Sec. 73.22 



FIXED-APPENDAGE DESIGN 



699 



be placed in the lines of flow, but the proper 

 position is easily determined from a tuft test. 



73.21 Design of Torque-Compensating Fins. 

 So far as known, there are no finished designs or 

 working shipboard installations of devices in- 

 tended to counteract the effect of the unsym- 

 metrical propelling-plant torque, described in 

 Sec. 33.19. 



The approximate heel angle resulting from 

 unbalanced propeller torque is readily deter- 

 mined by entering the righting-moment curve of 

 the ship with the propeller torque Q. In this case 

 it is assumed that the righting moments are not 

 altered due to forward motion of the vessel. Any 

 such effect is usually of the second order of mag- 

 nitude. 



As the first stage in such a study it is proposed 

 that this be accomplished by fitting abaft the 

 screw propeller a fixed fin traversing some con- 

 venient diameter of the outflow jet. This fin, 

 like the well-known contra-rudder described in 

 Sec. 37.16, is twisted on opposite sides of the 

 shaft axis to develop thrust while extracting 

 rotational energy from the outflow jet. It thus 

 combines a torque-compensating device with a 

 means of improving the propulsive efficiency. The 

 counterbalancing torque produced by this device 

 is a function of the velocity of flow over it, just 



Heelini^ Moment Due to Engine. 



Lateral 



Forces 



Exerted on 



Contra- Fins 



In Rotating 



Water of 



Propeller 



-Outflow Jet 



PLAN VIEW FROM ABOVE 



Fig. 73. P Proposed Scheme for Counteracting 

 Unsymmetric Engine Torque 



as the propeller torque is a function of the 

 resultant velocity at an average blade element. 

 The counterbalancing torque thus increases with 

 ship speed and rate of rotation, to match the 

 increasing propeller torque throughout the ship 

 speed range. 



To provide the necessary compensating torque 

 from one diametral fin, the contra-effect could be 

 somewhat larger than is customary for a contra- 

 rudder. Methods of developing the twisted shape 

 for the latter are described in Sec. 74.16. 



A set of torque-compensating fins, each having 

 three or more radial arms, symmetrical with 

 respect to the shaft axis, could be used if con- 

 venient in place of the single fin with two arms. 

 The problem here, as with the two-arm assembly, 

 is to support the contra-fins in their proper 

 positions without increasing the appendage resist- 

 ance by a disproportionate amount. Fig. 73. P is a 

 sketch of such a device, with a vertical fin worked 

 into a compoimd-type contra-rudder assembly. 

 A horizontal fin is supported at the center by the 

 rudder post and at its outer ends by two vertical 

 struts. An upward component of flow abaft the 

 single propeller might require a tilting of the 

 horizontal arm, downward and forward. 



Sec. 69.13 describes a method of using imsym- 

 metric spray strips on a high-speed planing craft 

 to achieve a measure of dynamic reaction to 

 unbalanced engine torque. 



73.22 Fixed Guards and Fenders. Pro- 

 jections from the fair surface of the hull in the 

 form of fixed guards or fenders are of two types, 

 those primarily vertical and those which lie 

 generally parallel to the direction of motion. Of 

 the first type are guards for external scupper 

 and drainage leads or, occasionally, for operating 

 gear which must be external to the shell. Of the 

 second type are fenders and projecting fender 

 strakes, extending over considerable portions of 

 the length but covering limited portions of a 

 transverse section. 



The vertical projections are an abomination 

 from every point of view and are to be avoided 

 wherever practicable. They throw spray at all 

 except the lowest speeds and they cause pressure 

 resistance and separation drag, with holes in the 

 water behind them. They cover up shell surfaces 

 hable to heavy corrosion and they are vulnerable 

 to damage from other craft or heavy objects 

 lying alongside. 



Longitudinal fenders, regardless of type of 

 section or construction, must often be placed at 



