Estimating Resistance And Propulsion For Single-Screw And Twin-Screw Ships In The Preliminary Design
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Estimating Resistance And Propulsion For Single-Screw And Twin-Screw Ships In The Preliminary Design


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machinery - primarily the prime mover, reduction gear, 
shafting, and propeller. Watson and Gilfillan proposed 
a useful separation of this weight between the main 
engine(s) and the remainder of the machinery weight 
(1), 
 
 WM = WME + Wrem [40] 
 
This approach is useful because in commercial design, 
it is usually possible to select the main engine early in 
the design process permitting the use of specific 
vendor\u2019s weight and dimension information for the 
prime mover from very early in the design. If an 
engine has not been selected, they provided the 
following conservative regression equation for an 
estimate about 5% above the mean of the 1977 diesel 
engine data, 
 
 WME = \u3a3 12.0 (MCRi/Nei)0.84 [41] 
 i 
 
where i is the index on multiple engines each with a 
Maximum Continuous Rating MCRi (kW) and engine 
rpm Nei. The weight of the remainder of the 
machinery varies as the total plant MCR as follows: 
 
 Wrem = Cm (MCR)0.70 [42] 
 
where Cm = 0.69 bulk carriers, cargo vessels, and 
container ships; 0.72 for tankers; 0.83 for passenger 
vessels and ferries; and 0.19 for frigates and corvettes 
when the MCR is in kW. 
 With modern diesel electric plants using a 
central power station concept, Watson (18) suggests 
that the total machinery weight equation 40 can be 
replaced by, 
 
 WM = 0.72 (MCR)0.78 [43] 
 
where now MCR is the total capacity of all generators 
in kW. These electric drive machinery weight 
estimates take special care since the outfit weight 
included below traditionally includes the ship service 
electrical system weights. 
 
11.3.2.3 Outfit Weight 
 The outfit includes the remainder of the Light 
Ship Weight. In earlier years, these weights were 
classified into two groups as outfit, which included 
electrical plant, other distributive auxiliary systems 
such as HVAC, joiner work, furniture, electronics, 
paint, etc., and hull engineering, which included the 
bits, chocks, hatch covers, cranes, windlasses, 
winches, etc. Design experience revealed that these 
two groups varied in a similar manner and the two 
groups have been combined today into the single 
group called Outfit. Watson and Gilfillan estimate 
these weights using the simple model (1), 
 
 Wo = Co LB [44] 
 
where the outfit weight coefficient Co is a function of 
ship type and for some ship types also ship length as 
shown in Figure 11.17. 
 
 
11-23 
 
Figure 11.17 - Outfit Weight Coefficient Co (18) 
 
11.3.2.4 Deadweight Items 
 The cargo deadweight is usually an owner\u2019s 
requirement or it can be estimated from an analysis of 
the capacity of the hull. The remaining deadweight 
items can be estimated from first principles and early 
decisions about the design of the vessel. The selection 
of machinery type and prime mover permits the 
estimation of the Specific Fuel Rate (SFR) (t/kWhr) 
for the propulsion plant so that the fuel weight can be 
estimated using, 
 
 WFO = SFR \u2022 MCR \u2022 range/speed \u2022 margin [45] 
 
Early general data for fuel rates can be found in the 
SNAME Technical and Research Bulletins #3-11 for 
steam plants (34), #3-27 for diesel plants (35) and #3-
28 for gas turbine plants (36). For diesel engines, the 
SFR can be taken as the vendor\u2019s published test bed 
data with 10% added for shipboard operations 
producing a value of about 0.000190 t/kWhr for a 
large diesel today. Second generation gas turbines 
might have a SFR of about 0.000215 t/kWhr. In 
equation 45, the margin is for the fuel tankage that can 
be an overall percentage such as 5% or it might be 
10% for just the final leg of a multi-leg voyage. 
Overall this estimate is conservative, because the 
vessel may not require full MCR except in the worst 
service conditions and there are margins both in the 
SFR and on the overall capacity. This conservatism 
can cover generator fuel that can be estimated 
separately in a similar manner as the design evolves. 
 The lube oil weight can be taken from 
practice on similar vessels. This usually depends upon 
the type of main machinery. Overall recommendations 
(37) include, 
 
