHP
Voith propeller ] ·] 5 ]·55
Open fixed pitch propeller ]·3 ] ·8
Az imuth prope llers in
nozzles (ahead) ] ·35 ] ·8
Fixed/controllab le pitch
propellers in nozzles ] ·5 2·0
[conve ntional tugs)
] ·7 - 2·0
] ·55- ]·8
]· 35 - 1·55
ASD tugs 1·]5 - 1·35
VS tugs 1·0 - 1·15
Conventional twin screw
tugs with fixed/controllable 1·25 - 1·5
pitch propellors in nozzles
Propeller performanc e is also shown in so-called
thrust vector diagrams. Several kinds of these diagrams
exist, all of them giving different information. Thrus t
ve ctor diagrams give information on pr opuls io n
performance with zero speed in different directions,
which is also importan t information to-assess the tug's
assisting performan ce. An example of thru st vector
diagrams with an indic ation of thrust forces is given in
Classification societies issue regulations for ballard
pull tests. For instance, according to the rules of Det
Norske Veritas (DNV) , the towline length should not
be less than 300 metr es, the water depth not less than
20 metres within a radius of 100 metres around the tug,
wind speed not more than 5m/ sec and current not more
than one knot. An instrument giving a continuous read-
out and a recording instrument representing ballard pull
graphically as a functi on of time should, according to
DNV, be connected to the load cell. The figure certified
as th e tug's continuous bollard pull will then be the
towing force re corded as being maintained without any
tend ency to decline for a duration of not less than ten
minutes.
Although conditions mentioned in the DNV
regulations do meet the requirements, unfortunately
several other regulations for ballard pull testing do not
sufficiently take into account the conditions required
for accurate and reliable ballard pull testing of modern
powerful tugs. In the report called 'Bollard Pull' (see
References) a new ballard pull trial code is proposed
that ensures obje ctive results
and repeatability as well
as comparability for trials performed at different
locations andlor with different tugs.
Bollard pull tests are carried out with engines ahe ad
and increasingly, especially for azimuth tugs, on astern .
Bollard pull tests should be carried out with sufficient
underkeel clearance, no current and waves and not too
much wind. The tug should pull straight ahead, or
straight astern when astern ballard pull is measured.
The towline should be of sufficient length to avoid the
tug's propeller wash having any influence on the towline
force. Bollard pull is m easured by a device inserted in
the towline. It can be a measuring device based on an
ordinary spring system , the 'clock', or an electronic
device.
Figure 2.37 gives an indication of the ratio BHP -
Bollard Pull for different propeller configurations. The
values shown in the tabl e are more or less the maximum
valu es. Because the relati on between bollard pull and
engine power depend s on several factors, such as hull
form , nozzle typ e and propeller load, the valu es may
vary as shown in figure 2.38.
The relationship between engine power and bollard
pull vari es considerably with the extent of engine power
and in such a way that a conventional tug with 700 BHP
and a fixed propeller can attain two tons/IOO BHP, whilst
for conventional tugs with about 6000 BHP with nozzles,
towing force may even be less than 1.3 tons / 100 BHP.
figure 2.39 . It gives propulsion performan ce at zero
speed for equal installed power. Side thrust an d the
influence of interaction of propellers on side thrust are
clearly shown in th e diagram.
In this thrust vector diagram the ahead values given
are also more or less maximum values. The astern thrust
of ASD-tugs may vary between 90% and 95% of ahead
thrust. In the diagram the astern thrust of conve ntional
tugs with controllable pitch pr opellers is given . The
astern thrust of conventional tugs .with fixed pitch
propellers is higher and around 65% of maximum ahead
thrust, but it dep ends strongly on th e nozzle typ e,
propeller/rudder desigu and configuration. For example,
the Towmaster system may improve ahead thrust to even
more than 1·50 tons BP/IOO BHP, while a very good
30 THE NAUTICAL INSTITUTE
-.
I
•.. ·f---- ·
"._-
-'
e
---·-..t: -··r- .. - . -_'-- .- -(j"t . • _.: -.. _," , : , . ,_ ~ , , '1- •. t-
r ,
.~-"_.~.,-
.---~
.....-_ .~ . _ . -T.- ..r. .. .; .. . . - ' - -l ·~ · · I ·
, . . +-:
Astern
+ -l
-r
1l0 'llo o f ." .,d 6" iol ~ I""~
,
, ,
,
, i
~ ,
\.
"-
'-
-'.
t "
".
~ I -I
;
"
1 • L
Figure2.39 ExampleofThrust v"ctor Diagrams
Legend a) Tractor tug: Voith h) Itaaor tug: azimuth: prapelkr in naw es c)Stem drivetug: admuthpropeller in naaJ<s
d) Conuetuinoal tug: twin screw (cpp) nowes and bow thruster e) Conventional tug: twin screw (epp) with noa,les
These diagramsshow the achiruable thrust at zero spud in diffirentdirections for a numberof tug types with equal power installed. The
athieuable ahead thrustper 100 BHP installed poweras shown in du diagrams is 1-1 tons for a VS tug, 1-4 tonsfor tugs with asimuth
thrustersand 1·5 tonsfor cont entional tugswith propellers in noWes
astern thrust of more than 70"10 of maximum ahead thru st
'an be achieved.
Note: Particularly for the more sideways thrust it is
fficult to say how accurate the thrust vector diagrams
e. Simulated or calculated performance diagram s
ould therefore, as far as possible, be validated in full
Ie trials,
Official full scale trials by Clyde Consultants UK
with a VS tug have shown that thrust in the mo re
athwartships direction may be much less than indicated
in the thrust vector diagram. The athwar tships thrust
measure d was less than 40% of the ahead thrust while
deve loping over 80% of the shaft horsepower.
O n the other hand the athwartships thrust of tractor
tugs with azimuth thrusters can be higher tban indicated
when thrusters are set at a small angle to each other.
TUG USE IN PORT 31
Convencional with nozzle
Athwarthship
ASD enullTRACTOR AZIMUTH
Ewrgrtm Marine Corp. (Taiwan) Ltd. tJrUi plwto:J Plug, Ltkko ITES
Figure 2.40 An assisting method asused in some USA ports. TIu container shipisassisted in the portofLosAngeles by twoconventional tugs. The
stern tug operates lih a rudder tug. The smaller photograph shows howthestern tug 'Pointe Vi'enle' ('Q1lven!ional tug, twin screw, length 32m,
bollardpull ahead 46·5 tons, astern 28-5 tons) pushes the ship~ stern towards theberth. The assisting method issimilar to tha: used in a "'rge
number ofmst Pacifi<ports, such astkose in]apan, Taiwan andHong Kong. However, in those ports reverse-tractor tugs are used andoperate in the
push-pull mode while mooring
32 THE NAUTICAL INSTITUTE
Chapter THREE
ASSISTING METHODS
3.1 Introduction
IN THE FIRST CHAPTER DIFFERENT TYPES OF PORT we re
discussed. In these ports tugs may render one of the
following services:
Tug assistance during a transit to or from a berth,
including assistance during mooring and unm ooring
operations.
Tug ass is ta nce mainly du ring m ooring and
unmooring operations only.
To what extent tug assistance is considered to be
necessary depend s on parti culars of:
The ship - including type, size, draft, length/width
ratio, loading co nditio n, windage and
manoeuvrability.
The berth and nearby manoeuvring area - including
type and size of berth, alignment, berthing space,
manoeuvring space near the berth, size of turning
circle, water depth,. influence of current and wind ,
and availability of mooring boats.
The transit route - such as width, length and depth,
the bends in that route, maximum allowable speed,
traffic to be expected and whether moored ships have
to be passed, plus th e influence of current, wind ,
waves , shallow water and banks.
The important difference between tug assistance
during mooring/unmooring operations and during a
transit lies in the difference in ship's speed, which is a
maj or factor of importance for selec ting the mo st
appropriate type of tug and method of tug assistance .
The various methods of tug assistance employed in ports
around the world are reviewed in this chapter, including
the types of tugs used. In the next chapter the large
influence of ship's speed on the performance of the
different tug types in relation to the assisting methods is
considered. Tug assistanc e as may be required in ports
is first addressed in more detail to obtain a better insight
into what tugs should be capable of doing.
Tug assistance during a transit may comprise:
Passage through a river or chann el.
En try manoeuvres into a harbour or turning basin
from river, channel or sea.
Passage through narrow harbour basins.
Passing narrow bridges or locks.
Over the larger part of a transit route the speed of a
vessel is mostly within the range of about three to six
knots and sometimes even higher. Atthese relatively
low ship's speeds the influence of wind , current and
waves is more pronounced, affecting the required path
width adversely due to the larger drift angle. Steering
ability is less at lower speeds, and is adversely influenced
by wind and current. O n the other hand, spee ds up to
six knots become rather high for effective tug assistance.
When port configuration is such that tugs are mainly
used for m oorin g and unmooring operations, then tug
assistance may comprise:
The approach phase towards turning basin or berth.
Turning in a turning b asin.
Mooring and unmooring operations.
Contrary to transit speeds, ship's speed during these
manoeuvres is normally very low or zero. Although tugs
should be capable of controlling a ship's heading and
speed and compensating for the influence of wind and
current while approaching the turning circle or berth,
the influence of ship's spee d on the performance of
different
tug types is less predominant.
Tugs assisting during transits, taking into account the
assisting method applied, should be capable of:
Giving steering assistance and controlling shiP's speed
Steering assistance while the ship has headway may
be necessary in narrow pa ssages, when passing bridges
or negotiating sharp and!or narrow bends in the fairway,
river or channel, or when en tering harbour or turning
basins under the varying influence of current and wind
conditions. Controlling ship 's heading and speed may
be required when approaching the harb our or turning
basin or when entering a lock.
Compensatingfor wind and current during transit while
a ship has speed
While transiting a channel, river or harbour basin a
ship under the influence of wind and!or current may
experience drift. This can be compensated for by
steering a drift angle or by a higher speed. A higher
speed is normally not possible in confined port areas
and due to the limited width in narrow passages only
small drift angles are acceptable. Tug assistance is then
necessary.
Tugs assisting ships during a transit normally also
assist during mooring/unmooring operations and the
final approach and departure manoeuvres as tugs used
for mooring/unmooring operations only. All th ese tugs
sho uld, with the assistan ce meth od appli ed, be capable
of effectively: .
TUG USE IN PORT 33
Prestarnull
Air resistance
Higher the speed more directional stability it has
Controlling transverse speed towards a berth while
compensating for wind and current during mooring/
unmooring operations
During moo ring operations a ship 's longitudin al
ground speed is prac tically zero and, when there is no
current, the ship has hardly any speed through the water.
The same applies when a ship has to be turned in a
turning basin. Mainly crosswise pushing and!or pulling
forces have to be applied by th e tugs.
The tug assistance requi red as outlined above has
been somewhat simplified. In any particular case the
complete tug assistance procedure may consist more of
a combination of the separate aspects that hav e been
described . Envir onmental conditions have a lar ge
influence . For instance, when tugs are used mainly for
mo oring/unmooring, the influence of currents can be
such that although ship's ground speed is low, say two
knots, th e speed through the water can be rather high.
With a bow curre nt of two knots, the speed through th e
water is already four knots. Situations th en become
comparable to tug assistance durin g a transit with the
higher ship's speeds and the associated requirements
for the assisting tugs.
Additional services such as mooring boats also affect
th e extent and method of tug assistance. When no
mooring boats are available the tugs must be stationed
and operated in such a way that the ship cao be pushed
up to a berth.
It cao be concluded that the port configuration, th e
influ ence of the en vironmental conditions and port
services have a prominent bearing on the requirements
for tugs aod the method of tug assistaoce, while ship'S
speed is an essential factor.
3.2 Assisting methods
3.2.1 Assisting methods in use
The different ways ships are handled by tugs in
various areas and ports around the world can indeed
mainly be traced back to large differ ences in local
circumstances. Methods of assistance that different tug
types are used for have alrea dy been mentioned bri efly
whil e discussing th e various types.
Assessment of assisting methods in use all over the
world shows only two m arkedly different methods:
Tugs towing on a line.
Tugs operating at a ship's side.
In Europeao ports towing on a line .is mainly used,
whil e in the USA and West Pacific ports tugs usually
operate "at a ship's side , altho ugh in different ways
dep ending on th e type of tug used. Parti cularly in
Europe and in the USA there is a tendency towards the
use of mo re flexibl e types of tug . This tend ency has an
34 THE NAUTICAL INSTITUTE
impact on the assisting methods used in Europe as well
as in the USA, which should be kept in mind when
reading this paragraph.
In some ports combinations of methods are used,
depending on th e local situation. For specific situa tions
or circumstances, assisting methods are applied other
than those in normal use. So it is possible that in ports
wh ere tugs normally work alongside, th ey will
occasionally assist while towing on a line, for example
when narrow bridges have to be passed or when ships
have to enter a dry dock. Changing the assisting method
can b.ecome necessary at seas ide terminals, where tug
assistance is affected by wav es. If in calm weather it is
norm al practice to assist alongside a vessel, it may be
considered safer to tow on a line when weather and
sea conditions de teriorate in order to avoid parting
towlines aod losing contro l of the vessel.
According to resear ch carried ou t in 1996 into
assisting methods in use in ports around the world, the
two method s are generally app lied in the following ways,
assuming two tugs assist a vessel:
Tugsalongside during approach to the berth andpushing
orpush-pull while mooring
This method is normally used in th e majority of ports
in the USA, Caoada, Australia, Malaysia, South Afri ca
and also at large oil terminals in Norway. While th e
method used in these ports is similar, the type of tug
differs. The way tugs are secure d using this method
depends mainly on th e type of tug. When using tugs
with omnidirectional propulsion they are mad e fast at
the forward and aft shoulder, generally with one bow
line from the tug in case of ASD/reverse-tractor tugs
and with a line from the tug's stern when tractor tugs
are used (see figur e 3.1) .
In the U SA tugs m ay be secured alongside a ship by
one, two or three lin es, depending on th e type of tug,
•
4 r::
Figure 3.1 Tugs alongside at approach andpush-pull
while mooring/unmooring
Mas rebocador amarrado não é o método europeu?
Não! O Metodo europeu opera puxando e o americano no costado
Figure 3.3 Alongside towing (USA)
Tug'l bow to S'tafbQ8Jd.
ship Win00 to port.
Tug's boWto port...nIp wtR
90 to starboard .
Figure 3.4 Forward tugsecured akmgside. Asshown the ship can
tum on the spot and when the tugapplitS hardportrudder and
engine ahead; the ship mootS crosswise. Ship'S aIlendpower tobe
equal to tug's aheadpower
Tug's engil1l!l astern, ship's
speed will decre ase .
Tug may be fastened with ona or two linea.
Figure 3.5 Alongsid< towing in Cape Town for a 'dead ship'
up to 700 metres in kngth
Figure 3.6 Rudder orsteering tug
In some ports in the USA and in the Panam a Canal
a stern tug is used as shown in figure 3.6. A rudder tug
can control a ship's spee d and a conventional tug can
steer a ship in th e require d direction by giving forward
thrust and applying starboard or port rudder. Other
types of tug such as VS tugs also use this m ethod. A
similar metho d is some times used on Dutch inland
wate rs.
astern can be steered by a tug pushing at the ship's bow.
Pushing at the port side of the bow will give the ship a
swing to starboard, pushing at th e starboard side of the
bow will give the ship a swing to port.
Figure 3.2 Conoentional USA tug secured with backing, spring and
stern lines. In situation 2 theshipmczesastem:Ifshipmoves ahead
the stem line will leadforward. Depending onthe assistance required
andlocal situation, ont, twoor three lines may he required
the local situation an d the assistance re quired.
Conventional tugs normally operate with two or three
lines made fast, though in some cases only one line is
deemed sufficient (see figure 3.2). The forward line is a
tug's backing line to be made fast to the ship. The spring
may come from the forward winch through a tug's most
forward bow chock or fairlead . O n other tugs bo th lines
may come
from a winch . The third line, the stern line,
is nee ded when a tug has to work at right angles to a
ship to prevent the tug from falling alongside when the
ship has forward or astern movement through the water,
or to compensate for the transverse effect of a tug's
propeller when going astern . This line may come from
a winch or be fastened on a bitt. It also compensates for
the influence of the ship's propeller wash when the ship's
propeller is going astern. A forward as well as an aft tug
may be secured in this way.
In the US A other m ethods are also used by tugs
operating at the ship's side. When breasted or alongside
towing, also called 'on the hip ' or 'hippe d up', tugs
forward andlor aft are lashed up solidly alongside a
vessel (see figure 3.3). This alongside towing is also
operate d in many other ports in the world, but mainly
when handling barges. When a tug is lashed up, tug
and sh ip wo rk lik e a twin screw shi p with two
independent rudders. Wh en lashed up forward to a ship
with the tug's bow facing aft, the tug's engine and rudder
combined act like a kind of steerable bow thruste r (see
figure 3.4). A ship can .then turn on the spot or move
sideways. Alongside towing is also used in USA ports
to handle a 'dead ship', and occasionally applied in a
similar way in some other ports - for instance in the
port of Cape Town ships up to 100 metres in length are
sometimes handled as a 'dead ship ' by a VS tug lashed
up alon gside (see figure 3.5).
O wing to their bette r manoeuvrability, twin screw
tugs or tugs with steerable nozzles normally operate with
fewer lines whe n assisting at a ship's side. Usually one
or two lines will then be sufficient.
In US A ports methods are also used that differ from
those discussed above. For example, in certain situations
tugs may work stem to stem with a vessel. A ship moving
TUG USE IN PORT 35
Para o rebocador não sair de posicao
ON THE HIP -bacaças
Starboard
9H
r'f;>i
'.'
, ,
'v'
I
\
Apart from th e count ries mentioned above this
method is applied in some other ports around the world
either with reverse-tractor tugs or with tractor tugs.
Furthermore, conventional tugs are sometimes used for
this method, as is th e case in some US A ports whereby
th e stern tug ope rates like a rudder tug . While berthing
this tug stays close behind the ship's stern and pushes it
towards the berth on the tug's bow line.
Figure 3.9 At approach, forward tuga/Qngside andstem tug on a
line; push-pull while berthing
tugs have to assist while towing on a line, for example
when assisting ships to ente r dry docks or floating docks.
Photo:Moran Towing, USA
Fz"gure3.7 Conventz"onal tugworking stem tostem
with a largepassengershz"p
Forward tug alongside and aft tug on a line during
approach towards a berth andpush-pull while mooring
This method, which does not differ much from that
mentione d abov e, is mainly found in the ports of]apan,
Taiwan and Hong Kong (see figure 3.9). The after tug is
mad e fast by a tug's bow lin e amidships or at th e
starboard or port quarter aft and follows the ship. The
forward tug is made fast at the forward sho ulder, also
with a bow line. The after tug is used for steering and
speed control. During berth ing manoeuvres the tugs
change over to the push-pull method.Tugs in these ports
are all of similar design, spec ifically constructe d for this
typ e of operation . They are rever se-tractor tugs or
sometimes ASD -tugs, with 360' steerable thrusters
under the stern and made fast with a line from the tug's
forward winch. For certain specific manoeuvres these
Photo:A. Jv. &mpt
Figure 3.8 Conventional twinscrew tug 'Espera1l</l' (l.o.a. 30m,·
beam 9·2m, bp ahead 401, bpastern 32t) operating as a steering tug in
the Panama Canal. The tug has fixed pitch propellers in no<.:dM with
three rudders behind andtwo fUlnking rudders infrontofeach noale
Tugs towing on a line during transit towards a berth and
while mooring
This is th e assisting m eth od used spe cifically in
Europe, most often when conventional tugs are assisting
vessels, but other typ es of. tugs are also used for this
method.The method is also applied in many other ports
of the wo rl d, especially in p orts working with
conventional tugs (see figur e 3.10). In many of these
ports, ships are assisted by tugs during transit towards
the berth , e.g. on th e river, from the river into the
harbour and thr ough harbour basins up to a berth . The
advantage of this meth od of assistance is that it can be
used in narrow waters. This m ethod is also used,
therefore, when passing narrow bridges or entering locks
an d dr y-docks. In such situations the forward tug
sometimes has two towlines, so-called cross lines or gate
lines or both lines may come from a double winch at
the tug's bowas can be the case on some reverse-tractor
tugs. The tug can then react very qui ckly and only a
little manoeuvring space is req uired (see figure 3.11).
The typ e of tugs used were originally conventional
tugs with a small engine and a streamlined underwater
body. These were very effective when a ship had some
speed, by making use of th e tu g' s m ass and the
hydrodynamic forces on the tug's hull. The increasing
size of ships required the introduction ofmore powerful
tugs. Modern conventional tugs are more manoeuvrable
and have mo re engine power and generally a smaller
36 THE NAUTICAL INSTITUTE
PROVA
W'
the better the capabilities are applied to shiphandling.
The method is, for instance, practised in the Europoort
area of Rotterdam and at the port of Coteborg, where
mainly tractor~ reverse-tractor or ASD·tugs are used .
Q
- '
3 , "-
· ,
· ,
· ,
· 2 \
"----EJ J
,
' /
Al a Vert10.... speeod a
corwenllonllllugcan move10
e.,_poslllon1 & 2 for$pHd
eontrcl and Sl-w.g or10
J:0:llaon 3 Ill!'st_rIo;.
Figure 3.72 Towing on a line at the approiUh
andpwh-pullwhik mooring
Figure3.10 Towingon a lineat tk approiUh andwhik mooring Combinations ofthe above systems
In many ports various tug typ es are operated and to
assist larger ships more than two tugs are often requir ed.
Moreov er, port entry or berthing manoeuvres can be
so complicated that not just one assisting method is used
but a combinati on. As an example of a combined
method the assisting method applied in an Australian
port for large bulk carriers entering the harbour is shown
in figure 3.13.
Figure3.11 Ship ispassing a narrow bridgeanda omxentional tug
forward is assistingwith two crossed towlines. The tug can react
quickly and only little manoeuvring space is required
length/width ratio. These tugs are still effective when a
ship has speed. Due to the limitation in capabilities of
conventional tugs, new tug typ es have been introduced
such as tugs with azimuth propulsion. Also, VS tugs have '
for m any years been used for towing on a line.
Figure 3.73 Combination ofdifferent assisting methods. Raxrse-
tractor tugs orASD-tugs awngside andona lineaft.A conventional
tugforward. A good configurationJor steering and, inparticular,
when only a short stoppingdistance isavailable. Nearer the berth: one
ofthe tugs alongside hastoshift totheother side topwh
When more than two tugs are used during berthing
the forward and aft tug will usually stay on the towing
line to control approach speed towards a berth while
the other tugs .push at the ship's side.
Tugs tawing on a line during approach towards a berth
and push-pull while mooring
This assisting method is becoming common practice
in ports where towing on a line is carried out with highly
manoeuvrable tugs such as tractor, reverse-tractor or
ASD-tugs (see figure 3.12).The more familiar pilots and
tug captains become with the capabilities of these tugs,
3.2.2 Relationship between type of tug and
assisting method
. As can be seen, th ere is a relatio nship between
type
of tug and assisting method used. An essentia l factor is
whether a tug sho uld be suitable to operate at a ship's
side, tow on a line, or bo th . For the attentive reader it
will also be clear that th e most suitable tugs are not
always available or used.
In the ports ofJapan, Taiwan and Hong Kong there
is one assisting method and mainly one type of tug.The
TUG USE IN PORT 37
PROVA 
Canal do panama
reverse-tractor tug with its omnidirectional propulsion
at the stern is well suited to operate the assisting method
in use - on a line at a ship's stern and alongside at the
forward shoulder. ASD-tugs are, however, also used for
this inethod. It is anticipated that for these ports the
reverse-tractor tug is the type that willusually be ordered
in the future.
There is often a steady development towards a
particular tug type. For instance, twenty years ago there
were still several VS tugs in the Port of Yokohama. This
type has now almost totally been replaced by the
reverse-tractor type.
In Europe towing on a line is general practice,
Originallyjust with conventional tugs but now for many
years with VS tractor tugs too. Due to the limitations of
conventional tugs, various tug types with omni-
directional propulsion are increasingly being used,
resulting in a change to more flexible assisting methods.
This is the case in many other ports where originally
mainly conventional tugs were used.
In the USA tugs operate at a ship's side most of the
time, and for many years the conventional tug was
practically the only type to be found. The limited
manoeuvrability and low astern power of these tugs is
partly compensated for by the use of extra towlines,
installation of high engine power, specific propeller/
rudder configurations and/or specific assisting methods.
In many ports of the USA and Canada there is a
tendency towards the use of more flexible tug types -
tractor tugs as well as reverse-tractor or ASD~tugs..As
in many ports elsewhere, conventional tugs will
nevertheless continue to be built in the future.
In Australian, New Zealand and South African ports
tugs mainly operate at a ship's side. The majority of the
tug fleet already consists of those with omnidirectional
propulsion and new buildings will mainly comprise this
type. .
The increasing variation in tug types offers an
opportunity to select the .most suitable tug for a port,
taking into account port particulars, existing assisting
methods and future developments in port and shipping.
(see also section 4.3.4: Towing on a line compared
with operating at a ship's side).
3.3 Tug assistance in ice
3.3.1 Introduction
During winter months, shipping traffic to and from
several ports in the world is impeded by ice. Ports are
kept open as long as possible by icebreakers so that
ships can be berthed. When ice is not too thick, ships
themselves may be able to break it. In other cases an
icebreaker, if available, or tugs otherwise, are required
38 THE NAUTICAL INSTITUTE
to do so. But all an icebreaker and tugs can do before a
ship's arrival is to break the ice. They cannot completely
remove ice from a berth, so certain procedures have to
be followed for berthing and unberthing. Depending
on a ship's size, strength and engine power, berth
location and ice conditions, ships may berth or unberth
with or without tug assistance. How tugs can be used
during berthing and unberthing in ice is considered in
this section. Further information about types of ice and
pilotage in ice can be found in books mentioned in the
references.
Mooring in icy conditions is usually time consuming.
Each port has its own method of assistance in ice
conditions. The methods discussed here are based on
experience in one of the larger Baltic ports, where
shipping is impeded by ice for several months each year.
Methods in other ice ports may not differ greatly.
3.3.2 'IYPes of ship for manoeuvring in ice
As mentioned before, ships may berth or unberth in
ice with or without tug assistance. It depends on the
size of ships, strength and engine power, berth location
and ice conditions. Regardless of a ship's size, strength
and engine power, not all vessels can pass independently
through ice owing to their construction and/or loading
condition. A vessel operating in ice should be so
ballasted and trimmed that the propeller and rudder
are completely submerged. If this cannot be done and
the propeller blades are exposed above the water or
are just under the surface, the risk of damage due to
propellers striking the ice is greatly increased. Such
vessels and other vessels which may damage their
propellers or rudders when they have sternway and/or
when a ship's engine is working astern and light draft
vessels with bronze propellers which cannotbe ballasted
or trimmed sufficiently require tug assistance.
With respect to berthing procedures ships can be
divided into two main groups:
Ships that can work with their engines on Dead Slow
on a spring line, without the danger of parting: e.g.
small vessels and ships with controllable pitch
propellers.
Ships with large engines, high starting power and high
propeller thrust at minimum propeller revolutions,
not able to work at Dead Slow without parting the
spring line, even when a double line is used.
3.3.3 Preparation before berthing or unberthing
Before mooring, a berth should be prepared by an
icebreaker or by tugs when ice is too thick for the ship
itself. Ice should be broken near the berth and an
approach route towards the berth should be made. Prior
to departure ice should be broken around a ship and a
departure route should be made.
3.3.4 Tugs and tug assistance
The way ships are handl ed by tugs in ice conditions
depends largely on the type of tug. Tugs need to he
adapted to work in ice conditions. Those with light draft
and prope llers fitted in no zzles have very limited
capabilities, because whe n they are moving astern the
nozzles immediately fill with ice. Even with tug engines
on ahead ice can fill the nozzles. Wh en this happens
the tug should imm ediately be stopped and the nozzles
cleared by repeatedly reversing propeller thrust. That
is why this type of tug, and other tugs having prohl ems
in ice, should not tow on a line. The assisted vessel might
not react fast enough and/ or not be abl e to stop
immediately to avoid danger of collision or worse.
