Sec. 7-f.S 



MOVABLE-APPENDAGE DESIGN 



721 



non-overhauling the effort or the time required to 

 take off rudder angle in an emergency may be 

 too great to justify letting the CP move ahead 

 of the stock axis under any condition. If the rudder 

 torque becomes excessively large, the fore-and-aft 

 length of the blade — and the actuathig moment 

 required to turn it — must be decreased and the 

 aspect ratio increased, as was done on the large 

 hand-steered sailing ships. This can, of course, 

 also be done on ships which are mechanically 

 steered. 



With certain types of hydraulic steering gears 

 it is possible for an overbalanced rudder to take 

 charge in an emergency and swing rapidly to its 

 hard-over position, causing the ship to circle out 

 of control as long as it is moving ahead. 



The necessity for going astern on mechanically 

 propelled vessels, and for much backing and 

 maneuvering on special-service vessels, often calls 

 for the least practicable steering rudder torque, 

 whether the craft are steered by power or by hand. 

 This generally requires that the center of pressure 

 CP when going ahead be forward of the stock 

 axis so that the CP when going astern mil not 

 be too far from that axis. Shifting the stock axis 

 aft on the blade takes care of this situation but 

 involves the additional disadvantage of possible 

 rudder instability and chatter at zero angle, 

 especially with slackness in the gear, and a 

 tiresome job of steering Lf the gear is of the over- 

 hauling type. The relative position of the two 

 CP's and the axis, for all service conditions, is 

 therefore determined by the most important 

 service operating requirements. 



The following design rules govern the degree of 

 balance on rudders for steering and turning. They 

 are predicated upon reasonably uniform flow over 

 the whole rudder blade: 



(1) The rudder definitely should trail at all angles 

 on any craft: 



(a) Designed for hand steering, and for pleasure 

 only 



(b) Designed for hand steering, and where, for 

 efficiency and safety, the rudder effort must be 

 "felt." Examples are racing sailboats and motor- 

 boats. 



(c) Steered by one man who must also attend 

 to other duties, such as a lone fisherman. 



(2) The rudder is best made slightly under- 

 balanced at small angles for ahead operation at 

 service speed on any vessel: 



(a) Designed to run for long periods with 

 small rudder angle, with a premium on steering 

 effort and power, and wear and tear on the steering 

 mechanism 



(b) Where a reduced rudder-angle rate may be 

 accepted for astern operation 



(c) Which may have to maintain speed ahead 

 in the event of a casualty to the power steering 

 gear which leaves the rudder free to swing. 



(3) The rudder may be slightly overbalanced at 

 small angles for ahead operation at service speed: 



(a) When the friction in the mechanical or 

 emergency gear is enough to prevent the rudder 

 from taking charge if power is lost and from 

 swinging to the hard-over position. 



(4) The rudder may be overbalanced for angles 

 not more than one-third of the maximum and 

 appreciably overbalanced for the small angles used 

 for steering: 



(a) On special-service vessels called upon for 

 much backing and maneuvering, or for hard-over 

 rudder shifts at the maximum rate, when it is an 

 advantage to have the ahead and astern rudder 

 torques equahzed as far as practicable, with 

 neither of them very large 



(b) On vessels provided with reasonably reli- 

 able power-operated auxiliary or emergency steer- 

 ing gears. 



Non-uniform flow over the control-surface 

 blade, whose general effects are described in Sec. 

 37.7, may influence the balance situation; at least 

 the possibility or probability of this flow requires 

 consideration in the design stage. The nature of 

 the flow may control the distribution of balance 

 area along the stock axis, generally normal to 

 the adjacent hull, and the position of this area 

 relative to the retarded water m boundary layers 

 or to the accelerated water in propeller-outflow 

 jets. 



For example, a spade rudder blade is often 

 made shorter (in the direction of flow) at the 

 bottom, well below the hull, than close to the 

 hull, in order to reduce the stock bending moment. 

 This means that the balance portion is longer 

 (fore and aft) near the hull than below it. In fact, 

 the length of the balance portion compared to 

 the blade length, defined as the length-balance 

 ratio in Sec. 37.2, may be greater near the hull 

 than the area-balance ratio. At the bottom of the 

 rudder it may be less. With the long portion of 



