Prévia do material em texto

� is book is speci� cally written for maritime 
professionals involved in the daily practice and 
training of ship handling with tugs, particularly pilots, 
tug masters and training instructors. It is also of value 
to towing companies, shipmasters and mates of seagoing 
vessels and all other people or organisations involved, one 
way or another, with tugs and ship handling.
First published in 1997, Tug Use in Port has been known 
for more than two decades as the authoritative book on tug 
use – the essential practical guide to port towage and escort 
operations. As such it is recommended by IMO as a reference 
for providing adequate tug assistance in ports and fairways for 
ensuring maritime and port safety.
 
Captain Henk Hensen is a Master Mariner FG. A� er his career 
at sea he became a Port of Rotterdam pilot for 23 years, during 
which time he started the � rst combined simulator training courses 
for harbour pilots and tug captains, and participated in many port 
studies, including simulator research. Following his piloting career he 
continued to work as a marine consultant on the nautical aspects of port 
studies, harbour tug advice and simulator training and research, and as 
an expert witness. 
9 789083 124346
ISBN 978-90-831243-4-6
CAPTAIN 
HENK HENSEN 
FNI, FITA
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TUG
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Tug Use in Port
A Practical Guide
Including Ports, Port Approaches 
and Offshore Terminals
Tug uSE IN PORT
Tug Use in Port iii
TUG USE IN PORT
A Practical Guide
Including Ports, Port Approaches and Offshore Terminals 
Fourth revised edition
by
Captain Henk Hensen FNI, FITA 
iv Tug Use in Port
TUG USE IN PORT
A Practical Guide
Including Ports, Port Approaches and Offshore Terminals 
Fourth edition
©2021, Captain Henk Hensen
Published by STC Publishing, a company of Pole Star Publishing b.v.
www.stc-publishing.nl
Image cover: © Kees Torn
All rights reserved. No part of this book may be reproduced in any form, by print, photoprint or any other means without 
prior written permission from the publisher. The content of this book has been composed with utmost care 
and effort.
We have attempted to contact and inform each and all titleholders. If you feel that a specific notification of ownership 
and/or copyright is incorrect or incomplete, please inform us and contact STC Publishing.
ISBN 978-90-831243-4-6
IMO REFERENCE
SHIP/PORT INTERFACE
Availability of tug assistance
IMO’s Facilitation, Maritime Safety and Marine Environment Protection committees recognise 
the importance of providing adequate tug assistance in ports for ensuring maritime and port safety, 
the protection of the marine environment and the facilitation of maritime traffic. IMO has circulated 
a reference to Tug Use in Port to assist port authorities and port operators in assessing the adequacy 
of tug services in their ports.
(MSC.1/Circ.1101)
Tug Use in Port v
FOREwORd vI
AUTHOR’S PREFACE vII
ACKNOwLEdGEMENTS vIII
GLOSSARY OF TERMS x
TUG USE IN PORT: THE OvERvIEw 1
Chapter 1: TUG dESIGN FACTORS 3
1.1 Differences in tug design and assisting methods 3
1.2 Factors influencing tug type and tug assistance 4
1.3 Types of tug 7
1.4 Assisting methods 7
1.5 Conclusion 8
Chapter 2: TYPES OF HARBOUR TUG 9
PART A: Classification of tugs and operational design aspects 9
2.1 Classification of basic harbour tug types 10
2.2 Important general requirements for good tug performance 11
PART B: Basic tug types 17
2.3 Conventional types of tug 17
2.4 Combi-Tugs 24
2.5 Tractor tugs with cycloidal propellers 26
2.6 Tractor tugs with azimuth propellers 30
2.7 Reverse-tractor tugs 34
2.8 Japanese tug concept 35
2.9 Azimuth Stern Drive (ASD) tugs 37
2.10 Uni-lever system 40
PART C: Related tug types 41
2.11 Rotortug 41
2.12 Z-tech tug 45
2.13 RSD tug 46
2.14 Carrousel tug 47 
2.15 DOT tug 49
2.16 The All-Rounder AR360T 49
PART d: FAST tug types 51
2.17 Introduction 51
2.18 SDM (Ship Docking Modules) 52
2.19 EDDY 56
2.20 Carrousel RAVE Tug (CRT) 59
2.21 Giano tug 61
PART E: Specific Tugs. Research. Performance 64
2.22 Tugs handling LNG carriers. LNG terminal tugs 64
2.23 Eco- tugs 66
2.24 Ice tugs 73
2.25 Research 77
2.26 Tug performance 78
Chapter 3: ASSISTING METHOdS 81
3.1 Introduction 81
3.2 Assisting methods 82
3.3 Tug assistance in ice 88
3.4 Assisting Navy ships 94
Chapter 4: TUG CAPABILITIES ANd LIMITATIONS 100
4.1 Introduction 100
4.2 Basic principles and definitions 100
4.3 Capabilities and limitations 114
4.4 Design consequences 127
4.5 Environmental limits for tug operations 127
4.6 Conclusions regarding tug types 132
4.7 Some other practical aspects 132
Chapter 5: BOLLARd PULL REQUIREd 134
5.1 Introduction 134
5.2 Factors influencing total bollard pull required 135
5.3 Bollard pull required 143
Chapter 6: INTERACTION ANd TUG SAFETY 149
6.1 Introduction 149
6.2 Interaction and shallow water effects 149
6.3 Tug safety 155
6.4 Summary and conclusions 174
6.5 Finally 175
Chapter 7: TOwING EQUIPMENT 176
7.1 Introduction 176
7.2 Additional towing points and gob ropes 176
7.3 Towing bitts, hooks and winches 179
7.4 Towline Safety Systems 187
7.5 Towlines 188
7.6 Towline handling 205
7.7 SWL ship’s towing equipment 207
7.8 Requirements for emergency towing equipment, 
 escorting and pull-back 209
7.9 New emergency towing concept 212
Chapter 8: TRAINING ANd TUG SIMULATION 214
8.1 Reasons for training 214
8.2 Various training objectives and tools 214
8.3 How specific training courses can be given 221
8.4 Assessment of further training needs 236
8.5 Developments 237
8.6 Conclusion 241
Chapter 9: ESCORT TUGS 242
9.1 The background to escorting 242
9.2 Studies on escort requirements 243
9.3 Developments in escorting 245
9.4 Escorting objectives and tug placement 246
9.5 Escorting by normal harbour tugs 247
9.6 Escorting by purpose built tugs 249
9.7 Escort tug regulations 266
9.8 Concluding remarks 272
Chapter 10: TUG dEvELOPMENTS 273
10.1 Special developments in the design of tugs 273
10.2 Autonomous tugs 278
10.3 Developments in general 285
Chapter 11: BALANCING SAFETY 288
11.1 Introduction 288
11.2 Safety 288
11.3 Risks 292
11.4 Safety Management Systems 299
11.5 To summarise 304
REFERENCES 305
APPENdIX 1: Guidelines for Owners/Operators 
on Preparing Emergency Towing Procedures 310
APPENdIX 2: Safety when handling tugs 312
APPENdIX 3: Stability Rules – Escort Tugs 315
APPENdIX 4: Standard Guide for Escort Vessel 
Evaluation and Selection 317
APPENdIX 5: Beaufort wind force scale 319
INdEX 320
CONTENTS
vi Tug Use in Port
FOREwORd 
One of the most delicate responsibilities of pilots occurs during berthing and unberthing operations. 
Carefully berthing a ship, often after a long pilotage mission, is almost a work of art. Of critical assistance 
in this task is the support of tugs and their skippers. Today, this is probably true more than ever, as 
ships become ever larger, e.g., ultra large container vessels with their huge windage, while often being 
significantly less powerful or able to manoeuvre themselves in confined waters. These vessels may have 
proportionately smaller rudders coupled with engines that, often deliver less power, notably as a result of 
software-managed fuel consumption.
Furthermore, margins in ports and fairways are often small and a continuing occurrence of ship engine 
problems in port and port approaches can be noted. There is also an increased need for escort, such as for 
LNG carriers, bulk carriers and sometimes large container vessels. This all means that both masters and 
pilots greatly rely on the presence of powerful, manoeuvrable tugs aptly handled by skilful skippers. 
In addition, tugs are becoming more and more powerful and also more complicated. A small mistake can 
havea nautical dashboard 
for the tug master and a technical dashboard for the chief 
engineer. Information is displayed on the bridge, main deck 
and switchboard locations to ensure safety and efficiency of 
operations. The screens – with day and night view – are clear, 
supporting the tug master and chief engineer by showing 
only what is needed during operations, whilst allowing 
operators to select more detailed data when desired.
The nautical dashboard shows relevant information for 
the navigation of the tug, such as heading and speed, echo 
sounding, fuel consumption and thruster and engine data.
The in-house developed Electronic Stability Protection 
system assists the tug master in assessing the tug’s 
limitations. This measures the tug’s performance against its 
capabilities and signal to the tug master when he approaches 
a critical limit. See also in paragraph 9.6.2 Stability. For 
more information see References for `Human Machine 
Interface for Tugs’. 
Raymarine DockSense Alert 
Using advanced stereo vision camera technology, DockSense 
Alert detects objects above the waterline and creates an 
intuitive map of potential hazards around the tug. Live video 
feeds enhance the tug master’s view while audible and visible 
alerts inform the captain when an object is close to the vessel. 
DockSense Alert also offers helpful notifications tuned to the 
captain’s preferred proximity alert distance. DockSense Alert 
uses several cameras and DockSense heading sensors. 
14 Tug Use in Port
Moulded modular or block fender 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.
A tug’s bow and/or 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 fenders on bow and stern which 
are intensively used, basic vertical block fendering is very 
suitable.
The main type of tug fenders are:
•	 Cylindrical tug fenders.
•	 D-shaped fenders.
•	 Block fenders.
•	 M-shaped fenders.
•	 W-shaped fenders.
Cylindric fenders form the main fendering installed on 
a tug’s stern and bow. These fenders are used for pushing 
against ship hulls of all types and in all sea conditions. 
D-shaped fenders are similar to cylindric fenders, but with 
one flat surface. They can be used on the main deck sheer 
lines, on the forecastle deck and stern of tugs to provide 
protection.
Block fenders can have better grip than cylindric fenders 
because of their shape and grooved surfaces. They have large 
contact surfaces that reduce contact pressure per m2 between 
ship and tug, which makes them more suitable than other 
fender types for heavy-duty applications in wave and swell 
conditions.
M-shaped fenders are usually fitted to the bow and aft 
section of tugs to protect the tug and ship from damage 
during operations. They have a low weight and a large flexible 
surface area that reduces the forces per m2 on the attended 
ship during pushing operations. M-shaped fenders can be 
fitted around tight curves and provide additional grip due 
to their grooved surface. They are suitable for heavy duty 
operations. 
Fenders are constructed of rubber or synthetic rubber 
products. Beyond the mechanical requirements of load 
versus deflection and energy absorption, which is given 
in curves, attachment methods and structural limits, 
consideration should also be given to the material used in 
the fender. The material used should have good resistance 
to polluted 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 stern 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 and/or by the bow.
•	 The size and engine power of the tug which are 
important factors for the horizontal load and kinetic 
energy transmitted during contact and pushing.
•	 Size of contact area.
•	 The type and size of vessels to be handled eg ships with 
large bow flare and/or overhanging stern. Tugs pushing 
near the bow or stern 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. 
These conditions will give rise to additional forces in the 
fendering, for which it must be able to compensate.
•	 The tug’s bow and stern construction.
Additional fendering might be needed for tugs handling 
submarines and aircraft carriers, such as:
•	 Underwater fendering.
•	 Fendering at the outside and top of wheelhouse.
Tug fendering varies enormously. One frequently used fender 
system is the extruded profile type. Extruded fenders are 
produced in different lengths and in a wide variety of profiles 
and sizes. They can have a hollow D-shape profile, 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 view of design. Extrusion 
is a manufacturing method whereby uncured rubber is 
forced through a die to produce the required profile and then 
the lengths of formed rubber are vulcanised.
Figure 2A5: Tug with cylindrical bow fenders. Large cylindrical fenders 
are usually used to push against flared hulls and in open sea conditions. 
Their round shape is ideal for working with large bow flares of eg 
container vessels. Cylindrical fenders are often used on the bow in 
combination with M or W fenders. Photo: Trelleborg Marine Systems
Figure 2A6: Tyres are often used in combination with fenders. 
Photo: Piet Sinke
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Types of Harbour Tug 15
Bow fenders should have a large contact area and radius to 
reduce the pressure on the ship’s hull. The same applies to 
the stern fenders of tractor-tugs since these tugs are pushing 
with their stern. Tyres are often used in addition 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.
However, tyres are basically not designed as fender and a 
suitable way of mounting is problematic.
The following is an indication of some permissible hull 
pressures, which vary by ship’s type and size:
General cargo ships of
20,000 dwt and less 400-700kN/m2
Oil tankers of more than 
60,000 dwttyres which are cut to a specific size and 
compressed on to steel supporting rods. This fender type, 
made in the USA, is suitable for bow fenders, stern fenders 
and side fenders. There is one specific type which has a large 
absorption ability and is very soft, thus having a large contact 
area and ‘sticking ability’ when under load.
Tugs may also be fitted with foam-filled or pneumatic 
fenders, especially when working in exposed areas. 
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 manila rope fenders 
or tarpaulins, in addition to the standard tug fendering, 
may be used or the tugs may be equipped with grey rubber 
fendering.
A new type of fender material is coming on the market. 
Fendercare offers fenders that are manufactured with 
polymer products. Note: The name polymer is probably 
too general a term as it does cover a multitude of different 
materials, including natural and synthetic rubber.
The materials from which the fenders are made are from a 
specially formulated polyurethane which has been developed 
by Polymarine in Holland  and proven  to outperform the 
generic polyurethanes  in that they are lighter and stronger. 
Trials have proven that this specific  product is more resilient 
and protects vessels better than either rubber copolymers 
or standard polyurethane and in terms of through life 
cost presents a significant advantage. The fenders are more 
resistant to chemicals and ultra-violet light. The fenders have 
a high elasticity and will not mark ship’s hulls.
Figure 2A7: Crew preparing a tarpaulin around the fenders 
so preventing the light coloured hull of the ship to be assisted 
get marked by the tug fenders. Photo: Piet Sinke
Figure 2A8: All type of fenders can be found. Photo: Ole Peter Dahl
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16 Tug Use in Port
2.2.5 Specific requirements for tugs operating 
in hazardous areas
Tugs that have to handle gas carriers and are operating close 
to LNG/LPG terminals are subject to additional and specific 
safety requirements. 
Specific requirements apply also to tugs operating in ice 
conditions.
The requirements for these tugs will be addressed later in this 
chapter when dealing with these tugs.
2.2.6 Environmentally friendly tugs
The need for more environmentally friendly tugs can 
found in many ports. These tugs are called eco tugs, E-tugs 
or hybrids. Different systems exist to achieve this. More 
information about this subject can be found in paragraph 2. 
22.
From a tug and ship handling point of view two aspects 
are most important, that is the ease of handling of such 
system. Changing to full power should be simple so that 
in stress situation, when full power is immediately needed, 
no mistakes will be made. Secondly, if full power is indeed 
needed it should be possible to have it available without any 
delay. 
The various tug types will now be discussed. 
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.
Specific types of fenders can be provided with water 
lubrication 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. 
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 
moment will be.
As mentioned already, harbour tugs assisting submarines 
may also have underwater fendering to avoid contact 
damage to the 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 the nozzles 
of the tug’s azimuth propellers never come in contact with 
the submarine being assisted, the so-called ‘propulsion unit 
protective sponsons’.
Suitable and reliable towing equipment is also important 
for good harbour tug performance and safe working. This is 
dealt with in Chapter 7.
Types of Harbour Tug 17
where tugs are used for towing on a line, conventional tugs 
can be found with a more forward lying towing point.
Some new innovative small tugs are coming on the market, 
such as the Smart tug (Super Silent Smart tug) built by 
Uzmar Shipyards, Turkey and the Container tug.
2.3.1 General
Conventional tugs can be seen all over the world and are still 
built but in decreasing numbers. Conventional tugs are used 
for push-pull assistance, alongside towing and in particular 
in European ports for towing on a line.
Th ere is a large variety of conventional tugs. Th e 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 mechanism and a quick release towing 
hook lower this risk, providing they work under the extreme 
condition of girting, which is not always the case. Th e astern 
power of conventional tugs is generally low. When 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 that can produce good side thrust, such 
as tractor-tugs. Girting and interaction are dealt with in 
Chapters 4 and 6.
Th e towing point of these tugs generally lies about 0.45 x 
LWL from aft , although shorter distances may be found. Th e 
aft towing point on American conventional tugs lies further 
aft , which allows the opportunity to extend the deckhouse 
further aft . A more aft placed towing point limits the tug’s 
eff ectiveness when towing on a line at speed but this way 
of towing is not normal practice in the USA. In USA ports 
Figure 2B.1: Conventional twin screw tug Stan tug 3011.LOA 30.66m, beam 11.13m, BP 70 tons. Damen Shipyards, The Netherlands 
Figure 2B.2: Container Tug which can be transported in a container. 
Photo: DutchWorkBoats
2.3 Conventional types of tug
Chapter 2 
PART B
Basic tug types
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18 Tug Use in Port
the wheelhouse. It has the great advantage that it can deliver 
any propeller shaft speed ahead and astern without delay. 
In the past initial costs of diesel-electric propulsion 
were rather high, however in recent years the differences 
have become small particularly due to the lower costs of 
generators and frequency drives as a result of larger mass 
productions. 
 
Operational costs depend strongly on the way of 
employment. In case of one constant diesel engine load 
(typically 85 per cent of MCR) as in cargo liners, diesel 
direct propulsion has the highest efficiency and the lowest 
operational cost, however in case of varying power loads, 
as is the case with tugs, the efficiency of diesel-electric 
propulsion increases and operational costs are reduced. 
The larger the variations in loads, the larger the advantages 
of diesel-electric propulsion. In case of varying loads, the 
environmental performance of a diesel-electric system is 
clearly superior to normal diesel engines. Diesel-electric 
engines are subject to more wear and tear due to the constant 
high engine revolutions. 
Other important points are maintenance and downtime. A 
diesel-electric tug has more redundancy and maintenance 
can be carried out quicker; for instance changing one or 
more generators can be carried out rather quickly.
Diesel-electric propulsion has additional advantages in tug 
use: 
1. Power is faster availablesince generators are already 
running at full speed in contrast to diesel engines 
which need to speed up. Especially important in critical 
situations where every second counts!
2. Power can be controlled from zero upward and reverse, 
offering smooth and convenient manoeuvring without 
constant clutch operations and instantly reversing.
Most common nowadays on harbour tugs are high and 
medium speed diesel engines with reduction gears and 
pneumatic-hydraulic couplings. (See References for 
‘Operational benefits of high-speed electronic diesel 
engines’). Other types of couplings can be used. On tugs with 
fixed propellers the propeller thrust is reversed by means 
of a reverse-reduction gear, while on tugs with controllable 
pitch propellers (CPP) thrust is reversed by changing the 
propeller pitch. Torque problems may arise when a fixed 
pitch propeller is reversed at high tug speeds. These problems 
can be reduced or overcome by proper design (= the correct 
combination of engine, propeller and gear) and tuning of the 
whole propulsion system. Shaft brakes are used, depending 
on engine and propeller type.
Engine revolutions and propeller pitch are remotely 
controlled from the wheelhouse. Manoeuvring, especially 
with a CPP, is very smooth and the tug can accelerate fast. 
When the CPP control system is equipped with a combinator 
control, propeller revolutions are regulated in accordance 
with propeller pitch. The pitch of a CPP is regulated by a 
hydraulic system. CPP control systems, including remote 
control systems, the hydraulic system and emergency stop, 
require regular check-ups and good maintenance. Failure 
in the hydraulic or remote control system can cause serious 
The Smart tug is a 20.4m long line-handling tug with a 
bollard pull of 19 tons. It is a twin screw tug with propellers 
in nozzles and triple high efficiency rudder systems. The tug 
is designed for a wide range of mooring operations, oil spill 
response included. 
The container tug is a new design of a very small tug which 
can be transported in a 20ft container by road, sea or even 
by air. It came on the market in 2016. The tug is 6m long 
and fitted with a single fixed pitch open propeller and a 
large rudder. It is designed by Ben3D BV Naval Architecture 
in The Netherlands. The tug has a large deck space and a 
bollard pull of maximum 1.5tons. It has a draft of 1.05m and 
a weight of 7.5 tons. It is an all-round mini support vessel 
able to push and tow, and can easily be transported wherever 
work is to be done. 
More ‘mini-tugs’ will be dealt with in Chapter 8.
 