For these tugs in part icul ar, but also in general,
towing on a line in ice conditions is not without risk, as
explained later.Towing on a line is only acceptable when
a ship is moving at a very controlled low speed on a
straight course or when taking easy bend s in a channel
or river and during berthing or unb erthing operations.
Assistance in ice conditions during arrival and departure
is then carried out mainly by pushing and includes
br eaking th e ice and swee ping away th e ice from
between ship and berth. Without the help of tugs it is
almost impossible, in mo st cases, to remove ice from
between a ship and berth.
Whil e preparing a berth location, tugs often work
very close to the dockside. Some objects may stick out
or overhang, so tug sides should be clear of overhanging
fend ers, etc. Tugs sho uld, of course, always be very
careful when working between a ship and the dockside.
With resp ect to tug towing wires or ropes, the y
sho uld retain their strength in low temperatures but
should nev er be allowed int o icy water because it will
then be very hard to handle them.
The most reliable tugs in ice conditions are nonmal
ice strengthened conventional tugswith open propellers.
Twin screw tugs are pr eferable because of their better
manoeuvring properties.
Propellers and rudders may have ice pro tection and
nozzles may be fitted with protection bars or ice knives
fore and aft of the nozzle. Alth ough nozzle construc tion
itself may be adapted to ice conditions,
in particular
shallow draft tugs with nozzles are very limited in their
performance when operating in ice, due to the fact that
nozzles are often blocked with ice. This does not mean
however th at thi s type of tug is worthless in th ose
conditions . They can create an effective surface stream
for moving ice in situations as explained later. Deep
draft tugs are more reliable during towing operations.
Based on experience gain ed in some of the larger
ice ports, the following tug types are not very suitable
for service in ice conditions:
VS tugs.
Tugs with propellers in nozzles.
In addition, full scale trials were carried out in 1984
in Finland with two ice-going tugs, one fitted with an
ope n propeller and the other with a steerable nozzle, to
investigate thei r performance in ice conditions. During
a twenty hour test the nozzle of the latter tug was blocked
twelve times and the tug had to be stopped each time.
Having said that, some tugs with azimuth prop ellers
in nozzles that have to operate in ice conditions have
been built recently e.g. for Finni sh and Danish owners.
Performance in ice of tugs with azimuth thrusters in
nozzles can be improved by a proper design such as
adequate clearance between the hull and the thrusters
and by short reaction times for pitch changes or for
turn ing the thrusters adequ ately to get the ice out as
quickly as possible when they are blocked.
3.4.4 Berthing in ice
A berth should be approache d at a small angle. As
soon as the forward spring is secured the engine should
be set to Dead Slow Ahead. Prop eller revolutions or
pr opeller pitch sho uld be incr eased gradually, jus t
avoiding breaking the spring. It is best to double the
spring and the rudder should be used to swing th e stern
of a vessel in and out and away from the dockside. Th e
water flow caused by the propeller will force ice out
from between the ship and the dockside and wash it
away astern of the ship. The engine should be kept
running until the propeller wash has swept away all loose
ice. The ship can then be berth ed. In this way, provided
it is weak ice, it can be removed co mpletely from
between the ship and berth . In the case of dense and
thick ice the assistance of tugs is requi red .
In some cases berth location could be such that a
berth can be approached parallel to the dock (see figure
3.14)_In this case ice may be pushed away by the bow.
If there is unbroken ice on the starb oard side it will
push the ship towards the berth and pr ev ent her
swinging out. Care should be taken to avoid any ice
getting between ship and dock. It may be necessary to
Figure 3.14 Shipapproad!es the herth nearly parallel to tk dock. Ice
ispushedaway by tk how. TMship ;, pressed towards tlu herth by
unlrroken iceonthe starboardside
TU G USE IN PORT 39
move the ship forward and astern a few times to move
the ice out or to press the ice together between ship and
dock. This can only be done in the case of young and
weak ice.
Sometimes, approaching parallel to the dock may
not be possible due to the presence oflarge pans of ice
or dense, thick ice directly in the ship's track. Other
methods should then be adop ted such as the use of tugs.
Several procedures for the use of tugs in ice during an
approach towards a berth while berthin g or unbe rthing
are now considered.
In general, while approaching a berth in ice, the bow
of the vessel should be kept as close as possible to the
berth with the assistance of a tug pushing at the bow
(see figure 3.15A, B). Th e ice between the bow and the
dock will tend to push the bow aside. After the forward
spring has been secured the tug can break the ice outside
the ship and then wash the ice away from between the
ship and the dock (see figure 3.15 C, D). The ship itself
can swing its stern in and out by rudder action and use
of the engine, as explained.
Figure 3.75 Tug assistance in ice duringapproach to the berth
andwhilemooring
Sweeping ice away from round the bow area can
also be done effectively by a tug just ahea d of the ship
(see figure 3.16). With its stern directed towards the ship's
bow, the tug can sweep ice away by putting its engines
ahead . In this case the ship should not pass any head
lines, which would prevent the tug working in this way.
Since ice at the bow is usually squeezed between
bow and dock,getting it out is very difficult, Good results
can be achieved when there are 20-30 metres of free
berth ahead of a ship's planned position. The ship should
approach its berth ahead of th e planned position
(position I of figure 3.17) . Breaking ice at the outer side
of the ship and sweeping ice away from between the
ship and dock are then carried out. The ship can then
be brought alongside and mov ed astern while the tug is
constantly pushing the bow towards the dock.
40 THE NAUTICAL INSTITUTE
Figure 3.76 Tug sweeping ice awayfrom between ship anddock
. - ---- .,.. .
Figure3. 77 Mooring in icewhen some 30m free oerth is
availahlt infron t of the bowposition
Figure 3.t8 Combination oftugandbow thruster whilt mooring
A bow thruster can also be very effective in sweeping
ice away (see figure 3.18). A ship should approach the
berth at an angle. After the forward springs and head
lines are ashore, the stern is taken as far as possible out
by rudder and ship's engine. The bow thruster should
then be set to take the bow off in order to create a water
flow between ship and dock. Th e bow should be held
to the dockside by the ship's ropes and by the pushing
tug. Th e water flow of the bow thruster will sweep ice
away from between the ship and dock.
Another method by which good results are obtaine d
is moving the ship astern towards the berth to moor
with its starboard side alongside (see figure 3.19). After
approaching the berth at a smal l angle and securing the
back spring, the engine should be set for astern. Th e
propeller stream is normally very strong and will move
the ice be tween the ship and dock quick ly in the
direction of the bow. The bow should be swung in and
out by tug or bow thru ster. Thi s method is used and
suitable for large r vessels, as prop eller thru st astern is
lower than on ahead and consequently the tension in
the sp ring line(s) will be less.
Figure 3.19 Good results when approaching the berth altern
and mooringstarboard side alongside
These be rthing pr ocedures whereby a ship uses
engine and spring lines is not suitable for ships with
large engines and high starting power and!or high power
on Dead Slow. All operations in ice with these ships are
norm ally carried out by tugs. After approaching the
berth at a small angle, a spring line and head line are
made fast forward (see figure 3.20). O ne stern tug on a
line is used to take the stern from the berth and a second
tug is used for pushing th e stern towards the berth. Thi s
tug will also clear the ice. Propeller wash is not used.
Berthing will, in general, take a long time.
Figure 3.20 1Ug assistance when mooring in ice with
ships and powerful engines
In some cases, when po ssible.ft is better to appr oach
the berth astern with a stern tug towing on a line (see
figure 3.21). By giving short 'kicks ahead' on the ship's
engine to stop the vessel, ice will be pushed away from
the dock in the direction of ship's move ment
Figure 3.27 Ship approachingtheberth altern. OMaft tug secured.
Occasional bursts ahead onthe engine blow away the ice
With large ships, good results in removing ice from
between ship and berth are sometimes obtained with
two tugs working stem to stem. These two tugs, moving
togethe r forward and astern between the ship and berth,
sweep ice away. The safety of these tugs is ensured by
an additional three tugs keeping the ship in position as
shown in figure 3.22. Obviously, a large number of tugs
is required in this case.
Figure3.22 Two tugs stem to sumdearing
ice betwem shipand
berthwhit. ather tugs keep theshipin position
3.4.5 . Unberthing in ice
Before unberthing, tugs should break ice around the
ship and in areas of about 20·40 metres distan ce from
the bow and stern .
Some vessels can be taken off the berth by the stern
with the assistance of a stem tug towing on a line (see
figure 3.23). At the bow the ice between bow an d dock
will prevent the ship from coming too close to the berth.
In addition, the stem tug "ill drift the ice between the
ship and dock, which again prevents the ship from
coming too close to the dock when moving astern.
Sometimes it may be necessary to unberth the ship
bow first (see figure 3.24). A second tug may th en be
need ed to br eak ice near the stern and to prevent the
stern from coming too close to the berth . Someti mes
even the assistance of a third tug may be required to
crush ice at the outer side of the ship.
TUG USE IN PORT 41
Figure 3.23 Ship ofmedium sb.e departing. Before departure tugs
have broken ice around he.' in areas some 20-4Om from bowandstem
Figure3.24 Unmooring howfirst. A stem tugis required when ice
near the stemneeds tohe hroken andwhen the stern may touch the
berth when thsbow ispulled off Sometimes a third tugis required to
break ice alongside the vessel
When a departing ship has to be swung around after
being unberthed this should be carr ied out in a pr epared
area or channel in the ice. This area or channel sho uld
b e prepar ed by large tugs or icebreakers pri or to
42 THE NAUTICAL INSTITUTE
Figure 3.25 Channelthrough the ice prepared by ice breakers or
strong tugs. A ship moving astern through the ice is safis t. M en the
stern tngisstopped in orby ice theship can immediately be stoppedby
propeller
departure. Tugs handling the ship can assist the ship in
swinging and break ice when necessary.
3.4.6 Safety of tugs in ice
Tugs are at great risk when towing on a line through
a channel in ice. As previously mentioned, when a tug
has to stop due to nozzle blockage with ice, the ship
should also be stopped immediately. The tug may also
en te r dens e ice and consequently lose speed very
quickly. The assisted ship , therefore, sho uld always use
engines with utmost care. Even the n the safety of the
tug is still at risk. It is for these reasons that the safest
method of towing on a line is m oving a ship astern (see
figure 3.25). The engine should at all tim es be ready to
go ahead. When necessary, th e ship can be stopped
imme diately.
Further practical and useful information regardin g
navigating and manoeuvring in ice can be found in
'Marine Towing in Ice-covered Waters ' by Peter E.
Dunderdale and in 'Ice Seamanship ' by George Q
Parn ell (see References).
Chapter FOUR
TUG CAPABILITIES AND LIMITATIONS
4.1 Introduction
Now THAT VARIOUS ASSISTING ~l ETHODS and types of tug
hav e been introduced 10 the read er the more practical
subject - effective shiphandhng with tugs - is addressed.
When a ship is stopped in the water, meaning she
has no spee d through the water, the effect of, let us say,
a 30 tons bp tug is th e same irr espectiv e of type ,
assuming that the tug operates in the most effective way.
Differences in tug performance mainly become apparent
when a ship has spee d through the wate r. Th e emphasis
in this chapter, therefore, is on tug performance while
assisting ships under way.
Wh en considering effective shiphandling with tugs
there are, apart from the essential issue of bollard pull,
two very imp ortant aspects to be considered:
Correct tug positioning.
The right type of tug.
Different tug operating positions are consid ered in
relation to their effect on a ship. The performance of
different tug types are discussed, taking into account
both the various assisting methods and the different tug
positions relative to the ship. With respect to type of
tug, specific aspects of various tug types are necessarily
discussed in a fairly gen eral way, since there are so many
variations in design within each type. Reviewing them
all individually goes far beyond the scope of this book.
4.2 Basic p rinciples and definitions
For a good understanding of tug performance and
shiphandling with tugs some basic principl es and
definitions are first considered. These include the pivot
point, towing poin t, pushing point and lateral centr e of
pressure, direct and indirect towing and tug stability.
4.2.1 Pivot p oint
The pivot point is an imaginary floating point,
situated somewhe re in the vertical plane through stem
and stern, around which a vessel turns wlren forced into
a directional change. The form of the submerged body,
rud der size and type, trim, underkeel clearance and
direction of movement all affect the position of the pivot
point of a vessel. The exact location of the pivot point
is therefore not stationary but variable.
For effective tug assistance the location of the pivot
point of the vessel to be assisted is very important. It
affects the choice of operating positions for the assisting
tugs. When a ship is dead in the water and forward thrust
is applied with port or starboard rudder, the pivot point
lies far forward. As soon as a ship gathers speed the
pivot poin t moves aft. O nce a ship is in a stea dy turn
with rudder hard over the pivot point settles in a position
approximately one third of the ship's length from the
bow (see figure 4.IA).
B
Figure 4.1 Location of 1Mpivotpointfor aship at speed
SituationA: Ship turning with starboard rudder. The pivotpoint lies
between how and midships
Situation B:A tug ir pushingforward. Althaughthepivot point lies
further aft, theeffidforwardis la» becauseof theopposing
hydrodynamuforces also centredforward. When stasboard rudder ir
also applied thepivotpoint movesfUrlkr forward
Situation C: A tug ir pushing aft. The Iaural resistanceforward
contributes to1M swing. Thepivotpoint lies farforward,partieu"'rly
when starboard rudder ir also applied
For a good understanding, figure 4.1 requires a little
expl anation . In this figure three ships are shown with
different forces working on the ships. A force applied
to a ship, for instance a tug force or a rudder force,
gives a transverse force and a turning moment, resulting
in a lateral velocity and a rate of turn. The arrow V is
the direction ship's centre of gravity (G) may move as a
result of the lateral velocity caused by the rudder force
or tug force, and th e forward velocity of the ship. The
lat eral movement of th e ship is op pos ed b y th e
hydrodynamic forces centred forward on the ship
having headway, which also creates a turning moment.
Thi s turning mom ent opposes (situation B) or assists
(situa tion C) the turning moments created by th e tugs.
The location of the pivot point (PPj results from the
motio n of th e sh ip cansed by the vari ou s forces
mentioned working on the ship.
TUG USE IN PORT 43
[r- ; - . - - - ~ "?1
1::O~ :-·· · - ·b---- :
<,
A ship moving astern has its pivot point som ewhere
between stern and midships when turning, e.g. by use
of a bow thruster. The exact position of the pivot point,
therefore, is different for each individual ship and ship
condition.
moderate speed ahead. In addition, the tug's underwater
resistance counteracts the turn.
Other forces of externalorigin that affect the position
of the pivot point are wind and current. In port areas,
wind and current may var y in speed and direction
depending on locati on . Relative wind and curr en t
directions may also vary during a transit to or from a
berth due to changes in a ship's heading. For instance,
wh en entering a h arbour basin from a river the current
gradually decreases but als o changes in rel ative
direction . As a result, the influence of wind and current
on a ship fluctuate . Dep ending on the angle of attack
and point of application, wind and
cur rent may decrease
or 'increase the rate of turn, moving the pivot point
furth er forward or aft, or may have only a sideways
effect.
A ship dead in the water (see figure 4.2A) with one
tug pushing (or pulling) forward and one with th e same
bollard pull, pushing (or pulling) aft, pivots around its
midship s when on even keel. For th e same size of vessel
and same conditi ons, rate of turn depends on the tug's
bollard pull and on the lever arms between tugs. The
longer the lever arm the larger the turning effect of the
tugs. Wh en a tug pushes at th e bow or stern of a ship
that is stopped in the water, the ship turns aro und a
point located approximately a ship's width from the
stern or bow respectively (see figure 4.2B).
When a tug starts pu shing a ship underway at a
position aft, the pivot point shifts forward. The pushing
force has a long lever arm and the lateral resistance
forward then contributes to the swing (see figur e 4.1C).
It is evident that the furth er forward and/ or aft of the
pivot point that tug forces are exe rted on a ship, the
longer the lever arm and hence the more effective the
assistance will be.
It should however be noticed that the effect of the
forward tug differs with ship's hull form, draft and trim .
For conventional ship form s, on even keel in deep or
shallow water, the opposing hydrodynamic force is
ind eed centre d forward, as mentioned in 'Performanc e
and effectiveness of omni-direc tional stern drive tugs'
(see References). When, for instance, taking a tank er in
b all ast an d trimm ed by the stern, the opposing
hydrodynamic force is centre d much more aft, resulting
in a much larger effect of the pushin g tug forward.
A
The pivot point also changes position wh en, in
addition to rudder for ce, othe r forces such as bow
thru ster or push/pull forces from an external origin, such
as tugs, are applied. When, in order to assist a ship under
speed and in a turn, a tug starts pushing at the bow in
the direction of the turn, the pivot point moves aft. This
is because the ship tends to turn around a point which
lies further aft than when only rudder force is app lied .
Although the lever arm of tug force would be rather
long the effect is not very pronounced, so there is
another aspect to be con sidered. As explained earlier,
a tug pushing for ward tri es to move the bow to
starbo ard, say. This creates an opposing hydrodynamic
force, also centred forw ard (see figure 4.1B). The
hydrodynamic moment counteracts the turning mom ent
exercised by the tug. The effect of the pushing tug is
very small. This is also one of the reasons why the effect
of a bow thruster is small on a ship making slow to
Turning diameter is independent of ship's spee d as
long as engine pr opeller revolutions or propeller pi tch
match a ship's spee d but is depend ent on rudder angle
applied. When in shallow water, such as in most port
areas, turning diameter increases considerably, due to
the larger hydrodynamic forces oppos ing the turn .
Beamy full bodied ships have a smaller turning
diameter and a furth er aft pivot point than slen der ships.
When a ship is down by the head turn ing diameter is
also less and the pivot point lies further aft than when
on an even keel.
4.2.2 Towing point, pushing point and lateral
centre of pressure. Direct towing and
indirect towing. Skegs
Figure 4.2 Location ofthe pivotpointin aship with$0 speed
Situation A: Tugsofequalpowerpushinglpullingforward andaft.
The pivotpoint lies amidships. The tugs towing on a line have a
longer lever andso a larger effict
SituationB: Forward tugpushing; the pivot point lies far aft.
When an after tug ispushing, thepivotpoint lies farforward
The relative positions of the centres of three different
resultant forces ar e mainly resp on sible for a tug 's
performance. These are centre of thrust, the tow or
pushing point and the lateral centre of pressure of the
in com ing water flow. In particular, th e mutual
relationships between towing or pushing point, centre
44 THE NAUTICAL INSTITUTE
Com grande boca
Estreito 
Tem a ver com a estabilidade direcional
of thrust and centre of pressure affect not only the
effectiveness but also the safety of a tug.
The towing point
.For tugs towing on a line, th e towing hook or towing
"linch is not necessarily the tov..i ng point. The towing
point is that point from where the line goes in a straight
line from the tug towards the ship. For tugs pushing at a
ship's side the contact point or pushing point is of
impor tance . Before discussing the cap abilities and
limitations of different tug types the towing and pushing
point in relation to the location of propulsion and centre
of pressure are considered.
The lateral centre ofpressure
The lateral centre of pressure is a non stationary
point. Its location depends on the underwater hull form
including appendages such as rudder and pro pellers,
on the trim of the tug and the angle of attack of the
incoming water flow. The influ enc e of rudder and
propellers on the location of the centre of pressure seems
to be rather high .
Tractor tugs and especially VS tugs hav e a large skeg
aft, resulting in an aft lying location of the centre of
pressure.
Incoming water flow exerts a force on the tug. The
point of application of this force is the lateral centre of
pressure . The direction and magnitude of the force
depends on the underwater lateral plane and shape, the
angle of attack, the und er keel clearance and on the
speed squared. Speed, therefore, is a dominant factor.
The exact location of the lateral centre of pressure
and the magnitude and direction of the resultant force
created by the incoming water flow for different angles
of attack and speeds can best be determin ed in a towing
tank. The locations of the cen tre of pressure mentioned
later are m erel y an indication and are based on
observations and information e.g. from Voith.
Wh en water flow towards a tug comes from abeam,
caused either by crosswise movement of a tug through
the water or by a current at right angles, the centre of
pressure generally lies behind midships in a positi on
about 0·3 to 0·4 x LW1. from aft. For conventional tugs
it is prob ably more often in the vicinity of 0·3 x LW1.
from aft and for tractor tugs closer to 0·4 x 1.WL from
aft. Reverse-tractor tugs and ASD·tugs may have a more
forward lyi ng centre of pressure, depending on the hull
design.
When a tug turns with its bow into the direction of
wate r flow, the centre of pressure moves forward, The
smaller the angle between incoming water flow and tug's
head ing the more forward the centre of pressure lies.
For conventional and tractor tugs the centre of pressure
does not generally move forward of amidships (0.5 x
1.W1.). Reverse-tractor tugs and ASD-tugs m ay
experience a position of centre of pressure forward of
midships with a forward incoming water flow. When a
tug is turnIng with the stem into the water flow the centre
of pressure moves aft and with an acute angle of
incoming water flow will lie far aft.
Figure 4.3 shows a tug moving ahead, towing on a
line, assisting a ship und er speed . The resultant force
created by incoming water flow is force F, assumed to
be centred approximately near amidships . Force F can
be resolved into lift force 1. and d rag force D,
comparable with the lift and drag forces on rudders or
aeroplane wings. Lift force 1. gives an additi onal force
on the towline and drag force D has to be overcome by
the tug' s thrust. Towing point T lies a little behind th e
ce n tre of pressure. T h e for ce in th e towline in
combination with force L creates a counte r-clockwise
turning moment.
L
F "" Resultanthydrodynunic force, on tugbulland appendages
L = Lift Icece
D = Drag force
C = Lateral centre of pressure
T - Towing point
Ps = Location of propulsion atstem
Pt ...Locencn
of propulsionfot ltact ,n tuV
Figure 4.3 Forces createdonassistingtug, moving ahead
Consider two locations of propulsion -
position Ps for stern driven tugs, a conventional
tug for example, and position Pt for tractor tugs.
The smaller the distance between T and C the
smaller is the turnin g moment. Thus less steering
power, b y eit he r rudder d efl ecti on or
om nidirectio nal prop ellers, is ne ede d to
counteract that turning mom ent. Consequently,
more engine pow er is avai lable for towing. If
propulsion is located aft at Ps, starboard rudder
is needed, giving a little m ore drag but also an
additi onal force in the towline. If propulsion is
located forward (Pt) then sideways steering power
is ne eded, but in the opposite dir ection. This
consequently decreases the towline force.
With increasing speed, force F increases
and consequently lift force 1.. The higher the
sp eed th e m or e steer ing effor t is needed.
Therefore, the higher the speed the larger the
TUG USE IN PORT 45
, F
Figure 4.4 Forces createdon assisting tug; moving astern
difference in towline forces between a conventional and
tractor tug. As a forward tug the tractor tug is more
effective if it is possible to operate stern first.
Towline forces also create list. Considering the
direction of steering forces it is evident that with the
propulsion located in position Ps the sideways steering
forces increase the tug 's list, while with propulsion
located in Pt steering forces counteract the list caused
by the towline force . When an ASD -tug is operating
like a conventional tug its high steering forces result in
larger heeling forces. This is also due to the fact that the
centre of pressure of this tug type lies generally
somewhat further forward, resulting in a larger turning
moment to overcome. The larger heeling moment is
more or less compensated for by 'the large beam of this
tug type .
Although the towline position discussed here is the
most effective for both conventional and tractor tugs
when operating as a forward tug on a line, the towing
point on tractor tugs is located further aft for safety
reasons and for better performance as stern tug . This is
explained later. The consequence of the further aft
towing point on a tractor tug is an even less effective
tug as forward tug. More sideways steering power is
needed to counteract the larger anticlockwise turning
moment, resulting in a further decrease in towline force.
By giving more engine power in order to achieve the
same towline force as a conventional tugwould exert,
the tug comes more in line with the towline, resulting
in higher turning moment and drag force to be
overcome. At higher speeds drag force may become so
large that a tug is unable to react SUfficiently to the force .
and swings around.
The consequence is that when working forward a
conventional tug is more effective when towing on a
line than a tractor tug. The better the omnidirectional
thrust performance of a tractor tug the more effective it
will be. Reducing the underwater resistance of a tractor
46 THE NAUTICAL INSTITUTE
tug would increase its effectiveness as a forward
tug, However, this would have consequences
for its effectiveness as stern tug when operating
.in th e indirect mode whereby use is made of
the hydrodynamic forces on the tug 's hull.
Therefore a compromise has often to be found
for the location of the towin g point and also
for the underwater profile of a tug.
In figure 4.4 th e tu g is m oving astern
through the water. The centre of pressure lies
much furth er aft e. g. at location C for
conventi onal tugs as well as for tractor tugs.
Tractor tugs are considered first. The towing
point T is very dangerous, not only because of
the large heeling moment caused by the
hydrodynamic force on the tug's hull, but also
because large crosswise steering forc es (at Pt)
have to be exerted by the tug in order to
compensate for the turning moment created by th e
incoming water flow, giving additional forc es in the
towline and additional heeling forces. At higher speeds
and /or too large angles of attack of incoming water flow
the resulting heeling forces may cause capsizing of the
tug. The large vertical distance between the propulsion
units and towing point also contributes to the high
he eling moment. Therefore although towline forces are
high for tractor tugs it is much safer to locate the towing
point aft at a small distance abaft C, the centre of
pressure for smaller angles of attack. (In VS tractor tugs
the towing point lies gen erally just above the middle of ,
the skeg.) The tug then comes in line with the towline
when its engine s are stopped and very little steering
power is needed to keep the tug in the most effective
position when the indirect towing method is applied.
Neither do conventional tugs operate as shown in
figure 4.4 because with higher speeds it is almost
impossible to steer the tug safely and is th erefore very
dangerous. If the angle of attack increases, the increase
in towline forces might cause the tug to capsize. At very
low speeds conventional tugs often operate broadside,
for instance as a forward tug steering a ship whi ch is
moving astern or as a stem tug steering a ship moving
ahead. Especially on single screw tugs, this can only be
done with a gob rope or by passing th e towline through
a fairlead situated aft, as is the case on som e cornbi-
tugs. The gob rope system is dealt with in more detail
in Chapter 7. Using a gob rope the towing point can be
shifted to a position somewhere between the after end
of the tug and the towing bitt or winch. By shifting the
towing point from TI to T2 (see figure 4.5), the tug can
stay broadside on and steer the ship by moving ahead
or astern using the tug' s engine. By shifting the towing
point to a position at the stern of the tug, the tug can be
pulled astern by a vessel without the danger ofcapsizing.
The tug can then use its engin e to control the ship' s
speed. Twin screw tugs often use the propellers instead
of a gob rope to keep the tug in the position as indicated
in figure 4.5.
Heel inclinação
/
I
I
pi;
-"y*T2=----j
T1
I
I
FrguTt 4.5 Tug WIlTking onagob rope
Ship has a "try lowspeedahead: Tug can steer the .,,,,1 bygoing ahead
orastern. on the engine. Corwentional twin SCTtW tugs don't always need a
gob rope; they can mate a (l)Uple bythe prop,llm tostay broadside
Direct and indirect towing method
The direct and indirect towing methods are
explained in figure 4.8 (overleaf). P is the location of
the propulsion, C of the centre of pressure and T is the
towing point.
The direct towing method is carried out by an after
tug on a line at low ship speeds . The tug pulls in the
required direction, either to give steering assistance and!
or to control the ship's speed. Tractor tugs assist with
their stem directed towards the sterJof the assisted ship
and ASD/reverse-tractor types of tug assist with their
bow towards the stem. Whether tractor tugs or ASDI
reverse-tractor tugs are more effective in steering control
depends on the relation between the distance P'T and
CoT, the tug's engine power and thrust performance in
the pulling direction, but also on the tug's underwater
plane . The smaller the distance CT in relation to PT the
better the tug's performance in the direct towing mode.
The indirect towing method is applied by an after
tug at speeds higher than five to six knots. With the
indirect towing method, the tug makes use of the
hydrodynamic forces created by incoming water flow
on the tug's skeg and/or underwater body. The aft lying
towing point of the tractor tug, and consequently the
small distance between towing point (T) and centre of
pressure (C), implies that only a little crosswise steering
power of a tug is needed to keep the tug in the most
effective position to exert the highest steering
forces to
the assisted ship.
