Sec. 74.10 



MOVABLE-APPENDAGE DESIGN 



723 



For a ship on which the rudder is required to 

 produce good steering and maneuvering but 

 behmd which the rudder region is filled with 

 slowly moving water, the usual symmetrical 

 hydrofoil section, tapering in the run, is not 

 adequate to give prompt rudder response. The 

 rudder section may then be carried aft from the 

 stock axis at full thickness, with a square ending, 

 or the sides may even be splayed outward toward 

 the trailing edge. This section shape gives the 

 necessary lift forces at small rudder angles for 

 adequate steering in the manner described in 

 Sec. 37.17 and illustrated in Fig. 37.L. 



There appear to be no design rules, or even 

 rules of thumb, to use for guidance in the selection 

 or delineation of splayed or fish-tail rudder 

 sections. In most cases these sections are employed 

 to compensate for extreme eddying or backflow. 

 As the poor flow conditions should not exist in the 

 first place there is little point in conducting 

 systematic research to find out how to correct 

 them by using unusual rudder sections rather than 

 by reshaping the run of the hull. 



Whatever the shape of the trailing edge of a 

 rudder (or diving plane) section, whether inside 

 a propulsion-device outflow jet or not, it should 

 be such that there is never any doubt in the mind 

 of the water as to just where it is going to separate 

 from the section. If the trailing edge can not be 

 fine, narrow enough so that the eddies abaft it 

 are insignificant, it should be cut off square, or 

 even splayed like the split tail of a weathervane. 

 It is never to be rounded enough to permit the 

 separation points on either side to shift backward 

 and forward, with consequent eddy buffeting, 

 rattling, vibration, and perhaps even more serious 

 consequences. This is one reason for not using a 

 TMB EPH section, vnth. its rounded section at 

 the trailing edge. This section can, if desired, be 

 terminated in a double-chisel shape, depicted in 

 diagram 2 of Fig. 70. P. 



It is doubtful whether any rudder section 

 suitable for practical use can be kept free of 

 separation and cavitation at extremely large 

 effective angles of attack, of the order of 30 to 

 35 deg. These are always encountered when a 

 rudder is swung rapidly to a hard-over position 

 while the ship is moving on a straight course or 

 perhaps is swinging in the opposite direction. 

 The probable maximum effective angle of attack 

 after the ship has started swinging, at a rate 

 approximately half of that in a steady turn, has 

 to be estimated, using the best known procedure. 



A nose shape is selected for which the lift coefficient 

 C L is still increasing at this attack angle. 



The maximum thickness of a rudder section, 

 occurring almost invariably abreast the stock 

 axis, and the variation of this thickness along the 

 depth of the rudder, is generally a matter of 

 structural design. As such it is not covered here. 

 When the blades of rudders and planes are made 

 removable from the stock it may be necessary 

 to increase the thickness ratio tx/c at the support 

 ends of spade rudders and cantilevered diving 

 planes to a maximum of 0.25. However, this 

 large thickness should not be carried too far 

 along the blade. Normally, the thickness ratio 

 tx/c does not exceed 0.20 or 0.167 at the stock 

 end of the rudder. Toward the free, unsupported 

 end of a cantilevered rudder or plane, where the 

 required section modulus is diminishing rapidly, 

 the thickness ratio can be reduced considerably. 

 The value of tx/c is usually only about 1/6 of 

 that at the stock end. 



The sections of a close-coupled simple rudder, 

 or of the close-coupled portion of a compound 

 rudder, form a continuation of the hull, skeg, or 

 fin to which it is attached. There should be no 

 enlargement and no more discontinuity at the 

 hinged joint than is required to give the rudder 

 clearance to swing from one side to the other. 



Tests of various rudder sections at the David 

 Taylor Model Basin indicate that a simple section 

 shape composed of a semi-circular nose and per- 

 fectly straight sides, tapering to zero thickness at 

 the trailing edge, possesses hit-drag ratios 

 superior to those of the NACA sections suitable 

 for rudders. The thickness ratios tx/c were 0.15 

 and less. Further, the breakdown or stalling point 

 is delayed to a greater angle and the maximum 

 hft is increased. So far as known, no confirmation 

 of these features in full scale is available at the 

 time of writing (1955). It is possible that the use 

 of a short elliptic nose to prevent cavitation there 

 would not detract from the advantages of the 

 section. 



The straight-sided shape of these sections, or of 

 sections smiilar to them, may be found valuable 

 in a study of dynamic stabihty of route, dis- 

 cussed in detail in Part 5 of Volume III. The 

 convex side of a rudder, exposed to the angled 

 flow on the "outside" of a ship in a yaw, may set 

 up undesirable — Ap's acting to increase the yaw. 

 Straight rudder sides of the proper shape would 

 develop +Ap's, acting to reduce the yaw angle. 



74.10 Structural Control-Surface Design as 



