Sec. 67.13 



UNDERWATER-HULL DESIGN 



519 



and pressure field, passes through the aperture. 

 An adequate and proper aperture clearance, 

 although undoubtedly a function of the pressure 

 distribution around the adjacent blade elements, 

 can not be defined within close limits on the basis 

 of present knowledge. This matter is discussed 

 further in Sees. 67.23 and 67.24. 



A modern form of whaleboat or canoe stern 

 for single-screw merchant vessels of normal 

 design is represented by the sterns of the five 

 TMB Series GO parent forms for block coefficients 

 from 0.60 through 0.80. The typical stern profile 

 is illustrated in a diagram published by F. H. 

 Todd [SNAME, 1953, Fig. 28, p. 562]. The 

 adaptation to a particular propeller is dehneated 

 by J. B. Hadler, G. R. Stuntz, Jr., and P. C. Pien 

 [SNAME, 1954, Fig. 2, p. 123]. 



In a modification of the normal single-screw 

 stern, such as that laid out for the ABC design in 

 Figs. 66. P and 66. Q, the transom leads forward 

 to a sort of shelf, worked at about the level of 

 the top of the propeller aperture and the top of 

 the rudder. One purpose of this shelf is to protect 

 the propeller from air leakage as long as the 

 shelf is submerged. Another is to permit fining of 

 the upper portion of the skeg ending ahead of 

 the upper propeller blades by making the skeg 

 more of an appendage than a part of the main 



hull. The arrangement then resembles that of 

 each side skeg in a twin-skeg design, discussed 

 in Sec. 67.21 and in the technical literature 

 [SNAME, 1947, pp. 97-169]. However, it is not 

 as easy as might be expected, with this arrange- 

 ment, to obtain the requisite fining of the upper 

 levels of the skeg ending while giving the skeg 

 an ample degree of lateral stiffness. 



The profile drawing of Fig. 66. Q is an elevation 

 of the aftermost quarter-length, drawn in orthodox 

 fashion. The centerline buttock is included as a 

 sort of construction line, indicating the profile 

 shape of the after main-hull sections on the basis 

 that the thin centerline skeg is treated as an 

 appendage. The transom-stern body plan. Fig. 

 66. P, indicates this feature in the form of broken- 

 hne continuations of the sections at Stas. 16 

 through 18.5, carried through to the centerline 

 without taking account of the skeg. 



The aperture clearance forward of the upper 

 blades is made exceptionally large and the aftfoot 

 of the skeg is cut away to save wetted surface 

 and to improve maneuvering. It is to be noted 

 that whereas the centerline buttock is horizontal 

 for about 11 ft forward of the AP, it was found 

 not possible to level out the lateral buttocks in 

 the same way without decreasing the slopes of 

 the lower transom edges or losing displacement 



Fig. 67.J Port Quarter View op Transom-Stern Model for ABC Ship 