The ASD-tug/reverse-tractor tug has a larger
distance between the towing point (T) and centre of
pressure (C). Consequently, more crosswise power is
needed to keep the tug in the most effective position,
thus decreasing towline force .
In the indirect towing mode tugs can give high initial
steering forces to a ship underway at speed, as can be
seen in some performance diagrams in section 4.3.2.
As soon as a ship starts turning she gets a drift angle
and speed of ship's stem, being at the outside of the
tum, increases initially, so tug's speed has to increase,
resulting in even higher steering forces. The indirect
towing method is further dealt with in
Chapter 9 - Escorting.
From this brief explanation of direct and indirect
towing it is apparent that the locations of the centre of
pressure and towing poin t are very critical. A more
forward lying towing point in a tractor tug results in
higher towline forces, but the safety of operations and
Plwro:SJutland Islo.nd.r Caunci1
FiguTt 4.6 Swivel fairlead on the afier endofa tug's d,,*
for the gob rope
Photo:AutWr
Figur« 4.7 The 11Jrg' fairlead isthe oft lying towingpoint
on a VS tractor tug
TUG USE IN PORT 47
a) The skeg on tractor tugs . This type of skeg provides
better course stability when free-sailing ahead (with
skeg aft). It generates additional towing forces when
operating as stern tug in the indirect towing mode
because it increas es the tug's lateral underwater area
and brings the centre of pressure more aft, closer to
the towing point. The skeg may have a specific form
to generate the highest possible lift forces.
b) An aft skeg on tugs not being tractor tugs: A vertical
fin attache d to the tug's und erwater hu ll in the
centreline of the after section at some distance before
As can be seen a skeg m ay be effective for one task,
bu t ineffective for other tasks. With regard to skegs it
sho uld therefore be well cons idered what is expected
from a tug. There is a large variety of skegs. Mainly the
following skegs can be found on tugs, of which some
have already been m entioned when discussing tug
types :
S1c£gs and their ejfed
The tug's underwater fonn should be such that
the tug can perform in the best possible way. Skegs
can contribute to a tug's perfo nnance and tugs are
often designed with some sort of skeg.
A pure harbour tug should in general be most
effective at ship speeds below six to seven knots,
when the assisted ship is slowing down and has to
stop its main engine, losing its controllability to a
large extent and during turning, berthing and
unberthing operations when hardly any use can
be made of the ship's own manoeuvring devices,
except for bow and stern thrusters (see paragraph
5.1). Such a har bour tug should be able to apply
the high est possible towin g forces in all the
required directions and with a short response time.
High pushing forces may be needed with the tug
operating at right angles to the ship still having
speed. A low underwater resistance is therefore
needed.
bulbous bow, can be found on a number of ASD -
tugs, which also brings the centre of pressure more
forward .
Pushingpoint
Wh en pushing at a ship's side, the larger the
distance between the pr opul sion unit(s) (P) and
the pushing point (Pu) in relation to the distance
between the centre of pressure (C)and the pushing
point (Pu), the better the tug can work at right
angles (see figure 4.17).
On the other hand, a tug may have to operate at
higher speeds, and escorting of ships may be one of the
tug tasks.Then a well designed under water body, which
may include a skeg, plays an important role in generating
high towing forces in the indi rect mode by making use
of the hydrodynamic forces working on the tug's hull.
®
I I ,
I I I i
I I I I\)' ~ I I .~ "~ , I '~ ,
IT-- ..., J
. --.:...1_
. ~
o
(j)
- ,/
---
-- ::==_.
- -=:::::
Figure 4.8 Direct and indirect towing methods
®
®
I I I
I I I •
: I l I i ........,;;:.-
"' ''J. I I IJ.l I
, ,J. I _
!'. ~ --
'.--- --
Top: Direct TowingMetJwd - A: Tractor tug
B: ASDIReverse-tractor tug
Position 1.' Steering and retarding Position 2: Retarding
Ii IIi r I I iI'1" '"H~HH
Bottom: Indirect TowingMetlwd - A: Tractor tug
B: ASDIRevtrse·traclor tug
Position 7: Steering and retarding Position 2: Retarding
as a result performance decreases. A more forward lying
centre of pressure in ASD/reverse-tractor tugs doe s not
affect tug,safety but increases the tug's perform ance as
a stern tug. To minimise steering effort in keeping a VS
tug in line with an escorted vesse l when no assistanc e is
required, a second towing point is installed at the after
end of some VS tugs, which pins the tug under the
towline and reduces the steering effort required. Wh en
steering assistance is,:required then the original towing
point more forward ' is used again , which should be
possible without releasing th.e towline.
In ASD-tugs, specific designs are used to bring the
centre of pressure more forward e.g. in the USA ASD-
tug Kinsman Hawk. This tug is designed with a deep
forefoot which results in a more forward position of the
centre of pressure and the stern is cut away significan tly
to provide a clean flow to the azimuth propellers and to
push th e tug's centre of pressure forward as well.
Forward skegs at the bow, or in combination with a
48 THE NAUTICAL INSTITUTE
Photo:]M. Voith GmbH, Gmruzny
Figure 4.9 VStugoperating in the indirect towing mode
the propellers, to give the tug a better course stability
when free-sailing ahead.
c} A flat vertical skeg, or box keel, in the centreline of
several ASD-tugs and reverse-tractor tugs, which
extends for some distance before the propellers to
the forefoot. It provides better course stability when
free-sailingahead and often, depending on skeg fonn,
particularly astern. It generates additional towing
forces when operating as stern tug in the indirect
towing mode and when ASD-tugs operate as
conventional tugs at a ship having speed.
d) Skeg at the bow of an ASD or reverse-tractor tug.
Such a skeg improves the 'course stability when free-
sailing astern (not ahead) and increases a tug's
perfonnance when operating as stern tug in the
indirect mode and to some extent as bow tug when
operating bow-to-bow at a ship having headway.
Combinations of the skegs mentioned can be found
as well, for instance of skeg types c and d.
When reading the following paragraphs and the
capabilities of the various tug types in the different
situations it is good to consider at the same time the
possible skegs and their effects.
4.2.3 Stability
Operational stability, one of the basic design
requirements, is of great importance for harbour tugs
due to the nature of their work. Conventional tugs, when
towing on a line as a forward or after tug, can experience
very large athwartships towline forces. The same applies
to ASD·tugs when towing on a line as a conventional
tug. High towline forces can also occur when
conventional tugs are operating in the way shown in
figure 4.5.
Tractor tugs and ASD/reverse-tractor tugs also
experience high athwartships towline forces when
indirect towing. At high speeds these forces can be far
in excess of a tug's bollard pull. Towline forces can
increase even further due to dynamic forces caused,
amongst other things, by irregular engine performance
and/or tug control, tug movements due to waves, and
when towlines are used with too little stretch, such as
steel wires.
Tugs with azimuth propellers may heel over
appreciably if thrust is suddenly applied
athwartships. These tugs tend to be powerful
with respect to their size and the deeply
immersed point of application of thrust,
implying a long heeling lever, results in a large
heeling moment. Whether the indirect or direct
towing mode
is applied this heeling moment
counteracts the heeling moment created by
towline force. When conventional tugs tow on
a line the heeling moment caused by transverse
steering thrust enlarges the heeling moment
created by towline force, as explained when
discussing lateral centre of pressure. The same
happens when ASD-tugs operate like conventional tugs
while towing on a line. In figure 4.10 heeling forces due
to towline force, lateral resistance and steering force are
shown for a conventional tug.
All these aspects should be taken into account when
tug stability requirements are considered. Means of
increasing stability and reducing the heeling effects of
external forces on a tug include the following:
High GM and good dynamic stability
Good static and dynamic stability is required because
of the high dynamic forces a tug experiences. A tug
needs considerable residual dynamic stability when, due
to a sudden force, she heels over considerably. The tug's
beam has a large influence on its GM (IuitialMetacentric
Height). Making a tug beamier results in a larger GM
and righting moment, assuming all other factors
influencing its stability are unchanged. The length/width
ratio of harbour tugs is decreasing and many modern
tugshave a length/width ratio of between approximately
2·8:1 and 3:1. Several harbour tugs with even lower
length/width ratios have also been built, such as the USA
tractor tug Sroward (l.o.a, 30m, bp 53 tons) with a length/
width ratio of 2·5:1 or the Canadian reverse-tractor tug
Tiger Sun [l.o.a. 21·7m, beam 1O·7m, bp 70 tons) with a
Tow5neforoe
51 ngloroe
"it"""-""'_lateral -reslstance
Figure 4.70 Heelingftrc<s working ona conventional tug when
towing on a line
M = InitialMetacentre COP = Centre ofPressure
COB = Centre ofBouyancy CG= Centre ofGravity
V = Transverse Speed
TUG USE IN PORT 49
B
Stability curve for tug
1,2 - ----- - - - - ---- "B'-- -------------
£ 1,0 ~llg.W.Yer cen.!.:_ _ _
~
E 0,8
L 0,6
o
0,4
N
l.:) 0,2
a-1"-----.-------''--r---,..L----,---'-'-..<!
a 10 20 30 40
Angle of' h ee l (deg)
Figurt 4.72 Tlu <jfict ofa radial hook on stability
HeelingItvercentre 'E' resultiugfrom anathwartships lowlint forct
applitd 10 a lowingpoim al the centre lint of tlu lug. Thetmolitu
forct causes a hulinganghof37°.
Htdingleoer radialhook :4' resultingfrom tht samtforce hUIin cast
ofa radial hook. Thelowlintfora caUStS now a huling anghof 78'.
How a radial hook, or a similar system , increases a
tug's safety and consequently safety of ope rations is also
shown in figure 4.12. The situation is for a specific
conventional tug and a radial hook with a radius equal
to half tug's width. A certain athwartships towline force
is applied to the towing point near the centre of the tug.
The towline force is such th at it almost results in
capsizing the tug, bec ause the maximum stability lever
is only a little more than heeling lever 'B'. No safety
margin is left. With a constant towline force, heeling
angle is approximately 31°. In case of a radi al hook, the
same towline force is appli ed initially at the same height
above the lateral centre of pressure. Th e heeling lever
'A'resultingfrom thisforce decreases fast with increasing
heeling angle, and in this specific case maximum heel
ang le caus ed by a constant towline force is
approximately 18°, with a.large safety margin left. The
system itself is further discussed in paragraph 7.2.
: ---_.
b
'"--"""""'lI~;....-;-----
. d c
:,a. G _
Reducing the transverse resistance ofthe hull
Making the lateral area smaller allows a tug to be
pulled more easily through the water instead of rolling
over. Low transve rse resistance of a tug's hull also
increases its capability of workiog at right angles to a
ship's side with a ship underway and reduces its heeling
moment. For tugs making use of the underwater body,
like conventional tugs towing on a line and tugs using
the ind irect towing method, this is contradictory to their
required perform ance.
For a good performance these tugs need a high lateral
resistance in order to be able to generate high towline
forces. A skeg may be added to increase lateral area
(which also lowers the centre of pressure) and lateral
resistan ce. The higher towline for ces that can b e
generated and the lower centre of pressure, result in
larger heeling angles and consequently in higher stability
requirem ents. A radial hook, as shown in figure 4.11,
reduces the heeling angle considerably.
Tugs are some times designed with sponsons, which
create larger righting mom ents at smaller heeling angles.
Fig. 4.1/ Tlu <jfict ofa radialMok
WlIha radial hook thth"ling leoerarm c isshoner than withths
lowingpoim in tht anne lint oftlu lug(leoerarm d). With a radial
hook tht righting leoerarm b ismuch wnger than without (leoerarm
a). Withan equalforu in tht lowline asshawn in thisfigurt, tht list
wiD bemuch less incast ofa radial lwok. A radial hook isa
substantial improvement
Reducing the height ofthe pushing point
The vertical distance between the pushing point and
lateral centre of pr essure should be as small as possible
io order to reduce the heeling moment created by lateral
resistance when a tug is pushing at a large angle to a
ship's side.
Reducing the height ofthe towing point
The height of the towing point abov e the lateral
centre of pressuse should be as small as possible in order
to reduce the heeling mom ent created by towline forces.
If a tug is equipped with a towing winch the lead of the
towline may be such that either it goes straight from the
winch towards the ship or it passes first through a towing
bitt or fairlead. In either case the height of the fixed
points from where the towline leaves the tug should be
as low as possible above the lateral centre of resistance .
Using a towing arm or radial hook (see figure 4.11) or
similar gear, a tug heels until the heeling moment is
counteracted by the larger induced righting moment.
A radial hook is a substantial improvement for tug safety
and performance.
A towline with good shock absorption characteristics
This is required to reduce sudden heeling moments
caused by high peak forces in the towline: Towin g
winches can be equipped with load reducing systems,
althou gh these are not suitable for narrow port areas,
when such a system would slacken the towline at high
load s, for instance, when the tug is close to a dock wall.
Tug freeboard being such that the deck edge is not
immersed at too small a heeling angle
According to the form er Briti sh Department of
Transport, Merchant Shipping Notice No. M.1531 ofJune
1993, thi s angle should not be less than 10° (see
Appendix 2). Openings in superstructures, deckhouses
and exposed machinery casings situated on the weather
50 THE NAUTICAL INSTITUTE
Projeções laterais
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Questão de prova
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C =B
deck, which provide access to spaces below deck, should
be fitted with watertight doors . Such doors should be kept
.closed during towingoperations.Air pipes,vents,exhausts
should be designed to be as high up as possible and/or
should be fitted with an automatic means of closure.
The International Maritime Organization (IMO) has
establishe d recommendations regarding static stability
curve requirements . These recommendations apply to
ships in international trade over 24 metre s in length. In
addition , recommendations on weather criteria have
been established for ships of 24 m etres in length and
over. These apply to reserve stability with respect to
wind, wind gu sts and waves. No specific
recomm endations for the stability of tugs, which take
into account towline forces) are given.
The IMO stability criteria and all related aspec ts are
speci fied in the 2002 IMO publication "Code on Intact
Stability for All Types of Ships Covered by I MO
Instruments". The publi
cation consists of the text of
Resolution A.749 (18) as amended by resolution MSC.
75(69).
National authorities or classification societies often
have their own specific regu lations or guidelines. For
example, the stab ility requirements of the Un ited States
Coast Guard (USCG) for towing vessels are much the
same as the static stability curve requirements of the
IM O. In addition, USCG requires tha t tugs shall either
meet the static towline pull criteria or the dynamic
towline pull cr iteria. The static towline pull criteria
include a required minimum G M by which no deck-
edge immersion will occur due to the heeling effect of
deflected propeller thrust at full helm, taking into
account the tow hook height above the centre of the
propeller shaft. The dynamic towline pull criteria require
a certa in residual righting ene rgy at the point of
equilib rium of the righting and heeling arm curves. The
hee ling arm curve sho uld be calcu lated on a given
formula which takes into account the deflected propeller
thrust and height of towing point.
T he American Bu reau of Shipping gives
recommenda tions for residual dynamic stability based
on a towline pull at 90° of 50% of bollard pull for twin
screw tugs with no rmal propellers and 70% of bollard
pull for tugs with azimuth or cycloidal propellers.
Heeling arm should be taken from the top of the towing
bitt to the centre of buoyancy or for an approximation
to half the mean draft.
Other semi-static methods are used, allowing for a
constant athwartships towline force acting on the hull ,
causing it to be dragged bodily thro ugh th e water. The
requi rements of the previously m ention ed British
Shipping Notice are such that the minimum GM of a ,
tug should be sufficient to limit the heel to an angle of
deck imm ersion when being towed transversely through
the water at a spee d of four knots. This results in the
following simple relationship . The GM in the worst
anticipated condition should not be less than:
0·076K
i.C.
Where:
K = 1·524 + 0·081. - 0-45r
L Length of vessel between perpendiculars
(in metres)
Length nf radial arm of towing hook (metres)
Freeboard (metres)
Block coefficient
The effect of a rad ial towing hook is included in this
formu la. The same kind of requirement can be seen in
Norway where a five knot transverse speed with a tow
of 65% of the bollard pull should be possible without
deck immersion.
Unfortunately, in a tug's working environment large
dynamic forces far in excess of static and semi-static
values may be developed and these are almost
impossible to estimate accurately. When designing tugs,
therefore, stabili ty and in particular reserve stability
should be considered very carefully, taking into account
all relevant factors including type of tug, required
assisting methods, propulsion system and working
conditions . It is clear that good stability not only
improves a tug's safety but to a large extent a tug's
capabilities and performance. With respect to escort
tugs, stability requirements are further discu ssed in
paragraph 9.5.1.
4.3 Capabilities and limitations
The capabilities and limitations of different tug types
are now considered, based on the two principal methods
of tug assistance:
Tugs towing on a line.
Tugs operating at a ship's side.
Fur thermore, the performance of different tug types
and th e effect of tug assistance on a ship's behaviour is
highlighted. Rudder tugs, mo re or less comparable to
tugs operating at a ship's side but able to assist in steering
to port as well as to starboard, are mainly dealt with in
Chapter 9, while discussing escorting with no rmal
harbour tugs.
4.3.1 Capabilities and limitations of tug types
Good cooperation between pilot and tug captain is
indispensable for smooth, safe shiphandling with tugs.
Safety applies both to the ship concerned and to the tug
and her crew. Goo d cooperation is based on a good
understanding of the capabilities and limitations of the
attended ship and, in particular, of the assisting tugs.
TUG USE IN PORT 51
t
Figure 4.73 Basicdijfimu;e betweentugtypes
The main difference hetween types of tugwithrespect toperftrmance
when towing on a line:
Conventionol types of tug.,. lowingpointlocaud forward of
propulsion.
Iiaaor types of lug- rowingpointIocaud aftofpropulsion
Tugs towing on a line
The capabilities and limitations of tugs towing on a
line are closely related to the location of the towing point
and the propulsion units, as explained in section 4.2.2.
That's why, in Chapter 2, tugs were classified according
to these locations. Of course, a tug's manoeuvrability
and stab ility are also factors of major importance when
considering capabilities and limitations, but that applies
to any situation and to any type of tug.
In figure 4.13 a conventional tug is shown with its
propulsion aft and towing point near midships. It could
also be an ASD -tug when towing on a line and using
the after winch or towing hook in the way conventional
tugs do .The other tug shown in figure 4.13is a VS tractor
tug. It may also be a tractor tug with azimuth propellers
or eve n a reverse-tractor tug.As can be see n, the location
of the propulsion and the towing point in a tractor tug
are opposite to those in a con ventional tug. The
consequences of this are discussed now.
Forward tugstowing on a line
Forward tugs towing on a line are dealt with first
(see figure 4.l4A, B). Irrespective of the type of tug, a
forward tug towing on a line can give steering assistance
or deli ver crosswise forces to an assisted ship to
starboard as well as to port. How ever, the re is a
difference in response times between the performance
of tractor and conventional tugs. When required, a
tractor tug can move easily and quickly from one side
to the other e.g. from starboard bow to port bow to
deliver steering assistance or to keep the bow up into
the current or wind. This is due to its ability to deliver
side thrust from th e forward located propulsion un its.
A conventional tug takes a little longer. In addition, to
manoeuvr e a tug from one side to the oth er, captains of
52 THE NAUTICAL INSTITUTE
conve ntional tugs often tum their tug
at the beginning of the manoeuvre
round the towing point on a tight
towline. It speeds up the manoeuvre
but is not neces sary and not
adv ocated , because it results in a short
pull in the wrong direction which may
adve rsely affec t the man oeu vr e,
especially for light ships .
A tractor tug (see figure 4.14A) is
less effec tive in giving steeringt assistance or creating sideways forceson a sh ip h aving speed than a
conventional tug. As explai ned in
section 4.2.2, a tractor tug lies more
in line with the towline a nd
consequ ently a relatively higher sideways resistan ce has
to be overcome at the expense of effective towline pull.
A conve ntional tug (see figure 4.l4B) can tum the
tug arou nd the towing point, has a lower resistan ce to
overcom e owing to the smaller angle of attack of the
incoming water flow and can make better use of the
hydrodynamic forces, all ofwhich contribute to a more
effective towline pull .
The effectiveness of a conventional tug increases,
depending on the angle (b),and of a tractor tug decreases
with increasing ship 's spee d. The higher a ship's speed
the larger the difference in effectiveness between tractor
and conve ntional tugs. The lower th e unde rwate r
resi stan ce of a tr act or tug and th e b e tt er the
omnidirectional thrust performance th e h igh er th e
effectiven ess . With respect: to thi s, it h as b een
experienced that for th e same ship's spee d an azimuth
tractor tug can operate at a larg er towing angle (a) than
a VS trac tor tug and consequently can apply higher
sideways and steering forces on a ship, owing to a beller
thrust performance in directions other than ahead or
astern.
With a
tractor tug care should be taken that, with
increas ing speed, angle (a) is not ge tting to o large
otherwise the tug cannot ov ercome sideways resistance
any more and will swing around on the towlin e secured
at th e aft towing point and will come alongs ide th e
vessel. If the towlin e is on a quick 'release towing hook
or on the winch, the line can be released by the quick
release mechanism. It can be concluded that a trac tor
tug forward is very limited by a ship's speed.
For a conventional tug angle (b) can be very large
without any problem. A conventional tug can create
large force s in the towline, even with a large towing
angle (b), by increasing angle (c). With increasing ship's
speed due att ention should be given to a tug's heading.
Wh en angle (c) between a tug's heading and incoming
water flow becomes too large the tug might not be able
to come back in line with the assisted ship an d, as a
Devida
When forward tugs towing on a line give
steering assistance, this generally results in a
force vector tending to increase ship's speed.
There is another important aspect to be aware
of when tugs operate on a line - they often
have a tendency to keep towlines tight when
no assistance is required. This also has an
unwanted speed increasing effect on th e
assisted vessel and should be avoided as much
as possible. Pilots therefore often order tugs
to keep the towline slack when no assistance
is required .
the tum as shown in figure 4.15. The effect
is greatest at low ship 's speed with not too
heavy ships. A similar method - ru dd er
hard over towards the berth, engine on dead
slow ahead and the forward tug pulling off
the berth - can be applied when unberthing
with just one tug. C are shou ld be taken not
to overtake the tug .
In figure 4.14C a tractor tug is shown again. At lower
speeds a trac tor tug can give steering assistance by th e
direct towing meth od (see position I a, Ib). Giving
steering assistance in po sition Ib will not increase the
ship's speed. On the contrary, in this po sition braking
force s are also applied. A speed increasing force vector
is applied in position 10. In position la a tractor tug is
less effective than the conventional tug of figure 4.14D
(position I). This situation is comparable to ' that of
forward tugs towi ng on a line, as previously discussed.
If required, a tractor tug can easily change from posit ions
I to posi tion 2 for speed control or to a position to give
steering assistan ce to port. Even at higher speeds (e.g.
seven knots) a tug can safely swing around from position
la to position 2 owing to the aft location of th e towing
point. In some ports position l a, instead of position 2,
is also used as a standby position.
Stern tugs towingona line
For tugs operating as a stem tug on a line the situation
is totally different. It depends entirely on the type of
tug and ship's speed whether steering assistance can be
given to bo th sides. From the po int of view of assistance
it is also very impor tan t whe ther a stern tug can control
a ship's speed. Whether th is is possible or no t depends
also on the type of tug and ship's spee d.
t
o
1t
c
With a good conventional tug forward on a line,
sideways forces on a ship can be exe rted by applying
rudder whilst at th e same time the tug is counteracting
When a ship's speed is very Iowa conventional tug
can give very effective steering assistance when
operating as shown in positio n Ib (see also the photo
of the tug Smit Siberie - figure 8.9). A tug's resistance
creates high steering forces wi thout increas ing ship's
speed. The tug itself uses most of its engine power to
stay free from a ship's hull and this results in additional
towline force .
It often happens that quic k release hooks canno t be
opened in case of emerge ncy, especially when towline
forces are very high an d the towline, if fastene d directly
to the towing hook, has a lar ge vertical angle with the
plan e of the tug deck. Towing winches with quick release
syste ms are safer. Nevertheless, ship's speed sho uld
always be carefully contro lled when tugs are towing on
a line forward and, as far as possible, the pilo t should
close ly observe the behaviour of the tugs.
When reverse-tractor tugs, and ASD-
tugs operating as reverse-tractor tugs , assist
as a forward tug on a line they operate in a
Figure 4.74 Comparison between tractor type tugs andconventional tugs when towing similar way to a tractor tug but with the tug's
on a line with aship having headway b ow direc ted towards the ship's bow. These
A: Tractor type oftug madefast asforward tug B: Conventional tug (or ASD-tug) tugs have a comparable performance to
asjorwaDrd;,:g , , t ra c to r tugs and the d ifference in
C: Traclor type of tug asafter tug : "011oenl,,,,= typeoftug asafUr tug effectiveness depends on the sam e factors
consequence, athwartships towline forces may get too as mentioned earlier when discussing the direct towing
high. TIlls may also be the case with an ASD-tug when me thod. See also paragraph 6.3.12, section operating
operating like a conventional tug. The high athwartships bow-to-bow.
towline forces might overturn th e tug if the towline
cannot b e released in time. This is called girting, which
also h appens when a ship's speed is too high in relation
to the tug's speed or po sition.
TUG USE IN PORT 53
Because of 
1c -Indirect method for high speed above 7kn
Para velocidade acima de 3 kn: pode emborcar a memos que use a gob rope quetransferirá o towing point 
To aft
<
At higher speeds th e indirect towing method is
normally used for steering control' (see position Ic).
Steering assistance at higher speeds can be given to port
as well as to starboard. At the same time the tug is able
to control the ship's speed.
ASD-tugs and reverse-tractor type tugs perform in a
similar way, but with the tug's bow now directed to the
ship's stern. An ASD /reverse-tractor tug will generally
be somewhat less effective than a tractor tug when using
the indirect towing m ethod for steering assistance. The
factors influencing performance and effectiveness of
these tugs in comparison to tractor tugs have already
been mentioned when discussing the indirect and direct
towing methods.
A conventional tug can only give steering assistance
to one side; in figure 4.l4D this is only to starboard.
When giving steering assistance a conventional tug
delivers longitudinal forces which may increase a ship's
speed. Moving to a position to starboard of the ship 's
stern, for instance, to give steering assistance to port or
to compensate for wind or current forces at that side is
impossible at speeds higher than one to two knots .
At speeds over about three knots, it is dangerous to
manoeuvre from position I to position2 in order to control
the ship's speed. A tug may come broadside on with too
high towline forces and may capsize unless the towline is
released in time by the quick release mechanism. When a
tug is equipped with a gob rope winch, by which the towing
point can be transferred to a position at the after end of
the tug, the tug can swing around from position I to
position 2 at somewhat higher speed.
At very low speeds, of not more than about three
54 THE NAUTICAL INSTITUTE
kno ts, conventio nal tugs can move from position 1 to a
position broadside astern the ship as shown in figure
4.5 (see also figure 7.5). The tug th en lyin g broadside
on can give stee ring assistance to both sides. Twin screw
tugs often don't need a gob rope to operate in a similar
way, owing to their higher manoeuvrability.
It is clear tha t at speeds abo ve about three kno ts,
only steering assistance can be given and only to one
side. At very low speeds steering assistance can be given
to both sides and a ship's spe ed can be controlled . A
conventi onal tug is ve ry restricted in its m ovements as
a stern tug owing to the location of th e towing point.
When a conventional tug is work ing close to or
behind a ship's stern, a ship should be very careful in
using its propeller or the tug migh t be overturned by
propeller wash. A tractor tug and ASD/reverse-tractor
tug, on the other hand, will in general not be hindered
by ship's propeller wash due to the location of the towing
point near the tug's stern or bow. If working on a short
towlin e, however, excessive vibration of the azimuth
propellers may be experienced, due to th e turbulence
from the ship's propeller. Lengthening th e towline will
redu ce this effect.
