Rec. It. 6 



MOVABLE-APPENDAGE DESIGN 



7J3 



operating in shallow as well as deep waters, 

 proves that they do not suffer frequent or serious 

 damage, as might be supposed. Furthermore, 

 provided these operations are planned in advance, 

 they do not represent serious inconveniences when 

 drydocking or hauhng out. 



A variety of types and shapes of rudder, hull, 

 and aperture are shown by K. E. Schoenherr 

 [PNA, 1939, Vol. II, pp. 224-226]. Of these, 

 however, the single-plate rudder of his Fig. 15 is 

 practically obsolete, except for inexpensive instal- 

 lations on small vessels. L. Troost, J. G. Konmg, 

 and W. P. A. van Lammeren show a larger 

 variety, including the screw-propeller position(s) 

 for each type and shape [RPSS, 1948, pp. 331- 

 338]. G. de Verdiere and V. Audren describe the 

 results of model tests on still other rudder shapes 

 and arrangements [ATM A, 1951, Vol. 50, pp. 

 491-514]. 



74.5 Design Procedure for Conflicting Steer- 

 ing Requirements. It is as necessary to know 

 the range of speeds for which optimum steering 

 is required as it is to know the speed for optimum 

 performance of the hull or the propulsion device(s) . 

 In this respect the saihng craft poses the most 

 difficult design problem. It must steer and maneu- 

 ver well throughout the entire range, from the 

 greatest speed of which it is capable down to 

 practically zero speed. 



It is interesting to note the manner in which 

 this is achieved, especially for vessels with flap- 

 type rudders hung directly on the main hull. If 

 the vessel is relatively small, say less than 100 ft 

 in waterhne length, the underwater hull is cut 

 away sharply at both ends and the remaining 

 portion is short compared to the overall size of 

 the vessel. The horizontal circulation path around 

 it is short, so that the response to rudder angle is 

 good at any speed. The rudder is hung directly 

 on the skeg or deep keel, so that steering is 

 adequate and reliable even at slow speeds. 



For the large sailing ship the slow-speed 

 steering is equally good, but the underwater hull 

 is much longer in comparison, and the response 

 is slower. In this case, however, the response 

 should be more dehberate. It gives the crew the 

 extra time necessary to trim the sails and perform 

 other duties connected with the sailing evolutions. 

 It would be dangerous, in many cases, to permit 

 the vessel to swing too rapidly. 



The mechanically propelled vessel poses a 

 different problem. The shape and position of a 

 rudder, and its relation with respect to the 



adjacent hull, is not necessarily the same when 

 quick response at moderate power is required 

 as it is when the vessel is called upon to steer well, 

 with the propulsion device rotating very slowly 

 or actually stopped. Since both requirements are 

 often included, and understandably so, the solu- 

 tion in such a case appears to embody two sepa- 

 rate features: 



(1) A large tail section of the rudder blade lying 

 directly abaft a sizable portion of the hull, or 

 abaft a horn or skeg of appreciable area compared 

 to that of the tail, with the smallest practicable 

 hinge gap 



(2) A sizable foil portion having ample clearances 

 ahead of and abaft it, to provide room for the 

 rapid setting up of the circulation needed to 

 produce immediate steering response. 



The designs of the rudders for both the transom- 

 stern and arch-stern hulls of the ABC ship were 

 carried out with these features in mind. A better 

 all-around solution is probably to use one or more 

 spade rudders of generous area, provided these 

 can be accommodated in the design. 



74.6 First Approximation to Control-Surface 

 Area. The discussion in this section is limited 

 to rudders mounted in the vertical plane, or 

 nearly so. Diving planes of submarines and other 

 control surfaces are omitted because of the widely 

 varying requirements for different types of service. 

 Although the rudders of mechanically driven 

 vessels have developed through the years in 

 the almost complete absence of specific steering 

 or maneuvering requirements, the resulting rudder 

 parameters and hull-rudder proportions lie within 

 not too wide a range. 



Based solely upon the ratio of the rudder area 

 Ar to the lateral area ^l of the ship, or to the 

 product of the length L and the draft B., the 

 relative size of steering rudders has increased 

 gradually but steadily during the past half- 

 century. In fact many ships built in the 1900's 

 or 1910's had to have their rudders increased in 

 area, often by as much as 20 or 30 per cent, 

 indicated by the listing in Table 74. a. A rudder of 

 given area, working in a propeller-outflow jet, 

 gives greater lateral forces as the propeller power 

 is increased, and may even provide better man- 

 euverability at a higher speed. The shift to pro- 

 portionately larger rudders appears to be a sign 

 of unconscious but actual stepping-up of maneu- 

 verabUity requirements. It may be expected that 

 this intensifying process will continue, and that 



