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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