726 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 74.12 



and experience. A generous portion of both is 

 made available to the profession in a paper 

 entitled "Some Notes on Steeling on High-Speed 

 Planing Hulls," by T. Grenfell, previously 

 referenced in this section. Grenfell recommends 

 that the aspect ratio of rudders for these ultra- 

 high-speed craft be of the order of 2, making 

 them twice as deep as they are long. 



74.12 Design of Close-Coupled and Com- 

 pound Rudders. Any close-coupled simple 

 rudder is in reality a compound rudder, but its 

 effect is not easy to predict because the greater 

 part of the quantitative data in existence are for 

 flap-type hydrofoils of rather different proportions. 

 However, sufficient information has been obtained 

 from special model tests [Abell, T. B., INA, 1936, 

 pp. 137-144 and PI. XV; van Lammeren, W. P. A., 

 RPSS, 1948, pp. 327-328] to enable a fairly 

 reliable estunate to be made of the part of the 

 total transverse force exerted by the movable 

 rudder and the part exerted on the fixed or ship 

 structure adjacent to the rudder. Part 5 of Volume 

 III contains a series of diagrams from which these 

 fractional parts can be estimated for any probable 

 rudder arrangement. From that point on the 

 design follows the procedure described in the 

 sections preceding. 



The indications from a series of rudder and 

 stern profiles and corresponding graphs of ship 

 turning moment given by W. P. A. van Lammeren 

 [RPSS, 1948, Fig. 221, p. 331] are that none of 

 the fixed fins or posts shown there contribute 

 much additional force to that exerted by the 

 movable rudder blade. 



When designing a compound-type control 

 surface, or one with a movable blade carrying a 

 flap, it is well to limit the angle of the movable 

 portion, or the flap, to the order of 20 deg either 

 way, especially if the length of the movable portion 

 or flap is less than 0.4 of the chord length of the 

 whole section [NACA Rep. WRL-419, Jan 1944]. 

 This eliminates any possibility of stall. However, 

 if the whole control surface is of low aspect ratio, 

 or if the movable portion is very large compared 

 to the whole, it is possible that the movable 

 portion or flap may remain effective up to angles 

 of 35 deg either way. 



74.13 Conditions Calling for Tubular Rudders. 

 When steering in shallow, fast flowing rivers, 

 prompt rudder action at just the right moment 

 may mean the difTerence between a safe passage 

 and disaster. For this type of service no single- 

 blade or multi-blade rudder is adequate. If 



s\vinging or steering propellers can not be used, 

 one or more tubular or box rudders or swinging 

 Kort nozzles may be employed, arranged so as to 

 change the direction of the whole propeller-outflow 

 jet. Fig. 74. J illu.strates an experimental rudder 

 of this type. 



Fig. 74.J A Pair op Tubular, Swiveling Rudders 



For proper mechanical clearance the stock axis 

 must pass close to or through the plane of the 

 propeller disc. Even then a constant small tip 

 clearance is not possible unless the tubular rudder 

 has a shape abreast the propeller that is approxi- 

 mately spherical. Against the efficacy of a proper 

 design of tubular rudder must be balanced the 

 liability of bending the tube on rocks or debris in 

 the stream bed and jamming the propeller inside it. 



74.14 Closures for Rudder Hinge Gaps. 

 Sec. 37.3 and diagram 9 of Fig. 37. D illustrate 

 the manner in which differential-pressure leakage 

 can take place between the +Ap and — Ap sides 

 of a rudder-and-support assembly, even when the 

 clearances are reasonably small. On old sailing 

 ships it was customary to keep this gap as small 

 as easy movement of the wooden parts would 

 permit. Clearance spaces left for lifting the rudders 

 far enough to get the pintles out of the gudgeons 

 were covered by filhng plates [Barry, R. E., Mar. 

 Eng'g., Sep 1921, p. 689]. In general, plates or 

 other closures should be fitted on the fixed portion 

 of the post-and-blade assembly. This is the reason 

 for the fixed lugs on the sternpost shown by Fig. 

 73. K of Sec. 73.14. 



