Sec. 74.20 



MOVABLE-APPENDAGE DESIGN 



735 



these Japanese authors pomt out, much more 

 knowledge is needed concernmg the details of 

 flow abaft an actual screw propeller before the 

 designer is able to shape properly the leading 

 edge of a rudder horn or a balanced rudder. 



74.18 Design for Rapid Response to Rudder 

 Action. The development of a given lift or 

 normal force when angled is only one of the things 

 which a rudder or other control surface is called 

 upon to do. It must also do this rapidly, so as to 

 provide quick response on the part of the ship. 

 An excellent example is the control surface in 

 the form of an active roll-resisting fin. This has 

 only 10 sec at the most in which to shift its 

 position from hard over one way to hard over 

 the other and to produce a useful force and rolUng 

 moment before it has to shift position back again. 

 Another example is the rudder of a ship traversing 

 a canal, for which rapid response is much more 

 important than the simple ability to steer or to 

 turn one way or the other. 



The shorter the circulation path around a 

 rudder the smaller is the mass of water to be set 

 in motion and the sooner are the circulation and 

 lift estabhshed. A hydrofoil which is called upon 

 to exert a normal force rapidly should be short in 

 the direction of motion and have reasonably large 

 clearances or apertures both ahead and astern. 

 A short, high spade rudder is the best answer to 

 this design problem. A balanced rudder hung on 

 a small horn, short in the fore-and-aft direction, 

 is the next best. 



For a simple, balanced rudder or a flap-type, 

 unbalanced rudder at the stern of a twin- or 

 multiple-screw ship (or at the stern of a ship 

 driven by side paddlewheels), the clearance for 

 circulation in a horizontal plane around the rudder 

 is provided by an aperture forward of the rudder. 

 This resembles the aperture in which a propeller 

 would be fitted if the ship had only a single screw. 

 Two such apertures are sketched in diagrams 3 

 and 4 of Fig. 74. D. An equivalent aperture of 

 large area hes ahead of the underhung foil portion 

 of the rudder in diagram 5 of Fig. 37. D. 



74.19 Utilization of Automatic Flap-Type 

 Rudders and Diving Planes. There appears 

 to be a definite application in the field of control 

 surfaces for the hinged hydrofoil with automatic 

 flap, depicted in diagram 2 of Fig. 14. U. This 

 device is called here, for want of a better name, 

 the automatic flap-type rudder or divmg plane. 

 There is incorporated in it some simple leverage 

 or equivalent mechanism to apply positive flap 



angle as the main control surface is angled. 

 Both the control-surface angle and the flap or 

 tab angle are in the same direction, whether the 

 rudder is right or left, or the plane is at rise or 

 dive. The flap action always increases the lift of, 

 and the lateral force on the control surface. 



This device has been utihzed successfully for 

 a number of years on the active fins of the Denny- 

 Brown roll-stabilization gear, referenced in Sec. 

 37.9. The linkage for applying flap angle auto- 

 matically is outside the watertight hull of the 

 vessel but in view of its extreme simplicity it has 

 operated well in service. 



Although no designs have been prepared, or 

 installations made, so far as known, it should be 

 possible to substitute this device for almost any 

 rudder or diving-plane installation which fits 

 closely against a fixed portion of the hull. The 

 simple Denny-Brown linkage or its equivalent 

 may be used for operating the flap. 



For a control surface subject to severe pounding 

 or slamming when in waves, the flap may have 

 to be restricted to only a portion of the rudder 

 height or the diving-plane width. The total 

 impact forces on the flap may then be small 

 enough to be withstood by the automatic angling 

 mechanism. 



74.20 Design Notes for Bow Rudders; 

 Rudders for Maneuvering Astern. It is pointed 

 out in Sec. 37.11, supplemented by Fig. 37. G, 

 that a bow rudder produces decidedly inferior 

 steering action with the ship going ahead. Bow 

 rudders are fitted, therefore, primarily on vessels 

 required to back for appreciable distances. Under 

 these conditions, the bow rudder becomes a stern 

 steermg rudder, in the normal sense of the term. 

 If it were suflBciently important to pay for the 

 additional complication and the added steersman, 

 a bow rudder might justify itself on a long, fast, 

 slender craft, operating m shallow and restricted 

 waters. With such a rudder it might be possible 

 to move the ship sideways, or to hold it against 

 wmd and other effects. It would act in this case 

 much as the bow planes on a submarine; in other 

 words, not as a turning mechanism but as a 

 transverse-force-producing device. 



The bow rudder, as a rule, has no induced 

 velocity to augment its effect, but neither does it 

 suffer from a reduced speed of advance because 

 of friction wake. 



The fitting of a bow rudder requires a forefoot 

 that is rather full in profile and thin in section. 

 There is little to be gamed by mounting the stock 