A tractor tug, approaching a sh ip stern wa rds ,
experiences the influence of a ship's propeller washon the
skeg. Careful steering is then required to keep the tug on a
straight course. This is also the case when the tug is secured
and has to stay straight behind the vessel, as mentioned
while discussing direct and indirect towing methods.
From the above it is clear that prior to secur ing tugs
forward or aft the position of the different tug types in
general and of conventional stern tugs in particular
should be well considered, taking into account the forces
of wind and current to be compensated, bends to be
taken, etc.
Each type of tug has several versions with varying
capabilities, which should be regarded as well when
positioning tugs. A twin screw conventional tug, for
instance , will gen erally perform better than a single
screw tug. The same applies to a conventional tug
equipped with a rad ial towing arm. This will increase a
tug's capabilities and safety compared to the same tug
without such 'an arrangement. In addition to what has
been discussed already, therefore, performance and
safety of a conventional tug depend largely on good
manoeuvrability and appropriate towing equipm ent.
Also, combi-tugs with their azimuth bow thruster have
better capabilities than ordinary conventional tugs,
esp ecially when a combi-tug's towing point can be
shifted to an alternative pos ition far aft. The capabilitie s
of these tugs were explained in paragraph 2.4 .
Tugs operating at a ship's side
Tugs operating at a ship 's side while the ship has
some speed are shown in figure 4.16. Three types are
BECAUSE OF
shown - a tractor tug (which can be a VS tug or one
with azimuth propulsion), an ASD/ reverse-tractor tug
and a conventional tug.
Pushing mode
Wh ether one type of tug is more efficient in pushing
th an an other dep end s on how well a tug can push
effectively without incteasing ship's speed. The bet ter a
tug can work at right angles to the hull of a vesse l
und erway, the more effective it is. It dep end s largely
on th e ratio a:b (see figure 4.l6A): the relationship
between the lever of propulsion {P- PU}and the lever of
hydrodynamic forces {C-PU}. The better a tug can
overcome the turning mome nt resulting from
hydr odynamic force by the mo ment created by sideways
thrust of the propulsion, th e better a tug can work at
right angles to the ship and th e more power is available
for pushing. In addition, the vertical location of the
centre of pressure, stability and freeboard are important
factors. Tug fendering should prevent a tug sliding along
a ship's hull, otherwise one or MO towlines are required.
Owing to its aft lying centre of p res sure a
conventional tug may find it difficult to come to or
remain at right angles when a ship has spee d through
the water. Conventional tugs generally have a large
underwater plane and an important consideration for
effective pushing is steering performance, which is less
tha n that of tugs with omnidirection al pr opulsion
systems. Depending on th e situat ion conventional tugs
use stern lines to stay at right angles to a ship's hull
when the ship gathers speed, as shown in figure 3.2.
However, excessive speed imp airs safety as the line may
part or result in capsizing the tug. Devices increasing
the steering performance of conve ntional tugs, such as
high lift rudders and Towm aster systems, increase their
pushing capabilities.
The ASD/reverse-tractor tug with its highly efficient
steering propellers and the far aft lying propulsion in
combination with a generally more forward lying centre
of pressure is very effective at pushing. Tug company
C.H . Cates & Sons of Vancouver claim s that th eir
reverse-tractor tugs can deliver a 90° side push at ship
speeds up to eight knots instead of the usual four knots
for conventional tugs. Three to four knots is gene rally
the maximum sp eed for effective pushing b y
conventional tugs, although it depends on their engine
power and prop eller/ rudder configuration.Tractor tugs
are also much more effec tive than conventio nal tugs
due to their omnidirectional propu lsion.
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Fig. 4.16 Comparison ofperftrmance oftugtypes when pushing or pulling
Comparison of different tugtypes when pushingor pulling at a ship"side. The ship has headway. Locations ofthe lateral centre ofpressure are
approximaud. Apartfrom the underwater resistance tugperformance depends on: a) maximum heel; b)propulrion performance - omnidirectional
propulsion systems are very suitahl. owing to1M possibility ofapplyingforces in any required direction; c) ratio a:b (a = distance between propulsion
andpushing ortowing point, b = distance between lateral centre ofpressure andpushing ortounngpoint. The kzrger the leoer a in relation tolsoerb
1M less side thrust required tok epposition and 1M more thrust avaikzblefor effective pushingorpuUing
TUG USE IN PORT 55
Fig. 4.17 Pushingforce createdby hydrodJ1lflmicforce
on a tug's hull
A tuglaepingposition at an angle with theship 'shull may also exert
rather highpushingforces caused by thewaterflow, depending on the
ship's speed andthetug's undenoater hull form
,
I
I
I
I
I
~
i
I
I
I
I
~
II Incoming waterflow
I
I
I
I
W
I
I
I
I
I
I
'I'
Hydr
Conventional tugs, due to their large underwater
plane, experience heeling mom ents which are more
difficult to compensate for by their lower steering forces.
Wide beam tractor tugs and ASD/reverse-tractor tugs
with their effective and - for tractor tugs - deep set
steerin g pow er, are in a much better position to
compensate for ' heeling moments. As said, they are
capable of remaining at right angles to a ship's hull at
much higher speeds than conventional tugs. At high ship
speeds, tugs can push at a smaller angle. Lift forces also
create pushing forces, which can be rather high (see
figure 4.17) . This effect can be seen in graphical format
in figure 4.20.
Whether tractor tugs are mare or less effective than ASD
reverse-tractor tugs depends on the ratio a:b as shown
in figure 4.16A, th e tug's underwater body, its engine
power and thru st perform ance in the requir ed direction.
There is ano ther aspect which determines a tug' s
capability for operating at th e ship's side, viz. th e
maxi mum heeling angle. In this respect the height of
the pushing point is important. The heeling mo ment
caused hy hydrodynami c forces incr eases by the spee d
squared. This is counteracted by sideways steering forces
and by a tug's stability. The higher the pu shing point
th e larger the heelin g mom ent and the less it can be
compensated for.
Pulling mode
Tugs operating at a ship 's side need good astern
power, which sho uld be about the same as their ahead
power. Tug s wi th om nidirectional propulsi on are
therefore very suitable for push-pull work. In figure
4.16Bi th e same thr ee types of tug are pulling, secured
with one line. The ship is und erway through th e water.
The situation doe s not differ very much from situations
when stern tugs are towin g on a line in the direct mode,
as discussed earlier. Only for conventional tugs is the
situation rather different.
The
longer tugs can pull effectively with increasing
ship speed the better. It is obvious that the conventional
tug will swing around.The tug needs a stern line leading
forward to be able to pull at right angles. For the situation
shown, the paddle-wheel effect of the tug's propeller
also add s to the swinging motion . Tugswith twin screws,
steering nozzles, a Towmaster system or flanking rudders
perform b ett er. The m aximum shi p 's speed with
conventional tugs pulling, eve n using a stern line, can
only be low.
Tractor and ASD/reverse-tractor tugs perform much
better, because while pulling they can apply forces in the
direction of ship's movement. That is a big advantage of
omnidirectional propulsion systems engaged in push-pull
operations. Whether one of these types is more effective
than another depends on the same factors mentioned
when discussing the direct towing method, nam ely the
ratio a:b, a tug's underwater size and profile, its engine
power and thrust performance in the pulling dir ection.
An imp ortant aspec t to take into account is loss of
pulling efficiency due to a tug's propeller wash hitting a
ship' s hull. This force can be as large as its ball ard pull,
some times even larger. The effect is far less if the
distance between tug propeller and ship's hull is
increased. Tractor tugs therefore push and pull with their
stern so as to keep their propellers as far away as possible
from a ship's hull. In addition, tractor tugs with azimuth
propellers, when pulling, can set their propeller thrusters
at an angle, thus diverting the propeller wash. The sam e
applies to ASD /reverse·tractor tugs. High er pulling
effectivenes s can also be achieved using a lon ger towline.
This can only be don e when only pulling is requi red,
not pulling and pushing, otherwise it length ens response
time. The effect of propeller wash is furth er discussed
in Chapter 6.
When changing from pulling to pushing tug captains
should be aware of the dynamic forces in a towlin e.
Particularly with a steep towline angle and in wave
conditions these forces may draw the tug quickly in the
direction of the ship when its engine is suddenly stopped.
When stern thrust is also applied a tug may hit a ship' s
hull with force (see figure 4.18). See also th e note at the
end of paragraph 6.3.2 regarding damage to ships caused
by tugs.
56 THE NAUTICAL INSTITUTE
Stopping assistance
From the foregoing it is also clear that ASD-tugs,
reverse-tractor tugs and tractor tugs operating at a ship's
side have better performance when braking assistance
is required than normal conventional tugs. This is due
to omnidirectional propulsion, which provides almost
the same bollard pull astern as ahead.
Summary
Many differences in performance, capabilities and
limitations of different tug types have been reviewed.
For the reader's convenience a brief summary follows
of the most important aspects . It is assumed that all tugs
discussed are suitable for their tasks and have the
required stability, sufficient freeboard, proper towing
equipment and manoeuvrability.
Conventional tugs
Conventional tugs can be very effective when towing
on a line a ship having speed through the water. They
can assist in steering and in compensating wind and
current forces, but often also deliver an unwanted force
which increases a ship's speed.
fu forward tug on a line a conventional tug can assist
in steering to both sides 'but as stern tug it has its
limitations .At higher speeds, steering assistance can only
be given to one side. Only at very low speeds is steering
control to both sides and control of ship'Sspeed possible.
As both a forward and a stern tug, capsizing (girting)
is possible as a result of the position of the towing point
in combination with induced strong transverse forces.
To minimise risk of girting a completely reliable quick
release system should be used. A radial towing hook or
equivalent system also decreases the risk of capsizing.
Figure 4.18 Effictofdynamic fOrces in the towline
Pulling with a short andsteep towline creates high fOrces in the
towline, which are very much enlarged hy waves andswelL As soon as
tugengines are stopped, the tugwill immediately be pulled backwards
towards the ship by force F caused by stored energy in the elastic
towline. So, when thetugcaptain isasked tostop pulling heshould be
aware ofthis effiet andwhen ordered tochange overfrom puUing to
pushing, astern thrust should be applied very carefUlly
The ability to provide stopping assistance is nil for
forward tugs towing on a line and limited to very low
speeds for stern tugs towing on a line. Ship's engines
should be handled with care when conv entional tugs
are clo se to the stern. Due to these limitations as a stem
tug, tug po sitions should be carefully pl an ned in
advance.
The pu shing effectiveness of conventio nal tugs
decreases quickly with increasing ship's speed; pulling is
only possibleat zero or low speeds, dependin g on whether
a stern line is used. Ship's speed should be carefully
controlled so as to take account of the limited capabilities
of a conventional tug operating at a ship's side.
Tractor and reverse- tractor tugs
Tractor and reverse-tractor tugs towing on a line as
forward tug are able to render assistance to both sides.
As forward tugs only steering assistance can be given,
and these tugs may also deliver an unwanted force which
increases a ship's speed. As forward tug these tugs are
not as effective as conventional tugs for a ship underway
at speed.
As stern tug, reverse-tractor and tractor tugs perform
very well. They can provide steering assistance to both
sides and control a ship's speed even at rather high
speeds, although a reverse-tractor tug is generally
somewhat less effective than (VS) tractor tugs in
providing steering assistance at higher speeds (indire ct
mode) . Risk of capsizing hardly exists during normal
port operations and when operating as stern tug, they
are hardly affected by a ship's propeller movements.
Tractor and reverse-tractor tugs operating at the side
of a ship at speed through the water are effective in pushing
and pulling and in applying braking forces. It should be
noted that tractor tugs have a relatively large maximum
draft, which can be a disadvantage in shallow waters.
ASD-tugs
ASD-tugs are multi-functional and can be effective
as a forward tug on a line when operating as
conventional tug. As forward tug, ASD -tugs can also
operate as a reverse-tractor tug . As stern tug on a line
ASD-tugs generally operate as a reverse-tractor tug with
the same high performance. When pushing and pulling
at the side of a ship at speed, ASD-tugs are very effective,
also in applying braking forces.
4.3.2 Effectiveness of tug types
Model testing and full scale trials have been used to
determine tug capabilities. Most tests focus on the
abilities of one specific tug or tug type. Voith has done,
and still does, considerahle work regarding VS tractor
tugs . Aquamaster has carried out several studies
regarding tugs with azimuth propellers. Recent studies
and full scale trials that have been undertaken mainly
assess specific tugs and tug type escorting capabilities.
TUG USE IN PORT 57
Fig 4.14null
Figure 4.79 Performance and behaviour ofa 40m conventional tug
increase very quickly at speeds above four knots. These
longitudinal forces increase ship speed.
According to the same study, the effectiveness of
conventional tugs with inferior rudder performance
decreases quickly at ship speeds of about four knots.
When no bow line is used the longitudinal forces but
also the transverse forces exerted at speeds higher than
five knots are less , so tug perfonnance is less. In waves
of approximately six feet high, tug performance drops
quickly at speeds higher than three knots .
40
20
70
80
50
30
80
10
o
85
Pushlllll Angle
90
234
Tank~r Spe~d Iknolsl
PlIshlng Forces (with bow 111\8)
o
0
- ~' ..
s " .' " - ' - -.~......
0 ,- - '.
5 <--, - •• .,
0 -,
... Transverse Force •5 : ~.
.... longitudinal Force
0
•• Angle •Pushing
5 ., ",
0 .-- ~_ .. - ~ -.-- - -
5 ' -
0
..
s
2
5
4
4
3
3
2
Simulation programs don't normally take into
account all factors influencing tug performance, such
as ship-tug interaction, flow field around a ship, influence
of water depth and confinement on the flow field, and
the influence of ship's wake on a tug's braking
performance, which are discussed in Chapters 6 and 8.
There may, therefore, be some inaccuracy in simulation
results, depending on the situation.
Desktop computer simulation programs exist, based
on a steady situation - equilibrium of forces - by which
the performance of different tugs and tug types can be
determined. With these simulation programs capabilities
produced by different variations of tug design can be
predicted.
Simulation programs provide the possibility of gaining
insight into a more extensive range of a tug's ability using,
for example, a full mission bridge simulator. When these
programs are carried out in close cooperation with pilots
and tug captains and are, as far as possible, verified in
full scale sea trials, the results give quite high reliability.
Simulations are mainly carried out for one specific tug or
for a very limited number of tugs of which all details of
rudder, propulsion , stability, maximum list,
hydrodynamic coefficients and so on are known .
Most of these studies and trials, therefore, only involve
some specific aspects of ship assisting manoeuvres
required during daily tug handling. Of course, several
variations in the design of a specific type of tug exist.
Figure 4.20 Poformance andbehauiour ofa30mASD-tugfirpushing
Tug Force P tlonnesl
60 .-
100
20
60
Hull
Pro pe lle r
B
-..--
~---:-,.;----_J BO
v
•.
40
_ .....~--....-L: C .. ...
" ,
".
'" Pushlnll.....g1.
b; DfftAnllla
c: Propeller A"II1ft
Drill Angle and Propeller Angle .
120
' ..
4 •
Speed V {knots)
2
p
........ Propellar Anlll• •
·- R · Drllt Angle
........ Tug Force
10
20
----.------ -.-. --._-
40 --
'0
Real capabilities and in particular limitations are, of
course, experienced during daily shiphandling only, but
the results of simulation programs can verify some of
what is explained in this book.
As indicated in the graph, the pushing angle becomes
smaller as soon as the ship gathers speed. The transverse
pushing forces exerted by this tug decrease with ship's
speed higher than five knots, but longitudinal forces
Performance diagrams
Performance ofa conventional and an ASD-tug when
pushingat a ship underway at speed
The graphs in figures 4.19 and 4.20 are based on
simulation studies and provide an insight into the
capabilities of a conventional tug and an ASD-tug when
pushing.
The conventional tug has twin screws and three
rudders, length overall of 40m, beam lim, 5750 BHP
(open propellers), SOt bollard pull and draft 17ft. The
maximum pushing forces of this tug were determined
at various ship's speeds taking into account, amongst
other things, maximum heel at deck edge immersion.
The graph shows maximum transverse pushing forces
and the longitudinal forces exerted at the same time. It
also shows the tug's pushing angle. The tug is pushing
with a bow line.
58 THE NAUTICAL INSTITUTE
Longitudinal 
Tranversal
ANGULO TUG- SHIP
90
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Figure 4.21 Petfbrmana graphs forfour andsixknots speed
TUG USE IN PORT 59
In practice a speed of five or even four knots is a
rather high limit for conventional tugs to exert transverse
forces effectively. The study results may be affected
because not all factors influenci ng tug perfo rmance
could be taken into account. Naturally, differences in
pe rformance exist be twee n va rious ty pes of
conventional tugs. In general, however, the upper limit
at which effective sideways pu shing forces can be
exerted is found to be about three knots. This is also
proven by full scale trials in the USA in 1982 with a
1700 HP twin screw tug with nozzles, two steering
rudders, four flanking rudders and without the use of
auxiliary towlines. The length of the tug was 30m. In
ad dition, effective pulling forces were po ssible at
maxi mum speeds ofless than one knot.
The main conclusion is that at ship speeds higher than
around four knots, and for less manoeuvrable tugs three
knots, the performance of conventional tugs is very poor.
At these higher speeds transverse pushing forces are
minimal, but longitudinal forces increase very quickly,
thus increasing ship's speed, which is not desirable.
Next the performance of an ASD-tug when pushing
is considered. Particulars of the tug are: 31m length
ove rall, beam 10·7m, 3600 BHP, SOt bolla rd pull ,
maximum allowable heel 6'.
As can be seen in the graph this tug performs very well.
The tug exerts only transverse forces and no speed
increasing longitudinal forces. Th e higher the spee d the
larger the hydrodynamic forces on the tug's hull and
the larger the lift forces create d by the hull. At about
eight and a half knots, 80% of the transverse pushing
force is developed by the lift force.
Tug stability, freeboard and height of the pushing
point have a large influence on maximum achievable
pushing forces. Limiting factors are maximum engine
revolutions, engine torque and excessive heel.
The two graphs show a large difference in pushing
effectiveness between ASD and convent ional tugs. An
ASD-tug is still effective at a much highe r spe ed while
no ship's speed increasin g longitudinal forces are
exerted on the ship.
Performance ofan ASD and VS tug while towing on a line
These diagrams (figures 4.21 and 4.22) have been
produced by the TUGSIM simulation program of
Damen Shipyards, The Netherlands. Tug performance
in the diagrams is limited by a tug's maximum list -
deck edge immersion, and maximum engine load is
acco unted for.
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FiguTt 4.22 Ptiformanet grapbsfortiglu knots speed
60 THE NAUTICAL INSTITUTE
Fig 4.20
The TUGSIM program operator actually steers the tug,
controlling the rudder and/or thrust angles and thrust
magnitude. This prevents theoretical solutions being
calculated in which a steady situation exists, but
situations in which the tug could never be manoeuvred.
The graphs show an ASD and a VS tug towing on a
line at different speeds - four , six and eight knots.
Particulars of the tugs are given at the top of the graphs.
The m ain obj ective of the graphs is to show the
maximum steering and braking forces which can be
achieved.
The ship is at the centre a of the graph, sailing in
the direction indicated by the arrow. The ASD·tug is
operating as forward tug up to a towing angle of 900 as
conventional tug and as stem tug the ASD-tug operates
as a reverse-tractor tug. As stem tugs the ASD and VS
tugs operate in the direct or indirect mode, whichever
is the most effective.
The following general characteristics can be seen in
the graphs. The performance of the VS tug in giving
steering assistance as forward tug towing on a line
decreases quickly with increasing speed, while up to a
speed of about six knots the performance of the ASD·
tug in giving steering assistance is decreasing much less
at small towing angles and is even increasing at large
towing angles. At eight knots the ASD·tug can still
produce high steering forces in contrast to the VS tug.
Normal conventional tugs often perform in a similar
way, but are generally limited more by the tug's stability.
At four knots the tugs operate as stern tugs in the
direct mode and are both effective.
At six knots the ASD·tug performs better in the direct
mode, while the VS tug starts to perform better in the
indirect mode in applying steering forces. The braking
performance of both tugs in the direct mode is high .
At eight knots and in the indirect mode high steering
forces can be applied by .both tugs . The VS tug is
somewhat more effective, although it is less powerful
than the ASD·tug.
Highest braking forces are achievable at speeds of
eight knots by both tugs operating in the direct mode
and towing at a small angle (lowest part of the curve).
Both tugs perform about the same when taking into
account the difference in bollard pull.
Thus the following generalities can be observed. As
a forward tug towing on a line the ASD-tug performs
better. As a stem tug on a line and at higher speeds the
VS tug performs rather better in giving steering
assistance and the ASD·tug and VS tug perform about
the same in applying braking forces . Another aspect is
clearly shown in the graphs: the speed increasing force
vector of forward tugs towing on a line. For example,
take the ASD·tug of the six knots graph while towing
on a line as forward tug. The tug as shown in the
indicated position develops a steering force to starboard
of 35 tons, but at the same time a force in the direction
of ship's movement of about 15tons. This force increases
the ship's speed, which is in most cases not welcome.
On the other hand, all the stern tugs as shown in the
graphs when applying steering forces also develop
braking forces. This normally has a large positive effect.
It keeps the ship's speed low and , in addition, enables
the ship to apply additional engine power for steering,
without increasing the ship's speed. All this is in line
with the capabilities and limitations of tugs as discussed.
Speedcontrol - brakingassistance
Tractor, ASD and reverse-tractor tugs perform very
. well as stern tugs for steering assistance and speed
control. This has resulted in competition between the
designers of cycloidal propellers (VS) and azimuth
propellers (Aquamaster) about which type of tug, VS
or ASD, performs best as stern tug at higher speeds .
This is mainly due to the discussions with respect to
escorting, dealt with in Chapter 9. However, one aspect
is briefly discussed here, the braking performance of
tugs equipped with azimuth thrusters, because it is
important for daily assistance in ports. In this respect
some new terms have been introduced by Aquamaster.
It should be noted that when stopping assistance is
required by a VS tractor tug or ASD/reverse·tractor
tug, for instance at speeds of more than five knots, the
braking force that can be applied is higher when the
tug is pulling at a small angle with the ship's centre line
rather than pulling straight astern, as can be seen in the
TUGSIM performance graphs.
When braking assistance is required at high speeds
by a conventional tug operating over the bow as stem
tug, it may not be possible to reverse fixed pitch
propellers due to the high propeller load which has to
be overcome by the engine, although the effect of it can
be reduced by proper design and tuning of the engine.
For the same reason, at a crash stop VS pitch levers
should be set in accordance with the ship's speed and
azimuth thrusters have to be rotated to astern but can
be set, with independently controlled thrusters, at an
angle with the tug's centre line to avoid stalling. In the
case of azimuth thrusters with control1able pitch
propellers, astern pitch should be applied in accordance
with ship's speed when a ship having a rather high speed
has to be stopped. Because of the low performance of
controllable pitch propellers going astern, turning the
thrusters like thrusters with fixed pitch propellers is more
effective in applying braking assistance.
In the direct assisting method, Aquamaster claims
that at speeds of up to eight knots braking forces can
reach values up to one and a half times the bol1ard pull
TUG USE IN PORT 61
astern with azimuth thrusters (of ASD/reverse·tractor
as well as tractor tugs) rotated 180°, the thrusters thus
working in line with the tug's centre line in negative
flow. At spee ds higher than eight knots braking forces
drop off dramatically, regardless of the power applied.
Engine load then also increases rapid ly to an overload
condition. This braking meth od is called the Rever se
Arr est Mode by Aquamaster.
A second way of applying braking force in the direct
assisting method is the so-called Tran sverse Arrest
Mode. Large arresting forces can be created by pointing
the thru sters outward at an angle of approximate ly 90'.
These forces result from momentum drag and are
ge ne rated when the prope llers acce lerate the
athwar tships component of the was h . The forces
inc rease with speed and excee d the astern bollard pull
at speeds higher than eight knots without overloading
the engine .
So, be low eight knots the Reverse Arrest Mode can
be used (thrusters rotated 180' in line with the tug's
centre line) and at speeds higher than eight knots the
Transverse Arrest Mode (thrusters at an angle of 90'
with tug' s centre line) can be applied. See figures 9.5
and 9.7 for the different terms used and the achievable
braking forces.
Although eight knots is a high speed for tug assistance
in port areas, it is good to know how thrusters can be
use d to deliver hig h re tard ing forces. This way of
applying br aking forces can be utilised by all types of
steerable thrusters, but is most efficient whe n using
propellers in nozzles.
4.3.3 Effective tug position
Positioning tugs depends on several factors. Firstly,
ship's' particulars such as type, size , draft, windage,
man oeuvrability have to be cons idered. Seco nd ly,
fact or s such as the in flu ence of environmental
cond itions, particulars of the passage or fairway towards
the berth, available
stop ping distance, size of turn ing
circle, berth location, and so on have to be taken into
account. Together these factors determine what should
be expecte d fro m tugs - steering ass is tance,
co mpensating ex ternal forces of wind and current,
assistance in stopping th e ship or a combination thereof.
Ship's berth ing side is also an important factor to be
tak en into account whe n pos itioning tugs. And, of
course , it is very important to know the number, type
and bollard pull of available tugs-,
In figure 4.23 different tug positions are given. A
ship has headway and has to make a turn to starb oard.
Tugs have to assist. Whether a particular type of tug is
more or less effective in one or more of the positions
shown has been discussed already and is summarised
in paragraph 4.6. Atten tion now turns to the effect on a
ship when tugs are ope rating in one of these positions .
62 THE NAUTICAL INSTITUTE
The location of th e pivot point is taken into account.
Forward tug no . I, towing on a line , is capable of
exerting quite high crosswise steering forces on a ship .
The effect can be limited because of the transver se forces
near a ship's bow to be overcome, as explaine d when
discussing th e pivot point. It is clear that for a par ticular
ship th ese transverse forces are proportional to the draft
and underk eel clearanc e. Also, the more th e tug is
pulling in line with a ship's heading th e mo re the tug
will increase a ship's speed.
Position of tug no . 2 is not so good for the steering
assistance required. The tug has to overcome th e same
transverse forces as tugno . 1, but the lever of crosswi se
steering forces exerted by the tug is much shorter and
the tug's underwater resistance opposes the turn. Also ,
when a tug is unable to push at right angles to a ship's
hull it will increase a ship 's speed.
Rega rding tug no . 2 it should be kept in mind that
this tug might even have an opposite effect. Simulation
studies carried ou t by, amongst others, Dr. Paul
Brandner and describe d in his thesis 'Performance and
effectiveness of omni-directional stern drive tugs ' (see
References) show that a tug pushing at the bow of a
loaded tank er on a steady course, with an initial speed
of four knots, the engine on Dead Slow Ahead and
rudder amidships, has a tendency to turn against th e
Figure 4.23 Different tugpositions
Of that
1
2
3null
Cp
6
5
pushing direction of the tug . The tests were carried out
with a depth/draft ratio of 1.2. This effect has also been
experience d during full scale trials. During these trials
a loaded tanker was on a steady course at five knots
speed, the ru dder amids hips , and the engine was
stopped. A conventional tug started to push on the port
shoulder.After an initial tum to starboard the ship started
to turn to port , while speed increased.
It does in no way say tbat for other ship types or
oth er loading conditions, th e same effect might be
experienced. The opposing transverse force at th e bow
differs by ship type, draft, trim and und er keel clearance
(see above for tug no. 1). In the report mentioned above,
test results of other loading conditions are given. If the
same tug is pushing at the sho ulder of the tanker when
in deep wate r, in hal last condition and trimm ed by the
stern , the tug does turn th e tanker in th e requir ed
direction and the effect does not differ much from a tug
pushing at the quar ter (tug no. 4).
Apart from what hasjustbeen mentioned, the positions
of tugs no. 1 and 2 are not always inadequate . It depends
on the situation and circumstances, because the tugs are
in a good position to compensate for drift forces caused
by wind and/or current from starboard. If required, tug
no. 1can easily compensate for the wind and current forces
from port as well. This flexibility in operation is an
advantage of the forward tug towing on a line.
