Sec. 66.32 



STEPS IN PRELIMINARY DESIGN 



499 



Fig. 66.T Selected Inclined Waterlines for the Two Principal Variable- Weight Conditions 



Correo. Table 66.g indicates that the estimated 

 mean drafts under these two conditions are 

 22.74 and 20.01 ft, respectively. For round 

 numbers assume that these are 22.75 ft and 20.00 

 ft, involving reductions in the 26.0-ft mean draft 

 of 3.25 ft and 6.00 ft. 



It is useful to have an idea of the changes in 

 trim for a given internal arrangement of the 

 principal weights. It is necessary at some stage 

 of the preliminary design to check on the trans- 

 verse metacentric stability for these two loading 

 conditions. However, the most important hydro- 

 dynamic feature is to keep the propellers well 

 under water, with a reasonable tip submergence 

 and as good shielding from air leakage as can be 

 obtained. 



The procedure followed in the ABC design is 

 first to establish two drafts aft, corresponding to 

 the two variable-weight conditions, which will 

 insure air-free flow to the propeller. Following 

 this, the internal weights are so arranged that 



the centers of gravity in the two loading conditions 

 coincide with the CB's for the displacements and 

 stern drafts selected. A check is then made to 

 insure that, if a bulb bow is used, as in the present 

 case, it will not be too close to the water surface 

 for open-sea running. The stern draft for the 

 heavier condition, 16,400 t - 2,425 t = 13,975 t, 

 is made 24.25 ft, to place the bottom of the 

 transom at the AP a few inches under water in 

 the at-rest condition. The stern draft for the 

 lighter condition, 16,400 t - 4,400 t = 12,000 t, 

 is selected as 23.0 ft, which gives a tip submergence 

 at the propeller disc of just under 2.5 ft. Fig. 66.T 

 illustrates these features. While this submergence 

 is admittedly small, both absolutely and relatively, 

 the propeller is running at a considerably smaller 

 thrust-load factor at this light displacement. 

 Furthermore, the stern- wave crest at 20.5 kt 

 should be sufficient to fill all the volume between 

 the at-rest waterplane and the under side of the 

 hull just forward of the transom. 



Projecting the traces of the two waterlines 



TABLE 66.J — Weight, Buoyancy, and Stability Data for Two Variable- Weight 

 The figures given apply to the molded shape and dimensions of the transom-stern 

 weight displacement, molded, in standard salt water is 16,400 tons. 



Mean draft, ft 



Tons less than designed weight, from Table 66. g 



Weight displacement, nominal, t 



Trim by the stern, selected, ft 



Volume displacement, from molded lines, ft' 



Corresponding weight, at 35.977 ft' per ton, t 



Tons less than designed weight, actual 



LCB, in fraction of L from FP 



Coefficient of square moment of area C, ^ of WL about i-axis 



Calculated BM, ft 



Inclined waterplane area, ft^ 



KB, estimated, from Normand formula, Sec. 66.14, ft 



KM, derived, ft 



KG, estimated, ft 



GM, probable, ft 



Conditions of the ABC Ship 

 underwater hull, for which the 



