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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In small boat designs, a weight classification system 
similar to the navy groups is often followed. The total 
displacement is then as follows depending upon the 
weight classification system used, 
 
 \u2206 = WLS + DWTT 
 
 m n 
 = \u3a3 Wi + \u3a3 loadsj + Wmargin + Wgrowth [32] 
 i=1 j=1 
 
Focusing on the large commercial vessel classification 
system as the primary example here, the Light Ship 
weight reflects the vessel ready to go to sea without 
cargo and loads and this is further partitioned into, 
 
 WLS = WS + WM + Wo + Wmargin [33] 
 
where WS is the structural weight, WM is the 
propulsion machinery weight, Wo is the outfit and hull 
engineering weight, and Wmargin is a Light Ship design 
(or Acquisition) weight margin that is included as 
protection against the underprediction of the required 
displacement. In military vessels, future growth in 
weight and KG is expected as weapon systems and 
sensors (and other mission systems) evolve so an 
explicit future growth or Service Life Allowance 
(SLA) weight margin is also included as Wgrowth. 
 The total deadweight is further partitioned into, 
 
 DWTT = DWTC + WFO + WLO + WFW 
 + WC&E + WPR [34] 
11-21 
where DWTC is the cargo deadweight, WFO is the fuel 
oil weight, WLO is the lube oil weight, WFW is the 
fresh water weight, WC&E is the weight of the crew 
and their effects, and WPR is the weight of the 
provisions. 
 
11.3.2 Weight Estimation 
 
 The estimation of weight at the early 
parametric stage of design typically involves the use of 
parametric models that are typically developed from 
weight information for similar vessels. A fundamental 
part of this modeling task is the selection of relevant 
independent variables that are correlated with the 
weight or center to be estimated. The literature can 
reveal effective variables or first principles can be used 
to establish candidate variables. For example, the 
structural weight of a vessel could vary as the volume 
of the vessel as represented by the Cubic Number. 
Thus, many weight models use CN = LBD/100 as the 
independent variable. However, because ships are 
actually composed of stiffened plate surfaces, some 
type of area variable would be expected to provide a 
better correlation. Thus, other weight models use the 
area variable L(B + D) as their independent variable. 
Section 11.5 below will further illustrate model 
development using multiple linear regression analysis. 
The independent variables used to scale weights from 
similar naval vessels were presented for each \u201cthree 
digit\u201d weight group by Straubinger et al (33). 
 
11.3.2.1 Structural Weight 
 The structural weight includes (1) the weight 
of the basic hull to its depth amidships; (2) the weight 
of the superstructures, those full width extensions of 
the hull above the basic depth amidships such as a 
raised forecastle or poop; and (3) the weight of the 
deckhouses, those less than full width erections on the 
hull and superstructure. Because the superstructures 
and deckhouses have an important effect on the overall 
structural VCG and LCG, it is important to capture the 
designer\u2019s intent relative to the existence and location 
of superstructures and deckhouses as early as possible 
in the design process. 
 Watson and Gilfillan proposed an effective 
modeling approach using a specific modification of the 
Lloyd\u2019s Equipment Numeral E as the independent 
variable (1), 
 
 E = Ehull + ESS + Edh 
 
 = L(B + T) + 0.85L(D \u2013 T) + 0.85 \u3a3 lihi 
 i 
 + 0.75 \u3a3 ljhj [35] 
 j 
This independent variable is an area type independent 
variable. The first term represents the area of the 
bottom, the equally heavy main deck, and the two 
sides below the waterline. (The required factor of two 
is absorbed into the constant in the eventual equation.) 
The second term represents the two sides above the 
waterline, which are somewhat (0.85) lighter since 
they do not experience hydrostatic loading. There first 
two terms are the hull contribution Ehull. The third 
term is the sum of the profile areas (length x height) of 
all of the superstructure elements and captures the 
superstructure contribution to the structural weight. 
The fourth term is the sum of the profile area of all of 
the deckhouse elements, which are relatively lighter 
(0.75/0.85) because they are further from wave loads 
and are less than full width. 
 Watson and Gilfillan (1) found that if they 
scaled the structural weight data for a wide variety of 
large steel commercial vessels to that for a standard 
block coefficient at 80% of depth CB\u2019 = 0.70, the data 
reduced to an acceptably tight band allowing its 
regression relative to E as follows: 
 
 WS = WS(E) = K E 
1.36 (1 + 0.5(CB\u2019 \u2013 0.70)) [36] 
 
The term in the brackets is the correction when the 
block coefficient at 80% of depth CB\u2019 is other than 
0.70. Since most designers do not know CB\u2019 in the 
early parameter stage of design, it can be estimated in 
terms of the more commonly available parameters by, 
 
 CB\u2019 = CB + (1 \u2013 CB)((0.8D \u2013 T)/3T) [37] 
 
Watson and Gilfillan found that the 1.36 power in 
equation 36 was the same for all ship types, but that 
the constant K varied with ship type as shown in Table 
11.VII. 
 
TABLE 11.VII - STRUCTURAL WEIGHT 
COEFFICIENT K (1, 18) 
 
 Ship type K mean K range Range of E 
 Tankers 0.032 ±0.003 1500 < E < 40000 
 chemical tankers 0.036 ±0.001 1900 < E < 2500 
 bulk carriers 0.031 ±0.002 3000 < E < 15000 
 container ships 0.036 ±0.003 6000 < E < 13000 
 cargo 0.033 ±0.004 2000 < E < 7000 
 refrigerator ships 0.034 ±0.002 4000 < E < 6000 
 coasters 0.030 ±0.002 1000 < E < 2000 
 offshore supply 0.045 ±0.005 800 < E < 1300 
 tugs 0.044 ±0.002 350 < E < 450 
 fishing trawlers 0.041 ±0.001 250 < E < 1300 
 research vessels 0.045 ±0.002 1350 < E < 1500 
 RO-RO ferries 0.031 ±0.006 2000 < E < 5000 
 passenger ships 0.038 ±0.001 5000 < E < 15000 
 frigates/corvettes 0.023 
 
11-22 
This estimation is for 100% mild steel construction. 
Watson (18) notes that this scheme provides estimates 
that are \u201ca little high today.\u201d 
 This structural weight-modeling scheme 
allows early estimation and separate location of the 
superstructure and deckhouse weights, since they are 
included as explicit contributions to E. The weight 
estimate for a single deckhouse can be estimated using 
the following approach: 
 
 Wdh = WS(Ehull + ESS + Edh) 
 \u2013 WS(Ehull + ESS) [38] 
 
Note that the deckhouse weight cannot be estimated 
accurately using Wdh(Edh) because of the nonlinear 
nature of this model. If there are two deckhouses, a 
similar approach can be used by removing one 
deckhouse at a time from E. A comparable approach 
would directly estimate the unit area weights of all 
surfaces of the deckhouse; for example, deckhouse 
front 0.10 t/m2; deckhouse sides, top and back 0.08 
t/m2; decks inside deckhouse 0.05 t/m2; engine casing 
0.07 t/m2, and build up the total weight from first 
principles. 
 Parallel to equation 38, the weight estimate for a 
single superstructure can be estimated using, 
 
 WSS = WS(Ehull + ESS) \u2013 WS(Ehull) [39] 
 
These early weight estimates for deckhouse and 
superstructure allow them to be included with their 
intended positions (LCG and VCG) as early as 
possible in the design process. 
 
11.3.2.2 Machinery Weight 
 First, note that the machinery weight in the 
commercial classification includes only the propulsion