Experience is an important factor in handling conventional 
tugs safely while assisting ships under speed. With a 
well qualified captain these tugs can be very effective for 
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 still equipped with diesel engines 
although an occasional old harbour tug with a steam engine 
may still be found somewhere outside a maritime museum. 
Although it does not yet apply to conventional tugs, more 
and more hybrid tugs can be found which can operate diesel-
direct, diesel-electric and on battery packages. Tugs which 
use LNG as fuel are also coming on the market. 
Note 1: It is good to keep in mind that some items discussed in 
this paragraph apply to other tug types as well, as is the case 
with propulsion systems, nozzles and bow thrusters.
Note2: The term LNG is somewhat misleading. LNG is a natural 
gas kept in liquid condition for storage and transport. Only if 
converted to gas – natural gas (NG) – can it be used as fuel. 
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 some conventional tugs. The engine has to 
be started on ahead and on astern. On some tugs engines can 
be controlled from the wheelhouse, 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.
Full diesel-electric propulsion systems can be found in some 
harbour tugs. The diesel engine(s) drives electric generators 
which in turn drive electric motors. These electric motors 
drive the propeller. This system is easily controllable from 
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Types of Harbour Tug 19
Conventional tugs with controllable pitch propellers in 
nozzles (nozzle type 37) achieve, when pulling astern, about 
45 per cent of maximum ahead bollard pull, while this 
figure is about 65 per cent for tugs equipped with fixed pitch 
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 3: 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 8-12 per cent 
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.
A nozzle often used is the Optima nozzle. It has an 
improved forward thrust performance comparable with the 
conventional 19A nozzle, combined with the good reverse 
thrust performance of the 37 nozzle. 
There is a continuous research towards ever better nozzles for 
various types of ship, including tugs. One of the latest nozzle 
type is Schottel’s VarioDuct SDV45 nozzle, shown in figure 
2B.4. Given the same propulsive power, it offers a greater 
bollard pull than previous nozzles such as the 19A nozzle 
and, at the same time, gives considerably greater efficiency 
in the medium and high speed range, if used in combination 
with an optimum designed propeller. The small outer 
diameter of the nozzle makes it also ideally suited to shallow-
water operations. The new compact nozzle is suitable for 
harbour tugs as well.
damage to tug, ships or berths. Modern CPP systems have 
reliable backup systems.
Propeller efficiency and manoeuvrability
The propellers of conventional tugs can be fitted in open 
frames or fitted in nozzles. Going full astern, an open fixed 
pitch propeller will, in general, develop about 60 per cent 
of its maximum ahead thrust. An open CPP going astern 
develops some 40-45 per cent of maximum ahead thrust. The 
lesser efficiency astern of a CPP has to do with the specific 
design and working of a CPP. Propellers are designed for 
maximum efficiency going ahead. A fixed pitch propeller 
will turn, when astern thrust is required, with the same 
pitch in the reverse direction as on ahead. The propeller 
blades of a standard CPP have a smaller width near the hub 
and therefore, 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 bollardpull 
significantly. 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 per cent in towing and pushing conditions 
and by about 30 per cent at zero speed..
Brief explanation: 
The working of a nozzle is mainly twofold:
Around a nozzle an increase of water speed is created, which 
causes the nozzle to work like a wing. An inward force is 
so created, that has a forward component. The nozzle itself 
therefore has a positive forward thrust.
In the nozzle the inflow water velocity is increased leading to 
a lower loaded propeller, which has a higher efficiency. 
Furthermore the small clearance between the propeller and 
nozzle reduces tip vortex, increasing also efficiency. 
As nozzle drag increases with increasing speed, at a certain 
speed it will become larger that the added thrust. Tugs often 
sail with low speed and heavily loaded propellers and are 
therefore fitted with nozzles. 
Various types of nozzles (figure 2B.3) have been developed. 
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 – about 70 per cent of 
the ahead value with fixed pitch propellers and special blades 
and 60-65 per cent with ordinary blades. Nozzle type 37 is a 
type of nozzle often used for conventional harbour tugs.
Figure 2B.3: Two commonly used nozzle types 
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20 Tug Use in Port
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:
Movable flap rudders
There are several types of movable flap rudders, such as 
Rolls-Royce, Stuwa, Barke, while the most used one is Becker 
(see figure 2B.6 and 2B.7, opposite page). 
At the end of the rudder blade is a movable flap, controlled 
by a linkage, comprising about 20-30 per cent of the total 
main rudder area. Maximum helm angle of the main rudder 
differs by type and is about 45-65°, while the flap can turn 
45° further, up to a maximum angle of 90-110°. Each type 
of flap has its own specific characteristics. The flap angle is a 
function of the helm angle.
Maximum lift, which is achieved at a rudder angle of 
approximately 30°, is increased by 60-70 per cent, compared 
with a conventional rudder of the same shape, size and area. 
Sideways thrust ranges up to 50 per cent of ahead thrust. 
Becker claims for his movable flap rudder that at maximum 
rudder angle the propeller stream will, depending on rudder 
size and balance, be reduced so that 95-97 per cent of the 
propeller thrust will be covered by the rudder and used for 
manoeuvring the vessel. 
At speed the vessel can turn very quickly and speed will 
drop fast. When dead in the water the vessel can turn on the 
spot and it may even happen that the tug moves backwards. 
Performance of the rudder when the tug has speed astern is 
in general lower as that of an unflapped rudder. Tugs may 
have more than one movable flap behind a nozzle. To protect 
damage to the flap in ice, the flap rudder of a Becker movable 
flap rudder will be designed with a special ice knife.
Schilling rudders (fishtail rudder)
Schilling rudders (figure 2B.8) can also be found on tugs 
eg, the tug Sayyaf at Abu Dhabi. Schilling Monovec 
As already said, 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.
Nozzles can be also be steerable. Their manoeuvring 
performance is superior to normal rudder 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 can be steered by the steerable nozzle. A 
vertical fin or a movable flap may be fitted at the end of the 
steering nozzle (see figure 2B.5).
Some twin screw tugs have two independently controlled 
steerable nozzles, so increasing the tug’s manoeuvrability 
further.
Conventional tugs can be single screw, twin screw and 
even triple screw, eg, the USA harbour tug Scott T. Slatten. 
Manoeuvrability of twin and triple screw tugs will, in 
general, be better than of single screw tugs.
In general tugs are equipped with balanced, semi-balanced 
or spade rudders. By far most tugs have balanced rudders. 
Single plate rudders are also still used. 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 steering couple 
and consequently a larger steering gear. The decrease of 
propeller efficiency is mainly caused by the flow separation 
at the leading (forward) edge of the flat plate resulting in 
additional turbulence and resistance. If the forward edge 
would be rounded some of the lost rotational energy would 
be recovered. 
Figure 2B.4: New nozzle type Schottel VarioDuct SD45. Test model.
Source: Schottel GmbH
Figure 2B.5: The working of a steerable nozzle with movable flap.
©Becker Marine systems
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Types of Harbour Tug 21
outboard and 40° inboard. A maximum side thrust of 70 
per cent 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.
Flanking rudders
Flanking rudders (figure 2B.10) 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 
rudders have no movable parts. Horizontal slip stream 
guide plates are fitted at the top and bottom of the rudder. 
The rudder itself has a high lift blade profile with a wedge 
profile, so-called ‘fishtail’, at the end of the rudder blade. 
The rudder develops 30-40 per cent more lift compared to 
a conventional rudder and maximum lift is obtained at a 
rudder angle of approximately 40°. The rudder can be used 
up 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 (see figure 2B.9). With a Schilling Monovec 
rudder, turning on the spot is almost possible while speed is 
dropping very fast.
Two Schilling rudders, called Schilling VecTwin, can 
be used behind a propeller and make the vessel very 
manoeuvrable. Each rudder has a separate steering gear. 
The rudders can be turned by joystick a maximum of 105° 
Figure 2B.6: The system of a movable flap rudder. ©Becker Marine systems Figure 2B.7: A movable flap rudder behind a controllable pitch propeller 
in a nozzle. ©Becker Marine systems
Figure 2B.8: Schilling rudder. ©Becker Marine systems Figure 2B.9: Comparison between fishtail rudder and conventional rudder
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22 Tug Use in Port
Other systems
Besides the rudder systems mentioned above, many other 
systems exist, such as different types of fishtail rudders, triple 
rudder systems (see figure 2B.12) and for instance the triple 
screw tug Scott T. Allen with her three rudders, of which 
the centre rudder can be operated independently from the 
outboard rudders.
Bow thruster
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 effectiveness of the bow thruster may already be reduced 
by 50 per cent. Seagoing harbour tugs operating in port 
areas as well as at sea for offshore work often have a bow 
thruster, which enables them to keep position better near oil 
platforms.
Conventional tugs may be equipped with a (retractable) 360° 
steerable bow thruster. These bow thrusters are much more 
effective and can operate in any direction. Tugs with this 
kind of bow thruster are the previously mentioned combi-
tugs.
2.3.3 Manoeuvring conventional tugs
Single screw tugs
Not many single screw tugs are left for ship handling in 
ports.
Three aspects are important in manoeuvring a normal single 
screw conventional tug:
•	 The aft location of the rudder and propulsion.
•	 The transverse effect of the propeller when turning for 
astern.
•	 The low astern power.
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.
Towmaster system
The Towmaster rudder system (figure 2B.11) 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 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. 
Figure 2B.10: Shutter rudder system with a fixed nozzle 
and two flanking rudders
Figure 2B.11: Towmaster rudder system of tug 
Hazam (LOA 38m, beam 11m, BP 70 tons ahead and 
50 tons astern. Photo: Damen Shipyards, The Netherlands Figure 2B.12: Triple rudder system. Photo: Nautican, USA
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Types of Harbour Tug 23
sideways to starboard without gathering headway, depending 
on trim, wind and current influence. The transverse effect of 
the inner propeller will enhance the side thrust although this 
effect will be much less with the propeller in a nozzle.
2.3.4 Conventional tugs in ship handling
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 point, it has severe limitations. When the ship has 
more than approximately three knots headway the after tug 
can only assist at one side of the 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.
When towing on a line, conventional tugs are not suitable to 
changing over to pushing at the ship’s side while the towline 
is still fastened. This 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 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 propellers will result in low propeller efficiency. 
In addition, the stern fendering 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 the 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 limited astern power. Specific 
rudder configurations, such as the Towmaster system for 
example, will increase astern thrust. Normal single screw 
conventional tugs can neither pull at right angles because of 
the transverse effect of the propeller, nor can a single screw 
tug pull at right angles with a cross current or strong cross 
winds. The same kind of problem arises when the assisted 
When ahead thrust is applied 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. This 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 of a single screw tug is possible, not even with 
high lift rudders, though sideways movement is possible with 
high lift rudders in conjunction with a bow thruster.
The transverse effect or ‘paddle wheel effect’ is caused by 
the propeller wash hitting the stern 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 propeller is set for 
astern, propeller wash hits the tug’s stern on the starboard 
side and the stern moves to port – consequently the bow 
turns 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 
wheel 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 Towmaster 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) screw tugs are much more manoeuvrable 
than single screw tugs. They can turn on the spot without 
making headway and can more easily manoeuvre straight 
astern. Turning can be done by reversing one propeller 
and setting theother for ahead while applying helm in the 
intended direction.
Propellers 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 
propellers is higher propeller efficiency. A disadvantage with 
fixed pitch propellers is the larger turning diameter, because 
the starboard propeller is left handed and the port one is 
right handed. When using the propellers as a couple, the 
transverse effect of the screws opposes the turn. This effect 
will be smaller in case of nozzle, but is still there.
With inward turning fixed pitch propellers a tug can move 
sideways (see figure 2B.13), so-called ‘flanking’. When the 
tug has to move sideways to starboard, one would think 
of setting the starboard propeller to ahead and the port 
propeller to astern. This works only when the tug is equipped 
with a bow thruster. However, without a bow thruster this 
propeller setting does not move the whole tug sideways, but 
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 will move 
Figure 2B.13: Twin screw tug moving sideways to starboard, also called 
flanking, by setting the port engine on ahead and starboard engine on 
astern while applying port helm. In the case of in-turning fixed pitch 
propellers the transverse thrust of the inner propeller will enlarge the side 
thrust to starboard, which is in particular the case with open propellers.
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24 Tug Use in Port
low, unless the tug is equipped with a special rudder and/or 
propeller arrangement which increases propeller efficiency.
By installing a conventional single screw tug with a 360° 
steerable bow thruster, also called an azimuth bow thruster, 
these disadvantages can be overcome (see figure 2B.15). Tugs 
equipped with such a bow thruster are the so-called combi-
tugs. The first combi-tugs appeared 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 spot, 
sail straight astern at a fair speed and move sideways as well 
(see figure 2B.16 opposite). 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 thruster is equipped 
with a nozzle and can be of retractable or fixed type. An 
azimuth bow thruster with a nozzle propeller below the keel, 
in contrast to a tunnel bow thruster, achieves high efficiency 
in any direction even when the tug is moving quickly. This 
provides an additional increase in the tug’s manoeuvrability.
As an example, an azimuth bow thruster of 400hp can 
increase the top speed of a tug of 27 metres length and 
engine power of 1,500bhp 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.
For older tugs this is a satisfactory and inexpensive way of 
improving manoeuvrability and bollard pull. As examples 
of converted tugs, in Amsterdam, the Netherlands, some 
older tugs were converted into combi-tugs and at San Pedro, 
California, USA, the tug San Pedro (now Pacific Combi) has 
been converted into a combi-tug. The San Pedro has been 
equipped with a 600bhp bow thruster, which has increased 
ship is moving ahead or astern while the tugs are pulling. 
It will then be impossible 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 direction. A bow thruster does not improve 
the situation as the conventional tug operates while pulling 
with the tug’s bow headed towards the ship’s hull. Steering 
nozzles, Towmaster and flanking rudders make it easier 
to keep the tug at right angles when pulling. Twin screw 
conventional tugs can make use of their propellers to keep 
the tug at right angles, although this will be at the expense of 
loss of effectiveness.
The capabilities and limitations of conventional tugs in 
relation to other tug types are discussed in Chapter 4. Some 
assisting methods with conventional tugs are shown in figure 
2B.14.
Capabilities of conventional tugs can be increased by a gob 
rope system and by a carrousel system. The latter system, 
which will be discussed later in this chapter, increases a tug’s 
capability considerably. The gob rope system will be dealt 
with in Chapter 7. 
Installing an azimuth bow thrusters will also increase a 
conventional tug’s capabilities. This will be discussed in the 
next paragraph. 
2.4 Combi-Tugs
2.4.1 Designing and manoeuvring combi-tugs
As discussed above, the manoeuvrability of single screw 
conventional tugs can be improved by the use of high lift 
rudders. However, the 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 
Figure 2B.14: Some assisting methods with conventional tugs
Figure 2B.15: Fairplay’s combi-tug Serwal 3, the former Petronella J. 
Goedkoop. LOA 28.5m, beam 6.9m. Main engine 900 bhp. One cpp in 
fixed nozzle and twin rudders. Retractable 3600 steerable bow thrusters 
of 420bhp, type Aquamaster UL 316/2600. BP of main engine 15 tons. 
Bollard pull of main engine + bow thruster 20 tons. Maximum speed 
ahead 11.9 knots, astern 10.2 knots when using both main engine and 
bow thrusters. The tug is equipped with a special fairlead at the stern and 
a towing winch. Line ‘1’ shows the towline in its 1 ‘normal’ position and 
‘2’ the towline passing through the fairlead.
Types of Harbour Tug 25
an eyelet or swivel fairlead at the tug’s stern. At the free end 
of the wire is a large shackle which can be put 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 further explained in paragraph 7.2.1. A gob 
rope arrangement normally needs two persons on deck. 
With the reduced numbers in tug’s crews a handier and safer 
system was developed by the former Goedkoop Harbour 
Towage Company of The Netherlands (now Wijsmuller 
Harbour Towage 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. With 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 
ship. The combi-tug makes fast aft and approaches stern first 
to the stern of the ship to pass the towline (see figure 2B.17 
position 1). The ship to be assisted may still have rather a 
high speed, eg 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 
the tug’s bollard pull by 40 per cent, 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 large 
fairlead aft. Some newer tugs are also equipped with azimuth 
bow thrusters, allof them of the retractable type.
If the azimuth bow thruster is not in use it causes extra 
resistance. 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 when 
the water depth is not sufficient for safe working of the bow 
thruster is strongly recommended.
2.4.2 Combi-tugs in ship handling
Combi-tugs can tow on a line forward as well as aft. As a 
forward tug the combi-tug operates like a conventional 
tug, but has the advantage of increased maximum speed, 
manoeuvrability and bollard pull. Also, the risk of girting 
is reduced and response time is less due to the higher 
manoeuvrability.
As a stern tug combi-tugs can operate as a conventional 
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 needs an additional towing point near the stern to 
prevent girting, especially when the assisted ship has a higher 
speed. On conventional 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 
Figure 2B.16: Free sailing manoeuvres with a combi-tug
Figure 2B.17: Some assisting methods with a combi-tug
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26 Tug Use in Port
Schneider propulsion system, was developed in the early 
1920s by Professor Frederic Kirsten of the Aeronautical 
Engineering Faculty at the 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 ship handling tug with the cycloidal 
propulsion under the tug’s forebody and the towing gear on 
the aft deck. The oldest Voith tug was the Biene, built at the 
Clausen shipyard in Oberwinter.
Many limitations of conventional tugs were overcome by the 
introduction of this totally new concept, which was called a 
Voith Water Tractor.
Note: All pictures in this paragraph have been generously 
made available by Voith Turbo GmbH & Co. KG, Marine 
Division, Heidenheim, Germany, unless indicated otherwise.. 
The cycloidal propulsion system is, in fact, a kind of 
controllable pitch propeller. The engine works at constant 
rpm and magnitude of thrust and the thrust direction is 
regulated from the wheelhouse.
New Voith tugs, like the VectRA 3000, may be equipped 
with a system having engine and thrust (better to say 
pitch) regulated in the combinator mode, comparable with 
controllable pitch propellers. This Voith Remote Control 
system regulates automatically engine rpm and thrust/pitch 
in the most effective way in accordance with a predefined 
graph.
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 and on the 
skeg (3). See figure 2B.19 below. 
Different engine rpm settings can be selected. The following 
pitch settings are recommended: 
•	 Maximum pitch pushing about 9. 
•	 Maximum pitch pulling about 8. 
•	 Maximum pitch free running about 10.
To avoid engine overload during pulling and pushing, 
the pitch restrictor, located on the control stand, must be 
engaged. Engine overload will be indicated by overload 
warning lights flashing at 110 per cent power and acoustic 
alarm on the VSP control stand.
The large skeg is typical for tractor tugs and in particular for 
VS tractor tugs. It gives course stability and brings the centre 
of hydrodynamic pressure further aft, which is advantageous 
to both safety and towing performance when towing on a 
line, especially towing performance when operating as stern 
tug at higher speeds.
The towing winch is located aft of midships. It may also 
be just a towing hook. The towing point, a large fairlead or 
towing staple (4), through which the towing line passes, lies 
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 the tug 
sheering out to port or starboard with the main propulsion 
going astern and the bow thruster working sideways. In 
positions 2 and 3 the incoming water flow creates lift forces 
on the tug and consequently a force in the towline. When the 
ship’s speed reduces, the effect of the tug in position 2 and 3 
will become less due to the reduced lift forces. The gob rope 
is then released or the towline taken 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 required from the tug to 
compensate for those forces, the tug is more effective when 
it proceeds with the assisted ship as a normal conventional 
tug (position 4) and thus can use its full ahead power. When 
required, the bow thruster can be used to increase bollard pull. 
The lift forces on the tug caused by the water flow increase the 
force in the towline.
If so required the tug can, even when the assisted ship has 
forward speed, shift to a position behind the ship’s stern 
by using the gob rope or fairlead, bow thruster and main 
propulsion (position 4 —> 5). This can be done faster 
compared to a normal conventional tug. Conversely, moving 
from a position abaft the stern to a position moving with 
the assisted ship is, because of the bow thruster, possible at a 
somewhat higher speed than with a normal conventional tug.
It has been made clear that the 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 combi-tug can also be used at the ship’s side, such as for 
push-pull operations.
When operating at the ship’s side, a combi-tug has many of 
the disadvantages 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 
bow thruster can be helpful to keep the tug’s bow in position 
and prevent sliding along the ship’s hull. The bow thruster 
will also give an additional transverse pushing force (see 
figure 2B.17, lower picture).
When pushing with the stern, the effectiveness of the tug 
is reduced due to the restricted water flow towards the 
propeller and it is more difficult to bring or hold the tug at 
right angles to the ship’s hull when the 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.
2.5 Tractor-tugs with cycloidal propellers 
2.5.1 Design
Tractor tugs have their propulsion under the forebody. 
Those with a vertical blade system, or cycloidal propulsion 
system, are the so-called Voith- Schneider or Voith tugs (VS 
tugs). The first vertical axis propeller, similar to the Voith 
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Types of Harbour Tug 27
the other one in the 
transverse direction. The 
propeller blades create 
a thrust in a direction 
depending on the 
location of the steering 
centre N. In sketch 1 
there is no thrust; the 
propellers are ‘idling’. 
In sketch 2 the steering 
centre is moved by one 
hydraulic cylinder to 
port. Thisoffset location 
of the steering centre N 
results in forward thrust. 
In sketch 3 the steering 
point N is moved by the 
two hydraulic cylinders 
to port and forward, 
which gives thrust in 
the indicated 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, 
except for the new CRT 
(Carrousel RAVE Tug) 
which has one propulsion unit forward and one aft.. 
The maximum draft, including the propulsion units, of a VS 
tug is relatively larger than that of conventional tugs, due to 
the weight and height of the propulsion units, the propeller 
location and dimensions. The location of the propulsion 
units is approximately 0.25 - 0.30 x LWL from forward. The 
towing point lies 0.1 - 0.2 x LWL from aft, although this may 
differ by tug depending on operational requirements.
2.5.2 Propeller control
The direction and magnitude of propeller thrust is remotely 
controlled from the wheelhouse. The remote control can 
be mechanically operated, which is a very reliable system 
for tugs and best when the distance between wheelhouse 
and propeller is short. With longer distances between the 
bridge/wheelhouse and propeller control and when several 
manoeuvring stands are installed other remote control 
systems are recommended such as an electronic control. 
How propeller thrust is regulated can be seen in figures 
2B.21, and 2B.22, on the next few pages.
Some attention will be paid to the specific Voith controls for 
steering and thrust. Transverse thrust is controlled by the 
wheel and longitudinal thrust is controlled by pitch levers. So 
thrust setting is a combination of transverse and longitudinal 
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, 
especially at the stern, because when pushing, the tugs push 
with the stern.
Most modem tugs have relatively small wheelhouses with 
optimal visibility. The same applies to modem VS-tugs, 
like the one shown in figure 2B.18. The small and optimum 
wheelhouse (6) often has one central manoeuvring panel for 
propeller control.
The principle of a cycloidal VS propeller is shown in figure 
2B.20. Links leading to the steering centre N are fitted to 
the vertical propeller blades. The steering centre N can be 
moved out of the centre O by two hydraulic cylinders. One 
hydraulic cylinder works in the longitudinal direction and 
Figure 2B.18: The total concept of a Voith tractor tug
Explanation of figure 2B.18:
1. Voith-Schneider propeller. 2 Propeller guard plate.
3. Skeg 4 Towing staple. 5 Second towing position.
6. Wheelhouse with specific Voith controls.
Note 4: A second towing point is only fitted in a small number 
of VS tugs, and is discussed further in Chapters 4 and 9.
Figure 2B.19: Voith propulsion units with protection blades. 
Photo: Andries Looijen, Multraship
Figure 2B.20: Principle of Voith propulsion.
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28 Tug Use in Port
ship’s hull is less compared to tugs with azimuth thruster 
which have a more concentrated propeller wash. 
The full bow of tractor-tugs and the flat and wide hull 
bottoms which are necessary to create sufficient room for the 
propulsion units sometimes 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.
Note 5:
There is a general problem of course stability often applying to 
modern tugs with a small length to beam ratio. For instance, 
such Voith tugs may veer somewhat to port and starboard 
– which had to do with the stern design, and caused by the 
water flow sticking to the tug’s hull and stern, but at a certain 
moment the flow becomes loose of the stern and forms a 
vortex. This may not happen symmetrically, causing the stern 
to be pushed from one side to the other and starting to sway. It 
may happen on one tractor tug, but not another. Voith, having 
wide experience with their tractor tugs, solved the problem in 
various ways, for instance by putting baffles on the stern side 
of the fin. Another system is mounting vertical strips on the 
thrust. Transverse direction has priority. For older tugs, 
when full transverse thrust is used (e.g. wheel hard to port) 
no longitudinal thrust will be available, even when the pitch 
levers are set in pitch position. For newer VS tugs, there will 
be some ahead thrust (see figure 2B.22). It can be seen that the 
full 100 per cent thrust cannot be applied in any direction. 
The two units of a VS tug can be controlled independently or 
together for longitudinal thrust but only controlled together 
for transverse thrust.
2.5.3 Manoeuvring
VS tractor-tugs are highly manoeuvrable, can turn on 
the spot, deliver a high amount of thrust in any direction 
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 some situations 
transverse 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.
Some Voith Water Tractors are specially designed for high 
speed escorting. These VWTs are of the Fin First Design 
(Fin first), ie, the fin (skeg) side is the bow and the free 
sailing is done fin first. This, however, does not alter the basic 
principles of the tractor tug. The standard VWTs have the 
VSPs in the bow (VSP first), the free sailing is generally done 
VSP first. Sailing ahead as well as astern is easily achieved by 
use of the wheel. Turning on the spot can be done by setting 
the wheel hard to port or starboard as shown in figure 2B.23.
A VS tug can be moved sideways, for example to port (see 
figure 2B.24). The port 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. Furthermore, loss of pulling 
effectiveness due to the propeller wash impinging on the 
Figure 2B.21: Voith mechanical control – steering wheel 
and thrust handles.
Figure 2B.23: Turning
Top: Wheel moves the bow to starboard.
Bottom: Pitch levers move the stern to starboard.
Figure 2B.24: Casting off; moving sideways.
stern of the tug so these vortices could no longer be formed, 
improving course stability. Voith patented this system and 
implemented the solution with Robert Allan Ltd – who call it 
StRAke stabilisers – which have also been applied to azimuth 
tractor tugs as well. The first application on Voith tugs was on 
VectRA tugs built at Sanmar Shipyard, Turkey. 
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Types of Harbour Tug 29
With both pitch levers set to full ahead (pitch 10), the wheel changes the direction of thrust.
With the pitch levers set to full ahead (astern) and the wheel set to approx 5 port (starboard), the thrust ahead (astern) 
is approx 55 per cent, the transverse thrust is approx 45 per cent.
With the pitch levers set to full ahead (10) (astern) and the wheel set to 10 port (starboard),the thrust ahead (astern) 
is 25 per cent and the transverse thrust is 75 per cent.
With pitch levers set to zero and the wheel set to 10 port (starboard), the resulting thrust to the side is 100 per cent. 
Lateral transition is generally done using the wheel which overrides longitudinal thrust.
See also: http://voith.com/corp-en/drives-transmissions/voith-schneider-propeller-vsp.html where a program can 
be downloaded to handle a virtual Voith tug. 
Figure 2B.22 : Wheel and pitch lever settings.
30 Tug Use in Port
VS tugs can also make fast directly to a ship’s side as push-
pull tugs, 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 speed, due 
to performance restrictions imposed by the location of the 
towing point, they are very suitable as after tug for course 
and speed 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.
Note 6: The different manoeuvres shown and also additional 
manoeuvres can be found in the Voith Water Tractor 
Maneuver Manual, including a very important guideline how 
to make fast at the bow of a ship having headway. 
2.6 Tractor tugs with azimuth propellers
2.6.1 Design
Tractor tugs with azimuth propellers have two 360° 
steerable thrusters under the forebody. There are several 
manufacturers of azimuth thrusters, including Rolls-Royce, 
Schottel, Niigata, HRP, Wärtsilä, Kawasaki, Veth, Caterpillar, 
etc. Some of the European manufacturers mentioned have 
merged. Different names are used for azimuth thrusters, 
such as Z-pellers, Rexpellers and Duckpellers, among others. 
Although the thruster systems are generally similar, each 
manufacturing company has its own specific design.
2.5.4 VS tugs in ship handling
To get some basic insight into the ship handling capabilities 
of a VS tug, various ship handling manoeuvres are shown 
on this page. These manoeuvres can be executed by well-
designed azimuth tractor tugs too.
Course control can be carried out at high speeds by the 
indirect method (see figure 2B.25A, situation b), making 
use of the hydrodynamic forces on the tug’s hull, or at lower 
speeds by the direct method (see figure 2B.25A situation a). 
Forces in the indirect method can be far higher than the 
tug’s bollard 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. 
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 2B.25, situations 
C,D). The forward tug can change to a 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 
control the length of the towline and to enhance safety.
VS tugs are used for towing on a line and for operations 
like push-pull (see figure 2B.25, situation E). 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 operations the disadvantages of conventional tugs, 
among others, 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.
Figure 2B.25: Various assist modes
Figure 2B.25A: Direct and indirect assist modes.
Figure 2B.25B: This special manoeuvre can be employed, if the pilot 
wants to have a light pull on the line. Normally, when braking forces are 
ordered, the tug should stay in line behind the ship, using pitch levers and 
a minimum of wheel. Pitch levers must be adjusted according to ship’s 
speed to avoid engine overload.
Figure 2B.25C and D: Fast forward/aft: Changing position to come 
alongside for pushing. To be done at low speeds, up to about 2 knots.
Figure 2B.25E: Push-pull while berthing.
Ship’s speed around 5 knots
B
A
C
D
E
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Types of Harbour Tug 31
hull openings for the VS units. An azimuth tractor tug of the 
same 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. Twin skegs are also used (see below). 
The 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 placed further 
forward increase a tug’s effectiveness while assisting. The 
thrusters deliver practically the same amount of thrust in 
any direction, though astern thrust might be about 5 per cent 
less. When the thrusters interact, as when producing side 
thrust, total thrust efficiency will be less. Thrusters should 
then be set at a small angle to each other.
For azimuth tractor tugs (ATD) various names can be found, 
such as the Azitrac designed by Offshore Ship Designers in 
the Netherlands.
Twin skegs
As mentioned before, azimuth tractor tugs may be equipped 
with a patented twin skeg The reason why will be explained 
now (see References for paper: Damen ATD Tug 2412 Twin 
Fin Concept).
The ATD Tug 2412 has unconventional main dimensional 
ratios, a L/B ratio of 1.89. An observation from full-scale 
trials and model scale trials was that the tug had to be steered 
(when sailing ahead) with about 5-10° of azimuth angle and 
allowing a drift of 3-5 degrees. Sailing astern showed the tug 
as directionally stable.
Opportunities to improve on this peculiar steering behaviour 
when sailing ahead were sighted in a study initiated to 
further enhance the design.
The first azimuth propellers were introduced into service 
in the 1960s. The first tug fitted with Schottel azimuth 
propellers was the German harbour tug Janus (1967). 
Azimuth propellers can be fixed pitch, eg, often with Niigata, 
or controllable pitch. Fixed pitch propeller revolutions can 
be regulated by a speed modulating clutch, which enables 
the propeller speed to be controlled in a stepless manner 
from zero up to maximum. This more or less eliminates 
the need for controllable pitch propellers and is much less 
expensive. A new system to modulate propeller speed of fixed 
pitch propellers of azimuth thrusters is the CAT Advanced 
Variable DriveTM (CAT AVD TM) which is installed on a 
RAmparts 2400 SX ASD-tug and which could probably be 
used on azimuth tractor tugs as well. The systems consist of 
a pair of dual input, continuously variable transmissions, 
located in the shaft lines between the main engines and the 
azimuth propellers. The AVD TM system can modulate 
propeller speed down to zero like a slipping clutch. See 
References for article ‘Flower Power’. 
On the other hand, controllable pitch propellers have, apart 
from the faster acceleration, the advantage that a fire-fighting 
system can be operated through the main propulsion unit 
without the need for a complicated coupling arrangement. 
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 front of the propeller and give only limited 
protection for the propellers. Protection plates serve also 
when docking.
The basic design of the tug itself does not differ much from 
VS tractor tugs. The displacement of a VStug is more than 
that of a comparable azimuth tractor tug of the same engine 
power, due to the higher weight of the VS propulsion systems 
and to the requirements for more stiffening due to the wider 
Figure 2B.26: 
Construction of a thruster
Source: Thrustmaster
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32 Tug Use in Port
Various alternative designs were tested in MARIN’s 
seakeeping and manoeuvring basin. 
The Twin Fin arrangements showed a nearly conventional 
directional stability. In addition, the turning velocity at 
higher rudder angles increased as well, which demonstrated 
that the Twin Fin gives a somewhat higher turning ability. 
The Twin Fin configuration enables a directionally stable 
compact tractor tug with perfect steering behaviour both 
sailing ahead and astern.
Some further information about the ATD 2412 Twin Fin: 
•	 Bollard pull maximum 70 tons.
•	 Maximum speed ahead and astern 12 knots.
•	 Tonnagebig consequences. Therefore, highly experienced tug masters are required with proper knowledge of 
the capabilities and limitations of their tugs, including the risks involved when assisting ships. 
Thankfully, this guide by the renowned author Captain Hensen is now available in its 4th edition for all to 
learn from and refer to. Such is its relevance and authority that IMO itself has recognised its significance.
I am delighted to write the foreword to such a distinctive publication which is of immense benefit to my 
profession and, I know, to tug and ship masters too and to all other professionals involved in ship handling 
with tugs.
Simon Pelletier
President, International Maritime Pilots’ Association 
Tug Use in Port vii
This book is specifically written for maritime professionals involved in the day-to-day practice and 
training of ship handling with tugs, particularly pilots, tug masters and training instructors. It is also of 
great value to towing companies, shipmasters and mates of seagoing vessels, training institutes, and all 
other people or organisations involved, in one way or another, in tugs, tug operations and ship handling.
The basic principles of ship handling with tugs do not change very much. However, the tug world is fast 
changing, with the ongoing development of new tug types, the growth of environmentally friendly tugs, 
the increase of operations in areas with unfavourable conditions, the progress of automation on board 
tugs, smart navigation and collision avoidance systems and more. 
At the centre of these developments are the practical people – tug masters and crews, pilots and ship 
captains – who have to deal with new developments and should be able to handle newly designed tugs in 
the locations where they have to operate in a safe and efficient way. Training and effective training tools, 
therefore, become more and more crucial. For the same reasons training institutes should stay abreast of 
new developments.
This is not the only reason for training. Accidents still do happen – more than once with dramatic 
consequences for the tug master and/or crew – which is of continuous concern. The reasons why these 
accidents happen should be investigated carefully and findings be included in the training programs. 
When reading accident reports it can be concluded that very often a lack of experience and knowledge 
is the cause. It is therefore incumbent upon towing companies, port authorities, training institutes and 
trainers to ensure that tug crews are fully aware of the capabilities of their vessels. Despite all the safe tug 
designs, they are worthless if the people on board do not understand their capabilities and limitations, 
along with the risks involved and the safe procedures to be followed.
If training is carried out in an efficient and purposeful way by professional and experienced instructors, 
tug masters, pilots and ship captains will benefit from it, as will towing companies, shipping companies 
and port authorities. In addition, dedicated training and interaction between practitioners and designers 
will help the industry to achieve an ever-higher level of safety and efficiency. 
Safety starts at the highest organisational level. Top management of a company and department directors 
set the organisational structure, allocate resources and budgets and create the safety culture, but often they 
are distanced from the operation both in time and locality. The causes of accidents are often embedded in 
the way a company is organised, work is prepared, and changes are managed. From this perspective, risk 
management is an important issue, together with essential risk assessment tools.
This book focuses on safe and efficient ship handling with tugs either normal tug operations, operations at 
terminals, or escorting. Not all the relevant aspects of tugs and ship handling with tugs can be addressed 
in detail in a single volume, but I hope this book will act as a guide to the reader, while at the same time 
encourages further research in topics of interest. The references at the end of the book may prove useful.
It is again the author’s earnest hope that this fourth edition of the book will contribute to an enhanced 
safety culture, an improved knowledge of tugs through nautical colleges, training institutes or by self study, 
and will lead to increasing safety during ship handling operations with tugs in ports, port approaches and 
offshore terminals around the world.
Henk Hensen
June 2021
AUTHOR’S PREFACE
viii Tug Use in Port
To complete a book like this, the knowledge and experience of people across a wide range of disciplines is needed. It is most 
surprising how many people were so helpful in sharing their knowledge and experience with me. It enabled me to create an 
update of the book Tug Use in Port in the best possible way. As the author I am very grateful for the generosity of all these 
people and their organisations.
I will start by thanking all those who have provided photos or permitted the use of their photos from the collections shown on 
internet. Their names are mentioned with the photos in the book. 
Many maritime professionals have helped in one way or another based on their knowledge and practical experience in ship 
handling with tugs. The marine pilots who have helped: John Betz, Los Angeles Pilot Service; Wim van Buuren, Rotterdam 
pilot, simulator instructor; Rafael Cabal Alvarez, Barcelona pilot, co-ordinator and instructor of the New Technologies course 
for Spanish pilots; Luke Felsinger, Gladstone marine pilot, Dir AMPI, and Sergei Shabal, pilot, St Petersburg, Russia. 
The harbour masters: Captain Paul Bryant, Deputy Harbour Master, Shetland Islands Council; Harbour Master, Associated 
British Ports Southampton, and Cor Oudendijk, former COO, Port of Amsterdam. 
I also acknowledge the help of tug masters and representatives from several towing companies:
Pierre Jourdain, tug master and ice pilot; Gregory V Brooks, tug master, Principal Towing Solutions Inc; Arie Nygh, FNI, tug 
master/instructor, Managing Director, SeaWays Consultants Pty Ltd; Daan Merkelbach, Manager Training and Consultancy, 
Tug Training & Consultancy BV; Roger Ward, former tug master and Operational Manager, Marine Consultant; Jarkko Toivola, 
Director/Vice President, Alfons Håkans Øy,Turku; Anna Fong, Senior Management Executive, Corporate Services Department, 
PSA Marine (Pte) Ltd.; Andy Perry, Regional Marine Manager-Fleet & Operations, Svitzer Australia Pty Ltd; David Mcinnes, 
Svitzer Fleet Training and Check Master, Fleet & Operations Australia; Scott Ward, HSEQ Marine Standards Officer UK, 
Svitzer; Hiroyuki Saito, President Tokyo Kisen Co Ltd; Sveinung Zahl, Fleet Manager Towage, Østensjø Rederi AS; Carl Pepin, 
MBA Director Operations, Towing and Navigation, Ocean Group, Québec; John Armstrong, Marine Advisor, Saam Smit 
Canada Inc, and Brent Lirette RG, Operations Manager, Edison Chouest Offshore Alaska. 
Of great importance for this book has been the contribution of naval architects, maritime researchers and research and training 
institutes. Therefore I am thankful for the help of naval architects Dr Markus van der Laan, Owner IMC Corporate Licensing; 
Arie Aalbers; Frans Sas, SASTECH; and of James R Hyslop, Manager, Project Development Principal, Robert Allan Ltd.
Likewise, the professionals of research and training institutes MARIN (Maritime Research Institute Netherlands) Johan H de 
Jong, MSc, International Co-operation; Dr Thijs Hasselaar, MSc, Project Manager, Trials & Monitoring and Jos van Doorn, Msc, 
Manager MARIN's Nautical Centre MSCN. Furthermore, Ismael Verdugo, Technical Director SiPort21; Vladimir Ponomarev, 
Vice President Solutions, Transas Marine Ltd, Ireland; Peter Jensen Schjeldahl, MScEng, PhD, Senior Specialist Simulation, 
Training & Ports, Force Technology, and Cliff Beazley, AM FNI, Managing Director, Port Ash, Australia.
The contributions of Gordon Meadow MSc, PgCLTHE, PgCERM, FHEA, Associateaft often lies 
too far aft to be effective if these tugs were to tow on a line at 
speed like a conventional tug. Sometimes the towing point 
lies nearly above the thrusters aft. The Japanese concept of 
tugs is among others such a tug and is further being dealt 
with in paragraph 2.8.
ASD-tugs are dealt with in paragraph 2.9. These tugs also 
operate more and more over the bow, and as a reverse-
tractor tug as well.
For the azimuth propeller systems in use, fixed pitch or 
controllable pitch, please see the tractor tug paragraph. 
Because the thrusters are fitted under the stern the 
maximum draft of reverse-tractor tugs is less than that 
of comparable real tractor tugs. Hull draft is less than the 
hull draft of a similar VS tractor tug, for reasons already 
explained when discussing azimuth tractor tugs.
The propulsion units are located approximately 0.1 x LWL 
from aft. The pushing point and forward towing point is 
at the forward part of the bow. Wheelhouse construction 
is completely adjusted to the assisting method. The 
manoeuvring station is designed in such a way that the tug 
captain has an unobstructed view of the forepart of the tug, 
the towline and the assisted ship, while seated behind the 
manoeuvring panel with the assorted instrumentation and 
control handles around him.
2.6.4 Azimuth tractor tugs in ship handling
The assisting capabilities of azimuth tractor tugs are 
comparable to those of VS tractor tugs. They are suitable 
either for operating at the ship’s side or for towing on a line 
(see figure 2B.32). Azimuth tractor tugs, and also VS tractor 
tugs if 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 operating in the 
indirect towing method at higher speeds.
 
On the other hand, because of their lower underwater 
resistance – mainly due to the relatively shallower draft – 
and the ability to provide nearly 100 per cent thrust in any 
direction, azimuth tractor tugs will be more effective at speed 
when direct towing as a stern tug and as a forward tug when 
towing on a line, again depending on a proper location of 
the towing point. The influence of the location of the towing 
point on the performance of tractor tugs is further discussed 
in Chapter 4.
Note 8:
Robert Allan Ltd designed a new tug concept of which the 
hull could be outfitted as either a 25m tractor tug (TRAktor 
2500-SX) with bollard pulls up to 70 tons or a 25m Rotortug 
(ART-60-25SX) with bollard pull up to 60 tons. The stern of 
the hull features the simple but effective StRAke stabilisers, 
a system developed by Voith and implemented by Robert 
Allan Ltd, which improve tractor tug course and directional 
stability, allowing for a reduced size skeg and improved 
manoeuvrability.
 
2.7 Reverse-tractor tugs
2.7.1 Design
Reverse-tractor tugs, formerly also called pusher tugs, are 
tugs with two azimuth propellers under the stern. They are 
more or less specifically designed for the assisting method 
Figure 2B.32: Some assisting methods with azimuth tractor tugs.
Figure 2B.33: Typical reverse-tractor tug. LOA 25.4m, beam 8.5m, 
BP 45 tons. Images courtesy: Cliff Chow and Jerry Low, HKST
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Types of Harbour Tug 35
the direct method reverse-tractor tugs might be some more 
effective because of the lesser draft. The effectiveness of tugs 
is dealt with in more detail in Chapter 4.
Please see note at paragraph 2.6.2 about thrusters’ controls.
2.8 Japanese tug concept
2.8.1 Design
A specific type of a reverse tractor tug is the Japanese tug 
concept, also adopted by Taiwanese, Chinese and South 
Korean designers. These tugs have some remarkable 
differences with, let us say, the European and American 
ASD-tug types. 
Note 9: The name Japanese tugs will be used here.
The most remarkable differcences are:
•	 The large flared bow hanged with tyres.
•	 The transom stern extending below the waterline.
•	 The sheer. 
•	 Long deckhouse.
•	 The winch located relatively far aft.
•	 The relative high working deck in front of the winch, 
which can be reached by a connection bridge.
•	 The relative small width.
•	 The relatively small draft.
•	 The high free-sailing speed of 15 knots and even more.
Some of these Japanese ASD-tugs have just one winch 
forward, others have a double winch forward, while there are 
also tugs of this type with three winches, two forward and 
one aft.
The tugs have two azimuth thrusters aft, often Niigata (see 
figure 2B.40), and no skeg except for a small skeg in front 
of the propellers. Bilge keels can be found on these tugs (see 
figure 2B.41). Several of these tug types have no funnels, but 
engine exhausts are then taken out via the transom, which 
creates a all around clear view from the wheelhouse.
Main reason for the typical bow is to reduce the wave 
making resistance by a fine and, more recently, a bulbous 
bow. This creates the possibility for a bow deck which can be 
opened widely with a big flare, which is necessary for braking 
bow waves at high speed and so creating a dry working deck. 
One of the other reasons is to keep the tug’s GT below a 
certain level. 
2.7.2 Propeller control, manoeuvring 
capabilities and ship handling
Propeller control with reverse tractor tugs is the same as with 
azimuth tractor tugs. Because of the two azimuth thrusters 
and the forward lying towing point reverse-tractor tugs are 
highly manoeuvrable and safe working tugs. They can turn 
on the spot and move sideways (see figure 2B.44). The astern 
bollard pull of these tugs is generally about 5-10 per cent 
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 assisted 
ship and the propulsion units away from the assisted ship. 
Reverse-tractor tugs do the same but are then heading in the 
reverse direction. That’s why these tugs are called reverse-
tractor tugs.
What has been mentioned about azimuth tractor tugs with 
respect to manoeuvring also applies 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 2B.35. 
They can easily change, when towing over the tug’s bow, to 
a pushing position at the ship’s side or for push-pull while 
berthing. A towing winch is useful to enable the towing line 
always to be set at a suitable length or to pick up any slack in 
the line. When operating at the ship’s 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 very 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 the indirect 
method reverse-tractor tugs are in general somewhat less 
effective in steering compared to a similar VS tug in the same 
situation, although much depends on skeg design, but in 
Figure 2B.34: Thrusters with cpp propellers on SD Stingray.
Photo: Jacco van Niewenhuyzen
Figure 2B.35: Assisting methods with a reverse tractor tug.
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36 Tug Use in Port
A bulbous bow will increase the waterline length as does the 
transom stern, resulting in a higher freesailing speed. This is 
needed because the high speeds needed for escorting of large 
ships and ships with hazardous cargoes in the inland traffic 
routes.
Tugs may have fire-fighting and oil recovery equipment. 
A shallow draft is required for areaswith a small waterdepth. 
2.8.2 Propeller control, manoeuvring 
and ship handling capabilities
Thruster control is often by a Uni-lever system. See 
paragraph 2.10. 
The tugs are very seaworthy and, as said, the typical bow 
design results in hardly any waves or spray coming on the 
foreward working deck. In case waves are coming from aft 
or from abreast, the transom and low aft deck may result in 
waves coming over the aft deck. 
There are mainly two tug types:
•	 Tugs for berthing and unberthing. The assising method 
is usually push-pull and direct towing at the bow.
•	 Tugs for escorting as well. These powerful tugs are able 
to operate even in stormy weather. As mentioned above, 
free-sailing speed of the escort tugs should be high and 
can be even be 15 knots or more. The tugs don’t make 
fast during escorting of ships. 
Figure 2B.36: FLNG Prelude; assisting tugs with long steep towlines.
Photo: Shell International Ltd
Figure 2B.37: Tug Oghi with forward working deck, no handrail.
Photo: Piet Sinke
Figure 2B.38: Tug Sagami with forward working deck. Deck crew with 
safety jacket and helmet. Photo: Piet Sinke
Figure 2B.40: An azimuth thruster from Niigata, Japan. 
Source: Niigata, Japan
Figure 2B.39: Tug’s transom extending under the waterline. 
Photo: Piet Sinke
Types of Harbour Tug 37
tractor tugs, they have two azimuth propellers fitted under 
the stern at roughly the same location, about 0.1 x LWL from 
the stern. 
Skegs of ASD-tugs have developed during the past years, 
consequently ASD-tugs have different skeg types. See for 
instance figures 2B.43a and 2B.43b and paragraph 4.2.2.
The azimuth thrusters of ASD-tugs are made by the same 
manufacturers as the azimuth thrusters of tractor tugs. 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. There is a large interest in this type 
of tug because of their manoeuvrability and multipurpose 
capabilities. Sometimes an azimuth bow thruster is installed 
– as is the case with the 4,000hp ASD-tug Erin McAllister of 
McAllister Towing and Transportation Company, USA. A 
retractable azimuth bow thruster of approximately 1,000hp 
was installed, so increasing the tug’s manoeuvrability, its 
position keeping abilities, maximum bollard pull ahead and 
astern and maximum achievable sideways thrust.
A very few ASD-tugs have only one azimuth thruster at the 
stern – for instance, the converted US Navy YTBs (yard tug 
boats) Kaleen McAllister and Donal G. McAllister. 
There is a large variety in ASD harbour tugs with respect to 
design, size and bollard pull. ASD-tugs may have different 
brand names, such as the RAmparts series of tugs designed 
by Robert Allan Ltd, of Vancouver. Another specific name 
for an ASD-tug is the Azistern tug designed by Offshore Ship 
Designers in the Netherlands. Azistern tugs can be delivered 
with lengths of 24-37m and bollard pulls of 60-120 tons. 
Glosten, Seattle, designed a new ASD-tug type, the HT-67, 
a 20.4m long tug for versatile service on inland and near 
coastal waters. The tug has an aluminium deck house, towing 
winch and stern roller. And Sanmar Shipyards, Turkey, has 
the Yeniçay class tugs – small tugs with a length of 18.7m and 
a bollard pull of 30 tons. 
A new concept is the Schottel SYDRIVE M which can for 
longer sailing distances, connect the port and starboard 
azimuth thruster, allowing both thrusters to be driven 
together by only one of the main engines. This leads to 
reduced operating hours of the main engines, resulting in 
lower maintenance costs, less fuel consumption, and lower 
Escorting is compulsory around Japan for ships with 
hazardous cargoes or a certain length. 
Exept for Tokyo Bay escorting can be carried out by any type 
of ship, provided the ship is equipped with FiFi equipment, 
which means in practice that all escorting is done by 
tugboats.
Escort tugs do not make fast to the ship but stay standby at 
short distance to the ship, unless tug services are needed. 
Indirect towing is not an option for the relative small beam 
tugs. It can even be dangerous. 
The specific design has a number of large advantages:
They can work under the large flares and overhanging sterns 
of particulary large container vessels.
The flared bow has bow has a large radius, which reduces the 
pushing forces on ships’ hulls. 
Due to the typical flared bow a large working deck is 
created enabling the crew to pass the towline to the ship to 
be assisted, or to retrieve the towline, in a handy way. See 
figures 2B.37 and 2B.38.
Due to the aft positioned forward fairlead the tugs can 
operate somewhat better than other tug types with steep 
towlines, which is often the case in dockyards. See figure 
2B.36.
2.9 Azimuth Stern Drive (ASD) tugs
2.9.1 Design
Conventional tugs have certain advantages and so do 
reverse-tractor tugs. ASD-tugs are nearly the same as 
reverse-tractor tugs but are designed in such a way that 
they can operate like a reverse-tractor tug as well as a 
conventional tug, thus combining the advantages of both 
types. ASD-tugs have a towing winch forward and a towing 
winch (can be optional) or towing hook aft. Some ASD-tugs 
have only one winch which is capable to operate over the bow 
as well as over the stern. When operating over the stern the 
towline runs via a trunk through the deckhouse to aft. This, 
for instance, is the case with the TundRA 3000 ice class tugs 
for Svitzer and the ASD 2312 tug Jupiter of Boluda Towage 
Europe. The winches of these tugs are located in an enclosed 
area in the superstructure. 
The aft towing point is at a suitable location for towing on 
a line, namely 0.35-0.4 x LWL from the stern. Like reverse-
Figure 2B.41: Photo clearly showing the bilge keels, bow fenders with 
tyres, and just a small skeg aft can be seen. Source: Tokyo Kisen
Figure 2B.42: Hong Kong tug braking ship’s speed. Source: Alan Loynd
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38 Tug Use in Port
 2.9.2 Propeller control, manoeuvring 
capabilities and ship handling
Propeller control is the same as with azimuth tractor tugs, 
either control of each thrusters separately or by Uni-lever 
or master pilot system. An example of the Uni-lever system 
is shown in paragraph 2.10. See also note about thrusters 
handling at the end of paragraph 2.62.
Th e manoeuvring capabilities of free sailing ASD-tugs and 
reverse-tractor tugs are shown in fi gure 2B.44, over page. 
Th ese tugs can deliver thrust in any direction, though 
maximum stern thrust is some 5 to 10 per cent less than on 
ahead.
Conventional tugs are eff ective as forward tugs towing on 
a line, while reverse-tractor tugs are eff ective aft and are 
also very suitable for push-pull operations. ASD-tugs are 
very eff ective and suitable for all kinds of ship handling, 
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 fi gure 2B.45, 1, opposite page) 
the ASD-tug is very eff ective, although the risk of girting 
exists. Th e risk is minimised when the tug is equipped with a 
reliable quick release system.
As a stern tug on a line an ASD-tug works over the bow 
(situation 1 and 2). Th is is eff ective for speed control and 
course control to both sides. Eff ectiveness when assisting in 
indirect mode (situation 2) is generally somewhat less when 
compared to VS tractor tugs, but ASD-tugs may be somewhat 
more eff ective when direct pulling (situation 1).
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). Th e forward 
ASD-tug should then assist like a reverse-tractor tug 
(situation2) which is very oft en the case. A bow thruster is, 
as for a reverse-tractor tug, useful for bringing and keeping 
emissions. Th e 30m long ASD-tug of Ramparts 3000 design 
with a bollard pull of 65 tons, equipped with this system, 
is ready for operation in the Port of Aarhus around mid 
2021. Please see: https://www.youtube.com/watch?v=b-
Ho4CULVvQ
Figure 2B.43a: ASD3212 – tug Mars, LOA 32.70m, BOA 12.82m, BP ahead 82.5 tons, BP astern 76.1 tons, fi xed pitch propellers. Source: Damen Shipyards
Figure 2B.43b. ASD-tug 2312 with twin skegs.
Length 23m; beam 12m; high freeboard.
BP ahead 70 tons, bp astern 65 tons.
One winch for operations forward and aft. Bridge with safety glass.
Twin skegs: increase course stability when sailing astern, increase 
turning moment, reduce interaction between thrusters and skegs, more 
forward lying point of application of hydrodynamic forces increase 
performance in indirect mode, increase performance when operating 
bow-to-bow. Note: black lines underneath the tug are pipes for engine 
cooling, this instead of box coolers. Source: Damen Shipyards 
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Types of Harbour Tug 39
Figure 2B.44, left: Free sailing manoeuvring capabilities of an ASD-tug and reverse tractor tug.
Figure 2B.45, above: Some assisting methods with an ASD-tug.
Figure 2B.46: ASD-tug 
Smit Seine (LOA 28.67m, 
beam 10.43m; bollard pull 
ahead 60 tons, astern 57 
tons) assisting Maersk 
McKinney Moller in the Port 
of Rotterdam. ASD-tugs are 
mainly operating over the 
bow; Smit Seine is 
operating over the stern.
Photo: Martin Menninga, 
KotugSmit
40 Tug Use in Port
This is in particular the case on tugs in Western Pacific ports 
– Korea, Japan, China and Taiwan. Attention will therefore 
be paid to the Uni-lever system often found on these tugs, 
namely, the Niigata Uni-lever control system. A cross section 
of a Niigata thruster with the most essential parts, such as 
input shaft coupling, rope guard, propeller, nozzle, etc, is 
shown in figure 2B.40. 
Figure 2B.47 shows the various settings of the Uni-
lever control handle and the corresponding thruster 
configurations and tug movements. On the right is the main 
engine speed control handle. 
Figure 2B.48 shows separate controls of the same make, 
which can be found on similar tugs not having a Uni-lever 
system.
the tug’s bow in position at the ship’s side. For this kind of 
operation 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 
360° steerable thrusters.
If an ASD-tug is equipped with an azimuth bow thruster, 
then the manoeuvres discussed can be performed faster and 
more effectively.
2.10 Uni-lever system
Although most often separate controls can be found on tugs 
with azimuth propulsion systems, many tugs are equipped 
with a Uni-lever or master pilot thruster control system. 
Figure 2B.47: Uni-lever system, Niigata. 
Source: Niigata, Japan
Figure 2B.48: Separate controls for each 
thruster: either two GSO Levers for fixed 
pitch propellers or two GSP control for 
controllable pitch propellers.
Source: Niigata, Japan
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Types of Harbour Tug 41
Most Rotortugs have standard winches and fixed pitch 
propellers. Only four have controllable pitch propellers of 
which the latest is Serco’s tug SD Tempest. 
Depending on the requirements, Rotortugs may have also 
have fire-fighting capabilities. 
The azimuth thrusters, engines and winches are 
manufactured by the same equipment manufacturers and 
the Rotortugs are built at the same shipyards at similar rates 
as ASD and tractor tugs. Their distinguishing features relate 
to active steering and braking forces which can be provided 
in both direct and indirect mode over the full 10-0 knots 
speed range, their versatility, and their ability to operate in 
confined spaces such as locks and bridges – as is to certain 
extent also the case with fast tugs and the present compact 
ASD-tugs of 24m length. 
2.11.2 Rotortugs in use
The first generation Rotortugs were RT Innovation, RT 
Pioneer, RT Spirit and RT Magic. These tugs are still 
operational in Bremerhaven and Mozambique today.
Rotortugs can be found in several large European ports, 
and also in ports in Australia (Port Hedland), some USA 
ports, and Africa. Sometimes the tugs are owned by separate 
towing companies or by a joint venture with Kotug. In mid-
2018 there were 44 Rotortugs operational and 12 being built. 
2.11.3 Propeller control and manoeuvring 
capabilities
Propeller control is similar to that of azimuth tractor and 
stern driven tugs, with individual thruster control units 
controlling respectively thruster steering and propeller 
revolutions or pitch (the latter if applicable). Rotortugs with 
fixed pitch propellers are fitted with speed modulating 
slipping clutches.
During fire-fighting duties the portside thruster is fully 
disengaged and the portside engine is used to drive the 
fire-fighting pump. This arrangement has a significant 
station-keeping advantage over other main engine driven 
fire-fighting pump arrangements.
Thanks to the third thruster, and in some cases also to the 
second towing winch, and consequently to the double-
ended control ability, capabilities of Rotortugs are superior 
to those of other tug types. The Rotortug’s manoeuvring 
characteristics enable tug masters to provide fast reaction 
times. The tugs can compensate safely for the interaction 
effects working on the tug and the towing points at both ends 
of the tug create safe working tugs. 
In this section the related tug types as mentioned in table 
2A.1 will be discussed:
•	 The Rotortug.
•	 The Z-Tech tug.
•	 The RSD tug. 
•	 The Carrousel tug.
•	 The DOT tug.
Why the name ‘related tug types’? This is because the 
capabilities of these tugs can to a large extent be compared 
with the basic tug types discussed in Chapter 2, part B. 
2.11 Rotortug 
2.11.1 Design
Rotortugs are triple azimuth thruster driven tugs with the 
thrusters arranged in a triangular configuration with a 
towing – or escort – winch forward and a towing winch 
aft. Basically it is a conventional tractor tug with azimuth 
thrusters, and the skeg replaced by a third azimuth thruster. 
This creates the possibility to design a versatile tug with 
unique capabilities for ship handling and with good active 
steering and braking capabilities.
Since the first Rotortug concept, Robert Allan Ltd has 
exclusively designed Rotortugs of various dimensions and 
designs. The company’s research capabilities mean they 
are able to optimise hull forms and course controllability 
characteristics for sailing ahead, astern and transverse. 
Combined with about 20 years of operating experience 
with the first Rotortugs, the latest generation of Rotortugs 
are called ARTs (Advanced Rotortugs) and provide a 
sophisticated design. 
Main dimensions of three ART tug types :
The ART(W) (Advanced Rotortug Wide body) is a tug 
with a larger beam, such as the ART 85-32W designed 
for operations in Port Hedland. Increasing a tug’s width 
increases stability. The point is, however, that with ASD-
tugs the propulsion units counteract the heeling angle in the 
indirect mode (see Tug Stability. A Practical Guide to Safe 
Operations), while with the Rotortugs the thrusters increase 
the heeling angle (see figure 2C.3). On the other hand if 
the heeling angle becomes too large it can be decreased by 
lowering the thrust. 
Type ART 65-28 ART80-32 ART 85- 32W
Length 27.50m 31.95m 31.5m
Beam 12.50m 12.6m 13.8m
Bollard Pull 65 tons 80 tons 85 tons
Chapter 2 
PART C
Related tug types
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42 Tug Use in Port
Rotortugs can operate in various ways, whether when towing 
on a line or operating at the ship’s side. The tugs can always 
operate bow-first, both as sternand as bow tug., They have 
good tug control capabilities when operating around the 
bow of a ship at speed or in the ship’s propeller wash. This 
feature makes them especially suited for center lead forward 
and center aft operations. Furthermore, the third azimuth 
thruster provides additional redundancy.
 
