Murthy 



be used to obtain an estimate of the motions of 

 the ACV in irregular seas within the limits of the 

 theory of linear superposition. 



I - INTRODUCTION AND SUMMARY. 



The purpose of this study is to develop ultimately a compre- 

 hensive hydrodynamic theory for the general motion of an Air Cushion 

 Vehicle (denoted, in short, by ACV and alternatively referred to as 

 hovercraft) in an arbitrary seaway. This general non-linear problem 

 will be formulated presently and, as may be expected, it will soon 

 become apparent that the solution will have to be carried through in 

 various successive stages, with some form of acceptable lineariza- 

 tion adopted at each stage in order to render the mathematical solu- 

 tion tractable and to keep the algebraic work within reasonable bounds. 

 Practical results can, however, be obtained from calculations based 

 on the lower order theory which can be relatively simple and a com- 

 parison can then be made with the results of full-scale trials and 

 model test data so that any differences pointing perhaps to a deficien- 

 cy in the theory may possibly be reconciled by invoking the higher 

 order theory. 



In a previous work (197 0) the author^) has considered the 

 case of an "ideal"hovercraft as a starting point for the larger study. 

 This amphibious craft is completely separated from the water surfa- 

 ce during its motions and oscillations and was assumed to be travell- 

 ing under a constant longitudinal thrust at a uniform speed in a uni- 

 directional seaway composed of regular waves with their long crests 

 normal to the direction of motion. Although the hovercraft was assum- 

 ed to be clear of the water surface, practical expressions for the wa- 

 ve resistance and side force in longitudinal and drifting motion over 

 calm water, the restoring forces and moments due to forced oscilla- 

 tion over calm water and the response functions for free oscillation 

 in a regular seaway have been derived. The effects of the compart- 

 mentation of the cushion and the overall cushion stiffness on the mo- 

 tion have also been presented. The mean increased resistance over 

 waves and the added mass and damping of water can also be calculat- 

 ed. In order to keep the algebra simple, the hovercraft was assumed 

 to undergo coplanar motion in the longitudinal plane with freedom in 

 pitch, heave and surge only. 



This restriction of the motion to a plane, although not a strict 

 requirement of the linearized theory, was considered as the only ty- 

 pe of motion which was likely when the craft was operating for a long 



100 



