Sec. 67.20 



UNDERWATER-HULL DESIGN 



529 



contra-guide stern or skeg ending, described in 

 Sec. 25.16 and discussed in greater detail in Sec. 

 67.22, is to change the direction of the water 

 flowing past the skeg ending so as to meet the 

 rotating propeller blades in the vicinity of the 

 12 o'clock and the 6 o'clock positions. The skeg 

 is deliberately twisted away from the centerplane 

 of the ship, for a half-propeller diameter or more 

 ahead of its termination, to cause the water to 

 flow to the propeller in the desired contrary 

 direction. This unsymmetrical construction more 

 than pays for its added resistance by the resulting 

 increase of incident velocity on the blade ele- 

 ments, the increased effective angle of attack, 

 and the higher efficiency of propulsion. 



The flow of water on a ship with a more-or-less 

 normal stern, carrying a right-handed propeller, 

 is generally upward and aft under the stern. It 

 meets the downward-swinging blades in the 2, 3 

 and 4 o'clock positions. However, the water 

 flowing aft and upward on the port side of the 

 single skeg follows rather than meets the blades 

 in the 8, 9, and 10 o'clock positions. The problem 

 now confronting the marine architect is to 

 increase the propulsive efficiency of a single-screw 

 vessel still further, possibly as much as 3 or 4 

 per cent, by changing the direction of the water 

 on the port side so that it flows downward and 

 meets the upward-swinging blades. If this full 

 change can not be made, because of prohibitive 

 drag and other reasons, it may at least be possible 

 to diminish the upward angle of flow on the port 

 side. 



There is no structural, machinery, or hydro- 

 dynamic reason why the stern of a single-screw 

 vessel with a single centerline skeg need be 

 symmetrical if there is a distinct advantage to be 

 gained by making it decidedly unsymmetrical, 

 much more so than the present contra-guide 

 stern. Indeed, there is no reason why, if the ship 

 is to benefit by the change, the axis of the single 

 propeller need be in the centerplane or even 

 exactly parallel to it. 



There are cases on record of tanker models in 

 which the flow near the end of a centerline skeg 

 carrying a single propeller is directed downward 

 as it meets the propeller. This may be due to 

 deflection from the under side of a separation 

 zone below the water surface and above the 

 propeller, or to downward flow on the insides of 

 two large longitudinal-axis vortexes coming off 

 the bilges, somewhat larger than the one dia- 

 grammed in Fig. 25. F. Unfortunately in these 



cases the downward deflection occurred on both 

 sides of the skeg so the propeller did not benefit 

 from it any more than it benefits from the normal 

 upward flow on both sides. 



It should not be necessary deliberately to create 

 a separation zone on the port side of the .ship to 

 deflect the flow downward on that side. Bulging 

 the stern out to fill the space which would be 

 occupied by such a zone means that, on a ship 

 of limited length, there would be another separa- 

 tion zone abaft the bulge, with its added drag. 

 It is not yet known how to accomplish this 

 downward deflection of water on the port side 

 without using up all or more of the energy to be 

 gained by the change but there is undoubtedly 

 some way of doing it. 



Any asymmetry in the stern in a scheme of 

 this kind has no appreciable effect upon main- 

 taining the upright position of the ship. Likewise 

 it should have no effect in steering or turning; 

 indeed, such a change might improve the steering 

 characteristics because of the present need for 

 carrying 2 or 3 deg of right rudder on a single- 

 screw ship with a right-handed propeller. 



67.20 Proportions and Characteristics of an 

 Immersed-Transom Stern. There is little reason, 

 at least as far as resistance, speed, and power are 

 concerned, for the use of an immersed-transom 

 stern unless, at some speed below the designed 

 value, the water clears the transom and leaves its 

 entire after surface exposed to atmospheric 

 pressure. Present knowledge indicates that this 

 speed depends mostly on the immersed depth of 

 the transom at its lowest point, and partly upon 

 the buttock slopes just ahead of the lower edge 

 of the transom. As described in Sec. 25.14, a 

 so-called "transom" or submergence Froude 

 number F,, may be set up, having as its length 

 dimension the greatest immersed draft Hu of 

 the transom below the at-rest waterline. For 

 reasonably flat buttock slopes at the stern, indi- 

 cated as Ib in Fig. 25.1, F,, may be as small as 5, 

 possibly as small as 4. Table 67.d gives a set of 

 immersed-transom drafts and corresponding 

 speeds, for a g value of 32.174 ft per sec^, at which 

 the Froude number F„ equals 5.0. 



In general, the immersed-transom draft H[/ is 

 selected upon the basis of the lowest speed at 

 which economical operation is desired, partic- 

 ularly when this speed lies below that at which the 

 underwater portion of the transom is completely 

 exposed to the air. The lower this speed, the 

 shallower should be the immersed portion of the 