A Rotortug’s aft towing point is located perpendicular above 
the centre line of the aft thruster, providing easy course 
control of the tug as bow tug. The tug’s large side-stepping 
capability is of great help for repositioning of the tug. When 
operating as stern tug, the tugs can provide relatively high 
steering and braking forces in the direct mode up to 10 knots 
speed, which is also called combination-arrest mode. When 
operating in this mode there is hardly any heel.
Rotortugs can provide active steering and braking forces 
between 6 and 10 knots using both direct thruster forces and 
hydrodynamic lift forces; the latter in the indirect mode. 
Performance in the direct and indirect modes are shown in 
figures 2C.4 and 2C.5, opposite page.
Figure 2C.1: Rotortug ART 80 32. Left: side view, middle: aft view, right: forward view. Source: Rotortug
Figure 2C.2: Forward tug applies the method of rotoring. Photo: Rotortug
The tugs use a specific manoeuvre at low ship’s speed – what 
is called ‘rotoring’. With rotoring, steering forces can be 
applied within the beam of the assisted ship which is of 
great help in confined spaces such as locks and when passing 
bridges (see figure 2C.3). This furthermore creates the 
possibility of operating with fewer tugs during most weather 
conditions. 
In this way the first generation Rotortugs assisted high 
windage car carriers through the locks in Bremerhaven. 
With rotoring with the towline fastened forward (at the end 
with the two thrusters) the tug can generate a force of 70 per 
cent of the tug’s bollard in transverse direction to the ship’s 
centre line with quick response to the pilot’s commands. 
Rotoring can also be applied during mooring operations. In 
doing so the ship can be brought as close as possible to the 
quay or jetty (see figure 2C.2). 
There are some aspects to keep in mind. Three thrusters 
within a relatively short distance of each other may 
cause some loss of effectiveness due to thruster-thruster 
interactions, depending on the thruster settings. Thrusters 
underneath the hull will, as with normal tractor tugs, 
increase the tug’s resistance.
Types of Harbour Tug 43
To benefit from the Rotortug’s capabilities it is highly 
recommended that tug masters follow a specific Rotortug 
training before they start operations. Tug handling with 
three thrusters can be challenging without prior instructions 
and training. 
Kotug has a Rotortug simulator with a 360° arc of view, 
where training is given in handling a Rotortug, as well as ship 
handling with the Rotortug. With this simulator an optimal 
training can be given to new and experienced tug masters. 
Furthermore, Kotug has a 15m long training Rotortug – see 
Chapter 8, Training and Tug Simulation.
Figure 2C.3: Various Rotortug manoeuvres with related thruster settings.
 Figure 2C.4: Comparison performance Rotortug with ASD-tug.
In the 4-8 knots speed range the Rotortug performs better and above 8 
knots the ASD-tug performs better. Note: For an explanation of various 
assist modes, see escort chapter. Source: Kotug
Figure 2C.5: Performance diagrams of Rotortug ART80-32, LOA 31.95m, 
beam 12.60m, BP 80 tons, operating as stern tug in direct (yellow) and 
indirect (red) mode at 8 knots speed.
44 Tug Use in Port
2.11.5 FLNGs and tugs providing in-field 
support
Three Rotortugs ART100-42, called in-field support vessels 
(ISVs), were specifically built for the Prelude FLNG 
(floating liquefied natural gas) project. Another ART100-
46 was built for the ConocoPhilips Bayu Undan Field. The 
ART100-46 tug RT Raven was launched in January 2018. 
The number of scheduled FLNG projects, and consequently 
ISV requirements, is gradually increasing with global energy 
demands.
The ART100-42 tugs have a length of 42m, width 16m, 
maximum draft 7.40m, bollard pull 100 tons and are 
equipped with FiFi1 with water spray. The ART100-46 (LOA 
47m, beam 16m) is fitted with DP2 capacity and has similar 
characteristics; an additional length of 5m provides, with 
a retractable towing fairlead, more deck space aft and gives 
additional support capability. 
ISVs provide a range of services and are generally customised 
to suit each project’s requirements. Their capabilities include 
(but are not limited to): 
•	 Assisting gas carriers coming alongside the FLNG for 
loading and when leaving.
•	 Emergency response. The ART100-42 is able to 
accommodate 85 people in case of an emergency on 
board the FLNG which involves people having to leave 
the vessel.
•	 Condensate hose-handling and maintenance.
•	 Safety stand-by and surveillance.
•	 In-field cargo and personnel transfer.
•	 Dive and ROV support.
Also in wave conditions, gas carriers have to be brought 
alongside. This can, in particular, be a problem when pushing 
at the ship’s side. The tugs should be able to push in such a 
way that the fenders don’t get damaged and the hull pressure 
on the gas carrier does not get too high. The Rotortug ISVs 
with their propulsion units arrangement below the bottom 
should be able to do so up to a certain wave height/period. 
When sea conditions worsen, the tugs can then tow on a line 
and use the method called ‘rotoring’, mentioned above. 
2.11.4 Summarising 
The advantages of the tugs are: 
•	 Excellent manoeuvrability, which includes among other 
things turning on the spot with a high rate of turn, 
astern speed equal to ahead speed and a side-stepping 
speed of approximately six knots. 
•	 Fast positioning and re-positioning and a large variety of 
assist modes with short response times.
•	 A high bollard pull or, alternatively, the same bollard 
pull with less draft, compared to a normal tractor tug 
with two azimuth thrusters. 
•	 High side thrust up to 95 per cent of bollard pull to assist 
vessels through narrow passages, locks and bridges. 
•	 Better reliability because two units bring sufficient 
manoeuvrability and bollard pull for day-to-day ship 
handling work; in case of breakdown of an engine, the 
tug is still operational and repairs can be postponed 
until a suitable time. 
•	 There is hardly any risk of damaging the azimuth 
thrusters on the bulb of an assisted ship as can happen 
with stern drive tugs, due to the thruster location and 
protection.
•	 Safe tug for operations near the bow of a ship having 
speed.
•	 Specific assist modes that can be utilised by a Rotortug 
enable them to operate within the breadth of a ship, 
reducing the number of tugs normally required, and 
bring ships as close as possible to the quay.
•	 Dynamic positioning systems can be installed for 
offshore operations. 
•	 Escort work with high towline forces is possible over the 
stern as well as over the bow of the tug at relatively high 
speeds. 
The Rotortug is a highly manoeuvrable and reliable tug 
and can generate high towline forces. But it is also a rather 
complex tug with its three thrusters and two winches, 
which does increase underwater resistance and may affect 
purchasing and maintenance costs compared to various 
other tug types. 
 Figure 2C.6: RT Evolution in the indirect assisting mode 
at 8 knots speed. Photo: Rotterdam pilot Marijn van Hoorn
 Figure 2C.7: RT Evolution in the direct assisting mode 
at 6 knots speed. Photo: Rotterdam pilot Marijn van Hoorn
Types of Harbour Tug 45
Since the first Z-tech tug based on practical experience, the 
tug has undergone modifications such as with the skeg which 
was first closed and is now open.
By mid-2017 Cheoy Lee Shipyards had built 45 Z-tech 
tugs, from 60-70 tons bollard pull, and 27-30m in length. 
At the same date, there were approximately 70 Z-tech tugs 
worldwide. 
As with theASD-tug, the skeg provides directional stability 
when sailing astern and does not impede ahead steering 
performance and or create too large additional interaction 
forces when coming alongside a ship at speed. 
The Z-tech tugs built for the Panama Canal are specifically 
adapted to the operational needs of the Panama Canal 
Authority. These include:
•	 A wider beam in order to ensure a higher clearance 
angle when working under the flare of ships.
•	 The height of the wheelhouse is lower than on other tugs 
of this class.
•	 Two independent winches, because the tugs use two 
towlines.
•	 A dayboat accommodation arrangement. 
Developments of Z-Tech tugs are continuing. In May 2017, 
USA-based Bay-Houston Towing Co. and Suderman & 
Young Towing Company awarded construction contracts to 
Gulf Island Shipyards for four Z-tech 30-80 class terminal/
escort tugs of 80 tons bollard pull for each company; tugs 
to be designed by Robert Allan Ltd. with sponsons to suit 
the Z-tech hull form. CFD (computational fluid dynamics) 
simulations demonstrate escort capabilities increase by 15 
per cent compared to the original Z-Tech. 
The Prelude is a 488m long floating unit which will be 
stationed 230km off the Australian cost in 240m deep water 
for 25 years and will operate continuously in terms of gas 
processing and the loading of LNG, LPG and condensate 
tankers. For such a project, specific tugs are needed, because 
the tugs have to operate during a long time in remote areas 
where redundancy is of great importance and local wind and 
sea conditions play an important role.
2.12 Z-tech tug 
2.12.1 Design
The Z-tech tug can be found in many ports and is an iniative 
of PSA Marine, Singapore. They consulted with the tug 
masters, crew and operation managers on their operational 
needs and specific requirements. It became clear that some 
had a distinctive preference for tractor-style tugs while 
others preferred the ASD type. The solution was to develop a 
single design that would incorporate the best characteristics 
of both the tractor and ASD type tug. In 2003 the first Z-tech 
tug was born, designed by Robert Allan Ltd. 
The Z-tech tug is especially configured to allow to work 
closely under the extreme flares of large container vessels, 
although the Singapore tugs handle all kinds of vessels, such 
as bunker barges, coastal tankers with very low freeboard, 
car carriers, bulk carriers, tankers and handle oil rigs as well. 
Still leaving sufficient room to install/withdraw the thruster 
units , the wheelhouse is situated well aft, which means 
towards the thrusters, creating a relatively large working 
deck forward and enabling the tug to work close to a ship. 
The working deck is furthermore free of anchor chains, 
because anchors are located at the thruster end of the tug. 
Because of the need to work in either push or pull mode 
under the flare ends of large container ships and car carriers, 
the Z-tech tug also has a flat forward sheer and a wide, 
heavily fendered bow, which is the skeg-end of the the tug. 
The stern of the the tug has a strong vertical sheer to creat 
good sea-keeping capabilities. According to PSA Marine, 
speed and bollard pull astern are at least 95 per cent of that 
going ahead.
Figure 2C.8: Z-tech tug Star Ruby, owned by PSA Marine. 
Photo: Piet Sinke
Figure 2C.9: Z-tech tug 6500 class tug built for service in the Panama 
Canal and approaches. LOA 27.40m, beam 12.20m, BP ahead 65 tons. 
See also double winches forward. Courtesy Cheoy Lee Shipyards 
46 Tug Use in Port
to some extent with a reverse-tractor tug. These tug types 
usually have only one winch and do ship handling only by 
the towing winch and towing staple at the skeg end, which 
is the bow for the Z-tech tug and reverse-tractor tug and the 
stern for the ATT. On an ATT and Z-tech tug the working 
deck is rather flat, and no anchor winch limits the working 
place. 
ATTs and Z-tech tugs sail ‘skeg first’ or ‘thrusters first’ 
depending on the situation. At somewhat longer distances 
and in sea conditions, the Z-tech and the tractor tug sail 
‘thrusters first’ with the higher deck forward. The Z-tech tug 
and several tractor tugs can manoeuvre well with the skeg 
end under the flare or overhanging stern of a ship. 
How Z-tech tugs operate in, for instance the Port of 
Singapore, can be seen in figure 2C.10 which shows how the 
tugs approach a ship and how they make fast.
2.13 RSD tug
2.13.1 Design and manoeuvrability
The RSD (Reverse Stern Drive) tug is a new tug type designed 
and built by Damen Shipyards. It is a new concept which 
combines the advantages of both ASD-tugs and ATD-tugs, 
as was the orginal idea for the design of the Z-tech tug. The 
forward end of the RSD tug is the thruster end. 
The skeg generates extra hydrodynamic forces on the tug 
when operating in the indirect mode and shifts the centre 
of hydrodynamic forces more in the direction of the towing 
point. When the tug is operating in the indirect mode the 
effect of a skeg on the steering forces that can be delivered is 
largest when the towing point is, horizontally seen, as close 
as possible to the location of the centre of hydrodynamic 
pressure on the tug’s hull. On tugs such as the Z-tech tug 
and the ASD-tug, due to their design and skeg location with 
respect to this an ideal situation is difficult to realise. This is 
further addressed in the Escort chapter.
Furthermore, the distance between this hydrodynamic 
centre of pressure centre and location of the propulsion units 
should be as large as possible. Of course, the vertical distance 
between the centre of pressure and the towing point should 
be as small as possible in order to reduce heeling angles. 
2.12.2 Propeller control and manoeuvring
All Z-Tech tugs built by Cheoy Lee Shipyards Ltd – and as 
far as is known by other yards as well – are fitted with fixed 
pitch azimuth thruster without speed modulating clutches, 
although Z-tech tugs built for the Panama Canal Authority 
have such a facility, called an MCD (marine control 
device). The thrusters have separate controls, one on each 
port and starboard wheelhouse console.
 
Seen from an operational point of view, the Z-tech tug can 
best be compared with an azimuth tractor tug (ATT) or 
Figure 2C.10: 
General Z-tech 
manoeuvres for 
approaching 
a ship and 
passing a 
towline.
Types of Harbour Tug 47
As indicated above, there are similarities with the Z-tech tug. 
Th e tug manoeuvres shown and mentioned for the Z-tech 
tug can be carried out in the same safe way by an RSD tug. 
2.14 Carrousel tug
Th e basic principle of a carrousel tug is a radial system. New 
with the system as applied on the carrousel tug is that it is 
not a half-circle, or less, but a full circle and has basically a 
diameter equal to the tug’s beam.
Th e radial system itself is not new. It has been applied for 
decades on several harbour tugs and in former times on tugs 
on the River Rhine. With a radial system, a tug’s heel due 
to a transverse towline force is limited. Performance and 
safety of several conventional tugs has thus been increased 
signifi cantly. Th e carrousel is initially situated above the 
lateral centre of pressure for a crosswise water fl ow. Th e 
advantages of the carrousel are:
•	 Th e tug can safely cope with large towline forces 
generated by the hydrodynamic forces working on the 
tug hull, while heeling angles are smaller than without 
such a system. Capsizing due to high athwart ships 
towline forces is not possible.
•	 It enables the tug to turn freely, in no way restricted by 
the towline coming in contact with the superstructure.
•	 In addition, it off ers the possibility to use highly effi cient 
and robust conventional shaft propellers and turn the 
whole hull including propulsion in line with the towline 
force instead of turning only the thrusters.
Th e fi rst aspect is related to speed. Th e higher the speed the 
higher the forces that can safely be generatedin the towline 
and applied to the ship to be assisted. Also, high braking 
forces can be achieved because the system enables a stern tug 
to operate safely broadside behind the ship.
Th e second aspect is not related to speed. It greatly enlarges 
the capabilities of particularly conventional tugs and combi-
An ASD-tug when sailing astern, for instance in the case of 
bow-to-bow operations, has the disadvantage that the stern is 
not designed as ‘bow’. Th e same applies when a tractor tug is 
operating as a stern tug on a line.
Bow and stern of the RSD are about of equal height. Th is 
enables the tug to operate safely as a bow tug. It then operates 
like a real tractor tug and also like a reverse tractor tug, 
which is an ASD-tug operating over the bow. Th e latter has 
the disadvantage of the low stern facing the incoming water 
fl ow and incoming waves.
Bow and stern of equal height also enables the tug to operate 
safely as stern tug on a line. As such, it will operate as a 
reverse tractor tug (ASD-tug operating over the bow) with 
the skeg forward and as a real tractor tug. Th is tug (ASD-tug 
operating over the bow or tractor tug) has the diasadvantage 
of having the low stern in the direction of movement, so 
facing the incoming water fl ow and waves with the low stern. 
It could be said that the RSD tug has two bows.
Th e tug has furthermore a twin fi n, as has been discussed 
with the azimuth tractor tug. It creates a highly 
manoeuvrable compact ship-handling tug with perfect 
steering behaviour both sailing ahead and astern. Th e tug 
has also a very good sea-keeping behaviour.
Tractor tugs are safe when operating near the bow of a ship 
at speed. Th is applies to the RSD-tug as well, because when 
making fast at the bow it will sail thrusters fi rst, like a tractor 
tug, and in doing so it can overcome the interaction forces.
It would be good to consider this tug type as a safe 
replacement for an ASD working over the bow. In addition, 
when the topmast is taken down, the tug is suitable to 
work under the overhanging fl are and stern of large ships, 
especially container ships. 
Th e fi rst RSD-tug is the RSD tug 2513, with a LOA of 24.75m, 
a beam of 13.13m, power 4,480bkW, bollard pull ahead 70 
tons, astern 67 tons. New designs are being developed.
Figure 2C.11: The RSD (Reverse Stern Drive) tug 2513. 
Source: Damen Shipyards
Figure 2C.12: Complete view of the RSD tug 2513. 
Source: Damen Shipyards
48 Tug Use in Port
in ship’s speed and a turning moment, which should be 
avoided.
Carrousel tug designs, accurate model studies for the 
optimum location of the carrousel in relation to the 
locations of centre of pressure at different angles of inflow, 
focusing on such aspects as the overall behaviour and 
optimum performance of the carrousel tug when towing, 
while appropriate reserve buoyancy, freeboard and hull 
shape, and in particular safety of operations and safe limits, 
should be studied as well. Further aspects to be considered 
are workable heel angles, safe abort manoeuvres and 
performance in wave conditions.
While high steering forces can be generated, attention is also 
needed to determine whether high and controllable braking 
forces can be delivered without giving the ship a rate of turn 
if the latter is not wanted.
tugs. It creates the possibility to turn the tug freely with 
respect to the direction of the towline, for instance enabling 
a stern tug to apply steering assistance to starboard as well as 
to port at a ship having headway. The lack of this capability 
is a large disadvantage of conventional tugs. If necessary for 
some reason, the tug can turn 180° with the towline attached.
Advantages with respect to stability will be dealt with in 
paragraph 4.2.3. 
Development of first carrousel tug Multratug 12 
In 2000 plans started to investigate the carrousel principle 
on a real-size vessel. As test tug, an existing conventional 
combi-tug was selected for conversion. In 2001 various 
model test were performed. 
In 2002 Multratug 12 was converted into the first carrousel 
tug by removing part of the accommodation, by adding a 
large rectangular deck box as support for the carrousel and 
additional buoyancy. 
Since 2002 the tug has been in operation and demonstrating 
the remarkable dynamic performance, however, limited by 
the small installed engine power and the tug age.
Model tests
As mentioned before, model tests have been carried out with 
a model of the Dutch combi-tug Multratug 12 (see figure 
2C.13). Right below the carrousel two vertical skegs were 
fitted, representing full scale skegs with a length of 6m and a 
depth of 0.4m. Each skeg was located at a quarter of the tug’s 
width from the side.
High towline forces were achieved during model testing at 
Delft University of Technology, as shown in figure 2C.14.
Full scale tests
Full scale tests have been carried out with the tug Multratug 
12 fitted with a carrousel and with skegs as used for the 
model tests. The tests confirmed the working of the system 
as well as the forces measured during the model tests. Even 
higher towline forces could be achieved due to a more stable 
position of the full scale tug. The photo (figure 2C.15, over 
page) shows the carrousel tug applying steering forces.
Aspects that require attention or further study
As the carrousel is now in a phase of further employment, 
some aspects require attention, although a number of the 
aspects to be mentioned have already been addressed in 
newer designs. Aspects requiring attention are: 
•	 The lead of the towline for all possible ships to 
be assisted, assist manoeuvres, conditions and 
circumstances, needs to be considered.
•	 Safety of deck operations, including efficient and safe 
towline handling under all working conditions and with 
minimum crew, requires attention.
•	 The possibility to install an appropriate towing winch, 
strong enough to withstand the high towline forces that 
can be generated, needs further study. A towing winch 
is of particular importance when the carrousel tug will 
also be used for escort operations.
•	 The carrousel requires a constant tension in the towline, 
because for most tug manoeuvres even a small constant 
tension in the towline may create an unwanted increase 
Figure 2C.13: Combi-tug Multratug 12 (LOA 28.5m, beam 6.6m, BP 21 
tons, retractable bow thruster 450hp).
Figure 2C.14: Towing forces based on model tests with a model 
of the 21 tons BP tug shown in figure 2C.13.
Types of Harbour Tug 49
The DOT and the Allrounder tug discussed in the next 
paragraphs have a comparable system.
As it is a very safe system that increases the capabilities of 
particularly conventional tugs, but also of other tug types 
such as the ones with one propulsion unit forward and one 
aft, more applications may be seen in the future.
2.15 DOT tug
The DOT (Dynamic Oval Towing)-tug is a conventional 
tug type with a movable towing point which is realised by 
a heavy rail around the superstructure along which the 
towing hook (or towing point) can travel. The system allows 
the tug to turn in any direction with the towline fastened; 
consequently the tug can pull either over the bow or over the 
stern without releasing the towline. Furthermore, the system 
prevents capsizing of the tug when the towline under high 
tension comes at right angles to the tug. Basically the DOT 
system is the same as the movable towing point system on a 
carrousel tug. As can be seen with Multratug 12 the original 
carrousel system is located at a lower height above the deck, 
which results in smaller heeling angles. 
A small number of DOT tugs have been built, mainly smaller 
conventional tugs.
2.16 The All-Rounder AR360T
A recently specifically designed conventional tug type with 
a movable circular towing point like the carrousel is the 
All-Rounder. 
The All-Rounder 360 Tug (AR360T) is a high-powered twin 
propeller shaft compact tug equipped with an all-around 
radial winch support anda new hull shape to enhance 
hydrodynamic forces. Hereby effective towing and braking 
performance can be achieved at bow and stern of ships from 
zero speed to making headway without risk of capsizing, 
apart from the pushing and pulling capabilities at the ship’s 
side. Aiming at port operations, escort tasks and ship to ship 
assistances.
Carrousel tug applications
Basically the carrousel tug makes effective use of the 
hydrodynamic forces working on a tug hull, which means 
that with increasing speed towline forces increase. When 
speed decreases the effectiveness of the carrousel tug 
decreases. This is in contrast with the requirements for 
tug assistance in many ports. In harbour operations tug 
assistance is generally needed at speeds below approximately 
six knots. Full tug power is then often needed for steering, 
braking and controlling a ship’s position. This means that for 
a carrousel tug, bollard pull is an important factor as well.
The carrousel tug is not specifically designed for tug 
operations at the ship’s side as applied in many ports 
around the world. For this operating mode, tugs with 
omni-directional propulsion systems and a towing point 
at the tug’s end are most suitable. However, the carrousel 
system can improve the capabilities and safety of operations 
of harbour tugs to a large extent and particularly of the 
conventional type of harbour tugs and of combi-tugs. As 
a forward or aft tug, among others, high steering forces 
can safely be handled, while the tug is not restricted by the 
direction of the towline.
Basically, the carrousel tug design can most effectively be 
applied in situations where tug assistance is required during 
a transit, such as in channels, fairways and port approaches, 
more or less as an escort tug. However, if sufficient bollard 
pull, it can be used during mooring/unmooring operations 
as well. The carrousel tug Multratug 12 showed remarkable 
dynamic forces, but was limited in bollard pull due to the 
small installed power. However, in a new carrousel tug the 
required power can be installed to achieve the requested 
bollard pull. This can be done with either conventional 
highly efficient propeller shaft lines or with thrusters to 
enhance manoeuvrability. For tug operations in confined 
spaces a short and wide-bodied twin propeller shaft tug can 
be used or, if considered necessary, with twin thrusters.
The carrousel system is also applied on the Carrousel RAVE 
Tug (CRT) which is dealt with in paragraph 2.20. The CRT 
has one Voith propulsion unit forward and one aft. 
Figure 2C.15: Combi-tug Multratug 12 modified as carrousel tug 
during full scale trials
Figure 2C.16: DOT tug Ugie Runner. 
Source: Macduff Ship Design, UK
50 Tug Use in Port
Operational safety is integrated in the tug design. On the 
outside of the accommodation a fixed walkway with hand 
railing is mounted. Hereby the crew can observe the deck 
situation before entering the deck zone with rotating winch 
and towline. Before the crew enters the deck area, e.g. 
during making the towline (dis)connection, the winch will 
be fixed in position by a parking brake. Simultaneously the 
drum brakes are opened to prevent significant loads on 
the towline. Before releasing this park brake all crew must 
leave the deck area. Both doors to the accommodation and 
wheelhouse are positioned at centerline to prevent early 
submersion and are clearly visible from the wheelhouse.
Further the tug is also equipped with SaferVents, (See book 
‘Tug Stability’ p71) ensuring that even at large heeling angles 
the engine room will not be flooded.
Operational training of the crew for this new tug type is an 
important factor and for instance simulator training centre 
in ALAM (MY) will provide the training including a special 
tug model using Wartsila Transas software. 
Note:
For more information about the effect of a carrousel system 
on stability, please see the book `Tug Stability’ (p47-50).
Summarising: This tug with conventional twin shaft 
propulsion, with towards the bow upward sloping central 
deck, the seaworthy hull shape, and in particular with the 
radial winch support, has become a tug with extensive ship 
assist capabilities in sheltered as well in unsheltered waters. 
In 2021, the first tug was ordered in Malaysia by owner 
Harbour 360 Sdn Bhd and built afterwards at Labuan 
Shipyard & Engineering Sdn Bhd.
 