Tug no. 3 can assist th e starboard turn by going
astern. In doing so, an additional starboard turning
couple is created by the tug's and ship's engines working
in opposite directions. By going astern the tug is slowing
down ship's speed, and thus increasing the effect of the
ship 's engi ne on th e rudder. The tug 's underwater
resistance contributes to the starboard swing. If tug no. 2
had a bow line, both tugs 2 and 3 are in a good posi tion
to take off ship's headway, if required.
Tug no. 4 is in an effective position to assist the
starbo ard turn by pushin g, because of the 10J;lg lever
and forward centred lateral resistance, which contribute
to the swing. The tug's underwater resistance gives
additional turning effect to starboard. When tug no . 4
cannot work at right angles, ship' s speed increases, but
as a result of the high er rate of tum caused by the
pushing tug and consequently the higher drift angle,
ship's speed is hardly affected. If the tug has a bowline
secured, it could also assist in the starboard swing by
going astern, in the same way as tug no. 3. In that case
the whole tug has to be pull ed crosswise through the
water by the ship's stern and hence opposes the turn .
Tug no . 5 is in a very effective position. The lon gest
poss ible lever for steering forces and the transverse
forces centred forward contribute to the swing. Also,
the tug does not increase ship 's speed. On the contrary,
the tug also provides retarding forces while app lying
steeri ng assistance.
Tug no. 6 is in a similarly effective position to tug
no. 5, but has the disadvan tage that this tug increases
ship's speed. The same would be the case with a rudder
tug (not shown in figure 4.23).
The difference in effectiveness between a forward
pushing and aft pushing tug can also be seen when a
ship gathers speed. For instanc e, assume that tug no. 3
and no. 4 are of same typ e and bollard pull and both
pushing at right angles. At zero speed the ship, on even
keel, moves crosswise. For reasons exp lained, as soon
as ship's speed increases, the effect of tug no. 3 is smaller
than that of tug no. 4 and the ship starts turning to
starboard.The same applies to tugs of similar capabilities
when towing on a line forward and aft.
For swinging, e.g. when the ship is stopped in the
turning circle , tugs no. 1 and 5 or 6 are in the best
position due to the long lever of exer ted tug forces.
The most effective tug positions have now been
reviewed. Which position s should be used during
passage towards a berth and while mooring/unmooring
depends on what is required from the tugs and this
depends on the ship, local situation, circumstances and
ship's berthing side.
If steering assistance to starboard is required during
passage towards a berth then tugs no. 3, 4, 5 and 6 are
in a good position. Tug no . 5 can even give steering
Pho/.(; :Author
Figure 4.24 Two amoetuional tugs assisting a tanker ha.ing headway in
mal<ing a starboard turn. The tugs are notin an effiai.e puslUngpas/tion
andare also inereasing 1M ship's speed due totJuir smallpushing angle
TUG USE IN PORT 63
O CP se desloca para frentenull
assistance to both sides. The same would be the case
with a rudder tug. If crosswise drift forces from port
have to be compensated for in a narrow fairway, tugs
no. 3 and 4 are in a good position and also tugs no. I
(when this tug shifts to port), 5 and 6. In case stopping
assistance is required tugs no. 2 and 3 (with bow lines)
and 5 will assist effectively.
If tug assistance is required dur ing mooring/
unmooring operations then several combinations are
possible, also depending on tug type . For mooring of
large ships even four tugs may be used. Often tugs nos.
3 and 4 are used for pushing and I and 5/6 for
controlling the approach speed towards the be rth.
If tug no.! is a tractor tug, reverse-tractor tug or ASD-
tug ope rating as a reverse-tractor' tug, then tug no .l
together with tug no.5 can easily push as well
as control
the ship's approach speed towards the berth during
mooring
Wh ether the required tug forces can be delivered
effective ly, depends on a correct assessment of the
required bollard pull and the right choice of the type of
tugs with respect to the tugs' positions and assisting
methods.
4.3.4 To wing on a line compared with operating at a
ship's side
In paragra ph 3.2, different assisting methods were
discussed. Which assisting method is most appropriate
for a particular port depends on the local situation and
circumstances. Nevertheless, it is good to have an idea
about the advantages and disadvantages of the two basic
methods. In paragraph 3.2 the small manoeuvring lane
within which tugs towing on a line are able to operate
an d the limitations of tugs operating at a ship's side due
to waves were mentioned. Taking into account the
capabilities and limitati ons of tugs, th e foll owin g
additional comments are given .
Different types of tug can be used for towing on a
line, some more effective th an othe rs. In a fairway
passage towards a berth tugs are normally positioned
so that the influence of wind and/or curre nt can be
compe nsated as mu ch as possible and changes in
heading can be made in a safe, efficient way. A ship can
also berth either side.using this system .
Towing on a line, therefore, has the adv antage that
tugs are normally positioned at the safe side of the ship
and are flexibl e regarding berthing side. Even in the
worst case, when wind and/or curre nt are getting too
strong, tugs on a line can assist up to the last moment,
minimising th e risk of severe damage.
Wh en omnid irectional propulsio n tugs are used for
towing on a line they are able to change over to the
push-pull method during berthing witho ut the need to
64 THE NAUTICAL INSTITUTE
release the towline. This shortens berthing time, because
no time is wasted in retrieving towlines or repositioning
tugs. In addition, a ship can be kept under better control
because towlines stay fastened while tugs eithe r push
or pull.
Tugs at a ship's side are positioned acco rding to
berthing side, to the forces of wind or current to be
compensated for and/or the changes of heading to be
made dur ing transit toward s a berth. Wh en pos itioned
to compensate for wind and/or curren t forces this may
be the wrong position for berthing. Tugs the n have to
be shifted before mooring takes place - common
practice in some po rts. However, this means that a ship
has no or little assistance during shifting of the tugs and
may start drifting.
When positioned to compensate for wind and/ or
current forces, risk is involved for both tugs and ship
when these forces are underestimated and a ship star ts
drifting. Wh en it becomes too dangerous for the tugs
they may try to get .out from between the ship and the
leeward or downstream fairway or cha nnel banks,
leaving the ship without any assistance.
4.4 Operational limits
Harbour tugs can operate in all conditions of cur rent
and wind . However, during fog the situation is different.
Fog in confined port areas makes tug assistance very
risky. In good visibility a tug captain assesses his position
and speed in relatio n to the speed and heading of the
attended ship, and also in relation to the surrounding
area, such as buoys, beacon s, rive r banks and quays .
Compared to ship movements, tug movements are
much faster, making it difficult to manoeuvre from the
tug's radar. In addition, tugs often operate close to a
ship's side, resulting in a distorted or partly blank radar
picture. Furtherm ore, during fog a tug captain m ay lose
a good view of his towlin e. Altog ether this makes tug
assistance during fog much m ore difficult than when
Visibility is good. For this reason restrictions on tug
assistance under poor visib ility conditions exist in a
number of ports.
Several ports lie close to open sea and jetties may be
situated in open waters. Consequently, tug assistance
may also be required in open sea. For harbour tugs,
passing towlines in wave conditions can be difficult.
Harbour tugs ope rating at a ship's side have short and
often rather steep towlines. Wh en tugs operate on a wave
exposed ship's side, dynamic forces in the towline may
reach high va lues and lines are liabl e to part in
deteriorating wave conditions. So, very strong and
sometimes double fibre lines of high stretch properties
are often used.
However, if circumstances permit, tugs can also
change over to towing on a line, allowing them to handle
a ship more safely since if towlines are longer tbey can
?..
In spite of that
Questão de prova
better absorb dyn ami c forces. When tugs are equipped
with lowing winches line can then be paid out as deemed
necessary and be sho rtened when conditions improve
or when entering port.
On tbe other hand, in wave conditions harbour tugs
can, instead of towing on a line, operate more effective ly
and at higher wave heights at the ship's leeside,if
circumstances and ship manoeuvres allow. It all depend s
on the local situation.
In wave conditions the risk of girting for conventional
tugs towing on a line is high er than in calm water and
passing towlines can be carried ou t more safely with
more highly manoeuvrable tugs. Tractor tugs will in
general, the refore, operate m ore safely and can provide
assistance in somewha t larger wave heights. It has been
reported that the movem ents of VS tractor tugs may b e
m ore violent in wave conditions. Anyway, waves limit
the operating effectiveness of harbour tugs when towing
on a line as well as operating at a ship's side when
exposed to waves . Perfor mance decreases with
increasing wave height An indication of the upper limits
for ope rations by harb our tugs is:
Maximum significant wave height:
Conve ntional tug types :1·5 - 1·8 m
Tractor types of tugs (incl.
reverse-tractor tugs), ASD tugs : 2·0 m
Visibility:
In several por ts a visibility of 0·5 mile is found to be
the limit.
4.5 Design consequences
What h as b een discu ssed with resp ect to the
performance of different types of tug has resulted in an
alternative design for some new VS tractor tugs. Th e
reason why is clear. A tractor tug is ve ry effective as a
stern tug on a line . It ope rates with the stern directed
towards the ship and the tug captain facing aft. Thi s is
also the direction of the assisted ship's movement. When
ope rating at a ship 's side a tug captain is also usually
facing aft an d the same applies during mooring and
unmooring operations.
So wh at can b e see n nowad ays is a totally new
concept VS tractor tug, as for instance in the Norwegian/
Swedish Bess an d Boss- the wh eelh ouse is turned 180°.
The stern is high er to give better pro tection against
incoming waves (see figure ·9.16). These tugs will be
considered when discussing escorting. Similar changes
to VS tug design can be found, amongst others, in the
VS tug Redhridge of Adsteam Towage, UK, where the
tug funn els are placed forward of the wheelhouse, giving
an optimum view aft for the captain . Th e stern in this
design is also raised . In addition, alternative towing
poin ts can be used , as mentioned in section 4.2.2.
Photo:Boh DoJ1tJUSQTt., Soutllampton, UK
Figure 4.25 VS tug 'Redbridge' ofAdsteam Towage, Southampton,
UK. (/.0.0. 33m,beam 112 m, hp43 tons). A newdesign, meeting
several operational requirements, it has an optimum v£ew of the after
ded: from thewheelhouse, unobstructedby fimnels and a much higher
sheer at thestern tokeep theaft duk: clear of water when running
astern at speed. particularly in wave conditions andwhen esC()fting
4.6 Conclusions regarding tug types
Assuming normal po rt operations with maximum
ship speeds of six to seven knots, it can be concluded -
with some reservations - that the suitability of different
tug types can broadly be ranked as follows:
As forward tug towing on a line:
ASD-tugs
Combi-tugs
Conventional tugs
Tractor tugs/ Reve rse-tractor tugs
As stern tug towing on a line:
Tractor tugs/ASD-tugs/Reverse- tractor tugs
Combi-tugs
Conventional tugs
When operating at a ship's side:
ASD-tugs / Reverse-tractor tugslTractor tugs
Co mbi-tugs
Conventional tugs
The above ranking is, of co urse, a general one.
Differen ces in design of a particular type can change
the ranking, especially tug types with more or less similar
characteristics such as tractor, ASD and reverse-tractor
tugs. Conventional tugs will never reach the high
m an oeuvrability of omnid irectional tugs. Bu t
conventional tugs also have many differences in design
and manoeuvring devices, making one much more
manoe u vrab le than ano the r. Wen chosen d e ck
equipment can improv e a tug's performance. For
instance, installation of a radial hook in a conventional
TUG USE IN PORT 65
To achieve or to execute
Escapamento
tug can make tha t tug supe rior to a similar conventional
tug witho ut such an arrangement. The same applies to
ASD-tugs, when they operate as a conventional tug.
It should be borne in mind, too, that the above bro ad
ranking refers to a tug's effectiveness. Wh en safety of
operations is the major requirem ent, then tractor and
reverse-tractor tugs are recommend ed. Although fire
figh ting is no t discussed in thi s book it is also an
important factor to be considered with respect to a tug's
manoeuvrability. Finally, the maximum draft of a tug,
e.g. of tractor tugs, can make them unaccept able for
certain ports regardless of their high manoeuvrab ility.
4.7 Some other practical aspects
There are other aspects which are important for safe
and efficient shiphandling by tugs.
Cooperation
As stated in section 4.3.1 pilots, ship masters and
t ug captains should know each oth ers capabilities and
limitations regarding ship and tug manoeuvres. This
knowledg e is the basis of good coo peratio n and
und er standing bet ween them. O n ly th en will
m anoeuvres go smoothly and a ship be hand led safely
and efficiently. Wh en manoeuvring, the pilot should
keep , as far as is possible, a close eye on the assisting
tugs. He will then see how the tugs are performing, can
take action when they don't act as expected, or when a
tug's safety is at risk.
Communications between pilots and tug captains
For good cooperation between pilots and tug captains
a good communication system is indispensable. Portable
radio-communication sets have been used for years by
pilots. When of a good mak e these sets are very hand y
and work satisfactorily. Radio sets should be tested prior
to a pilot boarding a ship and it is best tha t every pilot
has his own set.
Tug orders sho uld be given clearly and be open to
: onl y one interpretation . Tugs should be addressed by
• nam e or by operating position. Tug captains should
co nfirm and repeat the orders given, stating their tug's
nam e or position. Any possibility of misunderstanding
should be avoided.
Many ports prefer to use a standard system in
English, but it will take yea rs before such a system could
be intro duce d worldwide (see also paragraph 9.5.1
Communications and Information).
In near ly all ports the language between pilots and tug
captains is a kind of slang and is therefore not always
comprehensible to the master of a ship. Although pilots
and tug captains understand each other well enough, it
is a strange situat ion because th e ship master is still
responsible. It would therefore be bet ter if tug orders
were given in English, according to an intern ationally
66 THE NAUTICAL INSTITUTE
agreed stan dard vocabulary. Using und er stand able
English is fine in English speaking countries, but it does
cause problems in m an y non-Engli sh speaking
countries. In particular, tug captains often speak only
the local language. An international standard vocabulary
is, for that reason, hardly feasible. In addition, a standard
vocabulary canno t cover non-standard situations. In
critica l situations pilots and tug captains should be able
imm ediately to understand what is wan ted. A change
in com munication procedures m igh t re su lt in
misunderstandings. This sho uld be avoided.
Nevertheless, tug captains should always be inform ed
by pilots about the intended ship and tug man oeuvr es.
Furthermore, the use of a basic system for tug orders in
a port is necessary, even though only a local system,
but sho uld be standard for all local pilots an d tug
captains .
Tug use
Harbour tugs handling a ship should have a reserve
of power, be able to react fast and to handle a ship in
such a way that a minimum of space is.required for the
ship and assisting tugs. The slower tugs react, the longer
the towlines and the smaller the tug power, th e mo re
manoeuvring space is required for a ship and assisting
tugs. However, manoeu vring space is usuall y very
limited in port areas .
Tug size and power shou ld be relative to ship size.
Large and powerful tugs should normally not handle
small ships. Tug actions in that case could induce too
large moveme nts of the att ended ship, resulting in
inefficient shiphandling and in a worst case dam age to
th e ship . In additi on, bollard pull of the separate tugs
handling a ship should not mutually differ too mu ch,
Tug configuration should be planned we ll in
advance, taking in to accoun t availab le tu gs, the
capabilities and limitations of different tugs, manoeuvres
to be carried out, the influence of wind, current, and so
on . A nice example of tug configuration can be seen in
the photo of the bulk carrier at page xiii. Three different
tug types are used in an appro priate configuration . The
ship has to round a starboard bend. O ne conventional
tug is assisting the ship, position ed starboard forward
where it can be effective in app lying steering assistance.
In add ition, this tug can tow with a larger towing angle
than a VS tug. One VS tug is therefore pos itioned at
the port bow. At the port quarter aft is a powerful ASD-
tug, in the be st position for ass isting in a turn to
starbo ard. The ASD-tug can assist in the turn in the
dire ct mode without increasing ship's speed and the tug
can, if required, control the ship's speed. T he seco nd
VS tug with less bollard pull than the ASD-tug'is
therefore positioned on the starboard quarter aft.
Repositioning of tugs may sometimes be considered
necessary durin g a trip, bu t should be avoided as far as
possib le, part icul arl y if shifting the tu gs in volves
releasing and refastening towlines. This takes time,
especially with the limited number of crew .members
on board nowadays. During the time of shifting a tug,
the ship has less or no tug assistance and in the worst
case towlin es may foul ship's or tug's prop eller.
Speed
Ship' s speed sho uld b e carefully contro lle d in
relation to the limitations of the tugs involved. Thi s
gene rally means that speed should be low, taking into
acco unt the effect of current and wind . In any case , the
lower a ship's speed th e more effectively tugs can
operate. Also, other factors playa role with regard to
ship's speed, factors which can affect the tug assistance
required and tug safety, such as interaction and shallow
water effects, which 'are discussed in Chapter 6,
Decreasingeffectivenessoftugs when a ship gathersspeed
The difference in pulling effectiveness tha t arises
between a forward tug and stem tug when a ship gathers
speed has been mentioned earlier. In addition, an effect
to keep in mind is the decreasing effectiveness of tugs
in general when a ship, initially stopped in the water,
gathers speed . This has som etimes resulted in waiting
time for ships.
Example: A container ship has to depart from a
harbour basin with strong onsho re winds. Tugs are
ordered . Total bollard pull available
seems sufficient to
pull the ship off the berth. So far, no problem. However,
as soon as the ship's engines are started and she starts
moving, the tugs towing on a line take position to b e
able to keep pace with the ship, so their effectiveness
decreases. The ship may drift alongside the berth again
and addi tional or stronger tugs have to be ordered,
which takes tim e. When moored po rt sid e to and
departing astern out of the harbour basin with an
onshore wind the effect is worse due to the transverse
effect of the ship's propeller.
Ship pulled orpushed around by a bow tuggathersspeed.
A tug pulling at right angles to the bow of a ship
stopped in the water will give th e ship a lateral velocity
and a rate of tum, causing the ship to pivot around a
point somewhere near the stem (see for instance figure
4.2B ). As a consequence the ship' s lateral centre of
gravity follows a curved path. A body following a
circular path experiences a 'centrifugal force' , and such
a force also acts on the ship's centre of gravity moving
along the curved path. A 'centrifugal force' is always
directed outward and perp endicular to the curved path.
This 'force' originally acts almost in line with the ship,
thus causes the ship to gather headway. The fluid forces
also contribute to this effect.
TUG USE IN PORT 67
Entangle or jam
Chapter FIVE
BOLLARD PULL REQUIRED
5.1 Introducti on
TUG CONFIGURATION, THE NUMBER OF TUGS and total
ballard pull used are normally based on a pil ot 's
experience and may vary depending on port conditions
and circum stances. In general this system work s well.
However, with increasing ship size it is more difficult to
determine what exactly is needed to handle a ship safely.
Experienc e alone in such a case is too narrow a basis
and may not cover all situations and conditi ons which
might be exp ected . Information on wind, current and
wave forces may be essential. This could be particularly
the case when large co ntainer ships, car carriers, deep
dr aught tankers or bulk carriers have to be handled in
unfavourable environmental conditions and in confin ed
port areas.
Anoth er con sideration is that, because of economic
pressure, shipping companies often try to min imise tug
assistance costs.Thi s can easily lead to a dispute between
the pilot, master or shipping agency about the minimum
number of tugs to be used. Ships equippe d with bow
thrusters and/or stern thrusters often use one or two
tugs less. Side thrusters, however, have limitations to
their maximum power and effectiveness , which
decreases very rapidly when a ship gathers headway.
The tug assistance required is, therefore, often subject
to discussion about acceptable limits ofsafety. Pilot and
master, if well prepared, can avo id these discussions
and are in a better position to take the right decision.
Depending on the local situation, tug assistance on
arrival or departure gen erally comp rises three phases:
The phase whereby a shiphasreasonable speed
The ship can still use her engines and rudder to
compe nsate for drift forces caused by wind, current .
and!or waves, by steering a drift angle. Depending
on the situation, tugs may assist.
The intermediatephase
W"hen a ship has to reduce speed, entering a dock,
harbour basin, turning circle or approaching a berth.
The ship also has to be stopped within a certain
distance. When re ducing speed, a ship's steering
performance also decreases. The propeller has to be
stopped, the influence of wind and current increases
and tug assistance is needed more frequently and to
a larger extent.
The phase involvingthefinal part ofthe arrival
manoeuvre.
The ship is practically dead in the water, such as in
the turning circle and!or whe n berthing. The ship is
very restricted in manoeuvring performance and not
68 THE NAUTICAL INSTITUTE
able to compensate for wind and current forces. Tugs
have to assist fully.
For ships influ enced by wind, current and waves this
last phase, when a ship is stopped in the water, is most
important for assessmen t of ba llard pull requir ed. It is
this phase which will mainl y be considered, th erefore.
In considering ballard pull required , the availability
of side thrusters is sometim es taken into account,
be cause a side thru ster may replace part of the ballard
pull required. Whether this is the case depends on the
ship, th e local situation, the circumstances and por t
regulations .
5.2 Factors influencing total ballard
pull required
The following main factors influence tug assistan ce:
Portparticulars, including:
Restrictions in the fairway, port entrance, passage to
a berth, turning circle, manoeuvring space at a berth
or harbour basin, available stopping distance, locks,
br idges, moored ve ssel s, water d epths, speed
restrictions, and so on .
Berth construction, including:
Type of ber th: open, e.g. jetty, or solid.
Theship, including:
Type, size, draft and und erke el clearan ce, trim,
Windage, and factors such as engine power ahead!
astern, propeller type, manoeuvring performance,
and availability of side thru sters and specific rudders.
Environmental conditions, including:
Wind, current, waves, visibility, ice .
Method oftugassistance, including:
Towing on a line, ope rating at a ship's side or a
combination of methods.
The port is more or less a constant factor. Parti cular s
of port layout, such as fairway, port entrance, passage
to the berth, turn ing circle and berth location , determine
a basic number, type and total tug ballard pull for a
particular class of ship. This is based on local experience
and sometimes, for more difficult situations, on simulator
research. An indication of ba llard pull required for
tankers, bulk carriers and con tainer ve ssels is given
below. Berth construction has to do with the tran sverse
approach speed towards a berth, which is also dealt with
in this chapter.
In addition to tug assistance requirements following
from port layout and berth con struction, the varying
factors influencing the required total bollard pu ll for a
particular ship are:
Wind .
Current.
. Waves.
These factors have to be considered in relation to
ship details such as size, draft, und erk eel clearance , etc.
The man oeuvring performance of a ship may influenc e
required. tug assistance in a po sitive or negative '''lay.
The towmg method should also be taken into account.
Reduced visibility is also considered a factor of
im~ortance regarding tug assistance. This is true, but it
mai nly concerns specific safety procedures for tug
assistance during fog. Reduced visibility, therefore, is
not discussed further in this chapter. Tug assistance in
ice conditions was dealt with in Chapter 3.
The total force acting on a ship cou ld, in theory, be
compensated for by tugs wh en bollard pull equa ls the
total forces of wind, current and waves. However, there
are some important factors to be taken into accou nt:
Tugs must have sufficien t reserve power to push or
pull a ship up against wind and curre nt or to stop a
dr ifting ship quickly enough.
Tugs are not always pulling or pushing at right angles
to a ship . For instance, du ring arrival or departure
manoeuvres, a ship may have some forward or astern
movement. Tugs try to keep pace with a ship, and
thus use engine power in the direction of ship 's
movement at the expense of pull or push forces. The
same happens in situations where there is a current
and a ship has relative speed through the water.
Bollard pull actually available may, due to wear and
fouling, no longer be a full 100% compared to the
original bo llard pull tests .
Forward and after tugs often cannot pull or push at
full power simultaneously, even when the required
bo llard pull forward and aft is carefully considered ,
taking into account possib le yaw moments caused
by wind and/or current or trim . A ship
may start to
swing. At one end of the ship the tug then has to
reduce power in order to stop the swing.
The propeller wash of tugs towing on a line may hit
a ship 's hull an d decrease pulling effectiveness. This
can be infl uenced to a certain ex ten t by correct
towline length and towing angle, as exp lained later.
So , when calculating the forces of wind , current and
waves on a ship, a spec ified safety factor should be taken
into ~ccount for bollard pull required. In the graphs
showing bollard pull required to keep a ship up against
a beam wind , cross current and beam waves, a safety
f~ctor ?f 20% is included. For tugs pulling at a ship's
SIde th is safety factor is not sufficien t due to the large
loss of pulling efficiency, which is separately considered.
5.2.1 Wind forces
The forces on a ship ca use d by wind can b e
calculated by the formulae :
Latera l force:
FYw = 0·5 CywP V' AL Newton
Longitudinal force:
Fx- 0·5 CxwP V'~ Newton
Yaw moment:
Mxyw = 0·5 CxywP V' Ac L.pNewtonmetres
Cyw Lateral wind force coefficient.
Cxw Longitudinal wind force coefficient.
CXYw 'Vinci yaw moment coefficie nt.
p Density of air in kg/m' .
V \Vind ve locity in m/ sec .
A,. Longitudinal (broadside) wind area in m' .
~ Transverse (head-on) wind area in m2.
Lsp Length between perpendiculars in m. .
The lateral force, longitudinal force and yaw moment
coefficien ts depend on a ship's form, draft and trim,
superstructure such as bridge, deckhouses, masts and
ramp, and angle of attack of the wind. It should also b e
noted that deck cargo, as on container vessels, should
be included in calculating wind areas. The coefficients
Cyw' Cxwand Cxywdiffer by ship and can be determined
by means of model tests in wind tunnels.
For several ship types the wind coefficients are
know~ for all ang les of attack and certain loading
conditions. For tan kers they can be found in 'Prediction
of Wind an d Current loads on VLeCs'. Lateral forces
are largest and most important for calculating ballard
pull required. Cyw varies between approximately 0·8
and 1·0 for beam winds, dep ending on ship's type and
loading condition, but lies mostly between 0.9 and 1.0.
With value 1·0 for Cyw' 1·28 kg/m! for density of air
and calculating the outcome in kilograms instead of
Newtons, the formula for beam wind forces can be
simplified to: FYw= 0·065V'AL kgf.
To allow a safety margin of 20%, 25% should be
added to the previous formula, resulting in the following
handy formula for estimating ballard pull required for
beam winds:
F = 0·08 V' A kgfW L
The graph in figure 5 .1 is based on this formula,
whereby I mlsec = 2 knots' The calculated required
ballard pull in the wind grap h of figure 5. 1 is
approximately 5% higher when, for wind speed in knots,
a more accurate equivale nce in m/sec is taken. The
safety factor of 20% included is in some cases eve n
higher, b ecause for a lateral wind force coefficie nt the
value 1·0 is allowe d, which is sometimes only 0.8 or
0.85, although difficult to assess in daily practice . The
TUG USE IN PORT 69
...?
Figure 5.1
Bollardpull
requiredto
compensate
for beam winds
Note 1:
17m is equal to
lOOOkgforce
(= 9·8 leN) .
Nou 2:
For ltzrge gascarriers
see note in text
original data from
UK National Ports
Council 1977
8000
9000
500
7000
450400150 200 250 300 . 350
Required Bollard Pull in Tons
100
Longitudinal (broadside)Wind Area (Square Meters)
1000 2 3000 4000 5000 6000
50
:I
~ 0
i'l .e
11 60
10 50
9
-e 40
~ 8 ~~
"-til
-e
.S
~
-20
5
4
3 10
2
0
Figure 5.2 Windheighl velocity ratio
/
/
.I
/
/
V
.s->
V
o
0.5 0.6 0 .7 0.6 0 .9 1.0 1.1 1.2
Ralio Wind V.lo~Uy al IH) to V,lo olty _t 10 M Haight
s
'0
,s
20
2 5
30
3 5
vw V w (1O/h)'/1
. Haigh t IH) .bov. S.. Left' ln M. t r..
4 0
vw wind velocityat 10metres height (mls).
v. the wind velocityat elevation h (m/s).
h elevation above ground/water surface (metres).
