Linearized Potential Flow Theory for ACVs in a Seaway 



VI. STEADY MOTION IN CALM WATER 



When the ACV moves at a uniform speed in a longitudinal 

 direction under the action of a constant propulsive thrust the motions 

 of the craft and of the fluid are independent of time. The waves indu- 

 ced by the air cushion and by the side hulls travel with the same speed 

 as the craft and there is therefore no periodic disturbance. Although 

 it is not expected that the ACV would develop- periodic oscillations, it 

 is quite conceivable that it will take up a steady state trim on account of 

 the steady disturbance of the water surface appropriate to the forward 

 speed. 



Let us assume that the steady displacements of the ACV are : 



Surge x= 5 x 100 + % 10 + ^x 11() 



heave z= 55^ + fiz + S^ 1Q 



pitch e= *9 l00 + fi' 010 + W llQ 



(6-1) 



These are the displacements at and about the C.G. of the 

 vehicle. The first set of terms denote the displacement due to the 

 motion of the side hulls, the second due to the air cushion and the 

 third due to the interference between the motions of the air cushion 

 and of the side hulls. 



The only force acting on the ACV apart from its weight is the 

 thrust T which may be assumed to act in a direction parallel to the 

 deck of the ACV at a height h T above the C.G. This is on the assump- 

 tion that air propulsion is employed. In the case of water propulsion, 

 the thrust line will be below the C.G. and also, possibly, oblique to 

 the deck surface, but the principle of the discussion which follows 

 will be the same. 



The thrust is adjusted in such a manner that it is just suffi- 

 cient to overcome wave resistance as this is the only horizontal for- 

 ce (apart from skin friction, which is not considered in this study, 

 for the fluid has no viscosity) to enable uniform progression. When 

 the craft takes up a pitch trim the components of thrust along the x 

 and z axes will be T cos and -T sin d respectively, so that these 

 should equal respectively the longitudinal and vertical forces at the 

 C.G. of the ACV arising out of the action of the fluid pressure due 



139 



