Linearized Potential Flow Theory for ACVs in a Seaway 



time (long enough for the transients to have died away) in a regular 

 seaway with a uniform speed in a direction normal to the wave crests 

 and also capable of showing the essential features of a more general 

 type of motion. The extension of the theory to longitudinal or drifting 

 motion in a direction oblique to the regular seaway with six degrees 

 of freedom is straightforward and no major revision of the theory is 

 required as the beam/length ratio of present day hovercraft is of the 

 order of unity and the disturbance of the water surface due to the mo- 

 tion of the craft in the longitudinal or beamwise direction may be 

 considered to be of similar order providing that no water contact takes 

 place. The situation therefore is quite different from the case of con- 

 ventional displacement vessels. Also, the extension of the theory to 

 motion in an arbitrary course such as that during manoeuvering, to 

 accelerated motion in starting from rest and to motion in shallow and 

 restricted off-shore coastal waters can all be undertaken with suita- 

 ble modification of the results. The prediction of the motion in an 

 irregular, multi-directional, seaway can also be made by the method 

 of spectral analysis on the basis of the theory of linear superposition. 



The amphibious hovercraft free from water contact may be 

 considered as a special case of a more general type of ACV which we 

 take up as the subject of our present study. The ACV is now assumed 

 to be borne on air cushion contained by peripheral skirts at the bow 

 and the stern and by the side hulls which extend below the hard struc- 

 ture along the sides of the craft and which remain permanently im- 

 mersed in the water during the motion and oscillations of the craft 

 (see fig. l). 



It is however, assumed that the flexible extensions do not 

 contact the water surface during the motions and oscillations of the 

 ACV, but an extension of the present theory to take into account skirt 

 contact is straightforward if it is assumed that the flexible extensions 

 are rigid enough to retain their shape when contacting the water. A 

 later extension would be to cover the case of compliance to the pres- 

 sure of the water. 



It is assumed that the air cushion is bounded by thin hulls 

 along the sides and the air jets (or plenum air escape) at the front 

 and the rear. The theory can also be suitably revised to cover the 

 case of hulls (or skegs) which are located inboard of the lateral boun- 

 dary, the whole air cushion then being enclosed within peripheral 

 skirts. This configuration is sometimes adopted when water propul- 

 sion is used. The side hulls are assumed to be "thin" with different 

 "semi-widths" on either side. A vertical plane is sometimes used on 

 the inboard side of the hulls because of the relative simplicity in 



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