716 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 74.7 



A suitable and adequate rudder-design method 

 should produce, to meet a given set of steering 

 and maneuvering requirements, the proper type 

 of rudder, the rudder-blade area, the proportions 

 and shape of the blade, and the maximum rudder 

 angle. To meet unusual operating conditions it 

 should also produce the rate of angUng or laying 

 the rudder and other design features. Looking 

 at the rudder-design problem in its larger aspects, 

 it should also be coordinated with what might be 

 termed the maneuvering design of the hull as a 

 whole. 



Because the project is still incomplete, the 

 description which follows is simply a statement 

 of the various steps involved, \\ith some comments 

 on each. The project has not been carried through 

 to the point where a numerical answer, acceptable 

 by engineering standards, can be obtained by it 

 for a given situation. Presenting an outline only 

 is considered justified because of the need for 

 thought and study on this design problem along 

 new and logical lines. 



The starting point is a careful analysis of the 

 steering and maneuvering requirements of the 

 vessel for which the rudder is to be designed. These 

 are preferably expressed in numerical terms 

 somewhat more specific than those for the ABC 

 ship in Table 64. e, items (27) through (41), 

 especially items (28), (30) to (32), and (34). If 

 requirements for maneuvering have not been 

 laid down by the owner or operator they are pre- 

 scribed for the ship under study by the designer 

 himself, based upon his own experience in the 

 operation of similar types and sizes of ships. 



The rudder and other gear required for maneu- 

 vering, like the hull and the propelling machinery 

 necessary for propulsion, must as a rule be de- 

 signed to meet the most severe operatmg con- 

 dition. It is usually the case, although it may not 

 be taken for granted, that the lesser requirements 

 are then satisfied as a matter of course. 



Just what constitutes the most severe maneu- 

 vering requirement for a ship — the one calling for 

 the largest rudder area at the opthnum or specified 

 rudder angle — can by no means be determined by 

 inspection of the oAvner's and operator's specifica- 

 tions. For the average vessel, however, it is more 

 important to change direction quickly and de- 

 cisively in an emergency maneuver, to avoid 

 colhsion with obstructions or with another ship, 

 than it is to make a subsequent turn at a given 

 radius or speed. In other words, the rudder is 

 more valuable for sheering the vessel off its origmal 



course in the shortest possible time and distance 

 than for guiding it precisely around a steady turn 

 with a large change of course. The situation is 

 somewhat similar to that specified in item (32) of 

 Table 64. e where, to make a canal turn properly, 

 the vessel must accomphsh an offset of 400 ft from 

 the approach path extended in a curved arc of 

 2,100 ft, at a specified speed and rudder angle. It 

 is exactly the same as that faced by a diving 

 submarine. In an emergency, the submarine must 

 get under the water surface and point its nose 

 downward in the shortest possible time and dis- 

 tance from the dive or "execute" point. 



To be sure, every ship must be able to change 

 its course at will, to reverse its direction of swing, 

 and to follow a tortuous path, but there is reason 

 to believe that the initial sheering maneuver is 

 the most demanding and the most important. 

 This means that, vnth the vessel proceeding at 

 steady speed on a straight approach path when 

 a sharp turn is ordered, the rudder must be angled 

 to its full amount very rapidly and a certain 

 maximum swmging moment A'', to be discussed 

 presently, must be applied by it to the ship. 



The sheering maneuver at the beginning of a 

 turn is a logical as well as a practical assumption 

 Avith which to guide the rudder design because: 



(a) The forces and moments exerted on a ship 

 in the first few seconds of this transient stage 

 largely determine how rapidly, and in what ahead 

 distance, the vessel can clear its approach path 

 extended, change its heading, and sheer off to 

 one side or the other 



(b) For an analytic solution, the velocity mag- 

 nitude and direction of the water flowing toward 

 the rudder when beginning a turn may be assumed 

 to be substantially the same as it was a few 

 instants before, when the ship was proceeding 

 along the approach path with zero rudder angle, 

 in a steady-state condition. With this simplifica- 

 tion, only the effect of the angled rudder need 

 be evaluated just after the initial point of the 

 turn. 



(c) Once the ship is m the turn, the determination 

 of the swinging moment produced by the rudder 

 and the moment produced by the ship itself, 

 acting as a hydrofoil -with an angle of attack, 

 becomes exceedingly complex. This is true for an 

 experimental as well as an analytical attack on 

 the design problem. 



(d) As an incidental argument, it is possible in 

 many model basins to measure the transverse 



