480 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 66.15 



These exclude the shapes of stems made blunt 

 for construction or functional purposes only. 

 Past practice, and good performance as well, 

 indicated by the spots in the region of T„ = 0.4 

 to 0.75, has embodied — and justified, in a way — 

 the use of large ie values in these low-speed 

 ranges. Further study of the large deflection 

 drag undoubtedly associated with these blunt 

 stems, plus consideration of the equally large 

 wavegoing drag in head seas, calls for a reduction 

 in these slopes, if practicable. The design lane 

 in Fig. 66.1 is therefore lower than one laid out 

 to fence in most of the spots. 



Because of the lower L/B ratio of short vessels, 

 explained in Sec. 66.7 and indicated graphically 

 in Fig. 66. E, their is values are necessarily larger. 

 Fig. 66.1 contains therefore a branched design 

 lane for vessels of low L/B but high V/\/L 

 ratios. The lane for vessels of L/B = 6.0 to 10.0 

 rises sUghtly at high T,'s because of the straight 

 or slightly convex designed-waterline shapes used 

 in the forebodies of these craft. For the ABC 

 waterhne entrance an is value of 7 or 8 deg 

 appears suitable for the present, until the whole 

 waterhne is laid out and its characteristics are 

 checked with various requirements. 



Hydrodynamically, and for easy driving, any 

 parallel waterlines at and near the surface are 

 to be avoided, for the reasons given in Sees. 4.7 

 and 24.13. If parallel middlebody is used, parallel 

 waterlines of course come with them. When 

 vessels are built on slips or in docks of limited 

 width, or when they have to pass through canal 

 locks, some parallel waterhne is inevitable, 

 even without parallel middlebody. The lane on 

 Fig. 66.J is an indication of what has been found 

 acceptable in the past on vessels with varying 

 Cp . Judging by this the ABC ship could have a 

 parallel portion of the DWL up to about 0.22L, 

 but to keep the longitudinal waterhne curvature 

 more nearly constant a value of O.OL is selected. 

 This is also within the lane. 



The fore-and-aft position of the maximum 

 designed waterhne beam Bwx , which may or 

 may not be opposite the maximum-area section, 

 is determined by the position of the latter to 

 some extent. Nevertheless, for easy-driving ships 

 these positions are well related to the Cp value, 

 and hence are shown logically on different dia- 

 grams. Fig. 66. K gives a lane of good positions 

 for a large range of Cp values. When there is any 

 parallel waterhne, the indicated position along 

 the ship length is for the midlength of that 



10 20 30 40 50 60 70 



Lenqth of Parallel Designed Vi/oterline in Percentoqe- 

 of Ship Lenqth 



Fig. 66.J Design Lane for Percentage of 

 Parallel Waterune 



portion. On the ABC design the optimum position 

 appears to be about 0.54L; the exact position is 

 not too important. It will probably depend upon 

 subsequent adjustment of the DWL to achieve 

 nearly constant curvature. 



Inspection of the many waterhne endings and 

 run slopes in on the available SNAME RD 

 sheets, for ships of normal form and with canoe 

 or whaleboat sterns, together with those shown in 

 Figs. 23. A, 24.G, and 51. C, indicates the extreme 

 difficulty of shaping such a stern with a slope ig 

 of 15 deg or less. Even the TSS parent form, 

 EMB model 632, with a Cw of only 0.66 and an 

 L/B ratio of 6.85, has a run slope at the stern 

 as high as 22 deg; see Fig. 24.G. One solution, 

 and the one adopted here for the ABC afterbody, 

 is to use an immersed-transom stern, along the 

 hues described in Sec. 23.2 and illustrated in 

 Fig. 23.A. A conservative preliminary figure for 

 a not-too-wide transom beam B^ on the ABC 

 ship is 0.3B;r • This may have to be increased 

 later to keep the run slopes down to the order of 

 12 or 13 deg. 



Despite difficulties encountered with the wave- 

 going performance of certain full-stern vessels 