The tug features compact outer dimensions of 24 m length 
and 10 m width and offers a high bollard pull up to 65 ton by 
efficient and robust large sized shaft driven twin fixed pitch 
propellers in nozzles. The propellers are positioned wide 
apart to offer large steering lever in both ahead and astern 
conditions. Steering is further enhanced by independent 
high lift (flap) rudders and a strong bow thruster. 
The tug has a new specific hull design shape with lateral 
area centered at half length and upward sloping bow and 
stern lateral area in order to facilitate easy turning even 
under various flow angles and speeds. Below the tug, two 
large skegs are mounted to enhance the hydrodynamic 
towing performance and in addition offering protection of 
aft propulsion against ground contact, integrated engine 
foundation and finally docking support.
The towing installation is mounted on a slightly, towards 
the bow, upward sloping central deck enabling significant 
accommodation area under the bow deck, easy access from 
aft deck and large bow buoyancy (enabling working in 
unsheltered waters). For this tug also a new winch type has 
been developed with an instant automatic overload release to 
prevent high peak loads and prevent towline breaking. The 
winch moves around the accommodation on two rail tracks 
and distributes the towline load over two carriages. On both 
sides of the winch a hydraulic power pack is fitted for the 
winch motor and the brakes, operated by remote control. 
The whole deck and rail structure has been analyzed in detail 
with Finite Element Calculations (FEM) resulting in an 
efficient and strong steel structure for easy manufacturing, 
building and maintenance.
Figure 2C.17. The All-Rounder AR360T tug
Types of Harbour Tug 51
In both cases, and also when there is a failure with the 
thruster at the end where the towline is not fastened, the 
other thruster is still available and can be used to try to avoid 
an accident, although when operating as forward tug on a 
line, the situation might become risky. If the tug is equipped 
with a carrousel system, however, these aspects don’t play a 
role. 
The usefulness and benefits of such new designs have to be 
proven during daily practice, not only regarding the design, 
but also with respect to training needs. After the initial 
design, sometimes modifications take place – as has been 
the case with the SDMs andGiano tug. For both tug types, 
modifications of the original skegs or number of skegs were 
needed. 
Although the tugs that are already operational, such as the 
SDMs, EDDYs and Giano tug, perform satisfactorily, some 
aspects require attention:
•	 In the first place, as mentioned above, the consequences 
should be well understood of the fact that the large 
power a tractor tug/reverse tractor has at the forward 
end of the tug is much less with these FAST tugs.
•	 When free-sailing close to a ship, it should be well-
known how to compensate for the interaction effects 
working on the tug, especially near the bow of a ship 
having speed. If the wrong thruster is used, it may 
introduce the same risks as when a conventional tug is 
trying to enlarge the distance between tug and ship’s 
hull. 
•	 Seaworthiness is another important point requiring 
attention.
•	 Skeg-thruster interaction is always an item that requires 
attention. 
•	 Location of thruster handles: thruster handles should be 
positioned such that no mistakes can be made between 
which handle is for the forward thruster and which one 
for aft. 
•	 Quickly stopping a free-sailing tug or makinga crash-
stop: how this should be done in the most effective way 
while not diverting from the track or heading – which 
can be necessary in busy shipping areas – should be 
known for each particular tug type. One tug may set 
the CPP propellers on astern with the loss of about 55 
per cent propeller thrust (which can be less with specific 
propeller designs), or in the case of diesel-electric 
propulsion the fixed pitch propeller revolutions may be 
reversed, also with a loss astern thrust, or another tug 
may turn the thrusters so creating on unstable tug, or 
a tug has the same thrust ahead an astern without the 
need/possibility to turn the propulsion units. 
•	 Fail safe. When a forward tug is towing on a line to assist 
a ship having speed, will the tug be in danger in case of 
2.17 Introduction 
This section deals with tugs with Forward and Aft a Single 
Thruster or a single Voith propulsion unit (so-called FAST-
tugs), of the following types:
•	 SDM (Ship Docking Modules) / ATT (Asymmetric 
Tractor Tugs)( see paragraph 2.18).
•	 EDDY (Efficient Double-ended Dynamic) tugs ( see 
paragraph 2.19).
•	 CRT (Carrousel RAVE Tug)( see paragraph 2.20).
•	 Giano tug ( see paragraph 2.21.)
FAST-tugs differ from all foregoing tug types, although 
they could be compared to some extent with a combi-tug or 
maybe even more with a Rotortug. With the same power fore 
and aft, the handling of these tugs differs totally from the 
usual tug types, as does ship handling with these tug types. 
Other ways of ship handling are possible, but other risks are 
introduced when not being fully trained in handling these 
tugs.
There are some important facts to keep in mind with respect 
to this type of tug. This is because because at each end of 
the tug is only maximum 50 per cent of the total power 
available, while tractor tugs and reverse tractor tugs have at 
their forward end 100 per cent power available. Where tugs 
are under influence of interaction effects, as can be the case 
near the bow of a ship at speed, the tractor tugs and reverse 
tractor tugs with their 100 per cent power at the forward part 
of the tug, can much better compensate for the suction forces 
and turning moments working on the tug by steering away 
from the ship’s hull or by controlling the heading. 
If FAST tugs are designed as hybrids, the power can even be 
much less when sailing in economical mode with reduced 
power, eg diesel-electric. This can become particularly 
problematic when operating close to the bow of a ship 
at speed, for instance when preparing to make a towline 
connection, due to the interaction effects. With a tractor or 
reverse tractor tug redundancy will also be larger, because 
when one thruster fails these tugs can often still operate to 
some extent. 
There are, however, some other aspects. When towing on a 
line at an angle at a ship having speed, the less power at the 
tug’s end where the towline is NOT fastened has its effect on 
the operation. The tug will not be able to veer away from the 
ship as far as with a tractor tug or reverse tractor tug. The 
hydrodynamic force will become too large and the tug may 
swing around, unless other measures are taken to keep the 
tug operational, for instance by fast slacking of the towline. 
When operating aft in the direct mode, which will be 
explained later in the book, a similar effect may play a role. 
Chapter 2 
PART D
FAST tug types
Pgua_Supplay
Destacar
52 Tug Use in Port
Towing of Tampa acquired two SDMs Mark II in the years 
2000-2002, named Florida and Endeavor. Tug Florida has 
been renamed as Mobile Point and now belongs to Seabulk 
Towing, Florida. 
Main particulars
The main particulars of the SDM Mark I and Mark II are as 
follows: 
Mark I Mark II
Length overall 27,4 m 27,4 m
Maximum beam 15,2 m 15,2 m
Draft 4,90 m 5,0 m
Engine power 4.000 hp 4200 hp
Bollard pull 55tons 60 tons
The main differences of the SDM design compared to other 
harbour tugs are as follows: 
•	 Shallow flat bottom and an elliptical form.
•	 Very wide beam compared to the more or less normal 
length of a harbour tug.
•	 Two azimuth thrusters and two skegs in the following 
configuration:
 – One azimuth thruster is located at approximately 
a quarter of the tug’s length from forward and at 
some distance to starboard from the tug’s centre 
line,
 – and the other thruster is located at approximately 
a quarter of the tug’s length from aft and at some 
distance to port of the tug’s centre line.
 – In the centre line at each end of the tug one skeg is 
placed, in total two skegs.
The original idea of the SDM was to have a highly 
manoeuvrable harbour tug complying with the following 
requirements typical for harbour towage:
•	 Maximum bollard pull in all directions.
•	 Getting in position quickly.
•	 High side-stepping performance.
•	 The possibility to work under the flare of large ships.
•	 Ability to work in confined areas and in semi-sheltered 
waters. 
a breakdown of one of the thrusters, in particular when 
that thruster fails at the geatest distance from the towing 
point? How to handle such cases should be known. When 
the towing point is changed for this reason, as is the case 
on one tug type, one should be careful not to introduce 
other risks. In the past, the safe location of the towing 
point has been the subject of an extensive study with 
Voith tugs 
•	 To what extent the effect of the thrusters is reduced 
when for instance (sideways) pushing at a ship with 
speed or with no speed. The thrusters are then rather 
close to the vertical ship’s hull. 
•	 Tug handling is not just handling a free-sailing tug, 
but knowing how to use the tug in the most safe and 
effective way to handle all kind of vessels in a safe and 
efficient manner, including the risks involved. 
•	 Proper training, because handling of these tugs differs 
considerable from other tug types such as ASD or Voith 
tugs. 
•	 Not only tug masters have to be trained but pilots should 
also learn the capabilities and possible limitations of the 
new tug types. 
•	 Performce in the indirect mode of the FAST tugs 
could be compared with performance of, for instance, 
Rotortugs to get an idea of the escort capabilities.
•	 Capabilities of the EDDY,Giano tug and SDM could be 
increased by the use of a carrousel system as with the 
Carrousel RAVE Tug. 
 
Up until now only a few tugs have been built of these FAST 
type tugs. The various types are addressed below. 
2.18 SDM (Ship Docking Modules) 
2.18.1 Design
This type of harbour tug has been developed by Hvide 
Marine (USA), now Seabulk Towing in Tampa (USA), and 
Elliot Bay Design Group. 
Seabulk Towing has three SDMs Mark I and one SDM Mark 
II. SDM Mark II is a follow-up of the original SDM design 
with the same dimensions but somewhat higher bollard pull. 
The first SDM was the New River, delivered in 1997, followed 
by St ]ohns in 1998, Escambia in 1999 and the SDM Mark 
II Suwannee River in 2000. Towing company Marine 
Figure 2D.1: Forward view SDM Figure 2D.2: Side profile SDM
Types of Harbour Tug 53
release towing hook of 50 tons SWL is also fitted for 
secondary use. The towing winch is placed beneath the 
bridge deck, but can be observed by the tug master through a 
window in the deck. The towline, 150m Dyneema with a 50m 
long pendant of conventional fibre, can be deployed through 
fairleads, one amidships and one right aft. The whole tug 
concept is based on having a towing tug capable to produce a 
high and equal bollard pull in any direction.
Safety aspects
•	 High stability. 
•	 Large working deck aft increases safety of tug crew 
during operations.
•	 As indicate above, the deckhouse construction is well 
within the bulwarks, which enables the tug to operate 
under the flare and/or overhanging stern of ships. Also 
the distance of the superstructure to the towing point 
avoids the possibility of collision with the vessel be 
towed. 
2.18.2 ManningThe crew accommodation on the USA SDMs, and in 
particular on the Mark I, is minimal. They were initially built 
as two-man day boats, but that has never been the case. So, 
theoretically, two men can operate the tugs, although on the 
Spanish SDMs there is requirement from Flag Authorities of 
a crew of three persons in port and six persons outside.
2.18.3 Manoeuvring performance
Free sailing speed is approximately 12.5 knots and a side-
stepping speed of 6.5 knots can be achieved.
Due to the wide beam, stability of the tugs is large and 
consequently the tugs can operate safely.
The sides of the tugs are flared in order to provide also larger 
righting moments when heeling and to prevent contact 
between the tug’s underwater part and the ship’s hull.
The two skegs improve course stability and aid in dry-
docking. There is a hole in the skegs to reduce the difference 
in pressure between both sides of the skegs caused by the 
accelerated water flow into the forward nozzle and exiting 
from the aft nozzle. Without these holes the tug captains had 
to correct the tug’s track by steering the aft thruster 5-10° to 
starboard. 
Loss of effectiveness will be the case when one, or both 
thrusters, is operating close to the ship’s hull, which will often 
be the case in the USA, where the tugs generally operate in 
such a way. This loss of effectiveness may play a role when the 
tug is pushing due to the obstructed water flow towards the 
SDMs perform indeed well in confined areas as tight slips 
and dry-docks. 
In 2004 Remolques y Servicios Maritimos S.L. (Reyser), now 
P&O Reyser, of Barcelona, Spain, was licensed to construct 
five SDMs to be built in Spain. In Spain these tug types were 
also named ATT, Asymmetric Tractor Tugs. The five Spanish 
tugs are of two classes – see table at foot of page.
All the five units have the same dimensions as the SDMs 
Mark II.
The Spanish version is almost identical to the USA Mk II 
design with almost identical dimensions, but have a larger 
bollard pull and larger crew accommodation. The tugs built 
in 2005 have Caterpillar 3516B main engines and the tugs 
built in 2008 have MTU main engines . The Caterpillar 
engines produce 2 x 2,536 bhp at 1,600 rpm, giving a bollard 
peak pull of 74.5 tons (65 tons average). Power from each 
engine is transmitted to Rolls-Royce US 205 FP propellers in 
fully steerable propulsion units via Twin Disc slipping clutch 
units. The Twin Disc speed modulating clutches between 
each engine and thruster are of heavy duty type, allowing 
for precise propeller speed, even down to zero, irrespective 
of engine speed. This is of great benefit for manoeuvring but 
also when operating the FiFi1 system. The tugs with MTU 
engines have also Twin Disc clutches but Schottel propeller.
Towing winch 
The towing winch installed on the Spanish tugs has a 
maximum brake holding capacity of 135 tons and a quick 
 Figure 2D3: Large working deck aft of SDM Escambia with winch and 
two fairleads. See also the ‘soft loop’ fenders made of recycled tyres.
Photo: Seabulk Towing & Seacor Island Lines
Class Name
Zamakona 
Hull No
Year Built Operating Port
Main Engines
I Salvador Dalí 611 2005 Barcelona 2 x CAT 3516-1,840 kW @ 
1,600 rpm. BP 75 tons.I Ramón Casas 612 2005 Barcelona
II Eliseo Vázquez 651 2008 Ferrol 2 x MTU 16v4000M61
2,000 kW @ 1800 rpm.
BP 78 tons.
II Clara G 652 2008 Santander
II Willy T 653 2008 Barcelona
54 Tug Use in Port
View from the wheelhouse
Although the ATTs have an excellent view from the 
wheelhouse, a negative point is that the large funnels limit 
the all-round view of the tug master and especially in a more 
or less athwartships direction, which can create problems 
when coming alongside a ship or berth. 
This is a problem often seen in other tug designs as well. 
In the new ATT version, project ATT2020, this problem 
is solved.
Water on deck
One of the other disadvantages of the current design is the 
amount of overcoming water when sailing in waves, as the 
height at the bow is low, around 3m above the waterline. On 
the other hand, the superstructure protects the aft deck very 
well, keeping it completely dry. This disadvantage is also 
solved in the ATT2020 version.
propellers and in particular when pulling at close distance to 
the ship’s hull (see figure 2D.4). 
The original SDMs are mainly harbour tugs, which is 
included in the name, and operate successfully inside the 
ports of which prevailing conditions and circumstances will 
have played a role in the design. 
A tank test carried out by MARIN in the Netherlands in 
2008 showed that the ATTs are also capable of working 
outside the port in wave conditions and are able to carry 
out escort duties. Simulation studies were also carried out 
by Siport XXI in Madrid in 2010 to demonstrate the escort 
capabilities. 
For some specific ship assist manoeuvres, see paragraph 
2.18.6. 
2.18.4 Some disadvantages of the present design
As with all the tugs, also the SDM/ATT has some dis-
advantages, although it should be kept in mind that the tug 
was designed as a pure harbour tug. Some disadvantages are:
 Figure 2D.4: SDM pushing at ship’s side. 
Photo: Rafel Cabal Alvarez, Barcelona pilot
 Figure 2D.5: SDM as forward tug with towline through after fairlead.
Photo: Rafel Cabal Alvarez, Barcelona pilot
Figure 2D.6: SDM at dry dock. The two thrusters, skeg and hole in the 
skeg can be clearly seen. Photo: Seabulk Towing & Seacor Island Lines
Types of Harbour Tug 55
2.18.6 Ship assist manoeuvres
As explained before, the SDM/ATT has such dimensions 
and thruster-skeg confi guration because of the specifi c 
requirements of harbours with narrow and confi ned spaces, 
which is the case for example in Barcelona, where the ATT 
has been proven a good design. Th e pictures over the page 
show some specifi c assist manoeuvres with an SDM and 
ATT.
 
Because of the high side forces that can be applied, the 
tugs are very suitable to work in narrow areas, although 
when operating at the ship’s side the large beam can be a 
disadvantage when passing bridges, in locks and dry docks, 
where the available width is mostly at a minimum. Th e tug 
could then tow on a line, using either the centre staple or aft 
staple. Th e latter enables the tug to apply sideways forces to 
the ship within a smaller width, or working with a double 
towline as shown in fi gure 2D.10. 
More general assisting methods with SDMs 
In the USA the tugs are usually operating at the ship’s side 
(see fi gure 2D.11a; B,C,D) while in Spain tugs are usually 
made fast by the centre lead forward and aft (see fi gure 
2D.11a; A), although other assisting methods are used as well 
(see fi gure 2D.11b).
2.18.5 New ATT design
Th e Spanish owner, P&O Reyser, designed new ATTs without 
the present drawbacks. Th e fi rst tug Azabra of 75 tbp became 
operational at the end of 2020. Th e new ATT version is called 
ATT2020. Th e tugs would have following modifi cations:
•	 Foredeck will be made higher to improve the capability 
to operate outside the port.
•	 An additional deck below the bridge so increasing height 
and visibility.
•	 Stronger fenders of cylindrical shape all around the 
waterline. 
•	 New low emission engines.
•	 Oil recovery equipment.
•	 Improved accommodation with individual bathroom in 
each cabin.
Figure 2D.8: Water on foreship of Clara G sailing in waves.
Photo: Oscar Martinez Lezcano
Figure 2D.9: ATT 2020.
Figure 2D.7: Simulation 
of ATT’s escort 
performance.
Photo: Simulation Centre 
Siport21, Madrid
Figure 2D.10: ATT working in narrow space with double towline.
Photo: Oscar Martinez Lezcano (P&O Reyser Santander)
56 Tug Use in Port
Figures 2D.12 and 2D.13 show various common methods 
which can be applied by ATTs. The direct towing mode is the 
usually method in Spain. The figures also show the towing 
points used and the thruster settings.
 
Note: In figure 2D.13-A the aft towing point is used. The 
towline force creates with the hydrodynamicforce working on 
the tug a large turning moment. This limits the aft thruster in 
applying force in the towline, because it would then create an 
extra turning moment. If the indirect towing mode would be 
utilised more frequently it would then be recommendable to 
investigate if the aft towing point could not be placed above 
the aft thruster. Much higher towing line forces could then be 
achieved in the indirect towing mode.
In figure 2D.12, positions B and C, the aft towing point is also 
used. Using the midships towing point could probably create 
higher towline forces at a ship having speed, due to the effect of 
the hydrodynamic forces working on the tug.
2.19 EDDY
2.19.1 Design 
Typical for all EDDY (Efficient Double-ended Dynamic) 
tugs is the in-line thruster arrangement, with one thruster at 
each end on the centreline of the tug, and the towing point in 
between the thrusters. The designer claims this layout offers 
superior effectiveness and competitiveness for ship-assist 
and escort tugs in the range of 22-40m. The current EDDY 
tug portfolio ranges from 24-37m and 45-100 tons bollard 
pull. 
Other important characteristics:
•	 The standard propulsion system is a diesel-electric/
diesel direct hybrid propulsion system, with batteries as 
option.
Figure 2D.11a: Basic assisting modes in Spain (A) 
and the USA (B, C, D).
Figure 2D.11b: Various SDM/ATT tug positions as used in Spain.
Figure 2D.12: ATT made fast forward centre lead.
Figure 2D.13: A: indirect towing mode; B: direct towing mode.
Types of Harbour Tug 57
and easy tug handling. A subdivision of the hull in 
five compartments which are closed during operations 
(certified unmanned engine room) and double outside 
doors for the accommodation makes it virtually 
unsinkable. 
•	 From a builder’s viewpoint, EDDY tugs are developed 
for simple construction and require significantly less 
steel and only a minimum of equipment. Construction 
of these tugs is thus not only economical but also fast.
•	 The intrinsic simplicity of the concept and forward 
thinking during the development phase make EDDY 
tugs fit for the installation of LNG or CNG tanks, 
exhaust-gas after-treatment systems, all-electric 
propulsion systems (including high-power podded 
drives and rim-driven thrusters) and further automation 
of propulsion control systems, such as a single-joystick 
control system . 
2.19.2 Steering and thruster control
Standard bridge equipment is two separate thruster controls. 
Joystick, or master-pilot system, is an option.
Stopping the tug when having speed ahead can be done by 
turning the thrusters, or faster by reversing revolutions of the 
electromotor and consequently propeller revolution. Turning 
the thrusters at full speed creates some temporary course 
instability in tug’s behaviour. 
•	 The 30m EDDY, for instance, when sailing in the hybrid 
transit mode uses one 500kW generator for speeds up to 
9 knots and two for speeds of up to 11 knots. For higher 
transit speeds of up to 13.4 knots just one main diesel 
engine can be used. Both main engines will only be 
required during ship-assist operations or for maximum 
towline pull.
•	 Very low fuel consumption (overall reductions from 
30 per cent up to 50 per cent compared to tugs of 
similar size and bollard pull and executing the same 
manoeuvres at the same location, are proven). Two-
thirds of the fuel savings is attributable to the hull shape 
and the lower displacement, while the hybrid propulsion 
system counts for the other one-third of the savings.
•	 Low noise levels, mainly because the two thrusters have 
no interference, but also due to the hybrid propulsion 
system.
•	 Low wake: the absence of stern waves for speeds up to 10 
knots for the 30mversion (and up to 8 knots for the 24m 
version), allows the tug to maintain a high mobilisation 
speed, resulting in fast ship assist round-trips and lower 
fuel consumption.
•	 High static and dynamic stability, high freeboard, 
well protected and in the centre line located thrusters 
(avoiding steel-steel contact with the assisted vessel) 
Figure 2D.16: The EDDY tug 30-65.
Figure 2D.17: The EDDY 1 (EDDY 30-65).
Photo: Hans Hoffmann, Rotterdam pilot
Figure 2D.14: Diagram showing pushing effectiveness. 
Nett pushing force (bollard pull) is about 90 per cent of 
maximum pushing force due to the not optimal flow of 
the water towards the thrusters.
Figure 2D.15: SDM forward and aft handling an LNG carrier.
Photo: Rafel Cabal Alvarez, Barcelona pilot
58 Tug Use in Port
2.19.4 Ship handling
EDDY tugs operate very well in confi ned spaces. As indicated 
above, due to their characteristically forward-aft in-line 
thruster arrangement, EDDY tugs are capable of generating 
towline tension continuously, and in any direction. When 
operating in locks, the tug can move very close to the vessel, 
the tug master being completely free in selecting the best 
angle of attack for each situation including remaining 
parallel to the lock’s side, as can be seen in the various 
pictures below. Depending on the required towing force, 
these tugs can thus keep the assisted vessel under control at 
all times. Th ese manoeuvres can be executed either with a 
single towline, as well as in a twin-towline confi guration. Th e 
required space for EDDY tugs is thus lower compared to ASD 
or tractor confi gurations of the same size particularly with 
the twin-towline concept, because the EDDY can operate 
within the breadth of the assisted ship. See fi gures 2D.20 and 
2D.21.
Also, propeller wash can be directed such that it reduces 
interference with the assisted vessel’s underwater body, 
resulting in a more eff ective pulling power and consequently 
better control over the vessel. 
2.19.3 Manoeuvring
Specifi c EDDY ship-assist and escort performance features 
are: 
•	 High towline forces (close to maximum BP) are available 
over 270°.
•	 High side-stepping speed of 7 knots and ease (and 
safety) of side-stepping.
•	 High manoeuvrability in all directions. 
•	 High steering-forces in the indirect mode, partly due 
to the concept and hull shape but also due to the high 
stability of the relatively wide hull (above water) and the 
location of the thrusters.
•	 EDDY 1 has a SafeWinch, which will be dealt with 
in Chapter 7, while Telstar has a standard winch 
confi guration.
•	 Th e tug Telstar (EDDY type) has two separately driven 
winch drums, allowing the tug master to use two 
towlines if needed. See fi gure 2D.20.
•	 As an option the tug may be equipped with towing pins. 
See fi gures 2D.20 and 2D.21.
•	 An aft winch is an option.
Figure 2D.18: Hybrid propulsion EDDY 24-75 – diesel direct/diesel-
electric (yellow: main engines; blue: electro motors; silver: generators – 
two main generators for electric sailing and one is the harbour generator).
Figure 2D.20: Tug Telstar (EDDY 24-75) operating in the IJmuiden 
locks with two towlines. Notice the towing pins. The two towline gives 
a faster and better control capability than just one centre line.
Photo: Iskes Towage and Salvage
Figure 2D.21: Tug Telstar (LOA 25.45m, beam 12.20m, BP 75 tons), 
effectively pulling the ship alongside in the locks. Photo: Henk Hensen
Figure 2D.19: All-electric EDDY with podded drives.
Types of Harbour Tug 59
2.20 Carrousel RAVE tug (CRT) 
2.20.1 Design
The Carrousel-RAVE tug (CRT) concept is a novel 
development in the tug industry. The design is a joint 
co-operation of Robert Allan Ltd, Voith and the Dutch 
companies Novatug and Multraship. The prototype design 
is for a 70-ton static bollard pull tug with a carrousel system 
and two Voith propulsion systems. Two CRTs have been 
built for Multraship Towage and Salvage: Multratug 32 and 
Multratug 33. Both started operations in the first half of 
2018. 
 
The Carrousel RAVE tug is a RAVE (Robert Allan Ltd Voith 
Escort) type tug equipped with a carrousel system. The tug 
has two Voith Schneider propulsion units, one aft and one 
forward, both situatedProfessor Warsash School of Maritime 
Science and Engineering, Southampton Solent University; Salman Nazir, PhD, Associate Professor, Head of Training and 
Assessment Research Group (TARG), University College of Southeast Norway, and Sathiya Kumar Renganayagalu, Doctoral 
Research Fellow, Faculty of Technology, Natural Sciences and Maritime Sciences, Department of Maritime Operations Campus 
Vestfold, are also highly appreciated.
I am very grateful to the maritime organisations that have been willing to share their knowledge : Gijsbert de Jong, Marine 
Marketing & Sales Director, Bureau Veritas Marine & Offshore; SJ Banfield, Managing Director, Optimoor; John Vanezos, 
Technical Secretary, IACS; Rob Drysdale, Senior Technical Advisor, Oil Companies International Marine Forum (OCIMF); 
Cherian Oommen of Sigtto; Deborah McKendrick, Information Officer, The International Tanker Owners Pollution Federation 
Ltd (ITOPF); Naa Sackeyfio, Information Data Analyst ITOPF; Captain John Rose MNM, ExC, Director (Maritime), CHIRP; 
Linas Kasparavičius, Head of the Maritime Supervision, Division Maritime Department, Lithuanian Transport Safety 
Administration; Oessur Jarleivson Hilduberg, Head of DMAIB, and Svein Erik Enge, Emergency Preparedness Advisor, 
Inspection and Emergency Preparedness, Norwegian Maritime Authority.
Alan Sorum – Maritime Operations Project Manager, Prince William Sound Regional Citizens’ Advisory Council, Valdez, 
Alaska, was also very helpful in providing updated information, as were Bernabe Gallardo, Application Engineer II, 
SamsonRope; Sarah Padilla, Technical Director, Cordage Institute; Peter Solis, Senior Marine Consultant, Glosten, and Hans 
van de Veen, Consultant Ropes-Towing Gear.
ACKNOwLEdGEMENTS
Tug Use in Port ix
Justus Schoemaker, Dujam Desk KK, Tokyo, Japan, was a helpful intermediary, as was Alan Loynd, Managing Director, 
Branscombe Marine Consultants Ltd, Hong Kong, and Sandra de Koster, Regional Co-ordinator, E-commerce, MOL (Europe) 
B.V.
Dirk Degroote, Product Manager Tugs; Joop Jansen, Manager Research & Development; Jochem de Jong, Principal Research 
Engineer; Leo de Jong, Design and Proposal Engineer Tugs, and Tim van den Heuvel, Development Engineer, of Damen 
Shipyards, were always ready to answer my questions and provide me with the requested information.
Much work has been carried out by Karen van Vliet, MSc Offshore Engineering and MSc Sociology, and Jan-Hendrik Hensen, 
Mechanical Engineering (BEng), Quality and Safety Management University of Antwerp, Leadership in Crisis Harvard Kennedy 
School. They have made a unique contribution to this book.
Apart from the need for the information contained in this book to be correct, there was also the need to present the text in 
suitable English. Additionally, the link with the daily practice of ship handling with tugs is an essential aspect of this book. 
Therefore the work that has been done in checking all the chapters for these two aspects, initially by Ed Verbeek, FNI, MSc, 
retired pilot, simulator instructor ship handling, former manager operations and training 
 co-ordinator Amsterdam pilots, and later by Chris Stockman, tug master and SeaWays trainer, has been invaluable. 
For the fourth edition I am incredibly grateful for all those who were again willing to share their knowledge and experience like; 
LTZ2 A.A. (Bram) Kool of the Dutch Royal Navy, and Captain Max A. Newman D, Tugboat Operations Manager, Operations 
Vice-presidency, Panama Canal, about tug assistance for aircraft carriers and submarines. The same applies to Tony Bannister, 
Chief Admiralty Pilot QECP, HM Naval Base Portsmouth, UK and Captain Rob Hinton, tug master/instructor, UK, for their 
support and what they did within the restrictions.
The information provided by LTLT Emily Wilkin USN, News Desk Officer Navy Office of Information (CHINFO), Washington, 
USA; Captain Baykal Yaylali, Chief Pilot Port of Izmir, former pilot Turkish Straits and VTS operator Turkish Straits and Daan 
Uiterwaal, Area Sales Manager, Royal IHC, The Netherlands, is also highly appreciated. 
Most grateful to Alan Loynd, Managing Director Branscombe Marine Consultants Ltd., Hong Kong for his editing work and 
helpful information; Vince Hertog and his colleagues of Robert Allan Ltd., Vancouver; Sascha Pristrom, Technical Officer 
Subdivision for Marine Technology and Cargoes Maritime Safety Division IMO and her colleagues, experts of MARIN, OCIMF, 
Samson ropes, IACS and of course Dr. Markus van der Laan, Owner, IMC Corporate Licensing, for their contributions and 
answering my questions so generously.
Without the support of all those mentioned a book like this can not be written. Truly thankful to all those who helped in one 
way or another with this and former editions. 
x Tug Use in Port
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 stern 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.
BHP (winch) Brake Holding Power of the winch, which applies to the first layer on the drum.
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 stern tug in the indirect 
 towing mode. A box keel gives additional strength to the tug's hull and provides a better distribution of
 dock forces when in dry-dock.
Classification of ship types AHT – Anchor Handling Tug
 AHTS – Anchor Handling Tug/Supply vessel
 DVS – Diving Support Vessel
 ERRV – Emergency Rescue Response Vessel
 FS – Field Support Vessel
 IFS – Infield Support Vessel
 OMV – Offshore Maintenance Vessel
 OCV – Offshore Construction Vessel
 OR – Oil Revovery Vessel
 PSV – Platform Support Vessel
 SSV – Safety Standby Vessel
 SURV – Survey Vessel
 SV – Supply Vessel
CCTV Closed-circuit television. A TV system in which signals are not publicly distributed but are monitored, 
 primarily for surveillance and security purposes.
CFD Computational Fluid Dynamics (CFD) is the analysis of fluid flows along moving ship hulls using numerical 
 solution methods, including waves along the surface. CFD requires extensive computer calculations, but due 
 to the ever increasing computer calculation capacity, this method is increasingly being used in ship design 
 and prediction.
Classification Society The objective of ship classification is to verify the design, production and operation of a ship and all of 
 its components. Classification societies aim to achieve this objective through the development 
 and application of their own rules. The main classification societies are ABS (American Bureau of 
 Shipping), DNV GL (Det Norske Veritas – Germanischer Lloyd), LR (Lloyd’s Register) and BV (Bureau 
 Veritas). IACS is the International Association of Classification Societies.
Computational Fluid Dynamics 
 The use of applied mathematics, physics and computational software to visualise how a gas or liquid 
 flows, as well as how the gas or liquid affects objects as it flows past.
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 setsuch high fairleads is to have the towing line 
running horizontally from the winch through the staple 
without first running down from the winch and then 
upwards to the ship. However, during ship assistance the 
towline will often have an angle in the fairlead, either 
horizontally or vertically at high sided ships, or both.
2.21.2 Manoeuvring
The two skegs form a kind of tunnel. These skegs have 
underwater fendering, which enables the tug to push full 
power sideways at a ship stopped in the water, without 
getting a list (see figure 2D.31). When sideways pushing at 
a ship having speed, a part of the power is needed to move 
the tug forward with the ship, reducing the direct pushing 
force, unless ship’s speed is very low and/or friction between 
tug fendering and ship’s hull is such that the tug can be 
more or less pulled through the water by the ship. However, 
when pushing sideways flat against the the hull of a ship 
having speed no use can be made of the hydrodynamic forces 
working on the tug. When pushing at an angle to the ship, 
these hydrodynamic forces can create high pushing forces 
depending on tug’s pushing angle and ship’s speed.
There is another aspect to take into account. As with, for 
Figure 2D.30: Giano tug with high positioned winches and fairleads.
Photo: Captain Ugo Savona,Giano tug
Figure 2D.32: Giano tug pushing sideways. 
Photo: Captain Ugo Savona,Giano tug
Figure 2D.33: Giano tug in the indirect operating mode.
Photo: Captain Ugo Savona,Giano tug
Figure 2D.31: Giano tug.
Types of Harbour Tug 63
Furthermore, during normal ship handling usually 
alternatively pushing and pulling is needed. Then a towline 
is needed and pushing can be done by tug’s bow or stern, 
instead of sideways. 
Escort capabilities of the tug will be large. High steering 
forces can be generated due to the fact the forward towing 
point is situated only at a small horizontal distance away 
from the forward thruster. 
In the situation as shown in figure 2D.33 the centre of 
hydrodynamical pressure will be situated forward creating 
forces in the towline which increase with ship’s speed. The 
forward thruster can create additional forces in the towline. 
Figure 2D.34:Giano tug preparing to pass the towline at the bow of a ship having speed. Photo: Captain Ugo Savona,Giano tug
The aft thruster is needed to keep the tug in position with the 
most effective heading. Escorting over the stern might create 
higher forces because the aft towing point lies more directly 
above the thruster. Provided the tug is handled well, it can 
operate safely near the bow of a ship having speed. It can 
compensate for the interaction forces, such as suction forces 
and turning moments working on the tug, when taking into 
account what has been said in paragraph 2.17.
64 Tug Use in Port
Additional or other requirements may apply, such as for 
the new 80 and 85-ton ASD-tugs being operated by Svitzer 
Australia on the Gorgon project – the Svitzer Euro, Svitzer 
Boodie, Svitzer Dugong, Svitzer Perentie, and, at the 
Wheatstone project, Svitzer Kadala, Svitzer Mulga, Svitzer 
Dugite and Svitzer Gwardar. These tugs have been designed 
and equipped as follows:
•	 Double hulls.
•	 LNG alarms, remote closing fire flaps and pressurised 
accommodation.
•	 Dynamic escort winch with the capability to release the 
line under full towing conditions, minimising slack rope 
events and shock loads even in extreme weather.
•	 Side pocket ladder systems and wide opening bulwark 
doors on both sides to assist in MOB recovery.
•	 Capability for pilot transfer through a specially designed 
pilot-boarding platform.
•	 These tugs will also have the following eco-friendly 
features:
•	 Diesel-electric propulsion
•	 Non-hydraulic deck equipment to ensure no oil or liquid 
spillage (the equipment will be electric).
•	 Surfaces finished in a low-sheen silicon paint to reduce 
water reflection.
•	 Low-spilling sodium deck lights to reduce water 
penetration and disturbance during night operations; 
also turtle friendly.
•	 Double-walled fuel tanks to prevent leakage.
•	 Solar panels for water heating.
•	 Water recycling plant from the AC systems to be re-used 
for deck washdowns.
•	 Battery packs for electric driving – used for about 20 
minutes at a time to get in or out of the tug pens (a 
pen is a tug berth, well protected and sheltered, often 
constructed of floating pontoons).
•	 Wind turbine for loading batteries when at tug berth. 
Shell’s Prelude is a very special subject as mentioned earlier 
in the Rotortugs section; it is one of the first FLNG (floating 
liquefied natural gas) projects. The 488m (!) long floating unit 
is to be stationed 230km off the Australian coast in 240m of 
water for 25 years and will operate continuously, in terms of 
gas processing and the loading of LNG, LPG and condensate 
tankers.
KT Marine Services Australia supplies three 42m, 100 tons 
bollard pull ISVs (Infield Support Vessels) for the Prelude. 
These vessels are Rotortugs equipped with SafeWinches, type 
ART 100-42, and named RT Beagle Bay, RT Roebuck Bay 
and RT Kuru Bay. 
2.22 Tugs handling LNG carriers. 
LNG terminal tugs
Depending on the location and the service to be provided, 
ship handling tugs at LNG terminals should comply with 
certain requirements, apart from the requirements for 
proper dynamic stability, behaviour in sea conditions 
that are endurable for the crew, optimum fendering, all 
around visibility from the manoeuvring stations, etc. These 
requirements could also include: 
•	 Seaworthy and be able to operate in wave conditions 
appropriate to the client/terminal specific requirements 
for intended ships visiting.
•	 Sufficient bollard pull to handle in the prevailing 
environmental conditions of wind and waves the largest 
floating object that can be expected at the terminal.
•	 A powerful escort towing winch with quick response 
render-recovery capability.
•	 Escort capabilities.
Furthermore the following might be required:
•	 Gas detection alarm systems in the wheelhouse.
•	 Gas sensors with sensors around the deckhouse, at 
engine room intakes and accommodation ventilation.
•	 Explosion-proof motors for air intake fans on main deck 
and deckhouse.
•	 Main engines equipped with ‘rigsaver’ – a safety stop 
system if gas is sucked in. 
•	 Spark arresters in exhaust systems. 
•	 Remotely operated ventilation dampers.
•	 Fire-Fighting 1, or above, with water-spray.
•	 Fendering with water lubrication for pushing operations 
and with such characteristics that a very low contact 
pressure can be realised of, for instance, not more than 
14t/m2.
•	 Anti-spark fendering.
•	 All essential electrical equipment on deck of a certified 
safety standard, suitable for use in the specific LNG 
zone. 
•	 Intrinsically safe external electrical points.
•	 Anti-static synthetic towlines.
•	 Additional FiFi requirements
Note:
Clear instructions are needed for the tug master and his crew 
for how to handle events in case a dangerous situation is 
developing with gas, due to a leakage or explosion, for example. 
Should the tug release the towline and sail to a safe area, 
with all the possible consequences? Or should the vessel being 
handled be pulled away? Much depends on the source of the 
gas flow. If staying in the dangerous area, how to save the crew 
if engines are stopped with rigsavers? Clear instructions are 
absolutely needed for such dangerous situations. 
Chapter 2 
PART E
Specific Tugs. Research. Performance
Types of Harbour Tug 65
Hazardous areas classification
Other requirements may apply to tugs handling LNG 
carriers or operating in hazardous areas. Hazardous area 
are classified into zones based on an assessment of the 
frequency of the occurrence and duration of an explosive gas 
atmosphere, as follows:
•	 Zone 0: An area in which an explosive gas atmosphere 
is present continuously or for long periods. Unofficially: 
explosive atmosphere for more than 1000h/yr.•	 Zone 1: An area in which an explosive gas atmosphere 
is likely to occur in normal operations. Unofficially: 
explosive atmosphere for more than 10, but less than 
1,000h/yr.
•	 Zone 2: An area in which an explosive gas atmosphere 
is likely to occur in normal operation and, if it occurs, 
will only exist for a short time. Unofficially: explosive 
atmosphere for less than 10, but still sufficiently likely as 
to require controls over ignition sources.
Sources of ignition should be effectively controlled in all 
hazardous areas by a combination of design measures, and 
systems of work, such as:
•	 Using electric equipment and instrumentation classified 
for the zone in which it is located.
•	 Earthing all plant/equipment.
•	 Prohibition of smoking/use of matches/lighters.
•	 Control of maintenance activities that may cause sparks/
hot surfaces/naked flames through a Permit to Work 
system.
•	 Etc. 
 
