Sec. 74.11 



MOVABLE-APPENDAGE DESIGN 



725 



of mechanically powered small boats embody 

 those of Sec. 74.8 relative to balance and trailing, 

 plus the increased relative rudder area indicated 

 for small craft in Tables 74. b and 74. d. Whereas 

 formerly many of these rudders were operated 

 by hand, some of them now have power gear, and 

 a few of them automatic steering. 



For craft which run at speed-length or Taylor 

 quotients T^ of 3 or more, F„ > 0.89, the rudder 

 forces and moments are large compared to other 

 forces and moments. This applies to heeling as 

 well as swinging moments, so that whether or 

 not a boat banks properly (inward) on a turn may 

 depend as much on the rudder as on the hull. 

 As the Tj and the absolute speeds reach higher 

 values it is increasmgly important that the rudder 

 be of the proper size and shape. 



Moximuir Rudder 



Fig. 74.H Motoeboat Rudder with Streamlined 

 Sections 



Fig. 74.H indicates the outline and prmcipal 

 dimensions of a type of streamlined rudder 

 developed by Elliott Gardner for a 45.5-ft air 

 rescue boat, based upon systematic full-scale 

 tests and experience extending back to 1925. It 

 is reported to be suitable for all speeds from 5 to 

 50 mph, or 4.3 to 43.4 kt. 



Fig. 74.1 is a more modern rudder of somewhat 

 similar design, intended for use on a 52-ft air 

 rescue boat, but having parallel sides in the run 



Fig. 74.1 Blunt-Ended Parallel-Sided Motorboat 



Rudder 



The numerals may represent any units of measurement 



or serve as relative proportions. 



or tail portion and a wide, square trailing edge. 

 It is cut away at the after upper corner because: 



(a) The +Ap built up on the ahead side of the 

 upper after corner of an angled rudder with a 

 horizontal top, indicated in Fig. 74. H, exerts a 

 lift force under the stern and depresses the bow. 

 While this is an advantage in straight-away 

 running, it is liable to cause the bow to trip in 

 a turn [Grenfell, T., "Some Notes on Steering of 

 High Speed Planing Hulls," SNAME, Pac. 

 Northwest Sect., 27 Sep 1952; abstracted in 

 SNAME Bull., Jan 1953, p. 35]. 



(b) The portion removed permits a more-or-less 

 solid stream of water to pass over the angled 

 rudder, close under the hull. This acts as a shield 

 to prevent air leakage to the astern or — Ap side 

 of the rudder. Whether cut away or not, it is 

 preferable to place the rudder so that it lies 

 completely under the hull at any angle, to obtain 

 the maximum shielding effect from air leakage. 



The nominal aspect ratio of the horizontal-top 

 rudder of Fig. 74.H is 1.247 whereas that of the 

 more modern design in Fig. 74.1 is 1.224. 



The design of rudders for high-speed motorboats 

 of the planing type, especially those which travel 

 at values of the Taylor quotient T, = V/'vL 

 in excess of 5, is based largely upon experiment 



