710 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 74.4 



111 case the V-shape of the stern is only moder- 

 ately shallow, and the sides rise at an appreciable 

 angle from the edges of the flat over the rudder, 

 there is an opportunity to derive some lateral 

 pressure on and some swingmg moment directly 

 from the hull itself. This is because the hull is 

 near the top of the rudder and because of the 

 close fit there. At the designed speed, the crest 

 of the stern wave is expected to cover a sizable 

 area of the hull above the rudder, as projected 

 on the centerplane, in addition to that which lies 

 below the at-rest waterline. This increases the 

 area over which Ap's are exerted. 



A spade rudder may be tapered (reduced in 

 fore-and-aft length) toward the bottom to: 



(1) Increase the aspect ratio 



(2) Reduce the strength of the tip vortex at the 

 bottom when exertmg heavy lifts and lateral forces 



(3) Thin the lower part and reduce its drag 



(4) Diminish the bending moment at the head 

 of the rudder. 



Sometimes the possibility of air leakage to the 

 top of a spade rudder can not be prevented. It is 

 then wise to reduce rudder area at the top, 

 shortening the rudder and moving its contour line 

 farther from the adjacent water surface. The area 

 thus removed from the top of the blade is shifted 

 to the bottom, well below the surface, possibly 

 making the rudder longer at the bottom than at 

 the top. A change like this was found successful 

 two decades ago in the design of the U.S.S. 

 Farragut, a destroyer of the DD348 class. 



A centerline rudder on a twin- or multiple-screw 

 stern produces the greatest lateral forces 

 when it follows as closely as possible the contour 

 of the stern. There should be the minimum leakage 

 area between the rudder and the adjacent portions 

 of the ship upon which the rudder pressure fields 

 act. The lower the horizontal joint between hull 

 and rudder, the greater the hull areas above it, 

 projected upon the centerplane, which are acted 

 upon by these pressure fields. Whether this 

 enables the use of a smaller rudder depends upon 

 many other factors. 



A rudder hung from a skeg along its entire 

 leading edge, and preferably fitting the huU 

 closely along its top, has the advantage that it is 

 developing the maximum turning moment on 

 the hull. While it is completely unbalanced it can 

 be made smaller (shorter or narrower) than a 

 rudder not so closely fitted. The torques upon its 

 stock, and those to be exerted by the steering 



gear, are thus decreased. Nevertheless, such a 

 design is logical only when the vessel is intended 

 almost exclusively for ahead operation, as in 

 saihng craft. It is an advantage if backing is 

 only incidental and if there are no specific require- 

 ments about handling the rudder during backing 

 or when moving astern. 



The horn or compound-type rudder, illustrated 

 at 5 in Fig. 37.D, is not in the foregoing category. 

 Actually, its effective aspect ratio is usually 

 rather small. 



For an isolated rudder, not attached to a deep 

 skeg or keel, or to a horn, too high an aspect ratio 

 is to be avoided because of the breakdown in 

 hydrofoil flow — and in lift — which occurs at rela- 

 tively small rudder angles [van Lammeren, W. P. 

 A., RPSS, 1948, p. 323]. High nominal aspect 

 ratios in single rudders, say up to 3.5 or 4.0, can be 

 accepted if the rudder lies partly or wholly within 

 the outflow jet of a propeller. The fact that the 

 rudder is benefiting from the augmented outflow 

 velocity may make it unnecessary, in combination 

 with these high aspect ratios, to use rudder angles 

 beyond the stalhng value of 20 or 25 deg. 



Long experience with spade and horn-type 

 rudders and horizontal diving planes, on bodies as 

 well as on ships, indicates that for maneuvering 

 (rather than for steering only) a control surface 

 which is approximately square is as serviceable 

 as any. This is because the breakdown or stalling 

 point is deferred to larger angles. A greater total 

 lift or lateral force can be achieved, greater maxi- 

 mum control-sui'face angles can be used, and the 

 bending and torque moments in the stock are 

 more nearly balanced. 



Wherever practicable, a rudder deUberately 

 placed in an outflow jet to take advaiitage of the 

 induced velocity should span the whole jet close 

 to a diameter. Excessive underhang in spade 

 rudders, clearances to withdraw propeller shafts, 

 and the presence of swirl-core cavitation may 

 interfere. In this case spanning a portion of the 

 jet is of course much better than missing it 

 altogether. 



When one or more operative conditions of a 

 vessel involve large changes in draft in the vicinity 

 of the rudder, the actual immersed rudder area 

 must be proved satisfactory for all conditions. 

 As a rule, both hull and rudder come out of the 

 water simultaneously so that the smaller immersed 

 area of the rudder suffices to control what is left 

 of the hull in the water. However, adequate 

 performance in such a situation can by no means 



