Sec. 74.24 



MOVy\BLE-APPENDAGE DESIGN 



737 



neutral rudder positions on a full-scale vessel. 

 The friction forces in the steering gear and rudder 

 stock are large in proportion to the hydrodynamic 

 torque on the stock at small rudder angles. The 

 torques due to friction may even exceed those 

 due to water flow around the rudder. Furthermore, 

 unless it is known definitely that the rudder -will 

 trail if left to itself, one is reluctant to disconnect 

 the tiller or bypass the steering gear with the 

 vessel traveling at the speed for which the correct 

 neutral position is required. 



It oftens happens that a neutral position which 

 gives zero torque on the stock of an offset rudder 

 is not the one which results in minimum resistance 

 or shaft power. Either the ship designer or the 

 owner and operator must then decide whether 

 the neutral setting is to be for minimum rudder 

 torque or minimum overall resistance. 



For multiple rudders operated by a single 

 steering gear, with tillers connected by drag hnks, 

 it is sometimes possible to fit a temporary link 

 for the early sea trials and to replace it by a 

 permanent link having the proper length. 



74.24 Selection of Swinging Propellers for 

 Steering and Maneuvering. Swinging propellers, 

 described in Sec. 37.22, form perhaps the simplest 

 and most efficient of steering and maneuvering 

 devices. In fact, any propeller, large or small, 

 driven through the medium of a shaft that is 

 approximately vertical, lends itself admirably 

 to this means of steermg. The complete propeller 

 thrust, albeit somewhat modified by a large 

 degree of non-axial flow when the propeller is 

 first swung to a large angle, remains available as 

 an oblique force. A large force component normal 

 to the ship axis serves as the equivalent of the 

 transverse force which would otherwise be exerted 

 by a rudder. The mechanical steering mechanism 

 for a large or high-powered installation must be 

 non-overhauling, otherwise the torque reaction 

 from the vertical shaft takes charge and swings 



the propeller when steering or turning is not 

 desired. 



Efficient propulsion, combined with the swing- 

 ing motion of the horizontal shaft and the pro- 

 peller, reciuire that the inflow jet of water to the 

 angled propeller does not encounter undue inter- 

 ference from parts of the hull ahead of it. Unless 

 the speed of advance is small the axis of the inflow 

 jet follows the predominant flow in the vicinity 

 instead of the angled propeller axis. 



If the vertical drive shaft and its housing do 

 not project from underneath a portion of the 

 stern which is continually submerged, a horizontal 

 subsurface plate is required on the housing at 

 some point below the water surface to prevent 

 leakage of air from the atmosphere to the pro- 

 peller. A so-called "anti-cavitation" plate of this 

 kind, although usually much too small, is em- 

 bodied in the vertical-shaft housings of all out- 

 board-motor installations. 



If the propeller disc lies entirely below the keel, 

 a steering propeller lends itself to use as a tractor 

 propeller at the bow. It may operate either singly 

 or in combination with one or more other steering 

 propellers at the stern. A bow steering propeller, 

 however, is rarely able to take advantage of any 

 wake velocity due either to viscous or potential 

 flow. Instead of swinging aft and upward, like 

 an outboard motor installation when shallow 

 water is suddenly encountered, it must usually 

 swing outward and upward. 



For auxiliary propulsion as well as steering 

 there is available the so-called "active" or 

 Pleuger rudder developed in the early 1950's in 

 Germany [Hansa, 16 Jul 1952, Vol. 89, p. 918; 

 also p. 921]. This device, comprising a submer- 

 sible electric motor driving an auxiliary propeller 

 and swinging about a vertical axis like a rudder, 

 is described at some length in Sec. 37.22. At the 

 time of writing (1955) it is available in limited 

 powers only, not exceeding several hundred horses. 



