Sec. 67.2 



UNDERWATER-HULL DESIGN 



505 



crest, may neutralize much of the benefit gained 

 in careful shaping of the at-rest waterline at the • 

 designed draft. 



It is mentioned previously, but the caution is 

 worth repeating, that too much hoUowness abaft 

 the bow, resulting from an effort to fine the bow 

 waterlines to an extreme, may involve an exces- 

 sive and undesirable slope in front of the forward 

 shoulder. 



For the transom-stern ABC ship laid out in 

 Chap. 66 the first sketches of the designed water- 

 line are modified to suit the development of the 

 underwater hull described in detail in this chapter. 

 The final waterline shape, with its parameters and 

 0-diml offsets, is drawn in the lower diagram of 

 Fig. 67.A. That of the arch-stern alternative 

 design, described in Sec. 67.16, is drawn in the 

 upper diagram of the figure. 



For a stern shape which is wide and essentially 

 flat on its under side, like that of a scow, the 

 horizontal waterUnes at the stern close in toward 

 the centerplane at steep slopes with that plane. 

 For a truly flat stern with no rise of floor in the 

 sections this waterline slope Ir reaches 90 deg. 

 However, in cases of this kind, the flow upward 

 and aft under the stern is primarily along the 

 buttocks, or at least more upward than inward. 

 If so, the waterline slopes lose their significance. 

 Fig. 67. B reveals very steep slopes for the near- 

 surface waterlines at the stern of the ABC ship, 

 faired rather abruptly into the centerline-skeg 

 waterlines. However, inspection of Fig. 66. Q shows 

 easy buttock slopes in this region; Fig. 66.R 

 indicates that the actual flow under the stern is 

 more or less along these buttocks. 



The surface waterplane has a definite function, 

 not only in minimizing a surface-wave disturbance, 

 but in providing sufficient square moment of 

 area to insure the necessary transverse meta- 

 centric stability. As a rule, the best way to 

 change the metacentric height is to change the 

 maximum beam, assuming that this can be done. 

 Certainly, it is to be preferred to pulling the 

 designed waterline in and out, here and there. 

 This nibbling and padding usually changes the 

 moment of area only shghtly but it may have 

 major adverse effects upon the propulsion charac- 

 teristics of the underwater form. On the other 

 hand, so much attention can be devoted to the 

 hydrodynamic features of the designed waterHne 

 that its stabihty features are overlooked. 



Exactly this happened in the course of the 

 preUminary design of the ABC ship. It was 



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