Other tasks include escorting and berthing LNG carriers, 
condensate tanker tow-backs, pilot transfer, floating hose 
handling, as well as an integral part in security, emergency 
response, rescue and evacuation requirements. 
For terminal tugs, such as the above mentioned tugs 
operating in or near LNG terminals, other specific design 
requirements exist with respect to operations in open waters. 
These tugs usually operate over the bow and often proceed 
with the stern into the waves. Therefore the aft deck of such 
tugs should have a lot of shear and smooth upgoing lines 
in the underwater hull form aft which should create the 
tendency when proceeding astern the tug’s stern to climb 
out of the water. It should be said this requirement does 
not apply to terminal tugs alone, but also to all ASD tugs 
operating bow-to-bow at a ship having speed.
Note 10:
Winch considerations.
In open sea exposed conditions, the relative motions between 
tugs and vessel are relatively large and vary strongly due 
to wave spectra and hull reflections. Many operations are 
performed at a significant angle to wind and waves adding 
also heeling effects. The situation is further complicated by 
(very) short towlines with large vertical angles. Finally, many 
towlines consist of high strength Dyneema fibres without the 
use of a stretcher. The total combination requires a robust 
render-recovery winch with very high response speed to 
instantly release the towline at overload and instantly retrieve 
the line a slack line, and well trained winch drivers to support 
the Master.
FiFi
The possible need for FiFi (Fire-Fighting) classification has 
been mentioned already. FiFi classes as given by DNV GL are 
shown in Table 2E.1, below. 
Water Monitor System Capacities
Class notation Fire Fighter (I) and 
(II+)
Fire Fighter (II) Fire Fighter (III)
Number of monitors 2 2 3 4 3 4
Capacity of each monitor (m3/h) 1200 3600 2400 1800 3200 2400
Number of pumps 1-2 2-4 2-4
Total pump capacity (m3/h) 2400 7200 9600
Length of throw (m) 1) 120 180 150 180 150
Height of throw (m) 2) 50 110 80 110 90
Fuel oil capacity in hours 3) 24 96 110 90
1) For class notation qualifier I, measured horizontally from the monitor outlet to the mean impact area. 
For I+, II and III, measured horizontally from the mean impact area to the nearest part of the vessel 
when all monitors are in satisfactory operation simultaneously.
2) Measured vertically from sea level to mean impact area at a horizontal distance of at least 70 m from 
the nearest part of the vessel.
3) Capacity for continuous operation of all monitors, to be included in the total capacity of the vessel’s 
fuel oil tanks.
Table 2E.1.
66 Tug Use in Port
The forementioned actions to reduce air emissions of ships 
also have their effect on tugs. The reason why more and 
more Eco-tugs are built with the purpose of reducing fuel 
consumption and/or air pollution. How this is achieved will 
be discussed below. 
Three basic systems will be addressed:
A. The system of reducing fuel consumption by hybrid 
technique, which means the tug uses two or more 
distinct types of power.
B. The system of using fuel that causes less air pollution. 
C. Fully electric tugs causing no air pollution.
A – System of reducing fuel consumption 
by hybrid technique
The various propulsion systems will be classified as follows 
(based on the study Design and control of hybrid power 
and propulsion systems for smart ships: A review of 
developments. R.D. Geertsma, R.R. Negenborn, K.Visser, 
J.J. Hopman. Elsevier Ltd. 2017)
•	 Mechanical propulsion.
•	 Electric propulsion.
•	 Hybrid propulsion systems, which include: 
 – Electric propulsion with hybrid power supply. 
 – Hybrid propulsion with hybrid power supply.
 – Electric propulsion with DC hybrid power supply. 
The images shown on the following pages (also based on the 
above mentioned study) give an indication of the various 
systems. 
2.23 Eco- tugs
Tugs that have been designed in such a way that they can 
operate effectively in an environmentally friendly way by 
reduced consumption of fossil fuel and/or reduction of air 
pollution are referred to here as Eco-tugs. 
Many actions have been undertaken in recent years to 
significantly reduce air emissions from ships. These air 
emissions include carbon dioxide (CO2), sulphur oxides 
(SOx), nitrogen oxides (NOx) and particulate matter (PM)1. 
Most of these actions (for Preventing of Air Pollution from 
Ships) have been taken through Annex VI of MARPOL, 
an international instrument developed through the 
International Maritime Organization (IMO) that establishes 
legally-binding international standards to regulate specific 
emissions and discharges generated by ships. The IMO 
emission standards are commonly referred to as Tier l - lll 
standards.
In the IMO circular MEPC.1/Circ.778/Rev.2 of 6 April 2017 
a list is presented of special areas, emission control areas and 
particular sensitive areas in various parts in the world with 
the specific MARPOL amendments, protective measures, 
and the dates these amendments and measures come or 
came into force. 
See also Index of MERPC Resolutions and Guidelines related 
to MARPOL Annex Vl (see References). 
1 PM stands for particulate matter (also called particle 
pollution): the term for a mixture of solid particles and liquid 
droplets found in the air. Some particles, such as dust, dirt, 
soot, or smoke, are large or dark enough to be seen with the 
naked eye. Others are so small they can only be detected using 
an electron microscope.
Figure 2E.1: Smoky harbour tug – ASD-tug, LOA 29.95m, breadth 10.20m. Photo: Michael Cassar, Malta
Types of Harbour Tug 67
A. Mechanical propulsion (fi gure 2E.2)
Th e propulsion can be with fi xed pitch propellers, 
controllable pitch propellers, azimuth thrusters, VS 
propulsion, etc.
Mechanical propulsion has only three power conversion 
stages: the main engine, the gear box and the propeller, 
which leads to low conversion losses. 
However, mechanical propulsion is particularly effi cient at 
design speed, which is between 80 per cent and 100 per cent 
of the top speed.
Mechanical propulsion has a poor fuel effi ciency and high 
emissions when sailing at speeds below 70 per cent of top 
speed. 
Tugs only require 20 per cent of their maximum power 
requiring for towing during transit, which leads to poor 
specifi c fuel consumption and high emissions. Th us, electric 
or hybrid propulsion could be considered to improve part-
load fuel effi ciency. Nevertheless, this is yet only the case on a 
limited number of tugs. 
B. Electric propulsion (fi gure 2E.3)
Most electric propulsion systems utilise fi xed pitch 
propellers, because the electrical drive can run at every speed 
in forward and reverse direction and deliver full rated torque 
at every speed. As such, the speed of the ship or tug can be 
fully controlled without the need for a controllable pitch 
propeller.
When engines are running on part load, which is oft en the 
case with tugs, it will lead to poor fuelconsumption and 
a lot of emissions. Only a few harbour tugs have electric 
propulsion
 
Figure 2E.2: Typical mechanical propulsion system.
Figure 2E.3: Typical electrical propulsion system layout.
68 Tug Use in Port
C. Hybrid propulsion (fi gure 2E.4)
Ships that frequently operate at low speeds, such as tugs, can 
benefi t from a hybrid propulsion system. 
In hybrid propulsion, a mechanical drive provides propulsion 
for high speeds or with high effi ciency. Additionally, an 
electromotor (2), which is coupled to the same shaft through 
a gearbox (3), provides propulsion for low speeds, thus 
avoiding the main engines ineffi ciently in part load.
Hybrid propulsion is typically economical when the 
operation profi le has distinct operating modes with a 
signifi cant amount of time at low power, which is the case 
with tugs.
When the electric drive is designed to run parallel with the 
mechanical drive, it can be used to increase the top speed, 
or maximum towing power, and so reduce engine thermal 
loading and thus NOx emissions. 
D. Electric propulsion with hybrid power supply (fi gure 2E.5)
In electric propulsion with hybrid power supply, a 
combination of two or more types of power source can 
provide electric power. Th e power sources can be: 
1. Combustion power supply, from eg diesel engines.
2. Electric chemical power supply from fuel cells, and
3. Stored power supply from energy storage systems (see 2), 
such as batteries and fl ywheels. 
Fuel cells are still seldom used in the maritime environment. 
Research is focused on more compact storage of hydrogen, 
fuel cells that can use other fuels, such as LNG or even diesel 
oil. At present the main type of energy storage is the battery.
Th e energy storage can provide the required electrical power 
and enable switching off one or more engines when they 
would be running ineffi ciently at part load. Th e energy 
storage can then be recharged when the engine is running in 
an operating point with lower emissions. Th is can save fuel, 
reduce emissions, reduce noise, increase comfort and enable 
temporarily sailing without emissions, noise and vibrations 
from engines. 
Furthermore, the battery can enable peak shaving: the 
battery delivers power during periods where high power is 
required and recharges when less power is required. Th e 
battery can also provide back-up power during a failure of 
combustion power supplies (diesel generators).
Popularity of batteries is growing fast. For tugs and ferries, 
for example, the potential reduction of fuel consumption and 
emissions has led to investigations and application of electric 
propulsion with hybrid power supply. 
Figure 2E.4: Typical hybrid propulsion system.
Figure 2E.5: Typical electric propulsion system with hybrid power supply.
Types of Harbour Tug 69
E. Hybrid propulsion with hybrid power supply 
(fi gure 2E.6)
Th is system utilises the maximum effi ciency of the direct 
mechanical drive (1) and the fl exibility of a combination of 
combustion power from prime mover(s) (2) and stored power 
from energy storage (3) for electric supply. 
Hybrid propulsion with hybrid power supply has been 
researched extensively and has been applied in harbour tugs. 
Research at Delft University of Technology suggests that 
hybrid propulsion with hybrid power supply can deliver 
signifi cant savings in local emissions, partly by using energy 
from batteries that are recharged with shore connection.
If the control mode of the plant is determined in an optimum 
way by the operating mode of the vessel (towing, high 
speed transit, low speed transit or stand by) and the battery 
state of charge, positive results can be achieved because the 
operating modes of the plant lead to very distinct loading of 
the system. For example, in low speed transit or standby, the 
main engine loading is very low and, therefore, switching off 
the engine stops the engine operating ineffi ciently. 
Th e hybrid propulsion confi guration allows designs in 
which the main engines cannot deliver full bollard pull on 
their own. However, a design that for delivery of full bollard 
pull depends on an electro motor or batteries potentially 
introduces reliability and safety risks. Th erefore, it would be 
better to design the main engine such that it can deliver full 
bollard pull without additional power from the electromotor. 
F. Electric propulsion with DC hybrid power supply 
(fi gure 2E.7)
Th e most important reasons for applying DC systems 
are increased fuel effi ciency when running generators in 
part load and reduced power conversion losses. Th e DC 
architecture allows the diesel engine to run at variable 
speed, potentially leading to a reduction fuel consumptions, 
emissions, noise and engine mechanical and thermal 
loading.
Applications that utilise DC hybrid power supply systems 
are ferries, drilling ships, research vessels and wind farm 
support vessels. 
From the foregoing it has become clear how important 
batteries are for hybrid vessels and, obviously, for only 
battery-powered vessels. For that reason classifi cation society 
DNV GL started a Joint Development Project (JDP), designed 
to advance the understanding of the use of lithium-ion 
batteries in the shipping industry. More than dozen partners 
joined the initiative, including fl ag states, research institutes, 
battery and propulsion suppliers, ship owners, operators and 
yards. Including batteries in ships, whether as a hybrid or 
fully electric system, off ers the industry the opportunity to 
improve fuel economy, reliability and operational costs. Th e 
project started in 2017 and results will become available in 
2019.
Figure 2E.6: Typical hybrid propulsion system with hybrid power supply.
Figure 2E.7: Electric propulsion with DC hybrid power supply.
70 Tug Use in Port
Various hybrid systems have been briefly explained. 
Operating modes may have following names and functions 
(see paragraphs C and E above):
•	 PTI (power take in) – Booster mode: the shaft generator 
then functions as an auxiliary motor, and works 
concurrently to the main diesel engine. 
•	 PTI – Fully electric mode. In this mode batteries 
generate the energy for the propulsion. The main diesel 
engine and the generator sets are off.
•	 PTI – diesel-electric mode. This mode is used at low 
speeds and does not need the main diesel engine. The 
generator sets are functioning and feed the tug’s loads 
as well as the main propulsion. The shaft generator 
functions as propulsion motor. The system can also be 
used in the event of a main diesel engine failure (PTH: 
power take home). 
Other operating modes can be:
•	 PTO (power take of) – Parallel Mode. Additional 
power of main diesel engine in case power required for 
propulsion and vessel’s loads is higher than generator 
sets can provide. PTO – Transit Mode. The main diesel 
engine supplies power needed for the propulsion (and 
ship’s consumers). Generator sets off. PTO – Shore 
Connection Mode. All engines are switched off. 
Different systems have been mentioned that can cut fuel 
costs and make tugs more environment-friendly. Whatever 
system is used, an optimum working power, or energy, 
management system is needed to achieve the best results. 
Figure 2E.8: Engine control system of Kotug’s hybrid tug RT Evolution 
(LOA 32m, beam 12.6m, BP 83 tons). Photo: Piet Sinke
Figure 2E.9: Hybrid system of a Rotortug: hybrid propulsion system with 
hybrid power supply. Source: Kotug
Explanation (see also figure 2E.9): 
Top button left: selection between HYBRID and NON 
HYBRID. 
Middle button left: IDLE: On batteries only when moored, 
waiting for a ship or waiting until pilot is on board. Thrusters 
can be used, but only with propellers at very low speed. 
TRNST 1: One generator running producing electricity for 
the electro motors at the three thrusters (diesel-electric) and 
the tug can sail with a speed up to 6 knots.
TRNST 2: One main engine running. Thismain engine 
drives the propeller (diesel-direct) and the generator in the 
propeller shaft which provides electricity for the electro 
motors of the other two thrusters (diesel-electric). The tug 
can then sail with a speed of 9 knots. 
ASSIST: All main engines running, maximum speed and 
power available.
When a diesel generator is running the batteries are loaded 
at the same time.
Five aspects are of crucial importance for safe operations of 
an Eco harbour tug:
•	 Full power should be available in a minimum of time 
and preferably without extra handling of press buttons 
or switches. In emergency and stressful situations, 
the focus of the tug master is on the situation; extra 
handlings distract the tug master from the immediate 
actions to be taken and/or lead to mistakes.
•	 Full power should be available during at least some 
hours to enable the tug to, for instance, stay pushing 
over a long period to keep a large ship alongside during 
stormy weather. 
•	 Switching of the various drive systems should be simple 
so that in stress situation no mistakes will be made.
•	 The eco-system should be fully reliable. 
•	 Stability should not negatively be affected (applies to 
section B).
B System of using fuel which causes 
less air pollution
Apart from possible exhaust gas treatment systems, there 
are other options to reduce air pollution by tugs, even in 
combination with the above-mentioned systems:
•	 The possibility of running the engines on diesel and 
LNG or hydrogen, so-called dual-fuel engines.
•	 LNG, hydrogen or ammonia only as fuel.
•	 Compressed natural gas (CNG) for which storage 
requirements are less stringent than for LNG (see below). 
•	 Studies are ongoing to see if methanol could be used 
as fuel. Methanol is mainly produced by natural gas 
synthesis. The most used alternative fuel after LNG, 
it is available on a large scale. The RoPax ferry Stena 
Germanica was the first ship that uses methanol as fuel. 
Methanol is no more poisoning than diesel or gasoline. 
It has a low flashpoint of only 120 C. For this reason 
there are several requirements for storage and handling 
of methanol on board ships. Methanol as a ship fuel 
is interesting for ship operators because it does not 
contain sulphur and is liquid in ambient air conditions, 
which makes it easy to store on board ships. Converting 
engines to the use of methanol is not complicated, and 
Types of Harbour Tug 71
maintenance and noise level are lower. The problem with 
LNG as fuel is the space and the location. Conventional tugs 
store diesel fuel in tanks of all shapes, making use of tight 
spaces that can be of limited use for other purposes. LNG 
should be stored in specific tanks which have to comply with 
specific requirements.
LNG is a liquid and should be kept at a temperature of -162° 
C. The energy density of LNG is about half that of diesel. 
This means it requires twice as much storage as diesel for 
the same energy needed. In gaseous condition the volume of 
LNG is about 600 times as large. 
When it comes to the allowable locations of LNG storage 
tanks on board a tug, there are specific classification society, 
national authority, and international code safety-based 
restrictions. LNG tanks should be kept at a certain distance 
from the side shell, from the bottom, and from engine room 
spaces. These major location restraints, along with the space 
limitations, make it difficult to fit significant quantities of 
LNG on board a tug, especially a compact tug. There are 
furthermore specific mandatory and class requirements for 
safety and gas systems that cover the ventilation of the LNG 
system’s tank hold, tank connection spaces, gas regulation 
units, engines, double walled piping systems, bunker 
stations, air locks and the like. If an oil recovery system is 
also required it becomes even more complicated.
The characteristics of LNG and all the specific requirements 
make the storage of LNG on board a tug with rather small 
dimensions and high powered engines a complex issue.
The LNG storage tank should not placed at such a height 
that it will affect a tug’s stability in a negative way. The same 
applies to a partly filled LNG storage tank, due to the effect 
of the free liquid surface in the tank. Furthermore, work on 
deck, lead of the towline, visibility, etc, should not be limited 
by ventilation pipes and other constructions needed for LNG.
Contrary to what has been said above about the location of 
the LNG fuel tank, Mitsui OSK Lines’ (MOL) new tug has the 
fuel tank high up – on deck (see figure 2E.11). The tug, name 
Ishin, entered service in Osaka Bay in 2019.
Particulars of the tug are:
Tug with azimuth thrusters; LOA: 43.6m; beam: 9.2m; engine 
power: 2 x 1,618kW (2 x 2,200 hp); BP 50 tons. The tug can run 
on LNG and on diesel (dual-fuel engines). 
Locating the LNG in such a high position puts great demands 
on the tug’s stability, especially with this slender tug, having a 
length:beam ratio of approximately 5.
MOL declares that the size of the tank is such that good 
stability is ensured, which should also be the case when the 
tank is partly filled and the free liquid surface in the tank 
it is available in many ports around the world. Costs of 
methanol are lower than of hydrogen and ammonia.
•	 Ammonia as marine fuel is also being studied. One of 
the first working engines run on ammonia was built 
in 1935. Ammonia is liquid at a temperature of -34° C, 
but the volume needed for the same energy is at least 
three times as high as with conventional fuels. Costs 
of ammonia is higher than of methanol. Agreements 
are made between shipping companies, classification 
societies, engine manufacturers, etc. to study the use of 
ammonia as fuel for ships. In 2020 Japanese shipping 
company NYK line, classification society ClassNK 
and IHI Power systems signed a joint research and 
development agreement for the world’s first ammonia-
fueled tugboat. 
•	 Hydrogen – Ferries using hydrogen as fuel are scheduled 
to enter into service in 2021. It is a clean energy source, 
bountiful in supply, non-toxic and highly efficient; but 
it is expensive, there are storage complications and it is 
highly flammable. For the Port of Antwerp the dual-fuel 
Hydrotug will be built with engines that burn hydrogen 
in combination with diesel. It is the first hydrogen-
powered tug in the world and will be ready to enter 
service in 2023. Costs of hydrogen are considerably 
higher than of methanol.
•	 Lignin – with cellulose, the chief constituent of wood. 
Dutch scientific institute Chemelot InSciTe said in 2018 
that lignin will soon be processed on a commercial scale 
through a biobased process, and the oil produced (crude 
lignin oil – CLO) will be used as marine fuel that will 
be more durable and environmentally friendly than the 
bunker fuels that are currently used.
•	 Iron – Like some other metals, iron can burn and 
produce heat. The iron powder used for this purpose 
has a grain size comparable to the thickness of a 
hair. Eindhoven University of Technology has been 
researching metal fuels as a circular energy source for 
some time and sees iron as a promising option for three 
areas of application: power plants, the process industry 
and shipping. It has a relatively high volumetric density 
and meets the requirements for a future marine fuel. 
LNG will be addressed separately below.
LNG
LNG as fuel for tugs is a growing market: in 2017 six gas-
powered tugs were in service and eight on order. It complies 
with present and future emission requirements. Engine 
Figure 2E.10: Hybrid tug Ginga, with mechanical and electric 
propulsion (LOA 38m, width 10m, BP 55 tons, speed 14.8 knots), 
Tokyo Kisen, Japan. Photo: Piet Sinke
Figure 2E.11: MOL LNG tug. Courtesy Mitsui OSK Lines, Japan
72 Tug Use in Port
Since LNG fuel is rather complicated to control properly 
with rapidly varying loads, a practical solution is offered by 
a stable generation of main power byLNG generators and 
small fluctuation diesel generators. 
Several eco-tugs have been built. Examples are Carolyn 
Dorothy (Foss), Svitzer Gaia, Svitzer Boodie, Svitzer 
Mulga, RT Evolution (Kotug), Ginga (Tokyo Kiso), EDDY-
1, Fairplay XI, Noordzee (Dutch Royal Navy), BB Power 
(Bukser og Berging), Dux (Østensjø), etc, or the new eco-
friendly design Wärtsila HYTug series, launched in 2017. 
Apart from a modular hybrid propulsion these tugs have a 
wheelhouse design with a high visibility, semi-enclosure bow 
winch, side shoulders and low hull resistance. 
C: Fully electric tugs
There is an increasing interest in fully electric tugs. They 
have almost zero emissions, are quiet, both onboard and 
underwater.
 
The first operational all-electric tug in the world is the 
Zeetug Gisas Power of GISAS Shipbuilding in Turkey. `Zee’ 
stands for Zero Emissions Electric Tug. It is an 18.7m long 
tug with 32 tons bollard pull. Expectations are that the 
lithion-ion batteries will have a lifespan of 10 years. At a 
quick-charging station the batteries can be fully charged in 
one hour. The tug has two redundant battery rooms, one for 
and one aft, that are maintained at a constant temperature by 
a cooling system. More Zeetugs have been ordered.
In July 2020 keel-laying took place of the all-electric Damen 
tug RSD-E tug 2513 called Sparky, to be built for the Port of 
Auckland. See for description of RSD tug 2513 paragraph 2.13. 
For maximum redundancy, four identical and independent 
battery packs are situated each in an insulated, temperature-
controlled battery room. The lifetime of the battery system 
is expected to be approximately the same as the estimated 
working life of the vessel. 
The battery capacity is sufficient to perform at least two 
berthing operations fully electric in an average harbour with 
zero emissions. The tug can push-pull with 70 tons bollard 
pull for at least 30 minutes. It then takes two hours to fully 
recharge the batteries at the shore station. 
starts to play a role. The LNG tank will hamper any towing 
operations on the aft deck. However, these tugs operate over 
the bow. 
The 32m long harbour and terminal ASD-tug KST Liberty, 
whose naming took place in April 2018, also has its LNG 
tanks on the after deck.
With respect to LNG-fuelled tugs, a new mandatory safety 
code for ships using gases or other low-flashpoint fuels 
entered into force on 1 January 2017, along with new training 
requirements for seafarers working on these ships. The 
International Code of Safety for Ships using Gases or other 
Low-flashpoint Fuels (IGF Code) aims to minimise the risks 
to ships, their crews, and the environment, given the nature 
of the fuels involved.
There are various standards and guidances focusing on LNG 
bunkering, such as:
•	 ISO/DIS 20519 Specification for bunkering of gas fuelled 
ships. 
•	 SGMF (Society for Gas as a Marine Fuel) LNG 
Bunkering Safety Guidelines. 
•	 IACS Recommendations on LNG bunkering (Rec. 142). 
June 2016. 
•	 IAPH (International Organisation of Ports and 
Harbours) LNG bunker checklist.
•	 EMSA (European Maritime Safety Agency) 
Guidance on LNG bunkering to Port Authorities and 
Administrations. 
Note 11:
As mentioned earlier, although LNG is mentioned as fuel, it 
cannot be used as fuel. It has to be reconverted to gas, which 
can be used as fuel, if temperature etc is suitable 
Finally: Diesel-electric
Diesel engines are very efficient and robust energy sources. 
Their drawback is that efficiency and emissions are directly 
related to the revolutions and cylinder loads. For a ship 
sailing at one speed and power rating, diesel propulsion 
offers an attractive and efficient solution. And for this given 
speed and power rating, emissions can be further reduced by 
exhaust treatment. 
Tugs have large fluctuating revolutions and power rating 
with large variations. As a result diesel direct propulsion 
has lower efficiency and far higher emissions at non optimal 
speed and power ratings. Exhaust treatments for off-design 
conditions is very complicated.
For these varying conditions, diesel-electric propulsion 
offers interesting perspectives. Due to the diesel-electric 
configuration, revolutions and power can be better matched 
to optimal efficiency.
LNG can be used as fuel for a combustion motor directly 
driving the propeller shaft; however, for varying revolutions 
and powers, this still remains a difficult and complex system. 
For tugs, a more attractive solution is a LNG-electric system, 
whereby LNG is used as fuel for a combustion motor driving 
a generator. This also allows the inclusion of part diesel-
electric generators, whereby either LNG or diesel fuel can be 
used, or an attractive combination. 
Figure 2E.12: ASD-tugs Tornado, ASD2810 and Akmal working 
in broken ice in the Port of St Petersburg, Russia. 
Photo: Captain Sergei Milchakov, St Petersburg
Types of Harbour Tug 73
periods on arrival, departure or when berthed. With strong 
off shore winds tugs are sometimes needed to keep ships 
safely alongside the berth and this may take several hours. 
Th e same may be the case with large empty tankers. In case 
of groundings tug power may also be needed for hours. 
Maximum operating time may increase in the coming years, 
as batteries are improving continuously. 
Wireless charging technology
Wireless charging is already employed. Th is technology uses 
very high frequency waves.
Wärtsilä, for instance, has developed a wireless charging 
system for easy transfer from shore to ship. Th is technology 
is particularly suitable for fully electric vessels using 
batteries.
A technology which can be employed for fully electric tugs 
as well. 
2.24 Ice tugs
Ice conditions and requirements
In general, at least the following local ice conditions should 
be considered for requirements for tugs operating in ice, 
including local environmental conditions such as wind and 
current that may eff ect ice conditions:
•	 Type of ice, salinity, snow coverage.
•	 Level of ice thickness in normal and more extreme 
situations.
•	 Ice coverage.
•	 Amount of severity of ridging, consolidation of ridges.
•	 Ice drift and ice pressure; the local ice condtions. 
•	 Land fast and grounded ice.
Issues of importance for design or selection of ice tugs are:
•	 Required ice breaking capacity versus other required 
capacities. Ice breaking performance for a given level of 
power or, on the other hand, a reduced fuel consumption 
for a given level of performance. 
•	 Tug type: ASD or conventional, both being most suitable 
for severe ice conditions.
•	 Main engine and propulsion system. Direct mechanical 
fpp or CPP, or various diesel-electric arrangements. In 
this selection, the most important issues are: torque/
rpm curves and the machinery’s ability to be used for 
prolonged periods at high power, including tolerance 
Another important advantage of the all-electric system is 
that the extra oxygen in the engine room needed to run 
fossil fuel-driven engines is not required anymore, making 
it safer to operate in and around LNG terminals and other 
hydrocarbon/chemical facilities. Also, it decreases the need 
for large engine room air inlets and so increases tug safety in 
case of capsizing.
Th e tug can be operated by two men thanks to the degree 
of automation incorporated, the human machine interface 
and the centralised alarm, monitoring and control system 
connected to the Damen remote monitoring system. Please, 
see for two-man tug operations paragraph 6.3.14.
More initiatives can be seen for fully electric tugs. For 
instance, Vallianz Holdings has signed an agreement with 
SeaTech Solutions, both situated in Singapore, to develop a 
series of all-electric harbour tugs of the EVT-60 design with 
a `brand new battery-powered concept’. Th e tugs will be 26m 
long and have a bollard pull of 60 tons. 
In March 2021 LNGCanada ordered for their LNG terminal 
in the Port of Kitimat fully-electric tugs, the ElectRA2800battery-electric harbour tugs. Th e tugs of the ASD-type will 
have a length of 28m, a battery capacity of 5240KWh, and a 
bollard pull of approximately 70 tons. Th ey are expected to 
be operational around 2025. 
Also Crowley Engineering designed a fully electric tug. It is 
25m long and has a bollard pull of about 60 tons. Th e battery 
capacity should be enough for two ships assist jobs. 
Lithium batteries
Care should be taken with lithium batteries. In August 
2012, an explosion and fi re occurred in one of the l ithium 
batteries on the tug Campbell Foss. Subsequent to that fi re, 
Foss removed the remaining batteries from the tug, and it 
returned in diesel confi guration without batteries.
Th e investigation resulted in several structural changes to 
vessels and the installation of fi re suppression systems in the 
battery room.
More lessons about fi re suppression were learned in a later 
fi re on board the Norwegian ferry Ytterøyningen in 2019. 
Also, on 11 March 2021, the all-electric excursion catamaran 
MS Brim with two battery rooms with 790kWh of lithium 
batteries installed got on fi re.
Th e reader is referred to the DNV report of the maritime 
battery safety joint development project: ‘Technical 
Reference for Li-ion Battery Explosion Risk and Fire 
Suppression.’ (see References). It states, amongst other 
fi ndings, that ventilation alone is not enough to prevent an 
explosion if a very large number of battery modules (totally 
4,000 amp hours or more) fail in the same compartment 
at once. Th e battery design must have preventative safety 
barriers so that the fi re and gas emissions are limited to as 
small a part of the battery system as possible. 
Maximum operating time of fully electric tugs
Th e maximum operating time of fully electric tugs should be 
well considered with respect to the situation in the port, size 
and type of ships and wind conditions. For instance, during 
stormy winds full power of tugs may be needed for large 
container vessels, LNG carriers and car carriers during long 
Figure 2E.13: Stronger structural integrity is needed for ice tugs.
Courtesy Damen Shipyards
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74 Tug Use in Port
bays may also be required to ensure that the sea chest does 
not become blocked with ice. Most of the stronger ice classes 
require several forms of rudder and propeller protection. 
Two rudder pintles are usually required, and strengthened 
propeller tips are often required in the stronger ice classes. 
More watertight bulkheads, in addition to those required 
by a ship’s normal class, are usually required. In addition, 
heating arrangements for fuel tanks, ballast tanks, and 
other tanks vital to the ship’s operation may also be required 
depending on the class.
Tug type and equipment
Much depends on the local situation, as described above, 
and on the tasks to be performed. Tug type with respect 
to propulsion and propeller type is a specific issue. The 
following is based on designers and tug masters’ experience 
and research:
ASD-tugs
What the best tug type is depends totally on prevailing ice 
conditions and type of operations, although most versatile in 
general is an ASD-tug. Experience with ASD-tugs equipped 
with CPP propellers is optimal. CPP makes recovery of 
thrust after ice blockage very fast and easy to perform, and 
even reduces nozzle blockage incidents. 
Voith tugs
Regarding Voith tugs in ice, opinions and experiences 
differ. Some say Voith typically does not work in anything 
over very light ice conditions, mainly due to ice blocking 
and to some extent to vulnerability in real ice condtions. 
In ice conditions the normal coupled Voith control system 
limits the possibility of clearing ice of each propulsion unit 
independently with propeller washes. 
Experiences show that Voith tugs get easily blocked in 
anything but lightest ice, partly also due to the tractor tug 
concept which doesn’t have a hull ploughing ice away from 
the propulsion units, as is the case with azimuth tractor tugs. 
However, a recent report comes to a totally different 
conclusion (see Maritime Journal, 16 February 2012. A study 
of Voith tractors in ice). It was produced following testing 
at the Aker Arctic Ice Laboratory in Helsinki, and in the 
circulation tank at Voith’s headquarters in Heidenheim. It 
indicated that even under extreme conditions, tugs with 
Voith Schneider propellers have proved to be highly effective 
icebreakers.
The findings are consistent with the practical experience of 
Svitzer in Gothenburg, URAG in Bremerhaven and Bugsier 
in Rostock.
The impact point of the ice floes at the propeller blade is 
usually in the upper part of the blade near the bearings. This 
results in a lower bending moment. The magnitude of the 
bending moment has a significant influence on the service 
life of the propeller blades. 
1. Collision with the ice floes occurs at the leading edge of 
the blades. In this load situation, the section modulus of 
the blade profile is at its highest, which reduces the risk 
of damage. 
2. The ice floes are immediately pushed to the side after the 
first impact with the propulsion system and do not get 
caught in the VSP. 
for extremely rapid load variations. The complete 
propulsion line must be calculated, the propeller as 
weakest point and engine as the strongest.
•	 Nozzle or open propellers.
•	 Torsion loads and vibrations in propulsion line owing 
to propeller ice contact, ice between hull and propeller 
blade and blockage of nozzles owing to ice. 
•	 Engine cooling arrangements.
•	 Internal heating; noise control.
•	 Design and system solutions to minimise external ice 
build-up; means to remove ice from deck, deckhouse, 
winch, ladders, masts, aerials and superstructure.
•	 Towing gear operability to maintain workability of lines, 
winches and fenders; protection against freezing spray 
and snowfall. Towlines should be such that minimum 
breaking load will not be negatively affected by low 
temperatures.
•	 Extremely good searchlights, working lights, radar(s), 
and general visibility issues, including window heating 
and demisting arrangements, wiper heaters and 
washing-line purging systems. 
Furthermore:
•	 Stability requires specific attention because of the 
possiblity of ice accretion on board the tug, which can 
be very dangerous. Ice growth on superstructure, mast, 
winch, etc, can occur very quickly during freezing 
temperatures, quickly decreasing the tug’s stability. Ice 
accretion should be a factor to be taken into account for 
the tug’s stability.
Note 12: For those further interested in the severity of icing, 
the Mertins Diagram gives an indication of how much ice 
can be developed during various conditions of wind force, 
seawater temperature and air temperature. See: ‘Operations of 
ships in cold climates with emphasis on tankers and the new 
requirements’. DNV. November 2003. 
 