For calculating wind force in the equations, its
velocity at 10 metres height should be used. For wind
velocities obtained at a different elevation, adjustments
to the equivalent 10 metre velocity can be made with
this formula. On the oth er hand, wind indications given
Wind velocity also varies by height, as shown in the
graph in figure 5.2. The graph is based on the following
formula:Note:
For loaded tankers the outcome is too high, because
the lateral wind coefficient of fully loaded tankers is
approximately O·7. For fully loaded tankers, however, it
is generally more the mass that counts . Care should be
taken when calculating the required ballard pull for large
liquefied gas carriers. The lateral wind coefficient for
these ships varies between 1·05 for gas carri ers with
prismatic tanks and 1·2 for gas carriers with spherical
tanks (see References for 'Prediction of Wind Loads on
Large Liquefied Gas Carriers'). Therefore, for gas
carriers with prismatic tanks 5% and for gas carriers with
spherical tanks, 20% should be added to the outcome
calculated by the formula or indicated by the graph in
figure 5.1.
graph is only valid for tugs towing on a line or pulling
at a ship's side on a rather long towline .
For winds not coming from abeam the total ballard
pull required can roughly be derived from the ballard
pull required for beam winds. It can then be seen that
when the angle of attack of the wind is between abeam
and up to approximately 30 degrees each side of abeam,
the ballard pull required is nearly the same as for beam
Winds. In general, yaw moment is maximum for
quartering winds but depends, amongst other things ,
on type of ship, loaded condition, trim and deck cargo .
Wind does not blow constantly with the same force -
wind velocity fluctuates continuously. Therefore not just
mean wind velocity should be accounted for, but wind
that may be experienced in gusts and squalls. A wind
meter , properly installed with a recording device at a pilot
station, gives the best information . Ifconsidered necessary
gust factors, e.g. from PIANC, can be applied tentatively
to find the relationship between mean wind speed and
associated maximum speeds for shorter periods.
70 THE NAUTICAL INSTITUTE
1000
2000
3000
=0,8x1000x(25**2)
...??
Lateral current force coefficient.
Longitudinal current force coefficient.
Current yaw moment coefficient.
Density of water kg/m"
Current velocity in m/ sec.
Length betwee n perpendiculars in m.
Draft.
Longitudinal force:
Fxe = 0·5 CX, PV' r... T Newt on
Lateral force :
F 0 5 C P '" T T Newtony, . y, y - ..... •
Yaw moment:
Mxy, = 0·5 exy, P V' L.iT Newtonmetres
Cy , =
C", =
CXYe =
p
V
LBP =
T
The current coe fficients, CYe' C Xt and CXYe]
differ by a ship's underwater shape, draft, trim
and angle of attack, and are also affected by
underkeel clearance which has a very strong
e ffec t on the coefficie n ts . T hese are
determined by using ship mod els in test tank
studies .
l.oS
1.1
Current velocity is taken in metres /second, th e
outcome in kilograms. This formula is only valid for
deep water, i .e, more than six times ship's draft
The lateral force coefficient for cross currents in deep
water is around 0 ·6 . This is, amongst others, the
OCIMF-coefficient for loaded tankers. When Cy , equals
0·6, density of salt water is 1025 kg/m", adding 25% for
loss of tug's effectiveness and giving the outcome in
kilograms instead of Newtons, the following simplified
formula for calculating the approximate bollard pull
required for cross currents in deep water can be used:
For the bollard pull required the maximum
transv ers e forces exerted by a crosswise
current are important. Th e transverse force is calculated
using the formula: Fy, = 0·5 Cy , P V' r...T.
5000
,
,
.,. ----- -
,
,
I 3.0 .,g
--'------ -- .:
':i'
1.5 a
~
~
~
,;
12 •
~
4 "
__ _ _ _ _ 1 _
,
,
,
___ _ _ _ L _
,
-- -1-------
_ _ _ ,.J. _
_ _ L _
,
,
,
- r
,
,
- --1---- -- -
, 2
01 _
Un derwater Late ral Area (Square Metu .)
1000 2000 3000 4000
,
,
,
- - 1 -
,
,
, ,
_ _ __ L ~ _
, ,
, ,
, ,
___ _ __ L ~ _
,
,
,
___ '- _ _ J. _
,
,
, ,
---- --~ - -- - --T - ---, ,
, ,
, r ,
---- - - T------T------~--- -- r
1 I I I
I I I I
I 1 I I
-- - - - -~- -----T- - --- - r---- - -T---
I 1 I I
I 1 I r
I I I I
1.0
0.8
~
0 0.6!
~
0
~ 0.4
"•U
0.2
0
SO
•
•0
...
.S 100
'3
..
-e 150
"•
"•
".. 200
•
'3
"'•
'"
250
300
FIgure5.3 Bollardpull requiredina ooss-current
Note: Ton is'qual to l000kgforct (= 9·8 kN)
Original datafrom UK National Ports Council 1977
by a win d meter on top of a ship's mast give safe
approximations for evalua tion of the lateral wind force
and bollard pull required .
A ship drifts under the influence of wind whe n the
wind forces acting on her are not compensated for by
tugs. A factor influencing drift velocity is underkeel
clearance. A drifting ship has a relative spee d through
the water, as with curre nt. The drift spee d of a ship
decreases with underkeel clearance, because the forces
create d by the opposing water increase when underkeel
clearance gets smaller. This is considered later whe n
. discussing current forces.
Of course, a smaller drift speed does not imply that
less bollard pull is needed. A drifting vessel has to be
stopped and pulled back through the water. Stopping a
ship from drifting and pulling back also needs more
power in shallow water than in deep water. The amount
of water moving with a ship whe n drifting, the .added
ma ss, also in crease s with decrea sing underkeel
. clearance, requiring additional bollard pull to stop and
pull b ack a drifting vess el in shallow water.
5.2.2 Current forces
Since und erkeel clearance in port areas can be small,
the current forces in the se conditions are at least as
important as they are in deep water. With underkeel
clearance decreased to 1·5 x ship's draft , bollard pull
required increases conside rably to approximately:
r, = 110 V· r... T kgf
With an under keel clearance of 20% of ship 's draft,
the bollard pull requi red is rou ghly:
Fe = 150 V' LB, T kgf
The current force s acting on a ship can be calculated
in th e sam e wa y as wind forces . For th e sake of
co m p le te n ess, th e formulae u sed in O CIMF
publication s are given:
When underkeel clearance is further redu ced to 10%,
the bollard pull required is nearly five tim es as high as
in deep water, approximately:
TUG USE IN PORT 71
Fc = 185 V' LBP T kgf
25% has in all cases been included for safety reasons.
The graph in figure 5.3 gives an indication of ballard
pull required for cross curre nts and is based on th e
aforementioned formulae and OCIM F coefficients for
loaded tankers. The outcome includes a 20% safety
margin. The graph is only valid for tugs towing on a
line or pulling at a ship's side on a not too short towline .
Th e effect of reduce d underk eel clearance on current
force is also clearly shown in figure 5.4. Starting with a
current force of 10tons, the same current velocity causes
a strongly inc reasing force on the same ship whe n
underkeel clearance decreases. .
With a smal l underkeel clearance, current forces
decrease quickly when the angle of attack of the cur rent
b ecomes less th an 90° to a ship' s centre line .
Longitudinal forces th en increase. The effect of the
curren t forces on a ship may then even be in the opposite
dir ection to that expec ted, in particular when with a
sm all underkeel clearance the current is coming in at
about 20-30° on the bow. When, e.g. after unmooring,
turning with th e assistance of tugs a deep loaded bulk
carrier with a small underkeel clearance in a river with
current, the ship may gather headway and move against
the cur re nt direction coming in from th e port or
starboard bow. Pilots have experienced such effects and
while turning have constantly to apply astern power to
check the ship's headway.
Not only do current forces increase considerably with
d ecr easing underkeel clearance. Small underkeel
clearance also results in a larger turning diam eter, a
decrease in rudder effectiveness and an increase in
stopping distan ce. To compensate for these effects, the
assis ta n ce of tu gs might b e welcome for sa fe
shiphandling. Un derkeel clearance also conside rably
affects the du ration of swinging round a ship . The
transverse forces to be overcome fore and aft of midships
in crease with de cr easin g underke el clearance .
Consequently, the duration of swinging round increases,
unless more ballard pull is used.
5.2.3 Wave forces
Depending on environmental conditio ns in an d
around a port, wave forces may also be a factor to be
considered when establishing the ball ard pull required.
Harbour tugs can only operate effectively up to a certai n
maximum wave height (see Chapter 4), so only short
beam seas are considered. It is difficult to calculate wave
forces exactly. It is assumed that a ship's draft is large
enough to reflect the waves completely. Because of th e
relatively short wave period it is further assumed that
waves do not cause any ship motion. In practical terms
it means we are considering conditions such as those
found in windy but sheltered areas. The waves are sho rt
and steep and the wave length is small relativ e to th e
length of the ship. We are not considering ope n areas,
where ocean waves or swell might impinge up on th e
ship and cause it to heave, roll and pitch. The forces
per metre of ship' s length due to these short period
waves then amounts to approximately:
F = 0·5 P g r' Newton
wave 'e,
Because a ship's hull is not flat over its wh ole length
and draft, the total force on a ship caused by short period
waves is roughly:
F
w
... = 0·35 P g L 1;.' Newton
p Density of seawater in kg/m3•
L Length of waterline; assume length between
perpendi culars.
~a Wave amplitude, equal to 0·5 x wave height
(H,l.
H, Significant wave height from trough to crest,
as indicated by an experienced observer whe n
estimating visually.
A 25% safety margin is again added, and converting
to kilograms instead of Newton and wave amplitude in
~
o
E
111'fli!
0.5 x Draft
40 Ton
.·2 .·7.·? ·z. ;.z ·,·z·,·z.:·z·,
0.2 x Draft
Figure 5.4 Effietof underkeelclearanu on current force
72 THE NAUTICAL INSTITUTE
?..?
*
F = 112 LHs' kgfwave
Figure 5.5 Bolla rdpull requiredfor beam waves
significant wave height, th e simplified formula for
roughly calculating the bollard pull required to hold a:
ship up against short period beam waves reads:
O n the basis of this formula the bollard pull requir ed
is represented in the graph in figure 5.5. An example:
A sbip has a length between perpendiculars of 200m,
and estimated wave height is 1m.The force of the beam
waves on the ship is then (see formula):
112 x 200 x 1 x 1 = 22400 kgf = 22 tons
I
I
-7I
I
-++1-I I
I I
-I --l ----
---- I -r-r-r-- I
---
----,-1 il
---
"==:1-~_ f-----j=+=f----- - 1f--f---1--- ---
- - - ------~~~=1~ -1--->-- ------ - --- - 7
fc"::~ -~lI- - -- '----
>-- 1/
/4
Irr----+
./ I
I---- I
The larger a ship's displacement the more bollard
pull is needed to stop sideways movement, Not only
the displacement but also the water mass moving with
a ship influences bollard pull required. This is called
(added' or 'hydrodynamic' mass. Virtual mass is the sum
of displacement and added mass. The exact am ount of
added mass is difficult to determine. The added mass
increases w ith decreas ing underkeel cl e aran ce .
Furthermore, it depends on a ship'S underwater shape
and is very large with
a sideways motion. It then
normally varies be tween 25% to 100% of a ship's
displaceme nt. Many for mulae used for calculating
virtual mass of a berthing ship, especially for fend er
design, indicate values ranging from 1·3 up to more than
2·0 times the disp lacement.
As mentioned earlier, tugs should have sufficient
reserve power to stop a drifting ship. A comparable
situation exists during berthing. An arr iving ship is
stopped parallel to a berth or jetty and is then pushed ,
pulled or heaved alongside. Wind, current an d even
waves may also push a ship towards a berth . Due to
these forces a ship gains transverse speed which should
be slowed down by tugs to 'd ead in the water ' or to a
safe berthing speed at the mom ent a ship touches the
fenders . So, tugs have to oppose the forces of wind,
current and waves, and in addition have to reduce the
transverse appro ach speed of a ship towards a berth,
which requires additional bollard pull. Of course, the
wind may blow offshore and tugs may need full power
to push or pull a ship alongside. But even when there is
no wind, curren t or waves, bollard pull is needed to
control a ship's transverse speed.
5.2.4 The effect of sh ip's mass and berth
co nstructio n
Kil og ram Force per M length betwe en PP
500
480
460
440
420
400
38 0
36 0
340
320
300
260
260
240
220
200
180
16 0
140
120
100
80
60
40
20
o
o 0.2 0 .4 0 .6 0 .8 1 1.2 1.4 1.6 1.8 2
Sign ificant Wave Height (M)
REOUIRED BOLLARD PULL f OR BEAM WAVES
(Olll y valid for .hort per iod wavu!
Photo:PorIof Glatlsto1U, AlLIualia
Figure 5.6 Open berthconstructionfor bullc carriers
TUG USE IN PORT 73
.??
Berth construc tion also affects approach speed. Solid
berth s reduce a ship's approach speed because a water
cushion builds up between ship and berth. Open berths
or jetties do not reduce approach speed as the water
can flow away in any direction.
For fend er calculations it is gene rally recommend ed
to apply for a wate r depth of 1·5 times ship' s draft as
virtual mass 1·5 times the displacement and for a water
depth of 1·1 times ship's draft as virtual mass 1·8 times
the displacement.
Lo ad ed tanker s and bulk carrie rs wit h large
displacement nee d the largest tug power for controlling
transverse speed. T hese ships are less affected by wind.
When there is any current, ber th construc tion should
be such that the cur rent runs in line with the berth or
jetty, though unfortunate ly this is not always so. In any
case, tugs should have sufficient re serve power to
compensate for any current and /or wind effect. In
general, when handling heavy ships , tugs use a
substantial part of thei r power to control tr ansverse
approach spee d towards a berth.
As virtual mass 1·8 times displacem ent is taken and
be rth construc tio n is th en accounted for. A rough
ind ication can thus be made of tug forces required to
stop sideways movement:
For open berths:
For solid ber ths:
0·09 D x V',
--- tons
S
0·07 D x V ',
---- - - tons
S
As dra ft decreases the ball ard pull required for
contro lling transverse spee d becomes less, as indicated
in the examples for a loaded and ballasted tanker. Lateral
wind area increases and consequently available bollard
pull can be used to keep the ship up into wind, current
and/or wav es, if necessary.
Newer tugs are ahle to operate for a limited time at
l lOOfo MCR. This means that for a sho rt peri od these
tugs can deliver additional bollard pull, an advantage
in critical situations.
This formula is based on zero final speed and the
calculated force is in ton s. Final safe approa ch speeds
for VLC Cs are gene rally a maximum of 6-8 em/sec.
V,
D
S =
Initial speed in metres/ sec
Displacem ent
Stopping distance in metres
For ships affected by wind, current and!or waves a
safety margin is included in the graphs, also for the
purpose of controJling transverse speed. For load ed
vessels, tankers and bulk carriers, ballard pull required
for controJling tr an sverse speed is included in the
formula in section 5.3.1.
In th e following thr ee examples an initial speed of
0·5 knots (0·25 rn/sec) is assumed and tugs start pulling
when a ship is 30m away from a berth. Transverse speed
sho uld be zero when a ship touches a jetty or berth.
A 250,000 dun ballasted lanker
Length overall 340m, beam 38m, draft 9m (29·5ft)
and displacement 124,000 tons. She has to be berthed
alongside an open jetty. According to the. formul a
the total tug power required to stop the ship in 30
metres is approximately 23 tons.
A 250,000 dun loaded tanker
Draft 20·4m (67ft) and displacement nearly 300,000
tons - much mor e power, almost 60 tons, is needed.
A container ship
Length overall 294m, beam 32·2m and draft 12·2m
(40ft), displ acem ent 80,000 tons. She has to be
berthed at a solid berth. Based on the assumptions
above the bollard pull re quired to stop her in
approximately a ship's width i~ 12 tons.
These calculations give an indication of th e forces
required. In line with experience they show that large
di spl acement ships requite large stopping forc es.
Furtherm ore, berth construction is a factor influencing
approach speed.
74 THE NAUTICAL INSTITUTE
5.2.5 Thg wash effects
In certain pull ing situations, a tug's propeller wash
impinges on a ship's side, bow or stern, reducing pulling
effectiveness. The smaller a ship's underkeel clearance
Photo:Author
Figure 5.7 A tug', propelkr wash hilling a ship~ hull,
reducing lowingeffectiveness
the larger the negative effect of propeller wash hitting
th e hull. Increasing propeJler rev olutions or thrust
worsens' the situation bec ause counter e ffec t also
inc reases , caused by a larger, more co nc entrated
prop eller wash. Proper towline length and towing angle
Por que menor ?
Figure 5.9 'Coa7lJ1Jl ' <ffict
reduce thi s adver se effect. The less the underkeel
clearance and the more power needed, the longer a
towline shou ld be.
In figore 5.8, several towing positions are given for
a ship stopped in the wate r. In positions If (f=forward)
and l a (a=aft) there is a fair po ssibility that the pulling
tugs experience loss of pulli ng effectiveness due to
propeller wash hill ing the bow and stern almost at right
angles . A ship's hull form, shape of bow and stem and
whether she has a large bulbous bow, influence loss of
effectiveness. For th e pulling tugs, e.g. tug If, it might
even be poss ible that the tug's wash effect causes a
turning m om ent on the ship in an opposite direction to
that expected from the orientation of the tug. Such an
effect is shown in figore '5.9. The ship is loaded, has a
bluff bow a nd a sm all u nd erkeel cl earan ce . A
conventional tug is pulling at right angles to the ship' s
hull on a short towline . The consequence is an almost
total loss of towing effectiveness by the reaction force R
of the propeller wash hi tting the ship's hull. In addition,
the bulk of the accelerated water flow goes around the
bow of the ship and remains allached around the curved
surface. This is called the 'Coanda Effect'. The flow
So, loss of pulling effectiveness of forward and aft
tugs towing on a line can be minimised by appropriate
towline length, towline angle and/or thruster selling. A
towing winch is very useful for adjusting towline length
in accordance with circumstances. For tugs op erating
at a ship's side, when pulling, th e larger the distance
between propellers and ship' s hull the beller.
Compared to positions If and l a of figur e 5.8,
positions 2f and 2a may show'less loss of effectiveness.
Regarding loss in effectiveness due to propeller wash
towing positions and towing directions 3f and 3a are
considered the most effective. Tugs operating at a ship's
side , in positions 4f and 4a, have a large loss of
effectiveness when pulling. When operating in the push-
pull mode
to wline lengt hs are short and pulling
effectiveness can even be less than 50%, depending on
how close the tug's pro pellers are to a ship's hull.
In the following assessments of req uired bollard pull
it is assumed that equal tug powe r is required forward
and aft, which is not always the case. Yaw moments can
be caused by wind and depend on the wind force, angle
5.3.1 Bollard pull re quire d based on
environmental conditions an d displacement
5.3 Bollard pull required
For tugs operating at a ship's side and holding her
up into the wind, current or waves on short towlines,
the required pull in the graphs in figores 5.1, 5.3 and
5.5 should be increased by, say, at least 20%, resulting
in a total safety margin of 50%.
Tug propellers should be as far as possible away from
a ship's hull. Conventional tugs, towing on a line, have
their propell ers closer to a ship's hull compared with
tractor, reverse-tractor and ASD-tugs. The latte r two
types, when towing or pu lling over the bow, have their
prop ellers furthest away from the ship's hull. Thi s is of
special importance for tugs operating at a ship's side or
in narrow harbour basins where they often have to work
on short towlines due to limited manoeuvring space.
VS tugs have less pronounced propeller wash compared
with conventional tugs and tugs with azimuth thrusters,
in particular tho se wit h prop ell er s in no zzles .
Consequently, the negative effect of VS propeller wash
hitting a ship's side is less. Tugs with azimuth thrusters
can set their thrusters at a small angle, at least with
independently controlled thrusters, thus deflecting the
wash.
crea tes a low pressure resulting in a force F. This has to
do with the Bernoulli effect, which is explained in the
next chapter. The result is tha t the pulling force T is
opposed by the reaction force R and the only force left
is force F, giving the ship a forward and starboard instead
of port turning movement.
ta4a
F
Force due to Ooande Effect
"
Figure 5.8 DifJerrnl lowingposilions
"
TUG USE IN PORT 75
of attack and on the ship's profile above the water, which
varies with draft, trim and deck cargo . In addition,
current may cause a yaw moment, depending on the
current velocity, angle of attack and ship's underwater
profile which varies with draft and trim. Although with
beam win ds or currents a ship may experience a,yaw
moment, they are generally largest with quart ering
winds and currents. Yaw moments cause d by currents
even increase with decreasing underkeel clearance. Yaw
moments caused by wind and! or current may result in
a higher ballard pull requirement forward or aft.
There is another aspect to be taken into account.
When for example pulling a ship offthe berth, the lateral
underwater resistance becomes effective. On a ship
having a large stern trim, the centre of pressure of the
lateral resistance lies aft of midships. When forward and
aft the same amount of ballard pull is used, the after
tug(s) have to use more power than the forward tug(s) to
pull the ship parallel off the berth. A ship down by the
head may require more ballard pull forward than aft.
It is because these turning moments vary so much,
only the required total ballard pull is considered. How
much ballard pull or how many tugs forward and aft
are required should be carefully considered each time,
based on an assessment of the actual situation and
circumstances.
The onshore wind (figure 5.1) 117 tons
The crosswise current (figure 5.3) 42 tons
The waves 281 m x 30 kg (figure 5.5) = 8 tons
Total ballard pull required = 167 tons
To compe nsate for wind, current and waves, four
tugs with at least 40 tons ballard pull are needed. In the
total ballard pull required for wind, current and waves
a safety factor of at least 20% = 33 tons is included .
This reserv e pow er is also, am on gst other things,
sufficient to control approach speed towards the berth.
Without an y cu rren t or waves, four tugs of
approximately 30 ton s ballard pull would be need ed
Of, wh en available , two of 60 ton s, to compe nsate for
wind forces.
1\I105t container ships, car carriers, ro -ro ships and
so on are equipped with bow thrusters or b ow and stern
thrusters. 100 HP of a bow thruster is about 1·1 tons
force, 100 kWabout 1·5 tons force, for a ship 'dead in
the water'. The effectiveness of stern thru sters is
generally somewhat less. If th e ab ove m entioned
container vessel is equipped with a bow thru ster of 2500
HP (1840 kW) then 28 tons less ballard pull is required
forward . If just the influence of wind is to be
compensated for, this would lead to a reduction in th e
number of tugs of 30 tons from four to three, i.e . one
forward and two aft.
When towing on a line or pulling at a ship's side on
not too short a towline, the following compensatory total
ballard pull is required :
Therefore, experience is an indispensable factor. As
mentioned earli er, master and pilot are in a better
position to assess requirements for tug assistance and
unwanted effects to be avoided if they have a good
understanding of the forces and other factors influencing
a ship and of tug performance.
Shipsafficted by current, wind and/or waves
The graphs in figures 5 .1, 5.3 and 5.5 give an
indication of th e ballard pull required by ships affected
by wind, current and! or waves. As an example for using
the graphs:
Container ship: length overall 294m , length between
perpendiculars 281m, beam 32m, draught 12·5m, water
d epth 13·8m. Top of containers to waterline
approximately 22m . Onshore wind at right angles to
the berth. Wind speed 30 knots (7 Bft). The location of
the container berth is not too good, with a cross current
of 0·5 knots. Short period waves of 0·5m height are also
coming from a direction perpendicular to the berth.
Ratio draft/water depth 13·8: -12·5
Area above water, approx. 294 x 22
Underwater area, approx_281 x 12·5
Displacement
= [ ·1
= ±6500 m'
=±3500 m'
= 75,000 t
It is clear that whether a bow thruster can repl ace a
tug depends on the forces to be compensated for and
the ballard pull of the available tugs. It also depends on
the local situation, circumstances and port regulation s
as to whether side thrusters can replace on e or m ore
tugs. For certain situations, for example whe n passing
narrow bridges wher e tug assistance is required, it is
preferable to have a forward tug on a line. Regardless
of the fact that a ship is equipped with a bo;'" thruster ,
its effectiveness decreases very quickly as a ship gathers
forward speed. At a speed of two knots through the
water, effectiveness is usually reduced by 50% compared
to zero speed. At four knots the effectiveness of a bow
thruster is reduced almo st to nothing. At such speeds a
bow thruster cannot replace a forward tug.
It should also be noted that the effect of a bow
thruster on a ship becomes less with d ecreasing
underkeel clearance, due to the higher force s needed
to turn a ship, to move a ship sideways or to stop a
sideways movement and to compensate for the influence
of currents. Therefore a ship equipped with a bow
thruster, which normally uses no tugs, may require tug
assistance in shallow water conditions .
When tugs operate in push-pull mode and have to
hold a ship on short towlines up into wind, current and!
or waves, the required pull in the graphs in figures 5.1,
5.3 and 5.5 should be increased by at least 20%. In th e
case of the container ship with a bow thruster and an
76 THE NAUTICAL INSTITUTE
Number of Tugs TotalBollard Pull
4 200
3 150
2
o
AverageNumber of Tugs
Avcrage Bollard Pull
I I I
300 rn. Length o.a.
Figure 5.10 lbtalbollardpull in Ionsand average number of tugsfor containerandgeneral cargo vessels asused in a number ofportsaround the
world.Dependingon theportandlocal circumstances less tugs mo.y be usedwhenships are
equipped with sidethrusters
Number of Tugs
5
4
3
TotalBollard Pull
250
- 200
150
2 -
o
120 150 200 250 300 350 m. Lengtho.a.
Figure 5.11 Total bollardpull in tons and averagenumber oftugs for tankers andbulle carriers as usedin a number ofports around the world
(based on length overall)
Number of Tugs Total Bollard Pull
250
4 200
3
2
150
50
20.000
...;
100,ODO 200,000 300,000 Deadweight Tonnage
Figure 5.72 IbtalballardpuUin tons andaverage number oftugs1Mtankers andbulk carriers as used in a number ofports around the world
(based on deadweight)
TUG USE IN PORT 77
onshore wind of 30 knots, a total ballard pull would
then be needed of about 140 (117 tons +20%) - 28 (bow
thruster) = 112 tons: roughly a 40 ton tug at the forward
shoulder and two of 35 tons at the after shoulder.
ships, container vessels, tankers and bulk carriers in a
number of ports is shown. With regard to these graphs,
on departure and for ships partly loaded or in ballast,
fewer tugs or less ballard pull than indicated is
sometimes used.
5.3.2 Number and total ballard pull of tugs as
used in a number of ports
There is no uniform system in use in ports around
the world giving a relationship between size of ship and
numb er and power of tugs required. Calculations are
mostly based on len gth overall, but deadweight,
displacement or gross tonn age are also used as factors.
Ships with large displacements
Lo aded tanke rs and bulk carriers have large
displacements. For these type of ships the following
formula can be used, based on the displacement of the
ships:
Note:
Some times tugs have to assist in station keeping at
offshor e install ations, such as SPMs and F(P)SO s.
Although required ba llard pull as discussed generally
also applies to these tugs, the reader is further referred
to the information included in the OCIMF publication
'Recommendat ions for ships' fittings for use with tugs'
(see References).
The same applies to ships equipped with bow and/
or stern thrusters. Sometimes, if equipped with a bow
thruster, though not in every port, one tug less is used
than indicated in the graph and when equipped with
both bow and stern thrusters two tugs less than indicated
may sometimes be allowe d. Furthermore, ports or
terminals may have a limited number of tugs available
to assist ships varying in type and size.
When assessing ballard pull required, the assisting
mode - whether on a line or operating at a ship's side -
should be taken into account. For tugs operating at a
ship's side on short towlines the results of the wind,
curren t and wave graphs should be increased, roughly
estimated, by 20% when pulling.
The average bollard pull used shown in figures 5.11
and 5.12 for bulk carrie rs and tankers is more or less
compara ble with the ou tcome of the p reviously
mentioned formula based on displacement for ships of
deadweight up to about 230,000 tons.
For ships affected by wind, such as container vessels,
ro-r o vessels, car carriers, gas carriers, tankers and bulk
carriers in ballast, the ballard pu ll required can be
approximated using the wind graph for cross winds. The
influence of current and waves can be accounted for
using the current and wave graphs.
