Linearized Potential Flow Theory for ACVs in a Seaway 



fected by the naval architect, the "hull form" of the air cushion appears 

 to play an important part in the performance of the ACV. It is com- 

 monly assumed for want of more precise knowledge that thepressure is 

 uniform within the cushion. It is obvious that this cannot to so unless 

 the flexible extensions are also immersed in the water in the same 

 manner as the side hulls. There seems to be some evidence to show 

 that the pressure is reduced to atmospheric at the front and rear 

 boundaries of the cushion where there is leakage of air. This renders 

 the mathematical work slightly easier and enables practical results 

 to be obtained. This also gives the ACV cushion a "hull form" with 

 an acceptable deadrise and flare which may be considered by the 

 naval architect as very suitable for fast planing motions. 



It is also necessary to point out that we have gone into some 

 detail to study the hydrodynamic (including hydrostatic) effects on 

 the motions of the ACV. It is assumed for this purpose that the aero- 

 dynamic effects are known including, in particular, the stiffness and 

 damping of the peripheral -jet or plenum type of cushion. It may be 

 thought that such effects as that due to "wave pumping" should have 

 been taken into account, but this is purely a pneumatic effect as the 

 compression (or rarefaction) of the air cushion is due to the form 

 of the progressive wave underneath it and not due to the pressure of 

 the disturbed water and as such has not been treated here. No doubt 

 this important aspect will have to be taken into account along with 

 other aerodynamic effects in arriving at an overall picture of the . 

 motions of the ACV in a seaway. 



The higher order expressions for the forces and moment deri- 

 ved in this study are extremely complicated but their solution by nu- 

 merical methods with the aid of present-day high speed digital com- 

 puters need not present any serious problems. The finite element 

 method (FEM) which is receiving increasing attention in recent years 

 may prove to be valuable and powerful tool for the solution of pro- 

 blems of this nature. 



167 