Not all ships are built to an ice class. Building a ship or tug 
to an ice class means that the hull must be thicker and more 
scantlings (aggregate of girders, beams, and bulkheads 
resulting in stronger structural integrity) must be in place. 
Sea chests (openings in the hull for seawater intake) may 
need to be arranged differently depending on the class. Sea 
Figure 2E.14: ASD-tugs Tornado, ASD2810 and Akmal working in ice in 
the Port of St Petersburg, Russia. Photo: Captain Sergei Milchakov, St Petersburg
Types of Harbour Tug 75
thrust. However, bollard pull in relation to installed power is 
signifi cant lower with an open propeller. It should furthermore 
be well considered that tugs in ice might well be using over 
80 per cent of time over 80 per cent of MCR. You really need 
power, machinery built for that type of usage, and thrust 
to make you move and/or clear the ice with independently 
controlled propeller washes. 
Much also depends on tug design and really on how you 
operate the tug and plan the whole assistance operation. 
Various ice class notations exist. Table2E.2 gives an 
indication of the various ice class notations.For ice class 
vessels from diff erent countries/registers, corresponding ice 
classes are approximate and the fi nal decision on using a 
vessel is the owner’s responsibility and risk.
Purpose built ice tugs
A purpose built ice tug is the Yuribey,built for the extreme 
conditions at the Arctic Russian port of Sabetta, Yamal 
peninsula, Kara Sea. Th e tug has LOA of 39.54m, beam 14m, 
draft with skeg 7.1m, bollard pull ahead 97 tons, speed 14.4 
knots and Ice Class ARC6. It has been designed for a wide 
range of operations: ice breaking in the port and approach 
channel, escort of LNG carriers, escort operations at speeds 
3. Voith water tractors have the advantage of having VSPs 
positioned close together and rotating outwards. Th e 
general advantage of the tractor concept lies in the 
deeply immersed propellers, which are less likely to get 
in contact with ice. 
Comments from Captain Siegfried Kempe, a tug master 
aboard the Voith tractor tug Bugsier 16 since 1994, confi rm 
the results of the study. “I have carried out numerous towing 
jobs in ice. Quite frequently we have to clear the channel in 
Rostock harbour but we have also had successful operations 
in iced-up Swedish waters. Unlike jet (nozzle) propellers, 
which are quickly blocked, Voith Schneider propellers have 
no problem with ice fl oes. Th e VSP can push the ice aside 
without any trouble. Th e propeller slipstream can be steered 
in such a way that the ice is quickly and completely fl ushed 
away and what’s more, the wake of the Voith Water Tractor 
has another interesting eff ect, the channel clearance is much 
wider. Th is is benefi cial for the ships that follow us.” 
Further study might be needed to fi nd out why these 
diff erences in experience exist. 
Tugs with open propellers are favoured in very hard ice 
conditions, because blocked ice in nozzles results in zero 
Table 2E.2: Ice class notations. Source: mainly Worldwide Vancombe
76 Tug Use in Port
up to 10 knots, towing, pilotage of vessels and mooring 
to berths, fire-fighting, participation in rescue operations 
and oil spill cleaning, etc. The tug can also carry three 20ft 
containers.
The tug has seven watertight compartments, double hull and 
bottom, making it almost unsinkable. 
Propulsion is by Azipods. Total power is 9,400 hp (7MW) 
and the tug has four main engines.
Yuribey is designed for operating in temperatures down 
to -50° C. This has been achieved by various measures: 
forward and aft towing winches are located in enclosed 
spaces, coamings of doors, covers, fittings, handrails, 
Figure 2E.15: Tug Yuribey is engaged in ice breaking, escorting and 
towing, etc; it has Azipod propulsion. Courtesy Donmar
Figure 2E.16: Forward enclosed winch. Courtesy Donmar
Figure 2E.17: Aft enclosed winch. Courtesy Donmar
Figure 2E.18: Tug Calypso with ice breaking bow Saimaa. Courtesy: Ilari Rainio / Finnish Transport Infrastructure Agency
Types of Harbour Tug 77
communication and navigation antennas and other devices 
are electrically heated. The air take-in for the engine room 
and living quarters, passes heaters, so ensuring preheating. 
Two boilers are installed on the tug, providing heating of 
incoming air, heating the vessel’s spaces and tanks.
Tug ice breaking bow
The tug ice breaking bow is a new development designed by 
ILS Ship Design & Engineering. With such a removable bow 
a typical pusher tug can be converted into an ice breaking 
tug for inland waterway towage requirements. The removable 
bow has a length of 25.30m and for propulsion two engines 
of 800 hp each have been installed. See also figure 3.19 and 
3.39 for tugs with ice-scrapers on the bow. It is claimed that 
this first version will enable the tug to break 70cm ice at 2 
knots and 40cm ice at 6 knots.
Training
Training is a necessity for tug masters operating in ice and 
should focus on:
•	 Safe and effective handling of his tug in ice.
•	 The ability to read and predict ice and ice conditions, 
visually, on radar information and on all available 
preliminary information, in order to be able to find the 
easy spots and cracks in ice, to assess the risk of nozzle 
blocking, predict the possibility of passing through a 
ridge without ramming, etc. 
Much can be learned on simulators, but experience has to be 
built up in daily practice. 
Note: Much information comes from the paper Equipment 
and operational Issues for Terminal and Escort Work in Ice 
Conditions. See References. The reader is further referred to: 
The IMO Polar Code 1024(26) ‘Ships operating in polar waters 
and relevant classification rules’. 
2.25 Research
Various new tug types have been developed during the last 
years which have been addressed in the former paragraphs. 
Although ongoing research results in continuous 
improvements of modern tug types, only daily practice of 
ship handling shows to what extent the new tugs meet the 
expectations and what the capabilities and limitations really 
are. Nevertheless, research is essential to produce ever better 
tugs, be it for normal ship assistance, operations in waves, for 
escorting or for the living conditions of the tug crew. 
Research not only focuses on hull form, skegs, ropes, deck 
equipment, operations in waves, etc, but also on such aspects 
as liveability on board, automation and sensor information. 
A very wide range of interest, all focused on increasing 
performance and safety.
Apart from model tests and tank tests a method used in the 
design stage is CFD (computational fluid dynamics). CFD 
is the use of applied mathematics and physics implemented 
in dedicated software to simulate how a fluid (gas or liquid) 
flows, and how the fluid affects objects as it flows past. CFD 
has been around since the early 20th century and many 
people are familiar with it as a tool for analysing air flow 
Figure 2E.19: Tank tests for escort performance with Damen 
ASD tug 3212. Photo: Damen Shipyards
Figure 2E.20: Svitzer Deben ASD3212 escorting in the indirect mode 
in real-life situation. Photo: Damen Shipyards
around cars and aircraft. It is also often used in ship and 
tug design, for instance to optimise the tug’s underwater 
body, the inflow of the propeller disk and to optimise nozzle 
performance. 
At the start (around year 2000) potential flow calculations 
were possible, but since 2010 onward it has also been possible 
to make viscous flow calculations to investigate complex 
turbulent flows. Potential flow calculations can be valuable 
to enable a ship designer to study smooth hull shapes and 
slender bodies, particularly in the forward area. Gradually 
the flow turns into turbulent flow along the hull’s length due 
to friction and the formation of a boundary layer. For flow 
and distribution into the propeller, detailed viscous flow 
calculations are needed that can account for the frictional 
effects and a correct representation of the boundary layer 
around a ship.
Tug boats are typically full bodied vessels and introduce 
large, turbulent, sometimes separated, flows along the thick 
bodies. Today tugs are being calculated with CFD, and 
reasonably accurate resistance and flow distribution into 
the propeller area can be calculated for tugs; in recent years 
even the flow around hull with appendages in indirect modes 
can be determined. However, one has to consider that every 
single condition calculation requires a significant amount 
of computer power and time. And that analysing various 
conditions and speeds is very costly and time consuming.
78 Tug Use in Port
Figure 2E.21: CFD simulations of flow pattern around a tug at speed.
Courtesy Damen Shipyards
Figure 2E.22: CFD of wave pattern around a tug at speed.
Courtesy Damen Shipyards
2.26 Tug performance
With respect to tug performance it is good 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 withbollard pull conditions.
1) When 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 conditions. This is, for instance, 
when a bow tug is pulling a ship having headway. When 
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 stern 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 through 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 
revolutions are doubled, the force will increase by a factor of 
four, while the required engine power increases by a factor 
of eight. This relationship not only applies to bollard 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 not. Also, the type of 
propeller and nozzle fitted is important. To determine the 
towing force of a tug, bollard pull tests are carried out at 
different engine ratings, particularly at the manufacturer’s 
recommended continuous rating (MCR). Tests can also be 
carried out at engine overload conditions, for instance with a 
maximum rating that can be maintained for a minimum of 
one hour, and also with just one propeller working.
Bollard pull tests are carried out with engines ahead and 
increasingly, especially for azimuth tugs, on astern. Bollard 
pull tests should be carried out with sufficient underkeel 
clearance and line length to avoid the tugs’s propeller wash 
having any influence on propulsor performance and hence 
towline force. Other factors affecting the towline force are 
current, waves and wind, which should be minimal during 
testing. Bollard pull is measured by an electronic load cell 
connected in series with the towline. 
Classification societies issue different regulations for the 
bollard pull tests. In 2016 a Joint Industry Project was 
initiated by MARIN with the objective to derive requirements 
regarding towline length, water depth, test duration, 
environmental conditions and instrumentation based on 
scientific proof. Together with more than 30 companies clear 
definitions and procedures were developed to ensure the 
reported performance of a tug represents the tug’s maximum 
performance that can be realised in service conditions, 
irrespective of trial conditions, that can represent contractual 
performance obligations.  
Thus, CFD simulations provide the designer with high 
quality information on the entire flow around a tug. 
Among others, Lloyd’s Register gives guidelines for CFD 
Escort Performance (publication ShipRight. Design and 
Construction. May 2016). However, to successfully use 
CFD for practical calculations of complex turbulent flow, 
a thorough understanding of the errors involved in such 
simulations is required, and the relation to the computational 
set-up employed. Adding complexity to the simulations 
has an effect on the errors. Therefore, a procedure of 
verification and validation should be followed. Verification 
to quantify the numerical uncertainty can for instance be 
done by systematically reducing the cell size of the grid, and 
validation focuses on the modelling uncertainty.
Considering the ever increasing computer power, there is 
a bright future for CFD in the shipping and tug industry. 
However, there is also a clear need of good designers and 
users, as indeed the well known statement applies to CFD as 
well : Garbage in, Garbage out! 
Note 13: When seeing the flow pattern around the tug in figure 
2E.21 it can be observed that there is an increased water speed 
along the sides of the tug (the blue coloured lines) and a retarded 
flow (red lines) at the bow and stern. This is exactly in line with 
the water flow around a ship to be discussed in Chapter 6. 
Publication of research results are always very welcome so 
that all involved in one way or another in tug design, tug 
building and tug operations can learn from the results and/
or make use of them when needed and applicable. 
An example: Based, among others, on the results of the 
research carried out during the SafeTug JIP (joint industrial 
project) at MARIN, the Netherlands, which reported in 2010, 
Bureau Veritas published the Safety Guidelines for Design, 
Construction and Operation of Tugs in July 2014.
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Types of Harbour Tug 79
with controllable pitch propellers is given. The astern thrust 
of conventional tugs with fixed pitch propellers is higher 
and around 65 per cent of maximum ahead thrust, but it 
depends strongly on the nozzle type, propeller/rudder design 
and configuration. For example, the Towmaster system may 
improve ahead thrust to even more than 1.50 tons BP/100 
BHP, while a very good astern thrust of more than 70 per 
cent of maximum ahead thrust can be achieved.
Note 14: Particularly for the more sideways thrust, it is difficult 
to say how accurate the thrust vector diagrams are. Simulated 
or calculated performance diagrams should therefore, as far as 
possible, be validated in full scale trials.
These diagrams show the achievable thrust at zero speed 
in different directions for a number of tug types with equal 
power installed. The achievable ahead thrust per 100bhp 
installed power as shown in the diagrams is 1.1 tons for a VS 
tug, 1.4 tons for tugs with azimuth thrusters and 1.5 tons for 
conventional tugs with propellers in nozzles.
Note 15. A good indication of a tug’s ship assist performance 
over a range of speeds can be given by so-called polar diagrams 
as is shown in Chapter 4.
Based on a large matrix of model and full scale bollard pull 
tests it shows that for most tugs no performance degradation 
takes place when the water depth is approximately 6x 
the propeller immersion depth with a line length of 50x 
propeller diameter. In conditions lower than these thresholds 
performance degradation can be expected. Water density also 
affects bollard pull, whereby the thrust in high density water 
is higher for the same power compared to the thrust in fresh 
water. The largest uncertainty in bollard pull is, however, the 
sensitivity of the load cell measurement device for alignment, 
incorrect calibration procedures, temperature, torsion and 
instrumentation drift. Frequent recalibration and proven 
insensitivity to these factors is necessary for accurate bollard 
pull measurement. 
During bollard pull trials, the towline force should be 
recorded electronically using a sampling rate of 1Hz or better 
to avoid aliasing (= digital disturbance, distortion).The figure 
certified as the tug’s continuous bollard pull is the towing 
force recorded as being maintained without any tendency to 
decline for a duration of not less than 5 minutes for harbour 
tugs. Repeat measurements are recommended to give insight 
in the repeatability of the load cell and stability of the tug 
and towline force. The bollard pull trial code is schedule to be 
finalised and published in 2018.
Table 2E.3, opposite, gives a range of the ratio bhp - bollard 
pull for different propeller configurations. Because the relation 
between bollard pull and engine power depends on several 
factors, such as hull form, nozzle and propeller type and size, 
etc. these values do vary.
The relationship between engine power and bollard pull varies 
also with the extent of engine powerand in such a way that a 
conventional tug with 700bhp and a fixed propeller can attain 
two tons/100bhp, while for conventional tugs with about 
6,000bhp with nozzles, towing force may even be less than 1.3 
tons/100bhp.
Propeller performance is also shown in so-called thrust 
vector diagrams. Several kinds of these diagrams exist, all of 
them giving different information. Thrust vector diagrams 
give information on propulsion performance with zero speed 
in different directions, which is also important information 
to assess the tug’s assisting performance. An example of 
thrust vector diagrams with an indication of thrust forces is 
given in figure 2E.23. It gives propulsion performance at zero 
speed for equal installed power. Side thrust and the influence 
of interaction of propellers on side thrust are clearly shown 
in the 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 per cent and 95 per cent of ahead 
thrust. In the diagram the astern thrust of conventional tugs 
Tug propulsion type Bollard Pull in 
Tons/100kW
Bollard Pull in 
Tons/100bhp
Azimuthing thrusters
Stern drive 1.5 – 1.7 1.1 – 1.3
Tractor 1.3 – 1.5 1.0 – 1.1
Voith Schneider tractor 1.2 – 1.4 0.9 – 1.0
Conventional drive
Nozzled propeller 1.8 – 2.0 1.3 – 1.5
Open propeller 1.3 – 1.5 1.0 – 1.1
CPP* -2%
ICE* -2 - -4% according to ice class
*Not applicable on Voith Schneider propulsion
Table 2E.3 Ranges in relationship between brake horsepower and bollard 
pull for different tug types. Brake horsepower (bhp) is measured at the 
flywheel; shaft horsepower (shp) is measured at the propeller shaft (shp 
= ± 0.97 x bhp). Source: Damen Shipyards, the Netherlands
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80 Tug Use in Port
Figure 2E.23: Example of a thrust vector diagram.
Legend a) Tractor tug: Voith; b) Tractor tug: azimuth propeller in nozzles; c) Stern drive tug: 
azimuth propeller in nozzles; d) Conventional tug: twin screw (cpp) nozzles and bow thruster; 
e) Conventional tug: twin screw (cpp) with nozzles.
Assisting Methods 81
the performance of the different tug types in relation to the 
assisting methods is considered. Tug assistance 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 channel.
•	 Entry 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 3-6 knots and sometimes 
even higher. At these 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. On the other 
hand, speeds up to 6 knots may become rather high for 
effective tug assistance.
When port configuration is such that tugs are mainly used 
for mooring and unmooring operations, then tug assistance 
may comprise:
•	 The approach phase towards turning basin or berth.
•	 Turning in a turning basin.
•	 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 speed 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 passages, when passing bridges or 
negotiating sharp and/or narrow bends in the fairway, river 
or channel, or when entering 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 harbour or turning basin or when entering 
a lock.
Compensating for 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 
Figure 3.1: Port of Baltimore. Mortrac tug Harriet Moran (LOA 28.2m, 
beam 8.9m, 3,005hp) and reverse-tractor tug April Moran (LOA 26.5m, 
beam 10.5m, 5,100 hp) pushing at the stern of a ship dead in the water. 
A Mortrac tug is a combi-tug. Harriet Moran is a single screw tug with a 
triple rudder system and a retractable azimuth bow thruster of 640hp.
Photo: Pilot John Traut
3.1 Introduction
In the first chapter different types of port were 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 unmooring 
operations.
•	 Tug assistance mainly during mooring and unmooring 
operations only.
To what extent tug assistance is considered to be necessary 
depends on particulars of:
•	 The ship – including type, size, draft, length/width ratio, 
loading condition, 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 the 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 major factor 
of importance for selecting the most 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 
Chapter 3
ASSISTING METHODS
max allowable speed wtf
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82 Tug Use in Port
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 differences in local circumstances. Methods of 
assistance that different tug types are used for have already 
been mentioned briefly while discussing the various types.
Assessment of assisting methods in use all over the world 
shows only two markedly different methods:
•	 Tugs towing on a line.
•	 Tugs operating at a ship’s side.
Irrespective of tug type, both methods can be applied by 
almost all tug types, although for some tugs effective towing 
on a line can be somewhat problematic. The capabilities 
of modern and new tug types are large and this sometimes 
results in different ways of shiphandling. This has been 
shown in the former chapter but are still related to the two 
methods mentioned. 
In European ports towing on a line is mainly used, while 
in the USA and West Pacific ports tugs usually operate at a 
ship’s side, although in differentup 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, eg 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.
Cross lines/gate lines 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.
GLOSSARY OF TERMS
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Tug Use in Port xi
CRT Carrousel RAVE (Robert Allan Voith Escort) Tug – a tug with one Voith Schneider propulsion unit 
 forward and one aft. It has a ring around the wheelhouse along which the towing winch and towing 
 point travel.
Dead ship A ship which cannot use her own propulsion.
Density of air as used 1.28kg/m3
Density of sea water as used 1,025kg/m3
DOF Degrees of freedom refers, among other things, to the ability of simulators to reproduce ship and tug 
 motions, whether by the projection screen or by a hydraulic platform. Ship and tug motions are 
 expressed in sway, surge and yaw, being the motions in the horizontal plane, and heave, pitch and roll, 
 being the motions in the vertical plane. There are in total six degrees of freedom of movement for a 
 vessel that is unrestricted in its motions.
DOT Dynamic Oval Towing: A system whereby the towing point travels along an oval ring around the tug’s 
 superstructure. 
Escort tugs Tugs specifically built for escorting at high speeds.
Escorting tug Any type of tug escorting a ship underway.
FLNG Floating Liquefied Natural Gas
F(P)SO Floating (Production) Storage and Offloading Unit.
FSRU Floating Storage and Regasification Unit
Free sailing A tug sailing independently.
Girting Risk of capsizing, especially with conventional tugs, due to high athwartships towline forces. Also 
 known as girding, girthing or tripping.
Gob rope/gog line A rope or steel wire used on conventional tugs to shift the towing point.
HMPE High-modulus polyethylene fibre, under the trade names Spectra and Dyneema, used for high 
 performance ropes.
Hockle Kinking or twisting of a strand in a rope which makes it unfit for use.
IMO International Maritime Organization.
Lateral centre of pressure 
 The point of application of the hydrodynamic forces in the longitudinal centre plane of a ship or tug 
 This point changes with varying angle of attack of the incoming water flow and with draft, trim and 
 heel.
Lbp Length between perpendiculars.
LOA Length overall.
LWL Length at the waterline.
MBL Minimum Breaking Load (of a rope).
Maritime Autonomous Surface Ships (MASS) 
 Defined by IMO as a ship which, to a varying degree, can operate independently of human interaction.
GM Initial Metacentric Height.
Messenger A light rope attached to the towline in order to heave the towline on board a ship.
Norman pins Short iron bars fitted in the gunwales of the transom to prevent the towline from slipping over the side 
 gunwales. Sometimes called ‘king pins’.
Nozzle A tube around the propeller to increase propeller performance. The nozzle can be fixed or steerable.
OCIMF Oil Companies International Marine Forum.
PIANC Permanent International Association of Navigation Congresses.
Pendant/pennant A separate part at the final part of a towline 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 towline.
Pen (tug pen) A tug pen is a special protective berth from cyclonic winds and is enclosed on three sides.
Propulsion (types of) Azimuth propellers: 360° steerable propellers, which can deliver thrust in any direction. 
 Also called: ‘Z-pellers’, ‘Rexpellers’, ‘Duckpellers’ (azimuth propellers in nozzles).
 CPP: Controllable pitch propeller(s).
 FPP: Fixed pitch propeller(s).
 VS: Voith Schneider propulsion: propulsion system with vertical propeller blades, also called 
 cycloidal propulsion system.
PRT Prevention and Response Tug.
Significant wave height 
 The approximate wave height as seen by an experienced observer when estimating the height visually.
Snag resistance 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.
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xii Tug Use in Port
SPM Single Point Moorings.
Sponson 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.
Stemming A tug coming under the bow of a ship at speed.
Stretcher That part of a towline, between the original towline and pennant, which absorbs the dynamic forces in 
 the towline. Also called a spring and often made of nylon, polyester or a polyester/polypropylene 
 combination.
Towing point Point of application of the towline force. It is the point from where the towline goes in a straight line 
 towards the ship.
Towline A flexible hawser used for towing purposes.
Tripping 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 angle. The expression ‘tripping’ is also used for 
 girting.
Tug/engine power BHP Brake Horse Power: power delivered by the engine.
 SHP: Shaft Horse Power: power delivered to the propeller shaft (approximately 97% of BHP).
 BP: Bollard Pull, which in this book is expressed in the practical units of tons, equal to 
 1,000 kgf (= 9.80665 kN).
 MCR: Maximum Continuous Rating (of tug engine).
Ton The practical unit used in this book for force, eg for bollard pull, equal to 1000 kg force, and for 'weight', 
 equal to 1,000kg.
Tug simulation 
 Interactive tug: A tug simulated on a bridge manoeuvring simulator, able to interact with other 
 bridge manoeuvring simulators, which are simulating other tugs and/or the assisted 
 ship.
 Vector tugs: Tugs simulated by just a force vector.
UHMW polyethylene (UHMW PE) 
 Ultra High Molecular Weight polyethylene. Material used for dock fendering and for fenders of tug 
 boats at places where a low friction coefficient is required.
VS-tug A tug with VS propulsion.
The Overview 1
Figure 0.1: Reverse tractor tugs Seaspan Falcon (LOA 24.6m, beam 9.8m; BP ahead 39 tons) and Seaspan Eagle 
(LOA 28.2m, beam 12.6m, BP 71 tons) keeping bulk carrier alongside. Photo: SeaspanMarine, Canada
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 environmentalways depending on the type 
of tug used. Particularly in Europe and in the USA there is a 
tendency towards the use of more flexible types of tug. This 
tendency has an 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, or 
introduced, depending on the local situation or new tug type. 
For specific situations or circumstances, assisting methods 
are applied other than those in normal use. So it is possible 
that in ports where tugs normally work alongside, they 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 become 
necessary at seaside terminals, where tug assistance is 
affected by waves. If in calm weather it is normal practice to 
assist alongside a vessel, it may be considered safer to tow on 
a line when weather and sea conditions deteriorate in order 
to avoid parting towlines and losing control of the vessel.
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 these tugs should, 
with the assistance method applied, be capable of effectively:
Controlling transverse speed towards a berth while 
compensating for wind and current during mooring/ 
unmooring operations
During mooring operations a ship’s longitudinal ground 
speed is practically 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 the tugs.
The tug assistance required 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 have been described. Environmental 
conditions have a large influence. For instance, when tugs 
are used mainly for mooring/unmooring, the influence of 
currents can be such that although ship’s ground speed is 
low, say two knots, the speed through the water can be rather 
high. With a bow current of two knots, the speed through 
the water is already four knots. Situations then become 
comparable to tug assistance during a transit with the 
higher ship’s speeds and the associated requirements for the 
assisting tugs.
Additional services such as mooring boats also affect the 
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 can be pushed/pulled up to a berth.
It can be concluded that the port configuration, the influence 
of the environmental conditions and port services have a 
prominent bearing on the requirements for tugs and the 
method of tug assistance, while ship’s speed is an essential 
factor.
Figure 3.2: Combination 
of tug towing on a line 
and tug at the ship’s 
side (shoulder). Tanker 
Gener8 Ulysses; ASD-
tugs Smit Schelde (LOA 
28.67m, beam 10.43m, 
BP ahead 59 tons, astern 
56 tons) and Fairplay 
24 (LOA 34.75m, beam 
10.80m, BP ahead 52 
tons. Photo: Kees Torn
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Assisting Methods 83
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 the starboard side of the bow will give the ship a 
swing to port (see figure 3.10).
In some ports in the USA and in the Panama Canal a stern 
tug is used as shown in figure 3.8. A rudder tug can control 
a ship’s speed and a conventional tug can steer a ship in the 
required direction by giving forward thrust and applying 
starboard or port rudder. Other types of tug such as VS tugs 
and ASD-tugs also use this method. A similar method is 
sometimes used on Dutch inland waters.
Forward tug alongside and aft tug on a line during 
approach towards a berth and push-pull while mooring
This method, which does not differ much from that 
mentioned above, is mainly found in the ports of Japan, 
According to research carried out in 1996 into assisting 
methods in use in ports around the world, the two methods 
are generally applied in the following ways, assuming two 
tugs assist a vessel:
Tugs alongside during approach to the berth and pushing 
or push-pull while mooring
This method is normally used in the majority of ports in the 
USA, Canada, Australia, Malaysia, South Africa and also 
at large oil terminals in Norway. While the method used in 
these ports is similar, the type of tug differs. The way tugs are 
secured using this method depends mainly on the type of 
tug. When using tugs with omnidirectional propulsion they 
are made 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 figure 3.3).
In the USA tugs may be secured alongside a ship by one, 
two or three lines, depending on the type of tug, the local 
situation and the assistance required. Conventional tugs 
normally operate with two or three lines made fast, though 
in some cases only one line is deemed sufficient (see figure 
3.4). 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. On 
other tugs both lines may come from a winch. The third 
line, the stern line, is needed 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.
Owing to their better manoeuvrability, twin screw tugs or 
tugs with steerable nozzles normally operate with fewer lines 
when assisting at a ship’s side. Usually one or two lines will 
then be sufficient.
In the USA other methods are also used by tugs operating 
at the ship’s side. When breasted or alongside towing, also 
called ‘on the hip’ or ‘hipped up’, tugs forward and/or aft 
are lashed up solidly alongside a vessel (see figure 3.5). This 
alongside towing is also operated in many other ports in the 
world, but mainly when handling barges. 
When a tug is lashed up, tug and ship work like a twin screw 
ship with two independent rudders. When 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 thruster 
(see figure 3.6). 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 M in length are sometimes handled as 
a ‘dead ship’ by a VS tug lashed up alongside (see figure 3.7).
In USA 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 astern can 
Figure 3.3: Tugs alongside at approach and push-pull while mooring/
unmooring.
Figure 3.4: Conventional USA tug secured with backing, spring and 
stern lines. In situation 2 the ship moves astern. If ship moves ahead 
the stern line will lead forward. Depending on the assistance required 
and local situation, one, two or three lines may be required.
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84 Tug Use in Port
They are reverse-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 tugs have to assist while towing on a 
line, for example when assisting ships to enter dry docks or 
floating docks.
Taiwan and Hong Kong (see figure 3.11). The after tug is 
made fast by a tug’s bow line amidships or at the starboard 
or port quarter aft and follows the ship. The forward tug 
is made fast at the forward shoulder, also with a bow line. 
The after tug is used for steering and speed control. During 
berthing manoeuvres the tugs change over to the push-pull 
method. Tugs in these ports are all of similar design, more 
or less specifically constructed for this type of operation. 
Figure 3.5: Alongside towing (USA).
Figure 3.6: Forward tug secured alongside. As shown the ship can 
turn on the spot and when the tug applies hard port rudder and engine 
ahead, the ship moves crosswise. Ship’s ahead power to be equal to 
tug’s ahead power.
 Figure 3.9: Tug Cerro Picacho, tractor tug (LOA 28.9m, beam 13.5m, 
BP 82 tons), operating as steering tug in the Panama Canal.
Courtesy Panama Canal Authority
Figure 3.8: Rudder or steering tug.
Figure 3.7: Alongside towing in Cape Town for a ‘dead ship’ 
up to 100m in length.
Figure 3.11: At approach, forward tug alongside and stern tug 
on a line; push-pull while berthing.
Figure 3.10: ASD tug Gramma Lee T Moran (LOA 26.7m, breadth 9.8m, 
engines 5,100 bhp) pushing the bow of Queen Mary II to port on her 
maiden voyage to New York, April 2004.
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Assisting Methods 85
Apart from the countries 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 
the case in some USA ports whereby the stern tug operates 
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.
Tugs towing on a line during transit towards a berth 
and while mooring
This is the assisting method used specifically in Europe, most 
often when conventional tugs are assisting vessels, but other 
types of tugs are also used for this method. The method is 
also applied in many other ports of the world, especially in 
ports working with conventional tugs (see figure 3.12). In 
many of these ports, ships are assisted by tugs during transit 
towards the berth, eg, on the river, from the river into the 
harbour and through harbour basins up to a berth. The 
advantage of this method of assistance is that it can be used 
in narrow waters. This method is also used, therefore, when 
passing narrow bridges or entering locks and dry-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 bow as can be the case 
on some reverse-tractor tugs. The tug can then react very 
quickly and only a little manoeuvring space is required (see 
figure 3.13).
The type 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 the tug’s mass and the hydrodynamic forces 
on the tug’s hull. The increasing size of ships required the 
introduction of more powerful tugs. Modern conventional 
tugs are more manoeuvrable and have more engine power 
and generally a smaller length/width ratio. These tugs are 
still effective when a ship has speed. Due to the limitation in 
capabilities of conventional tugs, new tug types have been 
introduced such as tugs with azimuth propulsion. Also, VS 
tugs have for many years been used for towing on a line.
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 towing 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 3.14). The more familiar pilots and tug 
captains become with the capabilities of these tugs, the better 
the capabilities are applied to shiphandling. The method is 
practised where mainly tractor, reverse-tractor or ASD-tugs 
are used.
Combinations of the above systems
In many ports various tug types are operated and to assist 
larger ships more than two tugs are often required. Moreover, 
port entry or berthing manoeuvres can be so complicated 
that not just one assisting method is used but a combination. 
Figure 3.12: Towing on a line at the approach and while mooring.
Figure 3.13: Ship is passing a narrow bridge and a conventional 
tug forward is assisting with two crossed towlines. The tug can react 
quickly and only a little manoeuvring space is required.
Figure 3.14: Towing on a line at the approach and push-pull 
while mooring.
Figure 3.15: Combination of different assisting methods. Reverse-tractor 
tugs or ASD-tugs alongside and on a line aft. A conventional tug forward. 
A good configuration for steering and, in particular, when only a short 
stopping distance is available. Nearer the berth one of the tugs alongside 
has to shift to the other side to push.
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 3.15.
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86 Tug Use in Port
3.2.2 Push-pull versus towing on a line
The push-pull method has been discussed as well as towing 
on a line, and even the possibility of using a combination of 
both methods. 
Tugs operating alongside in the push-pull mode have 
the advantage that they can act rather quickly with short 
response times, which is of particular importance during 
berthing operations. The risks associated with tugs working 
in close proximity to the bow of a ship having speed, such as 
interaction forces and turning moments, are negated.
On the other hand, there are some drawbacks having tugs 
operating in this way:
•	 Towlines are often very short which presents a problem 
in wave conditions.
•	 When pulling with short towlines, there is a loss of 
pulling effectiveness caused by the tug’s propeller wash 
hitting the ship’s hull at right angles. Tugs may lengthen 
the towline if equipped with a towing winch and so 
decrease the negative effect.
•	 In case of conventional tugs, pulling effectiveness is low 
due to the low propeller performance when engines are 
running astern.
•	 Combined width of ship and tugs can be a problem in 
narrow passages such as locks and bridges.
•	 Tugs must be used on the opposite side to which ship is 
to berth. There is no flexibility in berthing side as soon 
as the tugs are fastened alongside.
•	 When the ship has to be turned, the turning lever is 
relatively short, namely, the distance between the two 
tugs at ship’s side. 
•	 With high windage ships and tugs fastened at the 
leeside, the tugs may become jammed between ship and 
shore or quay if the winds or gusts become too strong.
Speed control by stern tug
Omni-directional stern tugs are sometimes used to:
•	 brake the ship’s speed or; 
•	 to keep the ship’s speed at a low level of, eg, 3-4 knots 
which is lower than the ship’s Dead Slow Ahead speed,while the ship keeps its engine running in order to be 
able to steer.
The first can be necessary in case of an engine break-
down and the ship has to be stopped, or perhaps when 
approaching the turning circle to help in braking the 
speed. Simultaneously, the method can be used to avoid the 
transverse effect of the ship’s propeller when the engine is 
running astern which could have a negative effect on the 
manoeuvre. The time that the tug is pulling is usually only 
short.
The second method is utilised when approaching a lock 
(see figure 3.16), when passing a narrow bridge in windy 
conditions with a high windage vessel, or when moored 
ships have to be passed with a low speed. It is also used when 
the Dead Slow Ahead speed of the ship is too high, more 
than 6-7 knots, too high for the forward tugs to make fast 
safely, and continuously stopping the engine is not an option 
because of the wind or because poor steering performance 
with stopped engine. In these cases, the time that the stern 
tug is pulling can take much longer.
Keeping ship’s speed under control is becoming more 
commonly utilised. Tugs should be capable of doing so but 
vibration on board the tugs can be tremendous. For this 
reason tug masters may be unhappy with the speed control 
method when it is used for long periods of time. See also 
paragraph 4.3.4. 
 
Figure 3.16: Azimuth 
tractor tug Arion (LOA 
28.75m, beam 9.1m, 
BP 45 tons, pulling 
backwards on tanker 
Torm Loke and so 
enabling the tanker to 
keep engine running 
and ship steerable.
Photo: Port Towage 
Amsterdam
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Assisting Methods 87
In the ports of Japan, Taiwan and Hong Kong there is one 
assisting method and mainly one type of tug. The 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 method. It is anticipated 
that for these ports the reverse-tractor tug is the type that 
will usually be ordered in the future.
There is often a steady development towards a particular tug 
type. For instance, years ago there were still several VS tugs 
in the Port of Yokohama. This type has now been replaced by 
the typical Japanese type of tug, the reverse-tractor type.
In Europe towing on a line is general practice, originally 
just with conventional tugs. Due to the limitations of 
conventional tugs, various tug types with omni-directional 
propulsion are now 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 
•	 The tug masters have only little insight into the 
manoeuvres being undertaken by the pilot. 
 