5.3.3 Summary
The graphs in figures 5.10, 5.11, and 5.12 give the
minimum, maximum and average total bollard pull used
in a number of ports, including the average number of
tugs. For ballard pull used the upper line of th e graph is
assumed as the requirement for more difficult situations
and the lower line for normal and easier situations.
displacement )
--- -x 60 ) +40
100,000 )
Required ballard pull =
(tons)
Decisions on the number of tugs to be used in ports
and the ba llard pull required are mainly based on
experience. For the majority of ships and situations a
mo re or less standard number and/or ballard pull is
used. Large sh ips and mor e specific situations or
circumstances are generally assessed separately by the
pil ot and/or port au tho rities to determine the tug
assistance required and if necessary and possible this is
done in consultation with the master. Simulation studies
are sometimes needed to assess requirements for spe cific
situations or specific ships .
In most ports shipmasters or pilots are free to order
the numb er of tugs and/or ballard pull they consider
necessary to handle a ship safely. In some ports it is
compulsory to use a fixed number and power of tugs,
depend ing on the type, size and draft of the ship,
environmental conditions and location of the berth . It
.may also depend on whether a ship is to berth port or
starboard side to.
This ob ligation, though gene ra lly a mi nimum
requirement, exists in a numbe r of Far East and
Australian ports and at some large oil terminals. For
ships equipped with side thrusters, a reducti on in the
number of tugs or required power is sometimes allowed.
For ships with large di splacem ents, balla rd pull
required can be approximated using the formula based
on displacement. Th e graphs showing ballard pull used
in a number of ports give an indication of the ballard
pull require d for more difficult and more no rmal
situations.
Ships with side thrusters, partly loaded or departing
may use less ballard pull than indicated. However, this
depends on the local situation, circumstances and port
regulations.
Co ntrol of tr an sver se spee d to wards a berth is
included in the graphs and formulas. For a rough check
the formula as shown in section 5.2.4 can be used.
In figures 5.10,5.11 and 5.12 tug use for general cargo
78 THE NAUTICAL INSTITUTE
5.3.4 Influence of tariffs on availability and
number of tugs used
Shipping comp anies have to pay for the use of tugs,
though in some ports tug tariffs are included in port
dues . Tug tariffs are usually based on the size of ship
and number of tugs or total ballard pull used. In many
ports ships are charged extra for tug assistance during
adve rse weather conditions such as strong winds, ice or
fog. Th e same applies to tug services during night hours,
at weekend s and when tug assistance takes longer than
a specific basic time period.
Tug tariffs often affect the number or ballard pull of
tugs used. That is why some attention is paid to this
subject, bearing in mind that circumstances and tariffs
differ by por t.
Ship arrivals and departures have an irregular pattern
and may be influenced, amongst other things, by the
working hours of dock labour and tidal restrictions. This
means that ships may arrive dur ing peak hours, for
example during hours of slack or high tide. The number
of tngs in a po rt is to some extent determined by
shipping traffic during these peak hours. A numb er of
tugs rendering assistance during busy hours will be
un employed outside those hours, so peaks in shipping
traffic affect efficient employment of a tug fleet in a
negative way.
To run a fleet more efficiently and to redu ce costs a
tug company could consider redu cing the number of
tugs. However, this automatically affects the availability
of tugs during peak hours, caus ing waiting tim e or
resulting in fewer tugs being used for a particular ship
movement, thus affecting safety.
A category of ship with an irregular pattern of tug
use is those with side thrusters, twin screws and high lift
rudders, such as large container vessels, cruise vessels,
ferries, car carriers and fo-ro vessels having a large
windage. These ships often don't use tugs, or only a
minimum number, except when the wind is increasing.
This can happen after weeks of calm weather. These
ships affect the availability of tugs, especially du ring
adverse weather co nditions, resulting in a furth er
decrease in tug fleet efficiency.
In addition, ships are getting larger and consequently
tug power has increase d considerably in recent years.
Port dim ensions have ofte n no t expan de d
proportionately to the increase in ship sizes so large
ships in restricted manoeuvring areas plac e a heavier
demand on towing assistance. More powerful tugs also
mean higher costs for towing companies.
Due to the high costs of tugs and .their crews the
inefficiency of tug fleet use may result in the availability
of tugs coming under pressure, which is the case in a
number of ports. However, the availability ofa sufficient
number and ballard pull of tugs, especially during peak
hours, is an essential factor in good service to the
shipping industry and to running a port efficiently. But
considering the po sit ion of towing com panies ,
. availability alone does no t pay. Neither does inc reased
tug power, unless tug tariffs are also base d on total
ballard pull used.
Depending on shipping traffic in a port, a more
efficient tug fleet - witho ut affecting the availability of
tug assistance - can be achieved by the use of less units
but of higher power. Less tugs can thus be used per
ship, for example, for large tankers or bulk carr iers.
These types of ship normally use a standard number of
tugs of certain ballard pull. Beyond peak hours less tugs
will then be unused.
In ports whe re pr oblems eme rge regarding th e
availability of tugs and tug power a review of the existing
tug fleet may be necessary including a review of tug
tariffs. Thi s may result in less units of higher power, as
mentioned above.
Regular meetings between port authorities, towing .
companies, shipping compani es and pilot organisations
is nec essary , in ord er to keep port se rvices at an
acceptable level without raising tug tariffs too much. It
might be 'worth considering whether a basic tug tariff
could be included in a port tariff to ensure minimum
availability of tugs.
As indicated, in certain ports tug tariffsmay influence
the availability of tugs and consequently a pilot's work .
Pilots should be permitted to assess the minimum tug
requirement to handle a ship safely. On the othe r hand,
it is quite reasonable that the cost of tug assistance is a
factor taken into account by a shipping company when
ordering tugs, although economy should never have
priority above safety. The cost of tugs is frequ ently the
background of discussions between masters and pilots,
when the number required is discussed, except for ports
where the use of tugs is compulsory or strictly regulated .
A good contrac t between shipping companies and
tug owners, stating the numb er and ballard pull of tugs
to be used, and covering circumstances when additional
tug power might be neede d, e.g. adverse weathe r
conditi ons , is stro ng ly recommended. When tug
assistance is necessary it can then be expected that the
required number and b all ard pull of tugs will be
available without additi onal cost.
TUG USE IN PORT 79
Chapter SIX
INTERACTION AND TUG SAFETY
6.1 Introduction
PREVIOUS CHAPTERSDEALT \nTH THE CHARACTERISTICS and
effectiveness of various types of harbour tug. Another
very important aspect, sometimes mentioned in those
chapters, is the risks harbour tugs may encounter wh en
renderin g assistance. It is an essential point wh en
engage d in shiphand ling operations. Essential, because
it is not only the safety of a tug and her crew that could
be at risk but also the safety of a vessel. When rendering
assistance tug captai ns and pilots shou ld be fully aware
of the risks involved. Since a number of unsafe situations
can be traced back to interaction effects, attention is
first paid to this subject and also the influence of shallow
water on several interaction effectsand the tug assistance
requi red,
6.2 Interaction and shallow water effects
6.2.1 Interaction effects influencing tug
p erformance
There are different kinds of in ter action . Some
influence tug performance, others affect tug safety, some
both . Interactions influencing tug performance are:
Tugpropeller - tughull interaction
For example, the astern thrust of a reverse-tractor tug/
ASD-tug is 5-10%less than ahead thrust, as a result of
propeller wash hitting the afterbody of the tug and so
reducing bollard pull when astern thrust is applied.
Interaction oftugpropellers
This is espec ially the case with azimuth thrusters and
VS propellers. Depending on thrust direction, the
two propellers of tractor and ASD/reverse-tractoi
tugs interact to a certain ext ent and affect a tug's
performance.
Tug - ship interaction due to tugfendering
Fender character istics such as energy absorp tion
capabilities and friction coefficients influence the
interaction of forces between tug and ship and also
tug performance.
Tug - towline interaction
Tug reactions such as tug list and consequently tug
performance are in fluenced by towline
characteristics, especially by its dynamic load
absorption capabilities.
Tug propeller - ship hull intera~tion
The reduction in pulling performance due to tug
propeller wash hitting a ship's hull has been dealt
with in a pr evious chapter. In the case of small
underkeel clearance this effect is more pronounced .
Pu shing tugs are al so affec ted by thi s typ e of
interaction when prop ellers are close to a ship's hull,
due to interrupted water flow towards the pro pellers.
80 THE NAUTICAL INSTITUTE
Tug hull - shiphull interaction
The influence of this effect on tug performance is
particularly marked when a tug operates at a ship's
side . This kind of interaction is also influenced by
shallow and narrow waters and in parti cular by ship's
speed, affecting tug safety as well.
Shippropeller/ship hull - lug interaction
These interactions affectperformance when operating
as stern tug in the propeller slipstream or ship's wake.
The effect of ship's wake increases in shallow and
narrow waters.
The points above show that there are several kinds
of interaction affecting tug performance. Tug hull - ship
hull interaction affects tug safety as well. This effect and
ship propeller/ship hull - tug interactions are dealt with
in this chapter. The others have been discussed in
previous chapters. Sma ll und erkeel clearance affects
some of the interaction effects, as indicated. It is worth
considering som e other effects of shallow water.
6.2.2 Shallow wate r effects with respect to tug
assistance
Some effects of shallow water have already been
dealt with when discussing the bollard pull required in
relation to current forces and a ship's displacement. The
relationship between decreasing underkeel clearance
and increasing bollard pull required to hold a ship up
into a current or to stop a sideways moving ship has
been mentioned previously.
There are other shallow water effects necessitating
tug assistance and requiring the full attention of pilots
and tug captains. There are situations where these effects
occur and tug assistance is then very welcome: Shallow
water, meaning sm all under ke el clearance , has the
following effects amongst others:
Increase of banksuction and bow cushion effects
A ship proceeding to one side of a river or channel
and close to a bank experiences suction forces
towards the bank. These forces are no t uniformly
distributed over a ship'S length. Their resu ltant acts
somewhere abaft of midships. The overall effect is a
bodily attraction towards the bank - bank suction -
and a yawing effect away - bow cushion. A ship can
proceed in a stable situation parallel to the bank by
applying rudder towards the bank. But as soon as
this stable situation is disturbed e.g. by an irregu lar
profile of the bank, even a submerged bank, or by
careless steering, a ship may sheer away from the
bank. If this happens it is difficult to control the ship
and she may even sheer to the other side of the river
or channeL
T he sm aller the underkeel clearance the more
pronounced bank suction and bow cushion effects
are. They can be
kept under control by keeping away
from banks as far as possible and by adjusting ship's
speed. Bank suction and bow cushion effects increase
proportionately with ship's speed, viz. by the square
of the speed. At a speed of four knots attraction
towards a bank and yawing moment away from it
are four times as high as at two knots. Also, the lower
a ship 's speed the more reserve power is available to
give a 'kick ahead' with rudder hard over to
counteract a sheer. In shallow waters and when close
to banks tugs should be on the alert to counteract an
unexpected sheer. Bank suction and bow cushion
effects have all to do with a Mr. Bernoulli, which is
explained in the next section.
Decrease of rudder effict
Possible increment of transverse effict of thepropeller
Increase of turning circle radius
The turning circle radius in shallow water is much
larger than in deep water. The initial rate of turn is
much smaller. Manoeuvring a bend in really shallow
water is therefore more difficult than in deep water.
Tug assistance may be. required to take a bend
properly. The lower,a ship's spee d, the m ore reserve
power is available to control movement and the more
effectively tugs can operate .
Increase ofstopping distance due tolargervirtualmass
In shallow water a ship drags a large amou nt of water
along with her, increasing to as much as 40% of her
displacement when keel clearance reduces to 20'/0
of the draft. When unde rkeel clearance is small, more
astern power and consequently more tug power are
needed to stop a ship than in deep water.
When passing thro ugh a channel with little underkeel
clearance the large amount of water following a ship
occasionally leads to ano ther interesting effect. When a
ship comes to an abrupt stop in a basin at the end of a
channel the following mass of water needs time to slow
down and overtakes the ship. It may push her ahead,
may increase the rate of turn , or push her sideways when
she is turni ng. O ne has explained that this effect is
cause d by the so-called 'added mass '. However, it is
more likely to be the water flow in the channel following
the ship and filling the gap behind which causes the
delayed effect when a ship comes to an abrupt stop.
This effect has been expe rienced, for example, with large
tanke rs in a Caribbean port (see figure 6.1). The extent
of the effect is direc tly related to ship's speed.
It is clear that low ship 's speed is very important
whe n pro ceeding in shallow waters. Shallow wat er
effects also enlarge interaction effects between ships .
From the point of view of safety duri ng tug operations
one sho uld always be aware of the possible occurrence
of these effects.
Following warerfrom
channel turns and
moves ship aside
Bay
~ Wa G following snip up channel
Figurt 6.1 Effictoffollowing waltr whrn p=ing throngh a dumnel
with a dttp loaded ship andcoming to a stop at theend ofthe dumnsl
6.2.3 Interaction effects influencing tug safety
Flow pattern around a ship
The interactions which most endanger tug safety are
those happening when ships are sailing or manoeuvring
close to one other. It is the water flow arou nd a ship
which produces interaction effects. Wh ether a ship is
moving through the water or water is moving along a
ship does not make any difference; water speed relative
to the ship is the same.
For furth er exp lanation see figure 6.2, where the
actua l flow pattern th at could be experienced, for
inslance, by a tug stoppe d in the water is shown. In figure
6.3 the water flow relative to the ship's speed is given.
If a ship underway through the water had no beam
or draft, the water round the ship would have constant
speed relative to the ship - the relative speed of the
water would be the same as the ship's speed. Naturally,
however, a ship does have beam and draft, so water is
pushed sideways and downwards by the ship and still
has to pass along the ship from bow to stern in the same
time but on a path that is longer than the length of the
ship. Hence most of the time during its passage along
the ship the relative velocity of the water
flow is increased.
TUG USE IN PORT 81
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(nautical) breadth amidships
Bocanull
Reducedpres sure
- - --- -------- - ---- - ------ - - - - - -
--- -------
i
I
I
, \
I \
I \
I \ \I , ,
: /~_---- , I
! II '''\ ; ;
\ \ \ / _------------~-----------__ \ I / /
BQw eree outgoing flow /---- -------,Stern 'areaJlncoming flow
'. \ \ ', / '1/ / /'~ : ' '. \.....~/ns(antaneous particle velocity \ / / // I I<tf:\n\~ "<, .'" (. o---?- 8rnJn -r--> , I /./ ./ /
-, ~ , ..,~/
~- - -
- --- . - WMt1'iOri ~.. :
+-++ +-, - - --1 W~OIte, «~~ /\
-, rv v: " \!~ " V;
+ + "
Increasedpressure + +:--e::...-=-~=_:_--....,,....-"""';_=~===;====""bJ~~~:-. + + +
Ina eesed pressure
Figure 6.2 Schematicflow- unsteady flowfieldasfelt by an observer in a stationary tugseeing a ship approaching
-v
• Boundarylayer
Retarded flow
++
+2+++_ --1
+
Increasedpressure +
•
Acceleratedflow
uce pressure
Retard
FIgUre 6.3 Pressure pattern andrelativeflowfieldaround a bulkcarrier
This is where Mr. Daniel Bern oulli comes into the
picture . An 18th century Swiss philosopher, he
established the relationship between water speed and
water pressure. He showed that an increase in water
speed results in a decrease in water pressure and vice
versa, whereby the change in pressure is pro portional
to the square of the speed change. So, when water speed
is doubled pressure redu ces to a quarter.
Following Bernoulli ' s theory the re are reduced
pressure areas round a ship where the relative velocity
of water flow is increased. If stream lines were parallel
aroun d a ship the reduction in pr essure would be
uniform . Well ahead of a ship the stream lines are equally
spaced, but at a certain point they are wedged apart
and as they go round the body of the ship they are
compressed. At the stern the stream lines tend to spread
again in an effort to fill the gap astern of the ship .
Wh en stream lines diverge the speed of the water
reduces and, according to Bernoulli, pressure increases.
'When stream lines converge, water speed increases and
wa ter pressure reduces. This boils down to conservation
of energy in fluid flow. At low speeds a ship's wave
making resistance is minimal. Th e wave pattern
gen erated by a ship travelling at higher speeds causes
wave making and wave br aking (at the bow) resistance.
The wave length found in such a wave patt ern is a
function of the speed of the ship. Pressure field s caused
by the Bernoulli effect are the main cause of the wave
pattern arou nd a ship at low speeds.
It means that at the bow there is a high pressure area,
a bow wave, followed by a low pressure field around
the midsection while at the stern there is again a high
pressure area, altbough lower than that at the bow.
Due to viscous resistance or skin friction water is
dragged along with a ship, a little at the bow bu t more
and more towards the stern. It forms a fairly dead layer
of water, called th e boundary layer , in creasing in
th ickness fro m bow to ste rn. Abaft the stern the
boundary layer forms the frictional wake. This boundary
layer and wake astern of a ship result in a less marked
spreading of stream lines, resulting in a smaller high
pressure field near the stern than at the bow. Particularly
in the case of wide bodied ships, wate r speeds up round
the forward but less round th e aft shoulde rs, causing a
local wave trough .
In shallow water th e flow underneath a ship is
restricted and more water has to pass along the sbip
82 THE NAUTICAL INSTITUTE
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sides than in deep water. Consequently along the ship
sides the water has a higher speed and the reduction in
pressure is larger, while high pressure near bow and
stem increase,
assuming the same ship's speed as in deep
and open waters. When in shallow and in narrow waters,
the water flow between a ship and the banks is much
mor e confine d, causing an even higher water speed and
a much larger reduction in pressure along a ship side
and a further increased pressure near bow and stern,
with the highest pressure near the bow.
Thi s also explains bank suction and bow cushion
effect A ship proceed ing on one side of a channel has a
more confine d water flow at the side nearest the bank,
causing highe r water speed and lower pressure at that
side. The ship is forced towards the low pressure side.
Due to the boundary laye r, also formed along the ban k,
the space between bank and ship narrows towards the
ship's stem , causing the resultant force to act somewhat
abaft of midships, giving the ship a yaw moment away
from the bank. In addition, the high pressure near the
bow close to the bank increases and forms a pressure
cushion, causing the bow cushion effect. The effect of a
steep bank is bigger than that of a sloping bank, because
with a sloping bank some sideways inflow of water is
possible causing a smaller reduction in pressure.
The most relevant pressure fields around a ship have
now been explained. T he imp ortant role that the ship's
speed plays is clear. Besid es the importanc e of an
appro priately low speed, it is also important to keep in
mind that interaction effects will in crease when
underkeel clearance is small and when close to banks.
Interaction effects between ships or between a ship
and a tug are generated in th e same way as between a
ship and a bank. It is again the distance off and the
relative spee d of th e water between the ship and the
tug which causes the degre e of interaction .
Tug - ship interaction with respect to tug safety
In figure 6.4, a tug is slowly overtaking a bulk carrie r
an d tr avelling p ast th e sh ip . T he most rel ev ant
interaction effects on the tug are now considered. Th e
approximate stream lin es aro und the ship are shown.
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Wh en the tug approaches the stern from a position
behind tug no . I, it experiences an increase of speed
due to the relatively low water speed. Th e tug may be
pushed sideways to starboard as well by the incoming
waterflow (see also figure 6.2).
Wh en coming nearly abeam of the stern (position I)
the tug is sucked towards the ship because the speed of
water increases between tug and ship's hull causing a
low pressure field and con sequently a suction force
towards the ship. Since the tug's forepart is closer to the
ship than the stem the tug expe riences a starboard
turning moment. A lift force caused by a cross flow on
the tug also pushes the tug towards the ship.
As it pro ceeds the tug's bow reaches the trough near
the aft shoulder of the ship, causing an increased turning
effect to starboard and the tug needs more power in
order to maintain spee d due to the higher water speed
encountered.
Wh en abeam of the aft shoulder the tug is sucked
more towards the ship, du e to the local wave tro ugh. In
addition, there may still be some lift force experience d
du e to cross flow. As soon as the tug mov es further
forward and parall el with the ship's hull it exp eriences
a sudden outward turni ng mom ent, caused by the tug 's
bow cushion. In addition, the tug's stem is near the wave
trou gh at the aft shoulder (position 2) where the water
speed between the tug's stern and ship's hull is high. As
a consequence the stern is sucked towards the ship. The
tug is also sucked bodily towards the ship.
Near the ship's midship section the tug is still sucked
towards the ship with an outward turni ng mom ent
(position 3), all caused by effects identical to bank suction
and bow cushion effects. Near the bow the situation
changes quickly. Wh en th e tug reaches the forward
sho ulder, du e to the higher wate r spee d and the local
wave trough the tug needs more power to proceed at
the sarne speed. When passing the forward shoulde r
suction forces increase rapidly due to increased local
flow velocities. As soon as the after end of the tug reaches
the wave trough the outward turning moment increases
again (position 4).
)
Retardednow
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Figure 6.4 Itueraaion effects on a tug whenproce,ding along a ship
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TUG USE IN PORT 83
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When moving a littl e further forward (be twee n
positions 4 and 5) the outward turning moment suddenly
changes into an inward turn ing moment. This is due to
the cross flow near the bow of the ship acting on the
tug's rudder or skeg as a steering force. Due to the lift
force caused by the cross flow the tug drifts sideways
away from the ship.
Manoeuvres to pass safely past a ship, including the
positions where towlines are passed, are now considered
for two main types of tug. Conventional tugs with
propulsion and steering aft and ASD/reverse-tractor
tugs with steerab le propulsion aft are all considered
conventional tugs.Tractor tugs with steerable propulsion
forward are the other main type. The steerable bow
thru ster of combi-tugs tends to give a similar effect to the
propulsion of tractor tugs,but the power ofthe bow thruster
is low compared to the propulsion of tractor tugs.
Tugs approaching the stern to pass or pick up a
towlin e should be well aware of the increased speed
and possibly sideways movement to avoid a collision
with the ship's stem .
A conventional tug when in position I should apply
port rudder to counte rac t the turning moment.
However, port rudder also creates a sideways force in
the same direction as the suction forces. Therefore when
near this position conventional tugs should keep well
aw ay from th e shi p . Tractor tugs can direct th eir
propulsion away from the ship, thus counteracting the
starboard turn and the suction force, whichis safer. Position
I is also a position where towlinesare passed.Conventional
tugs should be particularly careful because of the turning
moments and suction forces in this position.
Between position I and 2 the situation changes. A
conventional tug sho uld, within a sho rt space of time,
change from port to starboard rudder. In doing so, the
sideways steering force created now points away from
the ship. Tractor tugs have to set their propulsion in the
direction of the ship's hull to counteract the turning
moment but at the same time a side ways force is
introduced in the direction of the suction forc e, which
is not safe.
At position 3 and 4 the rudder of conventional tugs
is still to starboard counterac ting the suction force.
Tractor tugs have to keep their propulsion to starboard
to compensate for the bow-out turning mome nt, and
still in the same direction as the suction forces. Especially
near po sition 4, suction force s and turning moments to
starboard may be marked.
A littl e furth er on, between positions 4 and 5, a
conventional tug should abruptly change from starboard
to port rudder. Ifnot aware of the turning moment the tug
might swing to starboard and end up under the bow of the
ship. A tractor tug should change the propulsion from
starboard to port to avoid coming under the ship's bow.
84 THE NAUTICAL INSTITUTE
Between positions 4 and 5 tug pow er can be red uced
to keep the same spee d since th e relative wate r speed
reduces. Tugs not aware of the change in tu rn ing
moment and maintaining their power setting run with
increas ing speed to starboard and possibly dramatic
consequences . Attention should also be given to the fact
that the cross flow acting on the underwater body of
the tug causes a decrease in effective stability.
Positions 4 and 5 are also positions wh ere towlines
are passed. A conve ntional tug can keep a steadier
position, because the application of rudder to counteract
turning moment also involves
counteraction of the
suction and lift forces. A tractor tug when cou nteracting
turning mo me nts sets th e pr opulsion in the same
direction as the suction and lift forces and at the positions
where suction forces occur the tug may come too close
to the ship's bow. For a tractor tug it is more difficul t to
keep a steady position close to the ship's bow to pass a
towline. Nevertheless, a tractor tug is safer because when
coming too close to the ship's hull the stee ring forces
with a tractor tug are directed away from th e ship.
From position 4 a tug generally steers somewhat
inwards to come closer to the bow to pick up or pass
the towlin e. It is evident that this should be done with
utmost care, due to the changing influences on the tug
near the bow.
The interaction effects described here only give an
indication of the influences on a tug. The effects differ
by ship type and loading condition. For instanc e, the
diversion of stream lines ahe ad of a ship is less with a
fine form ed ship, resulting in lower high pressure near
the bow and consequently a smaller bow wave. The
change in turning moment experienced on a tug near
the ship's bow occurs further aft at slender sh ips . These
ships also have less pronounced sho ulders, so effects in
these regions are less pron ounced . There is also a
shorter, flat area around the midsection , so changes in
interaction effects qui ckly follow each othe r whe n
passing along a slender ship , e.g. a containe r vesse l.
A tug's underwater body and appendages have th eir
influences as well, especially on th e turning moments.
Although interaction effects differ by ship and tug, these
do exist and one sho uld be aware of them. The smal ler
the distance between tug and ship the larger interaction
effects are . Shallow water and narrow waters have an
increasing effect on interaction between tug and ship.
Most imp ortant to keep in mind is that th e influ ence of
all interaction effects increase s sharply with speed an d
are mo st dangerous near a ship's bow.
Ship speeds can be rather high when tugs are coming
alongside or making fast. Speeds up to five kno ts are
quite normal for tugs taking or passing a towline near a
ship's bow or stern. Higher speeds are not uncommon ,
even up to nine or 10 knots. The interaction effects are
th en large, especially for tugs taking a line at the bow.
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With such high speeds highly manoeuvrable tugs with
a high, free sailing speed are required and, of course ,
very experienced tug captains.
6.2.4 Thg- ship in teraction with respect to tug
p erformance
The flow pattern a ro u nd a ship affects tu g
performance when operating close to a ship's hull,
altho ugh it is difficult to say to what extent due to the
interaction between flow patterns generated by both ship
and tug . To make it even more difficult, with changes in
tug position the situatio n may change rap idly.
It has been explained tha t the relative speed of water
along a ship's hull between bow and stem increases in
speed compared to a free stream. With wide body ships
the wate r speed near the forward and aft shoulders might
be even more than at the ship's midsection. A ship
stearning at, say, three knots through the water may have
a speed of four knots relative to the water flow along
the ship and relative ship's speed at the shoulders may
be higher still. A tug pushing at a ship's side i~ affected
by this increased water speed and tug performance is
adversely affected, particularly when operating nea r the
shoulders (see figure 6.5 positions I and 2). As already
explained, shallow and narrow waters increase water
flow spee d along the ship sides, further decreasing a
tug's effectiveness.
Fo r tugs towing on a line the situation is more
complicated. Firstly, tugs are operating in areas where
they are under the influence of different inte raction
effects as mentioned in sectio n 6.2.3. Secondly, tugs -
when in positions 3 and 4 and renderin g assistance -
frequently change position and heading. Thirdly,
interac tion effects differ by ship's hull form, loading
condition and speed. So it is hard to say whe the r
interaction effects affect the performance of a tug or tug
type whe n towing on a line in positions such as 3 and 4.
Apart from speed, an important aspect is towline
length and the distance to a ship's hull. With respec t to
tug no'. 3 the shorte r th e towlin e and the closer to the
ship' s hull, the larger the interacti on effects are. The
towing effectiveness of tug no . 4 decreases with a short
towline due to the reducing effect of propeller wash
imp inging on the ship' s hu ll. The effect is larger in tugs
with propulsion aft.
It is advisable for tugs towing on a line, like tugs
nos. 3 and 4, to use a somewhat longer towline length
and operate at a farth er distance from the ship's hull,
which is also safer. This reduc es interaction effects and
the negative effect of the tug's propeller wash impinging
on the ship's hull.
In position 5 a tractor tug, which could also be an
ASD/reverse·tracto r tug, is operating in a ship's wake
as well as in the prop eller slipstream. The wake and
propeller slipstream have opposite directions. It depends
totally on the assistance required whether or how wake
and/or propeller slipstream influence tug performance.
For instance, when retarding force s are required, a ship's
propeller is normally stopped or astern thrust applied.
