Sec. 67.17 



UNDERWATER HULL DESIGN 



525 



is 18 deg. This is about twice the maximum slope 

 previously recommended for tunnels between 

 skegs [SNAME, 1947, Rule 9, p. 130] but it is 

 made deliberately steeper in an attempt to slow 

 up the water passing through the tunnel and 

 increase the positive wake velocity at the pro- 

 peller position. 



The aftfoot on each skeg is cut away, not so 

 deeply as in the transom-stern design but extend- 

 ing farther forward. To eliminate unnecessary 

 wetted surface, to improve maneuvering charac- 

 teristics, and to insure a better flow of water into 

 the forward end of the tunnel between the skegs, 

 the lower edge of each is cut up for a considerable 

 distance to a height of 2.33 ft (28 in), above the 

 baseplane, equal to the depth of two docking 

 blocks. This horizontal skeg foot is 2 ft wide and 

 extends from about Sta. 16.25 to Sta. 18.3. 

 Between Stas. 15 and 16 there is another flat 

 horizontal region at the bottom of each skeg, 

 lying at a distance above the baseplane cor- 

 responding to the rise of floor at about the 16-ft 

 buttock. The fish-eye view in the lower part of 

 Fig. 67. M indicates the manner in which the 

 skegs converge slightly with distance, from their 

 forward to their after ends. Transverse sections 

 at the stations in the vicinity of the propeller 

 position are shown to large scale in the right-hand 

 diagram of Fig. 67. L. 



The twin rudders, lying close to the projected 

 tip circle on either side, resemble somewhat the 

 curved-blade, tilted-stock twin rudders of the 

 Thornycroft destroyers of a half-century ago 

 [INA, 1908, PI. IV, Fig. 6]. The tops of the rudders 

 are depressed 4 ft below the DWL to guard 

 against air leakage and possible rudder breakdown 

 on turns, with the ship heeling and diminishing 

 the submergence of the top of the rudder on the 

 high side. 



To avoid awkward fairing in the vicinity of the 

 rudder head the transom contour is dropped at 

 the sides to 3.5 ft below the DWL, on the basis 

 that some separation drag due to non-clearing of 

 this deep portion is preferable to irregular eddying 

 above the top of the rudder. Outboard of the skegs 

 the section lines fall naturally into a V-pattern 

 corresponding to that of a narrow ship with 

 centerline skegs such as would be obtained by 

 removing the center portion between the 14-ft 

 buttocks and bringing the two outboard sides 

 together. 



The forebody of the arch-stern design is exactly 



the same as that of the transom-stern design 

 back to Sta. 11, corresponding to 0.55L. 



67.17 Flow Analysis for the Arch Type of 

 Stem. Although it may involve some duplication 

 of material in the preceding section, there is given 

 here a brief analysis of flow conditions under the 

 arch type of stern. This analysis serves also as an 

 indication of the study that should be given to a 

 novel design of this kind in the preliminary- 

 design stage. 



What is termed here the arch stern for deep- 

 water vessels is distinguished from the tunnel 

 stern found on craft intended to operate in 

 shallow water, described in Sec. 25.19, by the 

 fact that, in the former, the tunnel roof never 

 extends above the designed waterline. Design 

 rules for tunnel sterns with roofs elevated above 

 the water surface are contained in Sec. 72.13. 



There has been no difficulty in keeping the 

 centerline tunnel full of water on ships with twin 

 skegs when proportioned as outlined previously 

 [SNAME, 1947, pp. 130-131]. Perhaps this is 

 because in a twin-skeg design, with propellers 

 carried by each skeg, only a part of the tunnel 

 area is occupied by propeller discs. In the present 

 case the propeller disc occupies nearly all of the 

 tunnel area at the propeller position. In a single- 

 tunnel or arch type of single-screw stern, the 

 selection and proportioning of the tunnel area, 

 starting from its forward end, therefore needs 

 great care. The propeller inflow jet, contracting 

 in area as it moves toward the propeller, prac- 

 tically fills the tunnel. Too small a tunnel could 

 be highly detrimental to propulsion. In fact, 

 there were indications when an alternative arch 

 stern for the ABC ship was being planned, that 

 tunnel-stern towboats and pushboats suffered 

 from excessive thrust-deduction forces, apparently 

 as a result of constrictions in the tunnels ahead 

 of the propellers. It was felt that this could 

 possibly be avoided in the ABC ship by keeping 

 the tunnel roof well up in the region just ahead 

 of the wheel. 



To diminish the tunnel-roof slopes on the ABC 

 design to values smaller than those indicated in 

 Fig. 67. M would involve loss of valuable dis- 

 placement volume, shifting part of the pro- 

 pelling machinery farther ahead, a longer exposed 

 propeller shaft, and a further widening of the 

 forward or entrance portion of the tunnel. Any 

 net increase in resistance, such as that caused by 

 thrust deduction, is of course only justified if the 