When tugs are towing on a line the risks of interactions do 
play a role, in particular when passing a towline near the 
bow. Response times may also be somewhat longer. There 
may also be a loss of towing effectiveness when the propeller 
wash impinges on the ship’s hull, depending on the length of 
the towline and towing direction.
Towing on a line has a number of important advantages:
•	 Towlines are longer, consequently effect of waves and 
even waves of passing ships are smaller.
•	 If necessary total path width can be decreased to 
approximately the width of the assisted ship, which is 
important for bridge and lock passages. The capability to 
do so is enhanced with conventional tugs and ASD-tugs 
working over the stern. Total path width can be further 
reduced by using two towlines ( see figures 3.9 and 3.13) 
and by specific tug types, such as the Rotortug and the 
new types of tugs with one propulsion unit forward and 
one aft. 
•	 With a tug towing on a line forward and aft, the ship can 
either moor with starboard or with port side alongside.
•	 Tugs operate at the safe side in case of crosswinds or 
cross currents and avoid getting jammed.
•	 In case where the ship has to be turned, the turning lever 
is as large as possible.
•	 Tug master has a better view of how the ship is behaving, 
eg, does it drift, turn in the wrong direction or too 
slowly, and can anticipate if necessary.
•	 During mooring tug masters have a good view of 
distances forward and aft and can inform the pilot if 
there is a risk of hitting any obstacles.
3.2.3 Locks and tug assistance
Large ships entering locks are assisted by tugs in various 
ways. In most cases just one towline is used (see figures 3.16 
and 3.18). However it might also be possible that the assisting 
tugs use two towlines from two independent winches as is 
the case in the new locks of the Panama Canal (see figures 
3.9 and 3.17). These tugs are well equipped for this type of 
operations. The advantage of using two lines is to enable 
the tug to be effective in the limited width of the locks and 
to reduce response times. However, securing tugs with two 
towlines can be problematic with the trend toward minimum 
numbers of crew members on board ships.
More modern tug types such as Rotortugs and the tugs 
with one propulsion unit forward and one aft have specific 
manoeuvring capabilities that are also able to assist within 
the ships width. See paragraph 2.11.3 and 2.19.3.
3.2.4 Relationship between type of tug 
and assisting method
As can be seen, there is a relationship between type of tug 
and assisting method used. An essential factor is whether 
a tug should be suitable to operate at a ship’s side, tow on a 
line, or both. For the attentive reader it will also be clear that 
the most suitable tugs are not always available or used.
Figure 3.17: Ship entering the new Panama Canal Lock with forward 
tug with two towlines. Courtesy Panama Canal Authority
Figure 3.18: Ship entering locks at Terneuzen with forward tug 
Multratug 14 (Voith Schneider tug, LOA 34.50m, beam 11.60m, 
BP 75 tons, with one towline. Photo: Adri van de Wege
Pgua_Supplay
Destacar
Pgua_Supplay
Destacar
Pgua_Supplay
Destacar
88 Tug Use in Port
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 
cannot be ballasted or trimmed sufficiently require tug 
assistance.
Note 1:
Tests in ice conditions in the North of the Bay of Bothnia in 
March 2016 with newly developed bronze propellers showed 
that these propellers can be suitable for vessels in 1A Super 
ice conditions. The testing conditions gave reliable results and 
input for the propeller design for 1A and 1A Super ice class 
vessels (for Ice Class notations, see paragraph 2.24).
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: eg, 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 handled by tugs in ice conditions depends 
largely on the type of tug. Tugs need to be adapted to work 
in ice conditions. Those with light draft and propellers fitted 
in nozzles have very limited capabilities, because when they 
are moving astern the nozzles immediatelyfill with ice. Even 
with tug engines on ahead ice can fill the nozzles. When 
this happens the tug should immediately be stopped and the 
nozzles cleared by repeatedly reversing propeller thrust. That 
is why this type of tug, and other tugs having problems in 
ice, should not tow on a line. The assisted vessel might not 
react fast enough and/or not be able to stop immediately to 
avoid danger of collision or worse.
For these tugs in particular, 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 bends in a channel or river and during berthing 
or unberthing operations. Assistance in ice conditions 
during arrival and departure is then carried out mainly by 
pushing and includes breaking the ice and sweeping away the 
ice from between ship and berth. Without the help of tugs 
it is almost impossible, in most cases, to remove ice from 
between a ship and berth.
Note 2:
In thick ice in the Port of St Petersburg, the ASD-tugs, when 
operating as bow tug, use ship lines which are taken on the aft 
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. Also 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.
As can be seen in Chapter 2, after the Voith tugs, ASD- 
and reverse tractor tugs, many new tug types have been 
developed with new and more extensive capabilities. These 
are sometimes in response to the needs of a specific port 
or just as a new idea. 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.
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 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. 
For tugs that are designed for operations in ice condition, 
please see Chapter 2. 
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 largest 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 Types 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 
Assisting Methods 89
In this paragraph the tugs referred to are ASD-tugs operating 
over the stern, because the percentage of conventional tugs in 
the world total tug fleet is becoming ever smaller. 
3.3.5 Ice blockage and nozzle clearing 
Performance of tugs with azimuth thrusters in nozzles 
operating in ice can be improved by proper designs such as 
adequate clearance between the hull and the thrusters and 
by short reaction times for pitch changes or for turning the 
thrusters adequately to remove ice blockages as quickly as 
possible. Nevertheless, the thrusters will inevitably become 
blocked now and then, although much depends on the ice 
condition.
The following is based on the experience of tug masters 
skilled at operating in ice conditions. 
 A key factor is that the tug master should basically operate 
in such a way that the risk of ice blockage is minimised. 
The tug master be able to recognize ice blockage problems 
quickly and react rapidly when needed.
A signal for ice blockage is that one or both propellers/
thrusters start to vibrate. It may also be seen on the meters 
which propeller is affected, or which one is most serious. 
Conventional tugs with fixed pitch propellers in nozzles, 
should reverse the propeller revolutions to clear the ice out of 
the nozzles.
In case of controllable pitch propellers, if possible, pitch 
should be reversed to get the ice out.
With ASD-tugs and comparable tugs possibilities are greater.
If the tug still has speed through the water, one thruster can 
be declutched and thruster turned to wash the ice out. When 
cleared, the thruster can be turned back, clutched-in and 
rpm be increased. If the other thruster is blocked as well, it 
can then be declutched and turned till the ice is washed out. 
Then it can be turned back and be clutched-in and rpm be 
increased. It may even happen that this has to be done three 
times in five minutes. 
Other methods often used are:
A. Thrusters with fixed pitch propellers (see figure 3.20).
•	 Tug master feels a vibration of the propeller. Propeller 
will be stopped and declutched.
•	 Both thrusters will be set crosswise in such a way that 
the working propeller can blow out the ice out of the 
blocked thruster. Care should be taken that the working 
propeller does not get blocked by ice. 
•	 In addition, the blocked thruster can be turned 180° 
several times, until the ice is out of the thruster.
B. ASD-tugs with variable pitch propellers.
1. As soon as the tug master feels vibration of the propeller, 
the clearing procedure starts. 
2. The loss of power will be first compensated by increasing 
the power of the other propeller.
3. Then the propeller of the blocked thruster will be set for 
30 per cent astern. The tug’s speed will drop unless the 
power of the other propeller is sufficiently increased, 
especially important during operations.
hook (no aft winch available) and then proceed with the bow 
into the ice to protect propellers and nozzles. 
While 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 fenders, etc. Tugs 
should, of course, always be very careful when working 
between a ship and the dockside.
With respect to tug towing wires or ropes, they should 
retain their strength in low temperatures but should never 
be allowed into icy water because it will then be very hard to 
handle them.
The most reliable tugs in ice conditions are normal ice 
strengthened conventional tugs with open propellers. 
Twin screw tugs are preferable because of their better 
manoeuvring properties.
Propellers and rudders may have ice protectionand nozzles 
may be fitted with protection bars or ice knives fore and aft 
of the nozzle. Although nozzle construction 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 that this type of tug is 
worthless in those 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.
The best type of tug for operating in ice conditions are:
•	 Conventional twin screw tugs, preferably with open 
propellers and 
•	 ASD- tugs and comparable tugs.
Voith tugs are sometimes used as well. 
With regard to open propellers, full scale trials were carried 
out in 1984 in Finland with two ice-going tugs, one fitted 
with an open propeller and the other with a steerable nozzle, 
to investigate their 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.
 Figure 3.19:Tug with ice scraper on the bow to clear ice from dock 
and lock walls. See also figure 3.39. Photo: René Beaumont, Canada
90 Tug Use in Port
The risks mentioned above are further illustrated in the 
pictures in the following paragraphs. 
3.3.6 Berthing in ice
A berth should be approached at a small angle. As soon as 
the forward spring is secured the engine should be set to 
Dead Slow Ahead. Propeller revolutions or propeller pitch 
should be increased gradually, just avoiding breaking the 
spring. It is best to double the spring and the rudder should 
be used to swing the stern of a vessel in and out and away 
from the dockside. The 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 berthed. In this way, provided it is weak 
ice, it can be removed completely from between the ship and 
berth. In the case of dense and thick ice the assistance of tugs 
is required.
In some cases berth location could be such that a berth can 
be approached parallel to the dock (see figure 3.21). In this 
case ice may be pushed away by the bow. If there is unbroken 
ice on the starboard side it will push the ship towards the 
berth and prevent her swinging out. Care should be taken 
to avoid any ice getting between ship and dock. It may be 
necessary to 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 of large pans of ice or dense, 
thick ice directly in the ship’s track. Other methods should 
then be adopted such as the use of tugs. Several procedures 
for the use of tugs in ice during an approach towards a berth 
while berthing or unberthing 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 3.22 A, B). 
The 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 3.22 C, D). 
The ship itself can swing its stern in and out by rudder action 
and use of the engine, as explained.
Sweeping ice away from round the bow area can also be done 
effectively by a tug just ahead of the ship (see figure 3.24). 
With its stern directed towards the ship’s bow, the tug can 
sweep ice away by putting its engines ahead. In this case the 
4. Mostly it works, otherwise same system should be used 
as with fixed pitch propeller.
It is also possible that ice blocks become trapped between two 
blades of a controllable pitch propeller. The tug master is then 
not able to adjust propeller pitch affecting operations. This can 
lead to damage. In this case also, the same system should be 
used as with fixed pitch propellers.
It will be clear that ice blockage can be risky during tug 
operations, in particular when a tug is fastened by its towline 
to a ship having way on, depending on the location of the tug 
and the way the tug is operating. 
Regardless of good clearing procedures, nozzles and 
propellers often do become damaged when working in ice, 
or sometimes nozzles get even lost. Thruster removal at ship 
yards is sometimes considered, although this is not a simple 
process. 
Figure 3.20: Schematic overview clearing procedure.
Figure 3.21: Ship approaches the berth nearly parallel to the dock. Ice is pushed away by the bow. The ship is pressed towards the berth 
by unbroken ice on the starboard side.
Assisting Methods 91
Another method by which good results are obtained is 
moving the ship astern towards the berth to moor with its 
starboard side alongside (see figure 3.27). After approaching 
the berth at a small angle and securing the back spring, 
the engine should be set for astern. The propeller stream is 
normally very strong and will move the ice between the ship 
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 20-30m of free berth ahead of a 
ship’s planned position. The ship should approach its berth 
ahead of the planned position (position 1 of figure 3.25). 
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 moved astern 
while the tug is constantly pushing the bow towards the 
dock.
A bow thruster can also be very effective in sweeping ice 
away (see figure 3.26). 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. The 
bow should be held to the dockside by the ship’s ropes and 
by the pushing tug. The water flow of the bow thruster will 
sweep ice away from between the ship and dock.
Figure 3.22: Tug assistance 
in ice during approach to the 
berth and while mooring.
Figure 3.24: Tug sweeping ice away from between ship and dock.
Figure 3.23: Tug Tornado (ASD-tug type 2810; 4,250hp) sweeping ice 
away between ship and berth. Ship’s spring line is ashore.
Photo: Captain Sergei Milchakov, St Petersburg
Figure 3.25: Mooring in ice when some 30m free berth is available in 
front of the bow position.
Figure 3.26: Combination of tug and bow thruster while mooring.
92 Tug Use in Port
In some cases, when possible, it is better to approach the 
berth astern with a stern tug towing on a line (see figure 
3.29). 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 movement.
With large ships, good results in removing ice from between 
ship and berth are sometimes obtained with two tugs 
working bow-to-bow. These two tugs, moving together 
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 
3.30. Obviously, a large number of tugs is required in this 
case. In case of a small ship one tug could be use for clearing 
ice by setting one thruster on ahead and one on astern, so 
both thrusters in opposite direction. 
3.3.7 Unberthing in ice
Before unberthing, tugs should break ice around the ship and 
in areas of about 20-40m distance fromthe bow and stern.
Some vessels can be taken off the berth by the stern with the 
assistance of a stern tug towing on a line (see figure 3.32). At 
the bow the ice between bow and dock will prevent the ship 
from coming too close to the berth. In addition, the stern 
tug will drift the ice between the ship and dock, which again 
prevents the ship from coming too close to the dock when 
moving astern.
Figure 3.27: Good results when approaching the berth astern 
and mooring starboard side alongside.
Figure 3.28: Tug assistance when mooring in ice with ships 
and powerful engines.
Figure 3.29: Ship approaching the berth astern. One aft tug secured. 
Occasional bursts ahead on the engine blow away the ice.
Figure 3.30: Two tugs operating bow-to-bow clearing ice between 
ship and berth while other tugs keep the ship in position.
Figure 3.31: Tug sweeping ice away between ship and berth. 
A second tug, ASD-tug Akmal near the bow, is making room for the ice 
coming out. Photo: Captain Sergei Milchakov, St Petersburg
Figure 3.32: Ship of medium size departing. Before departure tugs have 
broken ice around her in areas some 20-40m from bow and stern.
and dock quickly in the direction of the bow. The bow should 
be swung in and out by tug or bow thruster. This method is 
used and suitable for larger vessels, as propeller thrust astern 
is lower than on ahead and consequently the tension in the 
spring line(s) will be less.
These berthing procedures 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 normally carried out by 
tugs. After approaching the berth at a small angle, a spring 
line and head line are made fast forward (see figure 3.28). One 
stern tug on a line is used to take the stern from the berth and 
a second tug is used for pushing the stern towards the berth. 
This tug will also clear the ice between ship and berth. Ship’s 
propeller wash is not used. Berthing will, in general, take a 
long time.
Assisting Methods 93
Sometimes it may be necessary to unberth the ship bow first 
(see figure 3.33). A second tug may then be needed to break 
ice near the stern and to prevent the stern from coming too 
close to the berth. Sometimes even the assistance of a third 
tug may be required to crush ice at the outer side of the ship.
Figure 3.33: Unmooring bow first. A stern tug is required when ice near 
the stern needs to be broken and when the stern may touch the berth 
when the bow is pulled off. Sometimes a third tug is required to break ice 
alongside the vessel.
Figure 3.34: ASD-tugs Tornado and Akmal preparing a turning circle in 
ice for the ship to be turned. Photo: Captain Sergei Milchakov, St Petersburg
Figure 3.35: Channel through the ice prepared by icebreakers or strong 
tugs. A ship moving astern through the ice is safest. When the stern tug 
is stopped in or by ice the ship can immediately be stopped by propeller.
Figure 3.36: Ship entering the port backwards through the ice with after 
tug fastened with ship’s line. Photo: Captain Sergei Milchakov, St Petersburg
Figure 3.37: Ship moving backwards with after tug fastened 
with two ship’s lines – ASD-tug Sevruga (3,200 hp). 
Photo: Captain Sergei Milchakov, St Petersburg 
When a departing ship has to be swung around after being 
unberthed this should be carried out in a prepared area or 
channel in the ice. This area or channel should be prepared 
by large tugs or icebreakers prior to departure. Tugs handling 
the ship can assist the ship in swinging and break ice when 
necessary.
94 Tug Use in Port
3.4 Assisting navy ships 
Navy ships do visit commercial ports regularly. In navy ports 
they know how to handle their ships and they often have 
purpose-designed tugs with well-trained tug masters. This is 
not the case in commercial ports. As the frequency of visits 
is low, the level of knowledge and experience to handle these 
ships is equally low while tugs are not specifically designed 
for handling all types of navy ships. 
Prior to an arrival of a navy ship, and particularly in case of 
large navy ships, berth location and arrival procedures will 
be discussed between navy officials, port authorities, towing 
companies, and pilots. Tug masters will be instructed how to 
handle the navy ships.
3.3.8 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 enter dense ice 
and consequently lose speed very quickly. The assisted ship, 
therefore, should always use engines with utmost care. Even 
then the safety of the tug is still at risk. It is for these reasons 
that the safest method of towing on a line is moving a ship 
astern (see figure 3.35). The engine should at all times be 
ready to go ahead. When necessary, the ship can be stopped 
immediately.
3.3.9 Finally
Assisting ships when berthing or unberthing is an important 
task in ice covered waters, a task which takes much time and 
effort.
However, engine and/or rudder failures may happen on 
board ships when proceeding through ice. In that case the 
available tugs should also be able to assist, as is shown in 
figure 3.38.
The tugs may have to carry out specific duties in ice, for 
instance scraping ice from the walls of docks and locks. 
Some tugs on the St Lawrence Seaway are equipped to do so; 
they have on the bow ice scrapers as can be seen on the tug in 
figure 3.39. 
Further practical and useful information regarding 
navigating and manoeuvring in ice can be found in:
Handling Ships in Ice by Captain J Buysse. The Nautical 
Institute
Polar Ship Operations by Captain Duke Snider. The Nautical 
Institute
Figure 3.38: Tugs operating in ice. Docking bulker Panagia in Quebec Harbour after steering failure. Photo: Jean Hémond, Canada
Figure 3.39: ASD-tug Ocean Serge Genois with ice-scraper on the bow 
(7 January 2018). Photo: René Beauchamp, Canada
Assisting Methods 95
Aspects tug masters should be aware of when approaching 
and handling submarines 
While discussing the various aspects requiring attention, the 
specific terms used will be addressed too. See figure 3.40.
a. Specific hull form. When a submarine is on the surface, 
due to the almost round form and deep draught the 
widest part is below water. 
b. Protruding parts
 i. Sail or fin
 No. 1 in figure 3.40 is called the sail (USA) or 
fin (Europe/Commonwealth), also known as a 
fairwater.
 When on the water surface, the sail serves as an 
observation platform. It also provides an entrance 
and exit point when enough freeboard to prevent 
being swamped. Under water, the sail acts as a 
vertical stabilizer. 
 With some submarines, the sail also supports diving 
planes (or fairwater planes). See figures 3.41 and 
3.47. 
 ii. Hydrovanes or vanes, also called diving planes or 
hydroplanes
 With the planes the vertical motion of a submarine 
can be controlled. It allows the submarine 
to pitch its bow and stern up or down to assist in the 
process of submerging or surfacing. No. 2 in figure 
3.40 shows the port fore plane. Tugs should be aware 
of these vanes, particularly in reduced visibility. 
 Instead of fore planes, planes can be installed at 
each side of the sail or fin as mentioned above. 
These planes can be a problem for tugs operating 
alongside.
 Aft, just abreast of the top rudder blade (see no. 3) 
are the after planes. They are underwater and can 
not be seen. Tugs should therefore take utmost care 
when approaching the after part of a submarine. 
 Several submarines have the rudder and aft planes 
combined in a crossway system as shown in figures 
3.41 and 3.45.
Several of the navy ship types can be handled in the same 
way as merchant navy ships. Although, information about 
allowable maximum hull force is important when tugshave 
to push. 
However, there are two types of navy ships that require 
specific attention, viz. submarines and aircraft carriers. 
These two ship types will be dealt with below. 
In some ports, tugs might not be needed, where aircraft 
carriers can safely anchor within the harbour limits, 
although tugs will then be stand by for in case they are 
needed.
3.4.1 Submarines 
General information
There are various submarine designs, and they differ in size. 
Generally, submarines can be classified as follows: 
•	 Diesel-electric attack submarine ranges from 60 to 90 
meters in length with a draught of approximately 8m.
•	 Nuclear powered attack submarine (SSNs) ranges 
from 90 to 120 meters in length with a draught of 
approximately 10m.
•	 Nuclear powered ballistic missile submarine (SSBNs) 
ranges from 100 to 180 meters in length with a draught 
of maximum about 12m. 
This information is very generalized and can differ by 
submarine.
An important aspect of all submarines to be kept in mind 
is that submarines are like icebergs. This means the largest 
part, about 90%, is below the waterline. For rather small 
vessels they have a very large draft. As with icebergs they 
should be approached with care, as will be explained below. 
Although there are twin screw submarines, most of them 
are single screw. They have generally good steerage at about 
two knots and more. Submarine turning circles are naturally 
large. 
Figure 3.40. HMS Astute. Source: UK Ministry of Defence
Figure 3.41. Australian submarine pushed along the berth.
1: Fairwater planes; 2: combined after planes - rudder system
Photo: Arie Nijgh
96 Tug Use in Port
Many submarines can tow a sonar (line-array) and have 
therefore equipment near the stern rudder (a tow-box); 
depending on the submarine an anchor light may also be 
placed there (see figure 3.45). Other submarines may have the 
anchor light on top of the rudder.
Care should be taken by tugs that the tow-box and/or anchor 
light are not damaged by a towline or a tug. This can easily 
happen when a towline under tension must be replaced 
from port to starboard and vice versa. However, such a 
tug manoeuvre is only possible with submarines that have 
the planes and rudder in an X-configuration that only just 
appears above the waterline. With all other submarines the 
rudder is in the way. 
Merchant navy ships have indicated on the hull where tugs 
can push. On submarine it is the opposite way. There are 
No Pushing/`No Push’ areas where sensitive equipment 
is located. These will be marked on the casing above the 
waterline and the information has to be passed by the pilot 
during their pilot/tug master exchange (see figures 3.44 and 
3.45). 
Note:
An anchor must be carried to meet international maritime 
regulations. Submarines rarely use them. The anchor is stowed 
beneath the hull in such a way that the flukes make a flat 
surface with the hull.
Tug assistance and requirements for tugs
In navy ports tugs are often designed for the work to be 
carried out. In most cases these are highly manoeuvrable 
tugs with good fendering. The profile of such tugs together 
with the underwater fendering these tugs have, fits well with 
the profile of the submarines to be assisted whether alongside 
or when pushing. Tugs in commercial ports do not have 
these features. 
How tug assistance is carried out in the various commercial 
ports depends on the local situation, environmental 
conditions, and available tugs and tug types. 
As tugs in commercial ports are not fitted out as tugs in well-
equipped navy ports, the way tug assistance to be carried 
out should be such that submarines will not be damaged by 
 iii Propulsor
 The propeller is called propulsor and is the utmost 
aft end of a submarine. Contrary to normal surface 
ships the propulsor is behind the rudder instead 
of in front of it. This means that a kick ahead with 
hard rudder has no turning effect. Tugs should also 
take care and keep away from the propulsor and the 
propulsor wash, which can be strong. Figure 3.42 
shows the propulsor and the after planes. 
 iv Tiles
 Tiles are another important aspect for tug masters 
to be aware of when handling submarines. Tiles 
on the hull of a submarine absorb the sound waves 
of active sonar from another vessel, reducing and 
distorting the return signal, thereby reducing 
its effective range. Furthermore, they reduce the 
sounds emitted from the submarine, particularly 
its engines, and reduce the range at which it can be 
detected by passive sonar (a passive sonar listens). 
Source: Wikipedia.
 It will be clear that these tiles are crucial for a 
submarine. As they are costly and can easily be 
damaged by tugs, care should be taken that they 
stay intact and not be damaged.
Deck equipment submarine
Deck equipment differs completely from merchant vessels as 
can be seen in figure 3.40 and 3.41. A submarine is equipped 
with bollards which are below the deck when at sea to create 
a smooth deck. For fastening tugs and for mooring, the 
bollards are lifted from below the deck. 
On the most forward and aft end of the deck are the so-called 
bull rings, a kind of panama fairleads. The sets of bollards on 
the submarines casings are usually referred to by a number 
and a letter, beginning with number 1̀’ forward, and an s̀’ 
for starboard or `p’ for port. 
Note:
The SWL (Safe Working Load) will differ by submarine size 
and by navy. Tugs should be informed about the SWL of the 
deck fittings which is most important when current powerful 
tugs handle submarines. 
Figure 3.43. HMSTriumph. Anechoic tiles on the hull. Two patches of missing tiles are visible 
towards the forward edge of the sail. Source: NetMarine
Figure 3.42. Rudder, after planes and 
propulsor. Courtesy of Paolo Ulivi 
Assisting Methods 97
Figure 3.44. Dimensions Dutch 
Walrus class submarine and 
indications of bollards and 
‘no-push’ points (coloured red)
Courtesy of the Royal Dutch Navy
the assisting tugs. This means that towing on a line, with 
tugs fastened fore and aft via the bull ring on the bollards 
of the submarine, will be the best, this in combination with 
the push-pull method. ASD-tugs are therefore preferred, as 
they push with the bow. If pushing is needed and the bow 
fendering is not suitable enough for the submarine, then 
extra fendering on the bow of the tug is required. 
Fastening tugs alongside should be avoided because of the 
risk of damaging the submarine. This method is applied 
where tugs have underwater fendering such as in navy ports. 
See figure 3.47 and read there how the specific tugs are 
fastened. The recommended tug types are Voith tugs, ATD-
tugs or ASD-tugs. 
When the submarine has no propulsion, the same method is 
applied often together with a tug forward on a long line. Tugs 
with underwater fendering can also be found in the Panama 
Canal and are also assisting submarines without propulsion 
alongside. The method is applied in the narrow areas of the 
Panama Canal and especially through the locks. 
With propulsion and when going through the locks with 
locomotives, tugs will only escort and only assist if the pilot 
considers it to be necessary. 
Figure 3.46 Tug ASD 2509 in profile with a submarine
Courtesy: Damen Shipyards
Figure 3.45: 
‘No push’ area 
between red 
lines
Courtesy Royal 
Dutch Navy
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98 Tug Use in Port
In Hong Kong tugs never attempted to go alongside, because 
most submarines would tie up to a mooring buoy in the 
harbour. So generally, the submarines were escorted by a tug 
with a towline attached. The tug did very little unless a gentle 
pull was needed to get the bow near the mooring buoy.
So, it can be seen that it depends on the port, the waterway, 
the tug types and how the tugs are equipped, how 
submarines will be assisted. 
In short the following tug assist methods can be applied forsubmarines:
•	 One tug fastened forward when submarine is mooring 
on a buoy.
•	 Towing on a line, with one tug fastened fore and one aft 
via the bull ring on the bollards of the submarine, this in 
combination with the push-pull method. Best suited tug 
type is the ASD-tug. 
•	 Fastening tugs alongside when these tugs have 
underwater fendering. Recommended tug types: Voith, 
ASD-tug, ATD-tug.
•	 Fastening tugs alongside and one on a long line forward 
in case the submarine has no propulsion. Recommended 
tug type: See above.
Berths 
Although beyond the context of this subject, a few words 
about submarine berths. When berthing at a commercial 
quay care should be taken that the berth is suitable for a 
submarine with her specific wide underwater design and 
deep draft. Large fenders might be needed. In tidal ports, 
also tidal range should be considered. 
Mooring can also take place alongside a pontoon. A 
submarine needs such a floating pontoon that the widest part 
of the submarine (under the waterline) is kept away from the 
shore or embankment. As said before, submarines may also 
be tied up to a mooring buoy (see figure 3.48). So, it depends 
on the port how submarines are moored. 
Figure 3.49. Aircraft carrier Kitty Hawk in Hong Kong 
with two tugs stand by. Courtesy of Michael Luk
Figure 3.48: Royal Australia submarine HAMAS Farncomb moored 
on buoy in Sydney Harbour. Source: The Mariner 4291_Shutterstock
3.4.2 AIRCRAFT CARRIERS
Introduction
There are about 45 aircraft carriers. Dimensions vary 
between approximately 180m and 335m in length and can 
be 76,8m wide (flight deck width). Steering and propulsion 
may consist of up to 4 propellers. Several aircraft carriers are 
nuclear powered. 
It is of importance that all involved in the handling of 
the aircraft carrier are informed carefully about the tug 
locations, procedures to be followed and manoeuvres to be 
carried out. So it is vital that information about the various 
tasks is passed to pilots and tug masters well in advance. The 
pilot should also hold a pilot/tug master exchange with the 
tug masters when he has boarded the submarine in order to 
confirm the intentions.
How aircraft carriers are handled by tugs in commercial 
ports depends, as with submarines, again on the local 
situation, fairway width and alignment, environmental 
conditions and available tugs and tug types. 
Figure 3.47. Ballistic missile submarine USS Alaska 
Please note: The submarine is heading towards you. Two tugs with 
underwater fendering are alongside for `Active Escort’. As it is ATD-tugs, 
they are moving backwards with the propulsion units close to the stern of 
the submarine to be able to give effective steering assistance, if needed. 
See also clearance between fairwater planes and tugs. Photographer: LPhot 
Stevie Burke/Source: US Defence Government
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Assisting Methods 99
Figure 3.51. Tug under de flare of a UK aircraft carrier 
HMS Queen Elizabeth with indications of clearance. 
Courtesy of: A P Bannister MBE | Chief Admiralty Pilot QECP
wind speed during 30secs wind gust is then almost 19m/sec 
(gust factor 1.21; see Port Designer’s Handbook). This results 
in a required total tug power of approximately 190 tons, 
which includes a safety factor of 20%. Four pushing tugs, 
or four tugs towing on a rather long line, of about 50 tons 
bollard pull would then be needed to keep the ship under 
control. Ship bollards and fairleads should be strong enough 
to cope with these forces. 
Note:
When tugs have to pull to keep the ship up into the wind, 
a large loss in pulling effectiveness can occur when the tug 
propeller wash is hitting the ship’s hull near right angles. The 
larger the distance between ship and tug the smaller the loss. 
This factor should be taken into account for the total required 
bollard if such as situation may happen. 
Summarizing: Tug assistance for aircraft carriers depends on 
port lay-out, available tugs, local tug procedures and wind 
conditions. The following is recommended:
•	 Usually four suitable tugs, which can be employed as 
follows:
 – Two tugs forward and two tugs aft on a line via 
fairleads on or near the centre line of the aircraft 
carrier.
 – One tug forward and one tug aft via fairleads on or 
near the centre line and two tugs alongside
 – Four tugs alongside. 
•	 When the aircraft carrier must anchor in the port, two 
passive escorting tugs.
•	 Tugs that have to assist alongside should be able to lower 
their masts. Extra fenders between tug and aircraft 
carrier might be needed. 
•	 For the required bollard pull of tugs the wind condition 
should be taken into account. 
•	 Aircraft carriers require much manoeuvring room. 
•	 SWL of bollards and fairleads should be known as well 
as maximum allowable hull pressure or pushing points. 
Tug requirements and tug assistance
In general aircraft carriers have sufficient bollards and 
fairleads on the mooring deck and sunken bollards in the 
sites to secure towing lines. It is again important for towing 
companies and tug masters to know the SWL of the bollards 
as well as the maximum allowable hull pressure. If the 
aircraft carrier would anchor within the port limits two 
suitable and powerful escorting tugs are recommended. 
If the aircraft carrier has to berth, four tugs are generally 
recommended for commercial ports. Depending on tug, 
the way tug assistance is carried out, and aircraft carrier 
design, additional fenders between tug and ship might be 
needed. Figure 3.50 gives an indication of the clearances of a 
tug under the overhang, although this differs as said by tug 
and aircraft carrier design. There is one basic requirement 
for tugs handling aircraft carriers and that is that due to the 
large overhanging parts of aircraft carriers, tugs should be 
able to lower their mast, especially in ports where tugs are 
operating alongside, although it depends again on aircraft 
carrier design. 
The recommended tug types are Voith tugs and tugs with 
azimuth propulsion and all with proper fendering.
Aircraft vessels are huge vessels and require much 
manoeuvring room. An indication:
For the Nimitz class aircraft carriers, being the largest in 
the world, a minimum entrance channel width outside the 
breakwater should be about 240m and inside the breakwaters 
about 180m; the optimal turning circle diameter is 670 m, 
which is two times the length. (See References for Ìnterim 
Technical Guidance – Facilities Homeporting etc.’) 
Aircraft carriers have a large windage. The Nimitz class 
aircraft carriers will be considered for total required tug 
power. A windage area of approximately 6000-6700m2 is 
estimated. A windage area of 6500m2 will be taken into 
account. How much tug power is then needed to keep the 
ship under control? 
Suppose the middle wind speed of 7 Beaufort which is 15.5m/
sec, being a 10-minute average wind speed. The aircraft 
carrier will most probably react on 30secs gusts. Average 
Figure 3.50. Aircraft carrier George H. W. Bush with tugs alongside 
with masts down. Source: Shutterstock
Main deck to 
waterline 1.7m
Tug accomodation to ships 
huil 9.7m
Airdraft main deck at 
towing staple 7.9m
100 Tug Use in Port
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 point moves aft. Once a 
ship is in a steady 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 4.1 A).
For a good understanding, figure 4.1 requires a little 
explanation. 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 
Figure 4.1: Locationof the pivot point for a ship at speed
Situation A: Ship turning with starboard rudder. The pivot point 
lies between bow and amidships
Situation B: A tug is pushing forward. Although the pivot point lies 
further aft, the effect forward is low because of the opposing 
hydrodynamic forces also centred forward. When starboard rudder 
is also applied the pivot point moves further forward
Situation C: A tug is pushing aft. The lateral resistance forward 
contributes to the swing. The pivot point lies far forward, particularly 
when starboard rudder is also applied.
4.1 Introduction
Now that various assisting methods and types of tug have 
been introduced to the reader the more practical subject – 
effective ship handling with tugs – is addressed.
When a ship is stopped in the water, meaning she has no 
speed through the water, the effect of, let us say, a 30 tons 
BP tug is the same irrespective of type, assuming that the 
tug operates in the most effective way. Differences in tug 
performance mainly become apparent when a ship has speed 
through the water. The emphasis in this chapter, therefore, is 
on tug performance while assisting ships under way.
When considering effective ship handling with tugs there 
are, apart from the essential issue of bollard pull, two very 
important aspects to be considered:
•	 Correct tug positioning.
•	 The right type of tug.
Different tug operating positions are considered in relation 
to their effect on a ship. The performance of different basic 
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 general 
way, since there are so many variations in design within each 
type. Reviewing them all individually goes far beyond the 
scope of this book. However, knowing the basic principles 
may also result in a better understanding of the variations in 
design of these tug types. The same applies to the Related tug 
types and the FAST tugs, the tugs with one thruster forward 
and one aft. The related tug types have a certain similarity 
with the basic tug types. Both tug types have been addressed 
in Chapter 2, along with their specific capabilities. 
4.2 Basic principles and definitions
For a good understanding of tug performance and ship 
handling with tugs some basic principles and definitions are 
first considered. These include the pivot point, towing point, 
pushing point and lateral centre of pressure, direct and 
indirect towing and tug stability.
4.2.1 Pivot point
The pivot point is an imaginary floating point, situated 
somewhere in the vertical plane through stem and stern, 
around which a vessel turns when forced into a directional 
change. The form of the submerged body, rudder 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 
Chapter 4
TUG CAPABILITIES AND LIMITATIONS
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Tug Capabilities and Limitations 101
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 applied. Although the lever arm of tug force would 
be rather long the effect is not very pronounced, so there is 
another aspect to be considered. As explained earlier, a tug 
pushing forward tries to move the bow to starboard, say. 
This creates an opposing hydrodynamic force, also centred 
forward (see figure 4.1B). The hydrodynamic moment 
counteracts the turning moment 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 moderate speed ahead. In addition, the tug’s 
underwater resistance counteracts the turn.
It should however be noticed that the effect of the forward 
tug differs with ship’s hull form, draft and trim. For 
conventional ship forms, on even keel in deep or shallow 
water, the opposing hydrodynamic force is indeed centred 
forward, as mentioned in ‘Performance and effectiveness of 
omni-directional stern drive tugs’ (see References). When, 
for instance, taking a tanker in ballast and trimmed by the 
stern, the opposing hydrodynamic force is centred much 
more aft, resulting in a much larger effect of the pushing tug 
forward.
When a tug starts pushing 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 figure 4.1C). It is evident that the further 
forward and/or aft of the pivot point that tug forces are 
exerted on a ship, the longer the lever arm and hence the 
more effective the assistance will be.
A ship dead in the water (see figure 4.2A) with one tug 
pushing (or pulling) forward and one with the same bollard 
pull, pushing (or pulling) aft, pivots around its midships 
when on even keel. For the same size of vessel and same 
conditions, 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. When a tug pushes at the 
bow or stern of a ship that is stopped in the water, the ship 
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 the 
forward velocity of the ship. The lateral movement of the 
ship is opposed by the hydrodynamic forces centred forward 
on the ship having headway, which also creates a turning 
moment. This turning moment opposes (situation B) or 
assists (situation C) the turning moments created by the tugs. 
The location of the pivot point (PP) results from the motion 
of the ship caused by the various forces mentioned working 
on the ship.
Donkey effect
In addition to this explanation about the hydrodynamic 
forces centred forward, if in situation B of figure 4.1 the tug 
would push a bit more aft, it may push behind the forward 
lying centre of hydrodynamic forces. The result would be that 
the ship will turn to port instead of turning to starboard. 
This is called the Donkey Effect. If this happens, the tug has 
to push at a more forward lying position. The location of 
the centre of hydrodynamic forces depends on ship form, 
draught and trim.
Beamy full bodied ships have a smaller turning diameter and 
a further forward lying pivot point than slender ships. When 
a ship is down by the head turning diameter is also less and 
the pivot point lies further forward than when on an even 
keel.
Turning diameter is independent of ship’s speed as long as 
engine propeller revolutions or propeller pitch match a ship’s 
speed but is dependent on rudder angle applied. When in 
shallow water, such as in most port areas, turning diameter 
increases considerably, due to the larger hydrodynamic 
forces opposing the turn.
A ship moving astern has its pivot point somewhere between 
stern and midships when turning, eg, by use of a bow 
thruster. The exact position of the pivot point, therefore, 
is different for each individual ship, ship condition and 
circumstances.
The pivot point also changes position when, in addition to 
rudder force, otherforces such as bow thruster or push/pull 
Figure 4.2: Location of the 
pivot point in a ship with zero 
speed
Situation A: Tugs of equal 
power pushing/pulling 
forward and aft. The pivot 
point lies amidships. The tugs 
towing on a line have a longer 
lever and so a larger effect.
Situation B: Forward tug 
pushing; the pivot point lies 
far aft. When an after tug is 
pushing, the pivot point lies 
far forward.
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102 Tug Use in Port
•	 carrousel system and DOT system (see Chapter 2);
•	 azimuth friction free towing point, auto position escort 
winch, certain staple designs. 
The last three systems mentioned shift the towing point to 
the lower side of a tug heeling due to the towline force and 
so decrease the heeling lever, as will be shown in the stability 
paragraph below. These systems have a positive effect on a 
tug’s performance and on a tug’s stability. In particular the 
effect of a carrousel system is largely positive on safety of 
operations regarding stability and performance. Some tugs 
have more than one towing point, eg, some Voith tugs have 
an additional towing point at the tug’s aft end.
The gob rope system allows the towing point to be shifted in 
a longitudinal direction, increasing tug’s performance. If the 
towing point is shifted to the after end of the tug, it has also a 
positive effect on safety of the tug. 
Deck equipment will be further addressed in Chapter 7.
For tugs pushing at a ship’s side the contact point or pushing 
point is of importance.
Before discussing the capabilities 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 of pressure
The lateral centre of pressure is a non-stationary point. Its 
location depends on the underwater hull form including 
appendages such as rudder, propellers and skeg, on the trim 
of the tug and the angle of attack of the incoming water flow. 
The influence of rudder and propellers on the location of the 
centre of pressure seems to be rather high.
Tractor tugs and especially VS tugs have 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 
turns around a point located approximately a ship’s width 
from the stern or bow respectively (see figure 4.2B).
Other forces of external origin that affect the position of 
the pivot point are wind and current. In port areas, wind 
and current may vary in speed and direction depending 
on location. Relative wind and current directions may also 
vary during a transit to or from a berth due to changes in a 
ship’s heading. For instance, when entering a harbour basin 
from a river the current gradually decreases but also changes 
in relative direction. As a result, the influence of wind and 
current on a ship fluctuate. Depending on the angle of attack 
and point of application, wind and current may decrease 
or increase the rate of turn, moving the pivot point further 
forward or aft, or may have only a sideways effect.
4.2.2 Towing point, pushing point and lateral 
centre of pressure. Direct towing and indirect 
towing. Skegs
The relative positions of the centres of three different 
resultant forces are mainly responsible for a tug’s 
performance. These are centre of thrust, the towing or 
pushing point and the lateral centre of pressure of the 
incoming water flow. In particular, the mutual relationships 
between towing or pushing point, centre 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, the towing hook or towing winch 
is not necessarily the towing point. The towing point is that 
point from where the line goes in a straight line from the 
tug towards the ship. In most cases it is the staple or fairlead. 
The location of the towing point is extremely important with 
respect to stability, safety and performance of a tug.
The towing points shown in figures 4.3 and 4.4 are fixed 
points.Towing points can also be movable, as is the case with 
the:
•	 gob rope system; 
•	 radial towing hook;
Figure 4.3: Staple on Voith tractor tug Velox – see red arrow (LOA 37.0m, 
beam 14.0m, BP 65 tons, steering force at 10 knots 131 tons). Source: 
VoithTurbo Schneider Propulsion
Figure 4.4: Double fairleads on ASD-tug Venus (LOA 24.47m, beam 
11.33m, BP ahead 67.4 tons, astern 65.6 tons). Photo: Piet Sinke
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Tug Capabilities and Limitations 103
the towline in combination with force L creates a counter-
clockwise turning moment.
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 turning moment. Thus less steering power, 
by either rudder deflection or omnidirectional propellers, is 
needed to counteract that turning moment. Consequently, 
more engine power is available for towing. If propulsion 
is located aft at Ps, starboard rudder or thrust is needed, 
giving a little more drag but also an additional force in the 
towline. If propulsion is located forward (Pt) then sideways 
steering power is needed, but in the opposite direction. This 
consequently decreases the towline force.
With increasing speed, force F increases and consequently 
lift force L. The higher the speed the more steering effort 
is needed. Therefore, the higher the speed the larger the 
difference in towline forces between a conventional and 
tractor tug.
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 
further forward than with conventional tugs, 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 tug 
would 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 in case the ship has speed on. The better 
the omnidirectional thrust performance of a tractor tug 
the more effective it will be. Reducing the underwater 
resistance of a tractor tug would increaseconditions and ships calling at 
the port.
•	 The 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 important subject is discussed taking 
into account the effects of wind, 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 simulator training and research.
•	 Risk assessment and safety management systems are 
important items with respect to safety and are dealt with 
in a separate chapter.
All subjects are, as far as possible, related to situations 
encountered in practice.
TUG USE IN PORT: THE OVERVIEW
2 Tug Use in Port
Figure 0.2: Tanker Hayon Spirit (95,000dwt; LOA 244m) entering the lock in IJmuiden with tug assistance. Aft ASD-tug Titan, 
LOA 30m, beam 9.50m, BP 50 tons. The whole waterflow can clearly be seen. Photo: Henk Koning, The Netherlands
Tug Design Factors 3
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. It may 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 and also terminal 
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.
1.1 Differences in tug design and 
assisting methods
Methods of assistance provided by tugs 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 among harbour tugs. New types have been 
designed with high manoeuvrability and considerably 
increased engine power. Modern steering devices, new 
towing appliances and new materials for towlines, to name 
just a few, have been fitted. These developments affect 
methods of tug assistance and the number of tugs used. 
Following the Exxon Valdez disaster, the requirement to 
escort tankers in certain port approaches has resulted in 
the development of specially built escort tugs and tugs with 
escort capabilities.
As a result of the improved manoeuvring capabilities of 
many modern ships on the one hand, and the improved 
towing performance of modern tugs on the other, the 
number of tugs required for assistance in port areas is 
decreasing. Due to the economic factors faced by shipping 
companies, captains and pilots are often under pressure to 
use the minimum number of tugs.
Figure 1.1: Atlantic Bridge entering the lock at IJmuiden with azimuth tractor tug Arion at the bow. Tug dimensions: LOA 28.75m, 
beam 9.1m, BP 45 tons. Tugs should be able to assist ships through locks. Photo: Port Towage Amsterdam
Chapter 1
TUG DESIGN FACTORS
4 Tug Use in Port
b. Ports with mainly terminals
The location of ports with terminals, such as container 
terminals, can be such that there is plenty of room available 
for manoeuvring and manoeuvres can then be more 
standardised. Such terminals are very well suited for the so-
called ‘push- pull’ method and tugs are specifically designed 
for this type of operation.
c. Ports with mainly piers and jetties
Jetties can be divided into those in the open sea and those 
in protected waters. A major difference between a jetty and 
a normal harbour or terminal is the method of mooring 
and unmooring. With jetties, mooring is mostly done on 
dolphins or on a finger- or T-pier, allowing tugs to operate 
on both sides of the vessel assisted. In harbour basins and at 
terminals, however, mooring is done alongside a quay where 
assistance is, in most cases, restricted to one side.
d. Mooring facilities at remote locations
For mooring locations offshore, such as jetties and LNG 
terminals, ships are often made fast under windy conditions 
and in waves. This demands special requirements for tugs, 
such as good stability, sturdy fendering, render-recovery 
winches and high engine power.
Tugs handling SPMs and particular F(P)SOs and FNLGs 
have to meet specific requirements with respect to engine 
power, manoeuvrability and towing winches, depending on 
the situation, conditions and size of ships.
Ports under development
In many ports, developments take place – such as new 
berths or harbour basins – and new ports are still being 
•	 The type of ships calling at the port.
•	 The services required in and around the port and, if 
relevant, at offshore locations, such as jetties, LNG 
terminals, SPMs, F(P)SOs, FLNGs, or oil rigs.
1.2 Factors influencing tug type 
and tug assistance
1.2.1 Categories of port and their approaches
Ports can, in general, be divided into four categories:
a. Conventional ports
Conventional ports make up the majority of ports around 
the world, often with a long history as such. Ships are 
berthed in harbour basins or docks, along river berths and 
often have to pass locks and/or bridges. This creates specific 
requirements for tugs and tug assistance. The development 
of these ports has taken place alongside that of shipping – 
including the tugs. Ports such as Rotterdam, Le Havre and 
Hong Kong have a good, short connection with the open sea. 
Others – like Antwerp, Calcutta and New Orleans – have a 
long and sometimes complicated connection with the open 
sea. In all these ports, tugs should not only be suitable for 
assistance in the harbour areas, but for assistance outside 
the harbour as well, i.e. on the river or at sea. Development 
of conventional ports does not always keep abreast of the 
increase in dimensions of ships that call at the ports. This 
can resultits effectiveness as 
a forward tug. However, this would have consequences for 
its effectiveness as stern tug when operating in the indirect 
mode whereby use is made of the hydrodynamic forces 
on the tug’s hull. Therefore a compromise has often to be 
underwater lateral plane and shape, the angle of attack, 
the under 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 determined in a towing tank. The locations of the 
centre of pressure mentioned later are merely an indication 
and are based on observations and information, eg from 
Voith.
When 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 position about 0.3 to 0.4 x LWL 
from aft. For conventional tugs it is probably more often in 
the vicinity of 0.3 x LWL from aft and for tractor tugs closer 
to 0.4 x LWL from aft. Reverse-tractor tugs and ASD-tugs 
may have a more forward lying centre of pressure, depending 
on the hull and skeg design.
When a tug turns with its bow into the direction of water 
flow, the centre of pressure moves forward. The smaller the 
angle between incoming water flow and tug’s heading 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 LWL). Reverse-tractor 
tugs and ASD-tugs may experience a position of centre of 
pressure forward of midships with a forward incoming water 
flow. When a tug is turning with the stern into the water flow 
the centre of pressure moves aft and with an acute angle of 
incoming water flow will lie far aft.
Figure 4.5 shows a tug moving ahead, towing on a line, 
assisting a ship under speed. The resultant force created 
by incoming water flow is force F, assumed to be centred 
approximately near amidships, location C. Force F can be 
resolved into lift force L and drag force D, comparable with 
the lift and drag forces on rudders or aeroplane wings. Lift 
force L gives an additional force on the towline and drag 
force D has to be overcome by the tug’s thrust. Towing point 
T lies a little behind the centre of pressure C. The force in 
Figure 4.5: Forces created on assisting tug moving ahead.
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104 Tug Use in Port
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 of capsizing. 
The tug can then use its engine to control the ship’s speed. 
Conventional twin screw tugs often use the propellers instead 
of a gob rope to keep the tug in the position as indicated in 
figure 4.7.
As can be seen at figure 4.7 the use of the ship’s engine on 
ahead can become very dangerous for the tug. The ship’s 
propeller wash will hit the underwater hull of the tug and 
so given it an extra heeling force, in addition to the heeling 
force created by the towline force. 
Relationship of movable towing points and centre of pressure 
is of even greater importance.
In figure 4.7 is shown how a gob rope works in relation with 
the location of the towing point. If the towing point would be 
shifted far aft, as can be done with a gob rope winch, the tug 
will swing with the stern towards the attended ship.
Therefore, the location of a carrousel system and DOT 
system is very critical. With a towline force at about right 
angles to the tug, the location of the towing point should 
be such that no turning moment is created with the 
hydrodynamic force working at the centre of pressure.
The other systems, such as the radial towing hook, azimuth 
friction free towing point, auto position escort winch and 
certain staple designs, don’t move or move only a little in 
a longitudinal direction. This means that the horizontal 
distance between towing point and centre if pressure do not 
change much. The focus of these systems is mainly on the 
crosswise movement of the towing point. 
 
