720 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 74.8 



TABLE 74.C, Ship-Turning Moments and Rudder Torques for a 400-ft Ship 

 Adapted from G. H. Bottomley, "Maneuvering of Single-Screw Ships," ICE, London, 1935, No. 175, Appx. I, p. 19. 

 The rudders and fins listed in the two left-hand columns are illustrated diagrammatically in Fig. 74.F. 



low because the value of ylfl/[L(H)] = 0.016 is 

 low for a modern vessel (1955). 



K. E. Schoenherr has plotted some of Bottom- 

 ley's test data in grahic form [PNA, 1939, Vol. II, 

 Fig. 13, p. 209]. Fig. 8 on page 13 of the Bottomley 

 reference indicates that a long cruiser-stern 

 profile lying close above the full-length rudder 

 gives a larger ship-turning moment than with 

 the same rudder and other profiles. Undoubtedly 

 much of this moment is being exerted on the hull 

 proper above the rudder. 



It is realized that the procedure suggested 

 earlier in this section requires considerable 

 development, to say nothing of practical use, 

 before it is ready for application in general ship 

 design. It is of interest to know that this method, 

 greatly simplified, was used with success when 

 designing the divmg planes of the large 3,000-ton 

 U. S. submarines of the 1920's. While the shape 

 of the main hull adjacent to the movable planes 

 was such that little if any vertical force was 

 exerted on the hull, the planes were sized and 

 fashioned by reciuiring that they were to hold the 

 vessel level at a given depth with a given resultant 

 vertical force, either in excess weight or in excess 

 buoyancy. 



The method outlined here is elaborated upon 

 in Part 5 of Volume III of this book. 



74.8 Positioning the Stock Axis Relative to 

 the Blade; Degree of Balance. In general a 

 control surface, working in relatively open water, 

 develops the same lift or lateral force — and drag — 

 for a given angle of attack regardless of the fore- 

 and-aft position of the stock and axis relative to 



the blade. The torque increases rapidly, however, 

 as the stock axis is shifted away from the center 

 of pressure. For rudders, planes, and fins which 

 must be rather closely coupled to a hull there 

 remains a considerable range of positions for the 

 stock axis to suit the available actuating torque 

 or to meet other requirements. 



Any rudder steered by a hand tiller must trail; 

 in other words, the center of pressure CP must 

 always be abaft the stock axis. There must be a 

 positive rudder torque, at any angle and in any 

 condition, which acts to restore the rudder to 

 zero angle. Examples are tiller-operated rudders 

 on small sailboats or yachts. The rudder must of 

 itself swing into line with the flow when going 

 ahead. For steering, it must be held forcibly in 

 position at any other rudder angle. A mechanically 

 operated rudder designed for hand steering must 

 also trail, especially when there is no holding 

 mechanism at the control end of the gear. This 

 corresponds to steering an automotive vehicle in 

 which the steering gear returns to zero — or 

 nearly so — if the hands are taken off the wheel. 



If a hand steering gear is used for emergencies 

 only it is still necessary that the torque be small 

 enough for manual operation. A large ship rudder 

 requires many hands to move it, using the emer- 

 gency gear, and may involve danger to the steers- 

 men if the rudder is overbalanced and takes 

 charge. Indeed, it may at times be imperative to 

 let nature bring the rudder back to a small or to 

 zero angle when the man or mechanical power to 

 move it are not available. 



Even though a hand steering mechanism is 



