Sec. 67.16 



UNDERWATER-HULL DESIGN 



52S 



tunnel between the side skegs is widened slightly 

 with distance forward of the propeller in an 

 endeavor to allow for the increased area of the 

 inflow jet with that distance. The tunnel is neces- 

 sarily deep in that portion of the length normally 

 occupied by a single centerline skeg because of 

 the need for providing docking support in that 

 region. 



At the time the first modern twin-skeg sterns 

 were developed, an easy slope of the tunnel roof 

 was considered an important feature, so much so 

 that the maximum value was limited to 8 or 

 9 deg. It is now known that separation does not 

 occur at these small slopes, even at the free- 

 water surface. Well below the surface, where 

 much more hydrostatic pressure and pressure 

 gradient is available to change the water direction, 

 the slopes can be considerably greater than the 

 12- or 13-deg surface hmit. For the ABC design, 

 it was believed safe to increase the maximum 

 slope to about 18 deg, provided the tunnel areas 

 ahead of the propeller were kept large enough. 

 In fact, unpublished model test data on the 

 twin-skeg Manhattan hull form [SNAME, 1947, 



p. 116] indicated a definite thinning of the bound- 

 ary layer in way of the longitudinal bottom con- 

 vexity at the forward end of the tunnel, with 

 fairly high transverse velocity gradients at the 

 hull surface. This is an additional guard against 

 separation farther aft along the sloping roof of 

 the tunnel. Incorporating these features into an 

 alternative arch-type stern for the ABC ship 

 produced the body plan of Fig. 67.L and the stern 

 profile of Fig. 67.M. 



The resulting stern arrangement is not unlike 

 one proposed in 1874 by Robert Griffiths [INA, 

 1874, pp. 165-178 and PI. XXIV, Fig. Sa). He, 

 too, was concerned about adequate flow of water 

 to the screw propeller. 



The profile drawing in Fig. 67. M shows the 

 starboard skeg, looking from outboard, with the 

 starboard rudder. The centerline buttock re- 

 sembles in shape that of the transom stern, 

 depicted in Fig. 66. Q, except that in this case the 

 transom immersion at this buttock is only 0.5 ft, 

 it extends horizontally up to the propeller position, 

 and it does not fair into the keel until Sta. 14.5. 

 The maximum buttock slope, at about Sta. 17.35, 



Fig. 67.L Afterbody Plan of Arch-Type Stern for ABC Ship 