found for the location of the towing point and also for the 
underwater profile of a tug.
In figure 4.6 the tug is moving astern through the water. The 
centre of pressure lies much further aft, eg, at location C for 
conventional 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 forces (at Pt) have 
to be exerted by the tug in order to compensate for the 
turning moment created by the incoming water flow, giving 
additional forces 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 heeling 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 generally just above the middle of the skeg) The tug then 
comes in line with the towline when its engines 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 4.6, 
because with higher speeds it is almost impossible to steer 
the tug safely and is therefore 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 may 
operate broadside, for instance as a forward tug steering a 
ship which is moving astern or as a stern tug steering a ship 
moving ahead. Especially on single screw tugs, this can only 
be done with a gob rope or by passing the towline through 
a fairlead situated aft, as is the case on some combi- 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 T1 to T2 (see 
figure 4.7), the tug can stay broadside on and steer the ship by 
Figure 4.6: Forces created on assisting tug, moving astern
Pt is centre of propulsion for tractor tugs; 
Ps centre of propulsion for conventional tugs and ASD-tugs. 
Figure 4.7: Tug working on a gob rope. Ship has a very low speed 
ahead. Tug can steer the vessel by going ahead or astern on the engine. 
Conventional twin screw tugs don’t always need a gob rope; they can 
make a couple by the propellers 
to stay broadside.
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Tug Capabilities and Limitations 105
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 generally a larger 
distance between the towing point (T) and centre of pressurein increasingly complicated manoeuvres on arrival 
or departure, and additional requirements for the harbour 
tugs with regard to engine power, manoeuvrability and 
dimensions.
Figure 1.2: Reverse tractor tugs Seaspan Osprey (LOA 28.20m, breadth 12.60m, BP 80 tons) and Seaspan Resolution (LOA 30m, beam 12.2m, 
BP 80 tons) assisting container vessel with push-pull method towards the container terminal. Photo: Seaspan Marine
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Tug Design Factors 5
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.
Tugs used should, as long as safety permits, always be capable 
of working effectively under the environmental conditions in 
and around the port and, if relevant, in the port approaches 
or at offshore locations, whether during strong winds, 
currents or ice, waves and/or swell. When water depth is 
restricted, manoeuvring space is limited, or locks or bridges 
have to be passed, the type of harbour tug used should 
obviously be such that it is capable of performing well in 
these situations.
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 
their experience of shiphandling with the available harbour 
tugs. Moreover, tug companies can take account of these new 
developments when ordering new tugs 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.
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 be capable 
of working in more open sea conditions with waves and/or 
swell. Following the Exxon Valdez 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.
1.2.2 Environmental conditions
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 constantly subject to changes. Differences in 
water depth, bridge passages and lock entries may require 
the adoption of time windows. The accessibility 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.
Figure 1.4: Depending on the location of the port, tugs should be capable 
to operate in wave conditions. Hybrid ASD-tug Multratug 28, type 
ASD 2810, operating in the Rotterdam area. LOA 28.67m, beam 10.43m, 
BP ahead 60.7 tons. Photo: Kees Torn, the Netherlands
Figure 1.3: Port Hedland: open jetty. Photo: Piet Sinke
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6 Tug Use in Port
1.2.5 Assisting method in use
The method of assistance used by tugs will depend on:
•	 Port, jetty, terminal layout and/or offshore installation.
•	 Types of ship.
•	 Environmental conditions.
•	 Navigational complexity of river, channels and port 
approach.
•	 Whether bridges and locks have to be passed.
•	 Whether escorting is needed.
•	 Often on tradition.
The type of tug used depends largely 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.
In a number of ports, tugs are used to brake and control the 
ship’s speed, eg when approaching a lock or passing a bridge. 
Tugs should be capable of doing so.
Blackouts do happen on ships arriving in ports. Tugs, tug 
type and power should be such that they are able to keep the 
ship under control as far as possible by braking ship’s speed 
and/or controlling the heading. 
1.2.6 Available experience
Pilots and tug captains are accustomed to the assisting 
methods used in the port and to the types of tugs in 
the port. They have built up their experience with these 
tugs and with the tug’s crews. They know the advantages 
and the shortcomings of their tugs and are thus able to 
anticipate. Changing over to a new system or to a new 
type of tug may introduce difficulties, will take time and 
should be weighed carefully. Training and instruction will 
1.2.3 The ships concerned
Another important factor is the type of ships to be assisted, 
such as large LNG or LPG tankers, FSPOs, FLNGs , bulk 
carriers, oil tankers, container ships, car carriers, ro-ro 
ships and so on. Different types of ship give rise to specific 
requirements on the assisting tug, with regard to such engine 
power, towing equipment, fendering, manoeuvrability and 
superstructure.
For example, in a port where only tankers moor alongside 
an oil jetty, it is not too difficult to determine what kind of 
tug, and what power, is required. However, a large variety of 
types and sizes of ships means the requirements of tugs will 
vary too.
1.2.4 Services required in and around 
the harbour
The most important activities of harbour tugs have been 
mentioned above. In many ports these activities also include 
assisting at dock yards. Harbour tugs are often used for 
activities other than assisting seagoing vessels, such as:
•	 Towing offshore material, such as oil rigs and crane 
barges.
•	 Towing inland barges, elevators, floating cranes.
•	 Assisting push-tow barges.
•	 Fire-fighting and pollution control duties.
These activities also demand a specific type and size of tug, 
as well as specific manoeuvrability, equipment and towing 
methods, as is the case with tugs that have to operate, for 
example, at SPMs, F(P)SOs, FNLGs, LNG terminals or at oil 
rigs.
Figure 1.6: Other services required. Transport of tunnel segments over 
the river and through bridges. The pulling tugs are two Rotortugs 
RT Claire and RT Stephanie. Dimensions: LOA 27.7m, beam 11.2m, 
BP 65 tons. Tugs should be able to do this kind of work as well, 
including bridge passages. Photo: Kotug
Figure 1.5: In colder areas tugs should be able to operate in freezing 
conditions and in ice. Tug Texas, diesel-electric single screw, on 
Lake Michigan. Built 1916 (!), 1,200bhp, LOA 24.7m, beam 6.1m. 
Still operational as ship assist tug. Photo: Scott Best,USA
Tug Design Factors 7
1.3 Types of tug
The factors mentioned above have resulted in the use of 
different types of tug all over the world. At present mainly 
the following types are used:
•	 Single screw tugs.
•	 Twin screw tugs.
•	 Tractor type tugs.
•	 Tugs with azimuth propellers aft.•	 Rotor®tugs
•	 Tugs with one propulsion unit forward and one aft.
Single and twin screw tug types are generally well known. 
To increase towing power, many of them are fitted with 
nozzles. They may be equipped with fixed or variable pitch 
propellers. In some cases, to improve their manoeuvrability, 
a bow thruster or, in particular on single screw tugs, a 360° 
steerable and often retractable bow thruster is installed, 
which also increases the towing force.
Tractor types of tug have propellers under the forebody of 
the tug. These propellers may be Voith propellers or 360° 
steerable azimuth propellers.
Tugs with azimuth propellers aft resemble twin screw tugs. 
However, because of the 360° steerable thrusters they are 
much more manoeuvrable.
Rotortugs have two azimuth propulsion units forward and 
one aft and are very manoeuvrable. 
Various tug types exist with one propulsion unit forward and 
one aft. 
The different tug types are discussed in more detail in 
Chapter 2.
1.4 Assisting methods
Depending on local experience and circumstances, the 
following assisting methods or combinations of these 
methods are mainly used:
•	 Tugs towing on a line.
•	 Tugs operating at the ship’s side.
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 necessary. 
All this should be taken into account when considering the 
introduction of a new tug type or assisting method.
1.2.7 Safety requirements
Sometimes tugs have to operate in hazardous areas, for 
instance when handling LNG carriers or operating at LNG 
terminals. Tugs should then often meet certain safety 
requirements. 
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.
1.2.8 Summary
No two ports are the same. Many factors influence the 
choice of tug type, such as local customs, environmental 
conditions, water depth, type of ships using the port and 
escort requirements. These factors can differ enormously 
from port to port and so will affect the type of harbour 
tugs. Requirements for harbour tugs that have to operate at 
offshore installations may be an additional factor to take into 
account. It should be noted that the factors mentioned not 
only influence the type of tug needed, but also the assisting 
method used and that method has, in turn, a strong relation 
with the type of tug.
An overview of the factors influencing the operational 
requirements for tugs is given in the table at the end of this 
chapter.
Figure 1.7: Tugs might be needed for other services than just ship 
handling.Three tugs transporting the FSPO Western Island over the river 
to the yard. Photo: Mark de Bruin, the Netherlands
Figure 1.8: Brand-new MOL Tradition, Geoje, Korea.
Tugs should be able to assist at dockyards. Photo: Ole Peter Dahl
8 Tug Use in Port
When a new tug is needed a simple answer to the question 
‘which type of tug and/or which towing method is most 
suitable for the port?’ cannot easily be given. Too many 
factors play a role. It takes reliable research, weighing all the 
advantages and disadvantages, to establish the requirements 
for the most suitable tug for the port. Most important is not 
just what forces have to be considered but how, when and 
under what conditions and circumstances do these forces 
play a role – for instance with regard to ship's speed, confined 
areas, environmental conditions and underkeel clearance. 
This is the way more and more modern ports and/or tug 
companies work nowadays. The outcome may be a tractor 
type with azimuth propellers or Voith propulsion or even a 
conventional type of tug. Escorting of tankers will involve 
additional requirements.
On the other hand, tug owners want to operate as few 
different types of tug as possible and prefer that the available 
types are put into action as frequently as possible. Harbour 
tugs should, therefore, be as versatile as possible.
When towing on a line, the towline is made fast at the bow or 
stern of the assisted ship. The tugs operate at a distance from 
the ship’s bow and stern on a towline length normally at least 
one and half times the tug’s length.
Depending on the assistance required, local situation and 
type of tugs used, tugs operating at the ship’s side can be 
made fast with one, two or three lines. Different methods 
are used, such as the push-pull method, whereby the tug is 
usually made fast with one line. Alongside towing is another 
method. Tugs are then securely lashed alongside the ship 
with a minimum of three lines.
Different assisting methods are discussed in Chapter 3.
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 
frequenting the port, the environmental conditions, local 
traditions and consequently the types of tug and the assisting 
methods.
Figure1.10: Tugs may be required to handle submarines, which 
sets specific demands for the assisting tugs. ASD-tugs 
SD Independent (type ASD 2509, LOA 25.14m, beam 9.44m, 
BP 40 tons) and SD Dependable (type ATD 2909, LOA 29.13m, 
beam 9.98m, BP 43 tons). Photo: BAE Systems, UK 
Figure 1.9: Tugs might be needed to handle navy vessels, such as 
aircraft carriers, which will result in specific demands for the tugs. 
Photo: Moran Tugs 
Figure 1.11: Summary of factors influencing harbour tug choice
Types of Harbour Tug 9
Chapter 2 
TYPES OF HARBOUR TUG 
PART A
Classification of tugs and operational design aspects
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10 Tug Use in Port
2.1 Classification of basic harbour 
tug types
Tug types are named after their main characteristics, ie, the 
type of propulsion, propulsion manufacturer, location of 
propulsion or steering system. Names include conventional 
tugs, Voith-Schneider tugs, Z-peller tugs, Kort nozzle tugs 
and tractor tugs, among 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 propellers aft? 
The difference does not seem so great, but considering tug 
performance while rendering assistance, it is. After 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.
Some history...
When discussing the various tug types it may give the 
impression that azimuth thrusters are a product of the 20th 
century. But that is not the case. The general principle of 
today’s azimuth thrusters was patented under the heading 
of ‘steering propellers’ in the early 1870s. Not only patented, 
but actually made and used in several applications. Colonel 
WH Mallory developed the ideas in the USA and was able 
to patent them in Britain, setting up the Mallory Propeller 
Co in London. In 1881 he patented an azimuth thruster with 
twin propellers, designed to balance out the torque reactions 
and requiring less power to turn the thrusters (see figure 
2A.1). There is even evidence that Mallory also experimented 
a with a podded thruster with an electromotor in the 
underwater unit. 
Even the Voith Schneider propeller had a predecessor. The 
USS Alarm, commissioned in 1874,was equipped with 
a Fowler propeller, which was a vertical axis propulsion 
resembling a feathering paddle wheel set on one end. It had 
some similarities with the well-known Voith Schneider 
propeller, but lacked the sophisticated linkage and blade 
design. See figure 2A.2. 
Through tug development, a large number of different tug 
types have emerged. These tug types can be categorised in 
three groups – as shown in Table 2A.1.
First, attention will be paid to the basic tug types, because 
the other tug types have to some extent a relation with 
these tugs. Classifying the basic tug types according to their 
thruster and towing point location results in the following 
two main groups: 
a) Tugs with their propulsion aft and towing point near 
midships. These are basically conventional types of tug.
This category includes all normal conventional types such as 
single screw and twin screw tugs.
b) Tugs with their towing point aft and propulsion forward 
of midships. These are tractor tugs.In this category are:
•	 Tractor tugs with Voith propulsion.
•	 Tractor tugs with azimuth propellers.
There are 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 (more and more also 
called ASD-tugs) – tugs with azimuth propellers aft 
and towing point forward, built to operate mainly over 
the tug’s bow, as can be seen for example in Japan, 
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 same 
way regarding the towing point and the propellers, 
consequently the tug itself lies in the reverse direction.
•	 Azimuth stern drive (ASD) tugs – multi-purpose tugs 
with azimuth propellers aft which are built to operate 
over the tug’s bow 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, or have the option to be fitted with a 
towing winch later. Because an ASD-tug can operate as 
a reverse-tractor tug, it is often mentioned together with 
reverse-tractor tugs. 
•	 Modified older tugs with a 360° steerable bow thruster 
(combi-tugs) and equipped with an additional towing 
point at the after end of the tug. These tugs can operate 
as a normal conventional tug or like a tractor tug when 
using their aftermost towing point.
Figure 2A.1: Colonel WH Mallory steerable propeller. 1870s Figure 2A.2: USS Alarm, 1874 Source: USA Naval History and Heritage Command
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Types of Harbour Tug 11
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
The manoeuvring space required by assisting tugs should, 
depending on the situation, be as small as possible. 
This can be achieved by a suitable tug type with good 
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’s wheelhouse should be placed and constructed such 
that, at his/her manoeuvring station, the tug captain has a 
good view of the tug’s fore and aft ends and tug sides. He/she 
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 operation.
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°.
In addition to the all round view, well designed wheelhouses 
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 with large windows and a 
nearly 360° view.
Manoeuvring stations
When making fast to a vessel and while assisting, a tug 
captain should be able, to see in one glance from his 
manoeuvring station, the most essential outside information 
needed to operate his tug in a safe and efficient way, without 
jumping from side to side in his wheelhouse and without 
getting painful legs, neck or back. The essential outside 
information comes 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 the 
applied tug forces. When pushing, essential information also 
comes from the contact 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 type of tug and the assisting method 
in use, this essential information may come from totally 
different or even 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 
So the following basic types of tug can be seen, all belonging 
to one or both of the above groups:
•	 Conventional tugs.
•	 Tractor tugs with azimuth propellers or Voith 
propulsion.
•	 ASD-tugs.
•	 Reverse-tractor tugs.
•	 Combi-tugs.
There are, of course, many differences in construction, hull 
design, propulsion and rudder configuration and so on 
within each basic tug type. The different basic types of tug 
are therefore discussed in more detail starting with some 
general aspects regarding tug performance and safety of 
operations.
The ‘related tug types’ as shown in Table 2A.1 – eg, Rotortug, 
Z-tech tug and RSD tug – have much common with the basic 
tug types. The Rotortug has three azimuth thrusters and the 
other two tug types have two azimuth thrusters under one of 
the tug’s ends.
A more recent development is the FAST (Forward-Aft-
Single-Thruster) tugs, which have thrusters, either azimuth 
or Voith, at each end of the tug. These are the SDM, EDDY, 
Giano tug and RAVE tug. The latter has Voith propulsion 
units.
More attention will be paid to all tug types later in this 
chapter.
2.2 Important general requirements 
for good tug performance
For good harbour tug safety and performance, the following 
factors are important:
2.2.1 Tug performance and safety
Response time
Harbour tugs should have a short response time and their 
manoeuvrability should be such that the tug can react in a 
minimum of time. It is therefore important 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 decreasing 
numbers in a ship’s crew, the time taken to make tugs fast is 
increasing. Thus the requirement for tugs regarding fast and 
easy handling of towing equipment becomes an element of 
increasing importance in order to improve their response 
time.
Effectiveness and safety of operations
It is not only manoeuvrability, but also bollard pull and 
underwater shape that make a tug effective and therefore 
suitable for the job. For example, large container vessels with 
containers stacked six high or more on deck need powerful 
tugs in case of strong winds. When a ship is underway at 
speed, loss of tug’s effectiveness due to the ship’s speed and/
12 Tug Use in Port
coming alongside a ship or berth. Modern tugs often have 
one central manoeuvring panel in an optimal designed small 
wheelhouse, like a kind of cockpit.
At the 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 lateron, should be within hand reach at all 
manoeuvring panels. Towing winch control from the 
wheelhouse is also recommended for harbour tugs. The 
towline length can then always be adjusted when required 
without calling a man to the towing winch. The number 
of crew members on modern harbour tugs is very limited 
nowadays.
Communication
Good co-operation between the pilot and tug captain is 
a basic requirement for safe and efficient ship handling 
with tugs. Such co-operation is only possible with good 
procedures and efficiently working communication systems. 
Radio communication systems on board tugs should 
therefore be reliable. A double VHF set is recommended. 
This is sometimes an advantage for the pilot as well. On 
ships with open bridges the pilot is, during manoeuvring, 
often standing on the bridge wing busy with his own VHF 
set on the working channel with the tug. He has then often 
no possibility to listen to the traffic control channel. The tug 
master, having one VHF on the traffic control channel, can 
then pass the information to the pilot.
tug, which is assisting from over the tug’s bow, nearly all 
the essential outside information comes from forward 
and should be available in one outside look from the 
manoeuvring station. This can be achieved with one forward 
facing station. If the manoeuvring station is well planned, 
the tug captain may have an unobstructed view in the 
working direction, even from a seated position, of the winch, 
working deck, bow and side fenders and the assisted ship.
For all other types of tug and/or other assisting methods 
the visibility requirements may be totally different. For 
instance, a tractor tug used for push-pull operations works 
over the stern. Then an aft facing manoeuvring panel 
is needed. When the same tug is free sailing a forward 
facing manoeuvring panel 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 panels may be required, of course, depending 
on the wheelhouse size and construction. Some 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.
Controls at the manoeuvring panels should be arranged 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 captain is facing should result in 
an increase of movement in that direction. Turning a wheel 
or moving a joystick to the left should turn the tug in that 
direction, regardless of whether the direction of movement 
is ahead or astern. Any illogical way of control or complexity 
in control easily leads to human control failures, particularly 
when under tension.
It is clear that the wheelhouse layout and the number, 
location and orientation of manoeuvring panels depend 
largely on the type of tug and the usual assisting method and 
should be carefully considered, also taking into account the 
optimum view needed from the manoeuvring station when 
Figure 2A.3: Tug RT Stephanie (Rotortug; LOA 28.3m, beam 11.7, 
BP 68 tons) having a wheelhouse with a clear overall view. A good 
view abreast is also important for when coming alongside a ship 
having speed or when berthing. Photo: Piet Sinke
Figure 2A.4: View of the wheelhouse with the tug master handling 
the tug with the Uni-lever system (right) and speed control handles. 
Photo: Piet Sinke
Types of Harbour Tug 13
FlyingView bird’s eye view 
This system which monitors a 3600 field of view around the 
tug is studied by Mitsui OSK lines and Oki Electric Industry. 
Four fish-eye cameras were installed on the tug Asaka Maru 
with a digital compositing system and display on the tug’s 
bridge. The system was able to automatically calculate the 
distance to nearby obstacles and other vessels. The project 
forms part of a wider ongoing initiative named Focus Eye, 
which tries to make marine operations safer and more 
efficient. 
Note: Cameras have a large drawback. They don’t work 
properly in case of reduced visibility, dust, smog, etc. A 
better alternative is 3D sonar sensors, which don’t have these 
drawbacks. 
Augmented technology (AR)
AR technology, an element of XR (extended reality), is dealt 
with in paragraph 8.5. New maritime AR applications are 
emerging all the time. The European Union (EU) funds 
a project for AR bridge systems, designed to improve 
navigation safety and efficiency on ships.
An AR headset having maps, radar, alarms, warnings and 
interacting with the real view out of the bridge window will 
be a valuable option regarding the improvement in safety 
and situational awareness. AR is not just a PC or virtual 
reality content; AR combines natural and digital experience 
along with the interaction between the objects of both worlds 
made possible in real time. When purposefully designed 
such a system might be useful for tugs and tug master’s as 
well.
These are just a few systems. The development of remote-
controlled tugs (see paragraph 10.2) and automation 
on board tugs will closely be associated with further 
developments of decision support systems, data sensing and 
remote control systems. Such systems may become more 
common on board tugs.
2.2.3 Tug superstructure and underwater 
design
Tugs regularly have to work near a ship’s bow or stern, where 
the flare and overhang are often fairly pronounced. 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 stern 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 the 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 when a tug is required 
to come alongside for assistance or for bringing the pilot on 
board. 
2.2.4 Fendering
Tugs should be equipped with good fendering. Appropriate 
fendering protects both the assisted ship and tug from 
damage and decreases the tendency to slide along the ship’s 
hull when the tug is pushing at an angle to the ship’s hull. 
Decision support systems 
Various systems are developed and/or introduced intended 
to support the tug master. A very welcome development. 
Some will be mentioned below. Daily practice is the real 
test of such systems whereby such questions as mentioned 
below are of value to verify if or to what extent these systems 
support a tug master. Focus should then not only be on the 
standard manoeuvres but in particular on the more critical 
situations a tug master has to deal with. 
Five crucial questions regarding the development and 
introduction of decision support systems for a tug master 
are:
•	 Does it affect the tug master’s workload?
•	 Does it distract the tug master’s eye from òut of the 
window’ visual clues which remain paramount for 
manoeuvring alertly, especially in the more critical 
situations. 
•	 Does the decision support system indeed support the 
tug master in critical situations and adverse weather 
conditions, such as in case of operating near the bow of a 
ship at speed or in reduced visibility?
•	 Does the decision support system warn the tug master if 
a critical situation with respect to speed, direction, rate 
of turn, distances off, etc. develops and tells him to take 
action and what actions he has to take?
•	 Does the system inform the tug master about the level of 
tug’s stability and safe heeling angels limits?
Damen Human Machine Interface
Damen Shipyards has developed a Human Machine Interface 
with a range of possibilities, comprisingits effectiveness as 
a forward tug. However, this would have consequences for 
its effectiveness as stern tug when operating in the indirect 
mode whereby use is made of the hydrodynamic forces 
on the tug’s hull. Therefore a compromise has often to be 
underwater lateral plane and shape, the angle of attack, 
the under 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 determined in a towing tank. The locations of the 
centre of pressure mentioned later are merely an indication 
and are based on observations and information, eg from 
Voith.
When 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 position about 0.3 to 0.4 x LWL 
from aft. For conventional tugs it is probably more often in 
the vicinity of 0.3 x LWL from aft and for tractor tugs closer 
to 0.4 x LWL from aft. Reverse-tractor tugs and ASD-tugs 
may have a more forward lying centre of pressure, depending 
on the hull and skeg design.
When a tug turns with its bow into the direction of water 
flow, the centre of pressure moves forward. The smaller the 
angle between incoming water flow and tug’s heading 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 LWL). Reverse-tractor 
tugs and ASD-tugs may experience a position of centre of 
pressure forward of midships with a forward incoming water 
flow. When a tug is turning with the stern into the water flow 
the centre of pressure moves aft and with an acute angle of 
incoming water flow will lie far aft.
Figure 4.5 shows a tug moving ahead, towing on a line, 
assisting a ship under speed. The resultant force created 
by incoming water flow is force F, assumed to be centred 
approximately near amidships, location C. Force F can be 
resolved into lift force L and drag force D, comparable with 
the lift and drag forces on rudders or aeroplane wings. Lift 
force L gives an additional force on the towline and drag 
force D has to be overcome by the tug’s thrust. Towing point 
T lies a little behind the centre of pressure C. The force in 
Figure 4.5: Forces created on assisting tug moving ahead.
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104 Tug Use in Port
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 of capsizing. 
The tug can then use its engine to control the ship’s speed. 
Conventional twin screw tugs often use the propellers instead 
of a gob rope to keep the tug in the position as indicated in 
figure 4.7.
As can be seen at figure 4.7 the use of the ship’s engine on 
ahead can become very dangerous for the tug. The ship’s 
propeller wash will hit the underwater hull of the tug and 
so given it an extra heeling force, in addition to the heeling 
force created by the towline force. 
Relationship of movable towing points and centre of pressure 
is of even greater importance.
In figure 4.7 is shown how a gob rope works in relation with 
the location of the towing point. If the towing point would be 
shifted far aft, as can be done with a gob rope winch, the tug 
will swing with the stern towards the attended ship.
Therefore, the location of a carrousel system and DOT 
system is very critical. With a towline force at about right 
angles to the tug, the location of the towing point should 
be such that no turning moment is created with the 
hydrodynamic force working at the centre of pressure.
The other systems, such as the radial towing hook, azimuth 
friction free towing point, auto position escort winch and 
certain staple designs, don’t move or move only a little in 
a longitudinal direction. This means that the horizontal 
distance between towing point and centre if pressure do not 
change much. The focus of these systems is mainly on the 
crosswise movement of the towing point. 
 
found for the location of the towing point and also for the 
underwater profile of a tug.
In figure 4.6 the tug is moving astern through the water. The 
centre of pressure lies much further aft, eg, at location C for 
conventional 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 forces (at Pt) have 
to be exerted by the tug in order to compensate for the 
turning moment created by the incoming water flow, giving 
additional forces 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 heeling 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 generally just above the middle of the skeg) The tug then 
comes in line with the towline when its engines 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 4.6, 
because with higher speeds it is almost impossible to steer 
the tug safely and is therefore 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 may 
operate broadside, for instance as a forward tug steering a 
ship which is moving astern or as a stern tug steering a ship 
moving ahead. Especially on single screw tugs, this can only 
be done with a gob rope or by passing the towline through 
a fairlead situated aft, as is the case on some combi- 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 T1 to T2 (see 
figure 4.7), the tug can stay broadside on and steer the ship by 
Figure 4.6: Forces created on assisting tug, moving astern
Pt is centre of propulsion for tractor tugs; 
Ps centre of propulsion for conventional tugs and ASD-tugs. 
Figure 4.7: Tug working on a gob rope. Ship has a very low speed 
ahead. Tug can steer the vessel by going ahead or astern on the engine. 
Conventional twin screw tugs don’t always need a gob rope; they can 
make a couple by the propellers 
to stay broadside.
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Tug Capabilities and Limitations 105
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 generally a larger 
distance between the towing point (T) and centre of pressure