Doctors 



VI. CONCLUDING REMARKS 



VI. 1. Present Work 



Turning firstly to the case of Rectilinear Motion in a Tank, 

 it is clear that the problem of interference from the side walls during 

 accelerated motion in finite depth water is considerably less than that 

 during steady motion. Model tests under such unsteady conditions 

 would be much easier to perform as a tank width equal to four times 

 the model beam essentially simulates the laterally unrestricted case. 



With regard to the yawing ACV, the great dependence of side 

 force at super-hump speeds on smoothing was an unexpected result. 

 So much so, that it would be unrealistic to model the pressure under 

 the craft with a sharp distribution. Even assuming practical values 

 of aa. - /3a = 40 (which has a negligible effect on unyawed wave resis- 

 tance) reduces the maximum predicted induced side force by almost 

 one half. A study of the expression for the steady-state forces, Eq. 

 (50), reveals that this difference is due to the high frequency oscilla- 

 tions in the integral for d just less than tt/2 . The effect is worst 

 for a yawed sharp distribution when the oscillations decay very slowly 

 and is further emphasized in the integral for side force which con- 

 tains a sin 6 factor, rather, than the integral for wave resistance 

 which contains a cos 6 factor. A particularly large number of sub- 

 divisions in the integration is therefore required under these condi- 

 tions. This probably explains the small discrepancies found at small 

 non-zero yaw angles and yaw angles just below 90" , when attempt- 

 ing to verify the theoretical wave resistance calculated by Murthy 

 (1970) and Everest (1969). 



In practice, these high frequency wavelets probably break 

 due to excessive theoretical steepness, and other practical effects 

 such as cushion air flow. 



The induced side force has nevertheless been found to be 

 significant, being of similar magnitude to the wave resistance. It 

 clearly plays a role in the control of ACVs. This force has been 

 experienced by drivers of air-cushion vehicles, who usually refer to 

 it as "keel effect". 



During a typical manoeuver, it has been found that the induc- 

 ed side force is almost equal to the steady-state value at the same 

 instantaneous yaw angle. 



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