500 HYDRODYNAMICS IN SHIP DESIGN Sec. 66.32 



TABLE 66.k — Longitudinal Weight Balance for 20-ft Mean Draft, 6-ft Trim by the Stern 



From Table 66.]', LCB is 0.513 L from the FP. This corresponds to a CG location at Sta. 10.26, or 0.26 station-length 

 abaft Sta. 10. With a weight displacement of 12,090 tons, from Table 66.], the corresponding after moment, in terms of 

 tons times station lengths, is -0.26 (12,090) = -3,143.4. 



Even with a very large capacity assumed for the after peak tanks this still lacks 2,178 station-tons of being sufficient 

 after moment to bring the CG to Sta. 10.26 and to give the ship a trim of 6 ft by the stern. 



forward to the midlength drafts of 22.75 and 20.00 

 ft, as on Fig. 66.T, gives drafts at the FP of the 

 ABC ship of 21.25 and 17.00 ft, respectively. 

 The entrance at the 17-ft WL has an Ie of only 8 

 deg, and the widest portion of the bulb is some 13 

 ft below the at-rest waterline at the bow. This 

 should be sufficient for good average performance 

 in the open sea. 



Working over the body plan of Fig. 66. P up 

 to the two inclined waterplanes in question gives 

 the weight, buoyancy, and stability values listed 

 in Table 66. j. It is to be noted that the weight 

 reductions for the displacements corresponding 

 to the two inclined waterlines of Fig. 66. T are 

 2,358 t and 4,310 t, as compared to the reductions 

 of 2,275 t and 4,400 t from Table 66.g. The 

 assumed variable-weight conditions are only 

 approximations, so the displacement reductions 

 as derived from the inclined waterlines are used 

 for the remainder of the analysis. 



The problem is now one of internal arrangement, 

 to insure that the CG's for the two variable- 

 weight conditions can be br ought to the calcu- 

 lated CB po.sitions. Since the LCG for the lighter 

 condition appears to be the most difficult to 

 achieve, Table 66. k is made up to find what can 

 be done with the internal layout of Fig. 66. S. 



Only by crowding a very large volume of water 

 into the after peak tanks, involving some wing 

 tanks abreast the machinery space not previously 

 contemplated, is it possible to obtain an after 

 moment at all. This value still lacks 2,178 station- 

 tons, or 55,539 ft-tons, of the necessary amount 

 to trim the ship the required distance by the stern. 



Another difficulty arises because of the large 

 hogging moment imposed on the ship by the 

 full capacity of package cargo forward, the heavy 

 water ballast way aft, and the nearly empty 

 liquid-cargo tanks near amidships. 



Regardless of the problems facing the designer 

 at this point, problems not directly involving 

 hydrodynamics which are not worked out here, 

 the method of attack for efficient propulsion in 

 the variable-load conditions is considered basically 

 sound. In this connection there are quoted the 

 suggestions of N. H. Jasper and L. A. Rupp for 

 new ship designs [SNAME, 1952]: 



Page 344. "(e) Give greater consideration to pro- 

 viding increased volume in after-ballast tanks of 

 cargo vessels to maintain a fully submerged 

 propeller, if practicable." 



Page 354. "5. Ships should be operated at a condi- 

 tion of trim which will insure immersion of the 



