Murthy 



production, but we have covered the possibility of having different 

 off- sets on the two hull surfaces on either side of a longitudinal plane. 

 We are, however, assuming that the surfaces on the outer sides of the 

 two hulls and those on the inner sides are respectively of the same 

 shape in order to have lateral symmetry very essential to the motion 

 in a straight line we shall be considering. 



The ACV is considered as a freely hovering (but partially 

 floating) rigid body in motion under the action of given external forces 

 (such as those due to wind, propeller thrust, etc. ) together with the 

 hydrostatic and hydrodynamic forces arising out of the "ground effect" 

 of the air cushion in depressing the water surface and from the immers 

 ed part of the side hulls. The equations of motion for the most general 

 type of motion in six degrees of freedom will include in addition to the 

 external forces and the forces due to ground effect, some types of 

 internal forces peculiar to ACVs, such as momentum drag, forces 

 arising from the uneven escape of momentum due to the leakage of the 

 air cushion through the air curtain at the front and the rear and, pos- 

 sibly, even through the troughs of the induced waves which may make 

 part of the side hulls run dry unless the hulls are of suitable draught. 

 There is also the pneumatic effect of the "wave pumping" of the air 

 in the cushion due to the passage through progressive waves. We shall 

 assume, however, that the only force which enables the uniform 

 translation of the ACV is the longitudinal thrust, leaving due account 

 to be taken of all the other factors when the occasion arises. 



In the earlier study, the hovercraft was replaced by its equi- 

 valent mathematical model, namely a "travelling pressure disturban- 

 ce" with a basic "hull form" for the craft dictated by the planform of 

 the air cushion and the two-dimensional distribution of pressure on 

 the water surface consituting the lower boundary of the cushion. All 

 the results were derived on this basis and without enquiring into the 

 actual mechanism employed for the generation and retention of the air 

 cushion, i. e. whether a peripheral jet system or plenum chamber 

 with or without compartmentation was used. This model will be retain- 

 ed for the present study with a separate examination of the effect of 

 the side hulls and the possible interaction between the two. 



It may be taken for granted that hull design has arrived at a 

 stage of perfection due to the efforts of naval architects over the cen- 

 turies, but a basic requirement for developing the hydrodynamic theo- 

 ry of the motions of the composite ACV, i. e. with the air cushion 

 enclosed along the sides by the hulls is a knowledge of the hull form of 

 the air cushion which plays usually the major role in supporting the 

 ACV above the water with a small contribution from the buoyancy of 

 the side hulls. It is commonly assumed for want of a more precise 



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