Compared to a free stream situation the wake causes a
decrease in the tug's underwater resistance and propeller
braking performance, assuming the same amo unt of
engine power is used, resulting in a smaller towline force.
Th e wake is a combined influence of potential wake
and frictional wake . In figure 6.2 the frictional wake
behind the ship's stern and the incoming water flow
near the stern, which causes the potential wake, are
shown. As rela tive water speed in th e ship's wake
decreases in shallow and narrow waters, the negative
effect of the wake on a tug' s braking performance
increases. The effect of the propeller slipstream
is opposite.
It can be concluded, as interaction effects differ by ship,
that so does the influence on tug performance when
tugs are operating close to a ship and in the wake or
propeller slipstream. It is difficult to assess what the
influence is on tug performance . The most marked
influence is experienced by tugs pushing at a ship's side
and tugs applying braking forces in a ship's wake .
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4r
Acce/8ratedflow Acceleratedflow Retaidein!u.....------
Figure 6.5 EfJict offlow pauem around aship ontugperfrrrmo.na
TUG USE IN PORT 85
6.3 Tug safety
6.3.1 Introducti on
The exp lanat ion of variou s interaction effects on a
tug when close to a ship underway at speed has already
showed some of the risks involved for the tug. Th ere
are, howev er, various other situations which involve risk
for an assisting tug.
Not all of the following situations are related to the
same kind of interaction as discussed earlier. Interaction
between ship's propeller and tug is considere d along
with several other situations related to tug safety. Some
have already be en addressed while discussing the
capabilities and limitations of various tug types, but are
also mentioned here for the sake of completeness. Most
situations are well known to experienced pilots and tug
captains. Still, it is worth paying attention to the risks in
which harbour tugs are often involved, because many
serious accidents have been reported. The more one
knows about these risks, the better one can anticipate
and take the right measures . Besides, pilots often hav e
only a limited view from the bridge on the assisting tugs.
They are not always aware of the critical situations a
tug may find itself in.
The following ri sk y
situations are just a few
examples; it is impossible to cover all situations. Wh at
is mentioned here may be representative for similar
situations encountered by pilots and tug captains.
Several of the situations to be discussed are related to
the method of tugs towing on a line . T his is
understandable, because with this method of assistance
tugs often operate close to the bow or stem of a ship
underway at spe ed, locations where interaction forces
can have large and alternating effects. On the othe r
hand, in ports wh ere tugs normally operate at a ship's
side, it is also possible that in specific situations these
tugs tow on a line as, for instance, in confined areas, in
dry docks or when passing bridges.
It goes without saying that readers could probably
name other critical situations from their own experience.
Critical situations a tug may be involved in can simply
be divided as follows:
While passing a towline.
While the towline is secured.
Next, attention is first paid to the m anoeuvre of a
tug coming alongs ide a ship at speed. This is a practical
example of interaction.
6.3.2 Coming a longside and dep arting fro m a
sh ip's side
Whe n considering tug-ship interaction it is safest,
when coming alongside a ship underway at speed, to
approach near th e midsection where a more uniform
86 THE NAUTICAL INSTITUTE
flow pattern exists. At positions furthe r forward or aft
the interaction effects are larger and less predictable.
Dep arting from a ship's side can some times be
problematic, as the following example shows, In some
ports the pilot boards a ship from a harb our tug that is
to assist a ship. The ship has headway and th e tug is
coming alongside near the pilot ladd er. After boarding
the pilot it can be difficult to manoeuvre the tug free
from the ship' s hull. This can happ en with twin screw
tugs having an und erwater body which is rather flat at
the sides. Trying to get free from a ship's hull by moving
to a far forward or far aft position along the hull does
not help. Thi s can be explained by the earlier discussion
on flow pattern s around a ship. Tug captains note from
experience that when they apply astern thrust with the
inner propell er, complet ely again st th e expected
manoeuvr ing procedure, the tug comes free from the
ship's hull . The explanation is that the water speed
between ship and tug hull decrea ses and consequ ently
pressure rises. The increased water pressure between
the two hulls, in combination with bow cushion effect,
force the tug to come off. A nice example of Bernoulli's
law!Another solution is to decrease ship's speed, because
the higher a ship's speed the larger the suction forces.
Tugs with azimuth propellers controlled in the way
shown in figure 2.30 have the thrusters pointed
somewhat outwards when proc eedin g at low speeds.
When coming alongside a ship having a low speed the
wash of the inward prop eller causes an increase in water
speed between tug and ship and the tug may be sucked
violently towards the ship .
This becomes more probl ematic for tugs with fixed
pitch azimuth prop ell ers not equippe d with speed
modulating clutches. Such tugs have a relatively high
minimum propeller speed, causing much prop eller wash
at minimum tug speeds. This has resulted in much
contact damage while landing alongside a stationary or
moving vessel and during berthing and unberthing,
which, however, can be avoided by proper tug handling.
The sam e may happen when the clutch-on/clutch-off
system of the separate azimuth propellers of tugs with a
single lever contro l are not in complete balance.
Note:
Operating close to a ship and coming alongside a
stationary or moving vessel should always be done with
care and in a controlled way. Approaching a ship with
an inappropriate speed has resulted in dents in ship hull s
and damage to tugs and even oil spills have occurred
on several occasions caused by mooring ass ist tugs
penetrating bunker spaces_
6.3.3 Passing a towline near th e b ow
The most risky situations for a tug when ope rating
close to a ship's bow have already been discussed while
conside ring interaction effects . Some oth er situations
o
f!
A
Figure 6.6 A: Tug is wailingjiJr the approadlingship tocom, do", 10
passthe towline. TMr, is riskofan untxpteted sheer 10 partdut to the
ship~ how pressure wavt. B: Conomtionol tugpreparing to take the tow
lineat ship's bow. Due to interaction effects andinadequate reactions from
the tug captain, the tug .comes under the ship's how
are now highlighted [see figure 6.6A and 6.2). A tug has
to make fast at the bow of an approaching ship and is
steaming at some distance ahead. Tug speed is less than
the speed of the ship to be attended and the tug iswaiting
till the ship gets close enough to pass a towline. However,
due to the changes in the stream pattern caused by the
overtaking ship the tug may experience a turning
moment. When the tug captain is aware of this effect in
time he can, irrespective of the type of tug, take measures
to counteract the turning moment.
A large turning moment can be experienced,
particularly when attending loaded ships with a full-
shaped bow and still having reasonable speed. With this
type of ship th e bow wave may also have another
specific effect ontugs awaiting the approaching ship. It
has been experienced by tug captains that when
attending VLCCs or large ore-carriers having a speed
of about four to five knots and a small underkeel
clearance, the bow pressure wave may be such tha t the
tug is pushed forward and the tug capta in may even be
forced to reverse thrust in order to come closer to the
ship'S bow.
Another example of interaction is shown in figure 6.6B.
A conventional tug approaches a ship under speed to
take a towline at the bow. At a particular moment the
tug captain considers his tug too close to the ship's hull
and tries to clear the ship 's side using engines full ahea d
while steering to port. Due to this action the tug is pushed
against the ship by the steering forces and moves steadily
forward along the ship's bow, unsuccessfully trying to
get free. Finally the tug comes broadside under the bow
and is run down. The only satisfactory manoeuvre in
such circumstances is to go full astern. Some damage
might then occur to th e tug, but the situation is not
disastro us. A tractor typ e of tug is safer in such a
situation, because the steering forces are directed away
from the ship .
Taking or passing a towline at the bow ofla rge loaded
wide bod ied ships is not so dangerous. Wh en abeam of
the fore part of the bow the tug is pushed aside by the
earlier mentioned cross flow. Tug captains leam from
experience that when near the fore part of the bow and
steering a little inwards towards th e bow, the tug does
not get closer. However, when the tug is moving further
forward it experiences the earli er mentioned turning
moment towards the ship . This effect will probably be '
largest with a small underkeel clearance.
Without going into furth er detail, it can be concluded
from the foregoing that operating a tug near the bow of
a ship under speed involves risks. These vary depending
on the typ e and loading condition of the ship and
increase with a higher ship's speed. As alr eady
mentioned, ship's speed can be rather high when tugs
are making fast. Therefore when approaching the bow
of a ship to pass or take a towline careful attention and
quick reaction is needed from a tug captain in order to
avoid dangerous situations developi ng. Skilful tug
captains know the inte raction effects and related risks
near the bow by experience. Therefore, not just good
tug manoeuvr ability but ex perience too is an
indispensable factor.
It isnot the tug captain alone who masters the situation
near the bow or is solely responsible for the extent of risk
into which his tug gets involved. As already
stated, an
important factor is ship's speed which isunde r the control
of the pilot or master.An experienced ship's crew standing
by forward in good time and keeping sufficient heaving
lines of the proper length and strength ready available is
important This can help to avoid a tug captain being
forced to come too close to a ship's bow. Sometimes quite
thick lines of insufficient length are lowered from the
forecastle, forcing a tug captain to come very close to a
ship's bow and involving increased risk.
On the other hand, when a tug is pushed away from
a ship and a too short messenger line is used by the tug
itself, this line may break during transfer of the towline
from the winch. The towline then drops into the water
and may foul a tug's propeller which brings about
another dangerous situation. When a tug has to make
fast on a ship's line , the line should be hung at a suitable
height above the water, ready to be paid out as soon as
the tug has got hold of the line .
6.3.4 Passing a towline at the stern
Whe n making fast, after tugs are often very close
astern of a ship - sometimes just abeam of the after end
of the stern in order to pick up or pass a towline. The
TUG USE IN PORT 87
interaction forces at these locations are not so large or
dangerous. However, when approaching a ship having
headway from astern, the tug captain should be aware
that when coming close to the ship's stern , the tug is
pushed towards th e stern , as has been explained earlier.
One should always be aware of the ship's propeller.
Wh en a tug is making fast at the stern a ship's prop eller
should always be sto ppe d in case of a fixed pit ch
propeller. A controllable pitch propeller should be set
for minimum pitch. A prop eller turning ahead disturb s
the water and m akes it more difficult for a tug to keep a
steady position behind the stern. This effect is also
experienced by tugs making fast near the aft sho ulder.
An unsteady tug position affects smooth hand ling of a
towline and in the worst case an unsecured towline may
drop in the water and foul a tug' s or ship'Spropeller.
3
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2
A critical situation also arises when a tug is passing
or taking a towline close behind a ship's stern, or is
preparing to do so, and suddenly the ship applies astern
thrust hy giving astern on the engine or by reversing
the 'pitch. Particularly when large ships with powerful
engines suddenly apply astern thrust a deep wave trough
is crea ted close behind th e ship's stern, sucking a tug
towards the ship. A tug may touch a ship's stern causing
dam age to the ship or tug. This kind of acciden t has
happened occasionally. Even with smaller ships this
effect is noticeabl e. Wh en, for one reason or another, a
ship's propeller has to be used for astern thrust, a tug
captain should be informed by the pilot to allow him to
manoeuvre his tug out of the dangerous area.
The conclusion is that when tugs are making fast at
or near the stern , a ship's propeller should be stopped
and in case of a controllable pitch propeller be set for
mimimum pitch. When for some reason or another the
propeller has to be used , the tug captain sho uld be
informed.
Now some critical situations are discussed when
towlines are secured. Some situations relate to speci fic
manoeuvres as used in some large ports.
6.3.5 Overtaking a bow tug on a line -
Girting - Tripping
In figure 6.7A a tug with propulsion aft is assisting a
ship in making a tum to starboard. Ship' s speed may
become too high for the tug (position I), for instance
because the tug is pulling too much to starboard or
because the pilot has increased engine power to improve
rudder effect in order to make the jurn properly. In the
given situation it is very likely that the tug will come
abeam of the ship's bow (position 2) and eve n in a
position furthe r aft with the towline coming under high
tension (position 3). It is almost imp ossible for the tug
captai n to man oeuvre his tug back in line with the ship
and the tug is liable to capsize. This may not only be
caused by the strong athwartships forces in the towline,
but while trying to bring the tug back in line with the
SS THE NAUTICAL INSTITUTE
C
Figure 6.7 Girting andtripping
Two examples ofgirting (A & B, both witha conventional tug):
A - due to excessiveship1speed with respect to tug limitations
B - due to misunderstanding
Exampk C slwws tripping with a tractor lug
ship, the tug captain appli es high steering forces, adding
to the heeling forces. With a reliably working qui ck
release system the tug captain can release the towline,
so avoiding capsizing. On the other hand, if the pilot
recognises the dangerous situation arising in time he
may be able to reduce ship's speed. In doing so the
towline force reduces, creating the possibility for the
tug captain to come back in line with the ship.
It is obvious that the more manoeuvrable tugs are,
e.g twin screw tugs, the less likely they are to get involved
in similarly dangerou s situations. In addition, proper
stab ility, freeb oard and deck equipment contribute
marked ly to safe operations and enlarge the capabilities
of a tug. Doors and other openings on deck should be
closed during towing operations.
The above situation is less dangerous for a tractor
tug because of the aft lying towing point, A tractor tug
swings around on the towline and comes alongside an
attended ship un less the towline is released in time -
so-called 'tripping' (see figure 6.7C).
Similar situations can arise with a tractor tug when
the towing angle - the angle between ship's heading
and direction of the towline - is getting too large with
respect to the forward spee d of a ship. The tug is unable
to come back in line with the ship and swings around.
Although the above mentioned situations do occur,
the following comparable situations are also possible.
The danger of 'girting' or 'tripping' does not only
exist when a ship round s a bend. Even when a ship is
proceeding on a straight course girting can OCCUI. In
that case excessive spee d of the ship is the main cause.
When a ship increases speed to a level which is rather
high for a forward tug towing on a line, the tug captain
prob ably does no t keep position right ahea d of a ship's
bow, because that is too dangero us. The tug steers out
towards a position more aside in order to keep well clear
of the ship's bow. It is understandable that if ship's speed
fur the r inc reases, a comparab le girting or tripping
situation will arise for the tug as indicated before.
Although pilots should be aware of the implications
oftoo high a ship's speed for th e safety of assisting tugs,
it is again an indica tion of the importance of good
communications between tug captains and pilots. Th e
pilot may not have a good view of what is happening at
the bow and the tug captain should therefore inform
the pilot in good time if he considers a speed increase
too high.
Anoth er example of how the danger of girting can
arise is shown in figur e 6.7B. A ship is making a turn to
port, say, to enter a harbour basin. Because the tug
captain has not been informed that the ship has to enter
head first in to the basin he starts pulling to starboard to
contro l ship'Sheading, assuming the ship is veering off
course. If the pilot is not aware of thi s, the same
dangerous situation for the tug as described above
develops, in particular when the pil ot observes a
decrease in rate of tum due to the tug captain's action
and in creases eng ine power while applying a large
rudder angle. This is ju st an example, to show how
imp ortant it is for tug captains to be well inform ed about
a pilot's intentions. O n the other hand , of course, the
tug captain could have asked the pilot what his
intentions were,
A furth er example. A tug has taken position right
ahead of a ship, waiting for the ship'S
crew to release
the towlin e. With the small number of crew members
on board ships nowadays , this may take some time. In
the meantim e the ship is already increasing speed. In
the case of beamy full-bodied ships it may happen that
the tug, with the towline still not yet released, gets
pushe d forward by the bow wave of the ship and thus
reaches a speed which can' be higher than the free
running speed of the tug. When the tug moves sideways
towards a position abeam the bow, due to the danger of
the increasing ship's speed, the forward pushing effect
of the bow wave diminishes. The tug may not be able to
keep pace with the ship while stillwaiting for the towline
to be released. A dangerous girting or tripping situation
may arise. This example shows again the importance of
appropriate speed and good communications.
Figure 6.8 Some speafic manoeuvres by conomtumal tugs towingona
lineincludingriskojgirtingor cap,i;jng when a ship" speed is too high
with respect to tuglimitations
6.3.6 Forward tug steering broadside
In several ports, ships enter harbour basins stern first
D eparture is then easier and in case of emergency most
ships are able to leave without tug assistance. Entering
a harbour basin stern first can be done with e.g. two
tugs of which the forward tug is a conventional tug
operating broad sid e as shown in figur e 6.8A. The
forward tug, acting as a drogue, steers the ship effectively
by going astern or ahea d on the engine and so applying
steering forces to port or starboard. The tug usually uses
a gob rope, although with twin screw tugs this is not
always the case. This metho d of tug operation has
already be en describ ed in Chapter 4. For small ships
often only one forward tug is used operating in th e same
way. The ship maintains sternway using its engine.
A dangerous situation arises when a tug's capab ilities
and limitations are not sufficiently taken into account.
Wh en a ship's astern speed is becoming too high, tug
heel caused by high athwartships towline forces may
increase until the tug capsizes. This may not only be
caused by the large transverse resistance of the tug as it
is pulled bodily through the water, but also by the water
acting on the tug speeded up by the wash of the ship's
propeller. Tug stability, freebo ard and deck equipment
determine the limits of safe operation.
TUG USE IN PORT 89
Care should be taken when using the engine ahead.
A ship sho uld take care not to gather headway, otherwise
she will collide with th e tug due to the small distanc e
between bow and tug.
6.3.7 Stern tug steering broadside
See figure 6.8 B. This situation is simila r to th e
pr evious one. Th e ship is now moving ahead and th e
after conve ntional tug is the steering tug operating in
the same way as the forward tug discussed earlier. Th e
main difference between the two situations lies in the
close presence of the ship's propeller. Wh en operating
in this way the ship generally has a very low forward
speed. However, it is essential that the ship' s propeller
is handled with the utmost care. A very dangerous
situation arises if the engine is suddenly set, say, to half
ahead. The water flow on the tug together with the wild
propeller wash may cause the tug to list severely and in
the most serious case the tug may capsize. This has
happened more than once.
6.3.8 Stern tug manoeuvring from a stand by
position on starboard or port quarter
towards a position astern the ship
See figure 6.8C. During a certain phase of
manoeuvring it may be nec essary for a ship with
headway to have the port or starboard position tug
(Position 1) move astern of the ship (positions 3 or 4) to
assist in steering or for speed control. This might be
necessary when a ship has to wait in a river, swing or
b e stopped . This manoeuvre is dangerous to
conventional tugs when carried out at too high a ship's
speed. This is at speeds of more than about three knots,
and depends on tug manoeuvrability, stability and
freeboard . In situations 2 and 3 risk of girting exists due
to the high athwartships towline forces that may occur.
If a tug capsizes it has been observed that the tug is
pulled underwater stern first.
The manoeuvre just described is no problem for
tractor or reverse-tractor tugs, even with a fairly high
ship 's speed. Conventional tugs with a gob rope system ,
whereby the towing point can be transferred towards a
far aft position, can also swing around at a higher speed.
The gob rope system should be strong enough and fully
reliable otherwise such a manoeuvre becomes really
dangerous for the tug.
A conventional tug manoeuvring from a position
astern of the ship (e.g. position 3) to a position on the
starboard or port quarter can only_do this at minimum
ship's speed, otherwise risk of girting may arise .
6.3.9 Stern tug manoeuvring from starboard to
port quarter or vice versa
See figure 6.8D . Sometimes it is necessary for a
conventional after tug to move from a position on the
port to starboard quarter or vice versa.This may happen,
90 THE NAUTICAL INSTITUTE
for instance, when assisting a departing ship. A ship has
just left her berth and has been turned around in the
turning basin by assisting tugs. Th e after tug is on the
port quarter. The ship still has to pass through a channel
and it may be necessary to have th e after tug stand by
on the starboard quarter to compe nsate for wind or
current forces . It m ay also be necessary to compensate
for the transve rse effect of the ship's propeller when
she uses engine astern to wait somewhere in the channel.
The tu g has to manoeuvr e from port to starboard
quarter, close underneath the stern . Because of the risk
of girting this manoeuvre sho uld be carried out while
the ship is nearl y stopped in the water. This kind of
manoeuvre also involve s great risk due to the ship's
propeller. A pilot not aware of the tug manoeuvre could
go ahead on the engine or apply ahead pitch while the
tug is near position 2. The conventional tug comes into
danger. This kind of tug manoeuvre, whenever
considered necessary, should always be carr ied out with
the utmost care .
Figure 6.9 Due to excessivespeeda tug at a ship's side may capsize
ifthestern line cannot bereleased
6.3.10 Tug operating at ship's side
Conventional tugs operating at right angles to a ship's
side may use quarter lines or stern line s as shown in
figure 6.9 to stay in po sition'when the ship moves ahead.
When the tug is secured in that or a similar way,
excessive speed should be avoided to pr event possible
parting of the towlin e or capsizing the tug.
6.3.11 Fog
All the situations mentioned above can cause a
critical situation for tugs. However, during dense fog
these situations may involve even more risk . A s
mentioned in paragraph 4.4, during fog it is very difficult
for tug captains towing on a line to orientate themselves
with respect to an attended ship and surrounding area,
in spite of the availability of a radar. Furthermore, the
pilot loses his view of the tugs. It is absolutely necessary,
therefore, that ship' s speed is kept very low during fog
and that tug captains are kept well informed about
intended manoeuvres. Communications between pilot
and tug captains should be optimal.
It should be noted that although the use of towing
bitts may be necessary for ce rtain specific m anoeuvres,
the ir use is not recommended for tugs towing on a line
during fog conditions. In case of emergency it may be
almos t impossible and certainly dangerous to release
the towline under tension rap idly. Th e same applies to
quick release hooks, unless they are one hundred per
cent reliable. A good towing winch with a quick release
system which can be operated from the wheelhouse as
well as at the winch is safest in th ese conditions.
O n the other hand, tug captains sometimes prefer
to have a ship's line on the
towing bitt or towing hook
during fog conditions. The ship's lin e can th en be
released by the tug crew as soon as the tug captain thinks
the situation is becoming critical. If he has to wait for
the ship's crew to release his towline in a developing
critical situation, it could well be too late.
6.3.12 Some other practical aspects
Bulbour bows
Although there is a mark on a ship's bow indicating
tha t she has a bulbous bow, tug cap tains canno t see the
bulbous bow when it is underwater. Even when only
partly submerged the ex act position is difficult to
determine. This is a prob lem for forward tugs when
taking position to pass or take a towline or when they
are assisting using a very short towline. It is most '
dangerous when the stem of the tug touches the bulb ous
bow, and the ship has a rather high forward speed. The
tug may be severely dam aged and lives may be lost.
Tug captains have to be parti cularly careful whe n
operating close to a bulbous bow, especially during fog
and darkness.
Releasing towlines
If a crew on board ship is not able to release a tug's
towline when requir ed, problems may arise if a ship is
already increasing speed. This is particularly the case
with heavy steel wire towlines on powerful tugs. The
towline has to be slacked offby a tug in order to make it
possible for a ship's crew to release it. The slack towline
is then dragged through the water. Wh en ship's speed
increases the resistance of the towline also increases,
creating more tensi on in the lin e. Rele asing it then
becomes almost impossible. Ch ain stoppers, when used,
may break. It is a difficult situa tion which can only be
avo ided b y proper co n tro l of sh ip 's speed, an
expe rienced ship's crew, sufficient crew members on
station and good cooperation between ship and tug crew.
The aforementioned situation can get very critical
for the tug when th e ship is furth er increasing her speed
and as a result overtakes the tug. Th e risks stemming
from these situati ons have been discussed already.
Finally, a tug's towlines sho uld not be dropped into
the water but preferably be lowered onto the tug's deck
guided by the tug's crew. When applicable, Norman
pins should be raised on board the tug to prevent the
towline slipping along the sides. This avoids the towline
fouling the ship or tug propeller. In the case of a fixed
pitch propeller the ship's propeller should be stopped
when the ship's stern towlines are released.
Underestimating wind and current fo rces
Underestimating wind and current forces can create
risky situations for a ship and have resulted in accidents.
Tugs operating at a ship's side can also be endangered
(see figure 6.10). Tugs can be jammed between ship and
shore when they don't get out in time. The situation is
particu larly d an ger ous when tugs are secured by
towlines. The bollard pull of tugs to compensate for wind
and current forces should be more than sufficient to
avoid such situations.
Sudden changes in a ship sheading and speed
While passing or taking a towline, tugs are very close
to a ship's hull and a tug captain's attention is fully
focused on keeping in position and on line handling. It
sho uld be understood that during th ese operations
sud den changes in ship's heading or spee d without
warning can create critical situations for a tug.
Engin e starts of the large high powered container
ships may seriously affect the controllability of a tug
operating behind the ship's stern.
It is necessary, therefore, as already m ention ed
earlier, to inform assisting tugs about intended ship's
engine/ propeller and course changing manoeuvres.This
applies too for tugs operating at a ship's side. In that
way tugs can anticipate expected manoeuvres.
( '"
Fzgure 6.10 DUl to lot» powered tugs anda strong beam wind,
a contain" shipisdrifting and thetugs are getting jammed
betuum theship and thegeneral cargo b"th
TUG USE IN PORT 91
Ship design consequences
Due to the use of tension winch es on board ships
the number of bo llard s at their foreca stle and stern may
be reduced. The location of the remaining bollards is
not always optimal for towlines. This may affect proper
securing of towlines and lengthen the time for securing
tugs, especially when more than one tug is used forward
and aft.
Th ere are specific ship type s, such as submarines
and aircraft carriers, whe re it can be probl em atic to pass
or secure towlines, due to the underwater form of the
hull or overhanging structures. With m odern merchant
ships such as fa-r o vessels it can sometimes be awkward
to secure a towlin e in such a position that a tug can
operate effectively. Ship designers should take into
account that, eve n when ships are very mano euvrable,
there will always be situations during a ship's life when
the assistance of one or more tugs is necessary. On the
other hand tugs should meet, as far as possible, the
requirements of ships calling at the port regarding safe
and effective towline handling.
Apart from the nee d to have sufficien t and properly
located bo llards and fairleads available for securing the
number of tugs that may be needed, it is furthermore
necessar y that bollards and fairl ead s are in good
cond ition and suitable for the towlines to be used, and
strong enough to withstand the forces that can be applied
by the modern powerfu l tu gs. Not meeting this
requirement has resulted in failures .
Regarding this important subject, recommendations are
given in the OCIMF pub lication 'Recommendations
for ships' fittings for use with tugs' (see Referen ces);
recommendations that app ly to tankers, but which are
also relevant for other ship types, particularly large gas
carriers, bulk carri ers and container ships. Additional
recommendations are included for escort tugs and tugs
engaged in station keeping at offshore installations, such
as SPMs and F(P)SOs.
Information exchange pilot- ship master - tug cap tain
Informing the ship captain about tug manoeuvres
by the use of tug ord ers in understandable English has
been addressed in paragraph 4.7, while at other locations
in this book, and particularly in this chapter, several
arguments are given why tug captains should be
properly informed by the pilot about the intended ship
manoeuvres. A proper information exchange between
pilot, ship captain and tug master is needed for a safe
and smooth handling of the ship by the attending tugs.
Information for the ship captain to be provided by the
pilot may include the number, type and bollard pull of
tugs to be used (including, if necessary, the reason why
the specific numb er and/or total bollard pull of tugs
has been advised], the rendezvo us position and time of
th e tug(s); whe re at the ship and how tug(s) to be
fastened; when tug(s) to be released and how to be done;
92 THE NAUTICAL INSTITUTE
transit spe eds and intended mano euvres. If the ship has
special manoeuvring devices or limitations regarding
mano euvring, tug securing, mooring and anchoring
equipment, the ship captain should inform the pilot.
With respect to the information exchange the reader is
al so referr ed to th e earlie r m entioned OCIMF
publication 'Recommendations for ship 's fittings for use
with tugs'.
Operating bow-to -bow
The relati vely low effective ness of tractor tug s,
reverse-tractor tugs and ASD -tugs (when operating as
reverse-tractor) as bow tug towing on a line with a ship
having headway, including the reasons why and the risks
involved, have been discussed in par. 4.3.1.
For reverse-tractor tugs and ASD-tugs th is way of
op erating is generally called 'bow-to-bow' . When
operating in this way with a ship hav ing headway th~
tugs are sailing astern . Directional stability of these tug
typ es when sailing astern is gen er ally rather low,
particularly at higher speeds. Pulling straight astern at a
relative high