'IfiS 



HYDRODYNAMICS IN SMTP DESIGN 



Sec. 66.21 



be expected in wavegoiiig. With a draft H of 

 26 ft, this gives a hmiting diameter of 18.2 ft. 

 For better-than-average efficiency the propeller 

 should be considerably larger, with a diameter of 

 the order of 20 ft. The latter size is selected for 

 the ABC ship, as the basis for further sketching. 



It is soon found that, even by working reverse 

 curvature into the buttocks ahead of the transom, 

 it is difficult to make room for such a large 

 propeller on the centerline. The situation is 

 eased somewhat by eliminating the rudder shoe, 

 carrying a semi-balanced rudder on a fixed horn, 

 and dropping the propeller disc almost down to 

 the baseplane. However, when enough fore-and- 

 aft room is left for the rudder, the horn, and the 

 propeller aperture abaft the upper blades, the 

 propeller-disc position is rather far forward, 

 where the buttocks are definitely curving down- 

 ward. Flattening the under side of the main hull 

 to fair into the transom leaves a sort of shelf of 

 considerable extent just above the wheel. The 

 latter is thus shielded exceptionally well from air 

 leakage but it is difficult to provide a large tip 

 clearance at the top center. 



An adaptation of the twin-skeg stern with a 

 single propeller mounted in the tunnel between 

 the skegs offers advantages which appear to 

 warrant the development of a preliminary-design 

 variation along these fines. This arrangement 

 eliminates man.y of the usual difficulties in single- 

 screw sterns by: 



(a) Removing from the vicinity of the centerline 

 the lower apex of the V-sections in the skeg 

 ending, the rudder shoe, and other obstructions 

 which normally have to be accommodated 

 abreast the propeller on the centerline 



(b) Moving the propeller farther aft, where 

 there is more vertical clearance between the 

 baseplane and the buttocks, by shortening the 

 fore-and-aft length of the rudders. There would 

 be twin rudders behind the two skegs instead of 

 a single rudder. 



(c) Providing room for a propeller of greatly in- 

 creased diameter because of the much smaller tip 

 clearance needed inside the tunnel. 



Further developments of this alternative stern, 

 called an arch form, are described in Sec. 67.16. 

 It becomes apparent, as the small-scale afterbody 

 sketches proceed, that it may require different 

 widths and shapes of the designed waterline in 

 the run than the transom stern. 



66.24 Molding a New Underwater Form. 



Within the framework of the selected ratios, pro- 

 portions, coefficients, and parameters, plus the 

 general shape tentatively selected in the preceding 

 sections, it is now required to fashion a good under- 

 water form. So far as propulsion is concerned, it 

 should have the smallest practicable shaft power 

 which will drive it at the designed maximum 

 speed and meet the remaining specification 

 requirements. It may or may not have a low 

 total hull resistance, but it must embody a 

 machinery plant that represents the minimum in 

 first cost and in operating expenses consistent 

 with durability and refiabihty. 



The creation of such a shape, as the best final 

 solution of a most complex flow and resistance 

 problem in hydrodynamics, is probably as much 

 a matter of unconscious understanding and of 

 inspiration as of the straightforward use of all 

 available hydrodynamic knowledge. The selection 

 of a good underwater form as a guide is contingent 

 upon the availability of a store of information on 

 ship forms, contained in the designer's own files 

 or in those available to him, in the technical 

 hterature, in the SNAME RD sheets, and in 

 similar sources. It is equally contingent upon the 

 availability, among those data, of a form re- 

 sembling the one wanted, and on a certain amount 

 of knowledge and good judgment, mixed with 

 experience, when working over that form. 



Many good shapes have been developed 

 through a long process of intelligent refinement, 

 as witness the Taylor Standard Series. For the 

 ABC design being carried through here the 

 Taylor Standard Series form has too low a 

 maximum-section coefficient, 0.923 as compared 

 to a range of 0.955 to 0.96, it has too low a water- 

 plane coefficient, 0.66 as compared to at least 

 0.71, and it does not have a bulb bow. Further, 

 as the TSS parent is essentially a twin-screw 

 form, it appears not suitable for the single-screw 

 project in hand, even though the B/H ratio of 

 2.92 is close to the ABC beam-draft ratio of 

 2.808. The TMB Series 60, block 0.60 parent 

 form, having a Cp of 0.614, has too full a maximum 

 section {Cx = 0.977) for the easy shallow-water 

 driving required of the ABC ship, too small a 

 B/H ratio (2.50), too much parallel waterUne 

 (15 per cent), and no bulb bow. Rather than to 

 follow some other well-developed form of good 

 performance, or to use it as a guide, a large-scale 

 body plan for the new ship is roughed out on a 

 clean sheet, both literally and figuratively, on 



