A Survey of Ship Motion Stabilization 



with Ref. 24 discuss actual performance at sea of specific installations, while 

 Chadwick, Bell, and DuCane deal with control theory and design. 



Various types of fins are used by the different manufacturers. There are 

 articulated-flap fins and simple fins, both tapered and untapered. The range of 

 aspect ratios selected depends on the method of retraction, or lack of retraction. 

 The hydrodynamic design of fins [25] is influenced strongly by maximum lift co- 

 efficient as limited by cavitation and the free surface, while lift curve slope and 

 low drag considerations are not very important. Unsteady effects on lift slope 

 are not significant, even considering the high tilt rates required. There is evi- 

 dence [26] that the maximum lift coefficient is augmented by unsteady effects, 

 but use of this phenomenon in design is not widespread. Unsteady effects must 

 be included in the computation of tilting torque. The torque loads proportional 

 to acceleration and velocity are significant, and if not allowed for, the slow re- 

 sponse of the fins to control system orders could be embarrassing. 



In those cases where flapped fins have been specified by designers, cavita- 

 tion must have been a principal consideration. For merchant ships, wherein 

 cruising speed and full speed are nearly the same, the design speed for the fins 

 is relatively high. To economize on fin size, the desired stabilization capacity 

 is provided with the fins producing nearly their maximum lift coefficient. Under 

 these conditions, the more uniform pressure distribution on flapped fins is bene- 

 ficial. For warships, or ships having a cruising speed much less than maximum, 

 the design condition for the fins is not as severe. Large lift coefficients are re- 

 quired only at cruising speed, and the fin angle is limited at higher speeds to 

 maintain the stress level in the stock. At speeds where cavitation would be a 

 concern, only small lift coefficients are required. For this reason, plain fins 

 may be used. The elimination of the flap actuator and flap hinge structure may 

 in turn save enough weight to compensate for the increased area of a plain fin. 



Stabilizer fins are usually located in pairs, port and starboard. If more 

 than one pair is to be installed, the downwash effects of the forward fins upon 

 the flow to the after fins must be considered. 



Most modern roll control systems use combinations of roll ai^le, roll ve- 

 locity and roll acceleration to generate ordered fin angles. The "Denny Brown" 

 tj^es order fin angle [21] while the Sperry type orders fin lift [13,23] using a 

 deflection gage within the fin to feedback the lift. Earlier control systems used 

 much simpler concepts, having been analyzed and designed to deal with regular 

 waves. As experience with real seas and the ability to analyze random seas has 

 accumulated, more sophisticated controls have been adopted [21]. 



It is possible to design control systems to minimize any of several parame- 

 ters of the motion. The most common index of performance is roll angle reduc- 

 tion, although roll velocity or acceleration could be the factor of most interest. 

 References 27 and 28 are two of many papers in the control system literature 

 which discuss designing to minimize various energy criteria. Minimum energy 

 demand on the stabilizer, or minimum energy of residual motions are but two of 

 the possibilities. How much benefit such refined techniques might give to ship 

 motion reduction remains to be seen. 



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