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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