 WLO = 20 t, medium speed diesel(s) 
 = 15 t, low speed diesel [46] 
 
As an alternative, an approach like equation 45 can be 
used with the vendor\u2019s specific lube oil consumption 
data with tankage provided for the total consumption 
in about 20 voyages. 
 The weight of fresh water depends upon the 
designer\u2019s intent relative to onboard distillation and 
storage. Modern commercial vessels often just carry 
water for the entire voyage and eliminate the need to 
operate and maintain water-making equipment with a 
small crew. Naval vessels and cruise vessels 
obviously have much higher capacity demands making 
onboard distillation more of a necessity. On the basis 
of using 45 gallons per person \u2022 day, the total water 
tankage weight would need to be, 
 
 WFW = 0.17 t/(person \u2022 day) [47] 
 
with perhaps 10 days storage provided with onboard 
distillation and 45 days provided without onboard 
11-24 
distillation. The weight of the crew and their effects 
can be estimated as, 
 
 WC&E = 0.17 t/person [48] 
 
for a commercial vessel\u2019s crew and extranumeraries, 
while a naval vessel might use 0.18 t/person for 
officers and 0.104 t/person for enlisted (33). The 
provisions and stores weight can be estimated as, 
 
 WPR = 0.01 t/(person \u2022 day) [49] 
 
for the provisions, stores, and their packaging. Naval 
vessel standards provide about 40 gallons water per 
person or accommodation \u2022 day and provisions and 
stores at about 0.0036 t/(person \u2022 day) (33). 
 
11.3.3 Centers Estimation 
 
 The estimation of centers of the various 
weight groups early in the design process can use 
parametric models from the literature and reference to 
a preliminary inboard profile, which reflects the early 
design intent for the overall arrangements. The 
structural weight can be separated into the basic hull 
and the superstructure and deckhouse weights using 
equations 38 and 39. The VCG of the basic hull can 
be estimated using an equation proposed by Kupras 
(38), 
 
 VCGhull = 0.01D (46.6 + 0.135(0.81 \u2013 CB)(L/D)2) 
 + 0.008D(L/B \u2013 6.5), L \u2264 120 m 
 
 = 0.01D (46.6 + 0.135(0.81 \u2013CB)(L/D)2), 
 120 m < L [50] 
 
The longitudinal position of the basic hull weight will 
typically be slightly aft of the LCB position. Watson 
(18) gives the suggestion, 
 
 LCGhull = \u2013 0.15 + LCB [51] 
 
where both LCG and LCB are in percent ship length, 
plus forward of amidships. 
 The vertical center of the machinery weight 
will depend upon the innerbottom height hbd and the 
height of the overhead of the engine room D\u2019. With 
these known, Kupras (38) notes that the VCG of the 
machinery weight can be estimated as, 
 
 VCGM = hdb + 0.35(D\u2019 \u2013 hdb) [52] 
 
which places the machinery VCG at 35% of the height 
within the engine room space. This type of simple 
logic can be adapted for the specific design intent in a 
particular situation. In order to estimate the height of 
the innerbottom, minimum values from classification 
and Coast Guard requirements can be consulted giving 
for example, 
 
 hdb \u2265 32B + 190\u221aT (mm) (ABS), or 
 
 hdb \u2265 45.7 + 0.417L (cm) (46CFR171.105) 
 
The innerbottom height might be made greater than 
indicated by these minimum requirements in order to 
provide greater doublebottom tank capacity, meet 
double hull requirements, or to allow easier structural 
inspection and tank maintenance. 
 The longitudinal center of the machinery weight 
depends upon the overall layout of the vessel. For 
machinery aft vessels, the LCG can be