Doctors 



In these experiments, the total drag was usually measured 

 with a dynamometer. The aerodynamic drag was then estimated from 

 the drag coefficient on the model, and the momentum drag was obtain- 

 ed from the mass flow into the cushion. The agreement between theory 

 and experiment was found to be best at speeds greater than hump. At 

 lower speeds, nonlinear and viscous effects become important and 

 there was a large scatter in the data. 



To avoid the troublesome wetting drag, Everest (1966a) 

 attempted to eliminate it using a thin polythene sheet floating on the 

 water surface. The resistance breakdown is further discussed by 

 Hogben (1966). The experiments only indicated the presence of the 

 first two (n = 1, 2) and possibly there (n = 3) humps. Hogben 

 (1965) showed that this result fitted in with the idea that the maximum 

 ratio of wave height to length is about one seventh. That is, wave 

 breaking prevents the occurrence of the additional humps. 



Further experimental work by Everest, Willis and Hogben 

 (1968 and 1969) dealt with an ACV at an arbitrary angle of yaw. This 

 problem was also studied numerically by Murthy (1970). In the expe- 

 riments, the wave resistance was measured directly from a wave 

 pattern survey, using the transverse cut method. There was consi- 

 derably less scatter in the data using this method, since the rather 

 doubtful technique of estimating the wetting drag was eliminated. In- 

 deed, the agreement was found to be somewhat better, particularly 

 for the lower of the two cushion pressures tested. 



In an attempt to get better agreement with experiments at 

 lower speeds, Doctors and Sharma (1970 and 1972) used a pressure 

 distribution which essentially acted on a rectangular area but had a 

 controlled amount of smoothing - both in the longitudinal (x axis) 

 and in the transverse (y axis) direction. The distribution used was : 



P(x,y) =X P o 



tanh |a(x + a)[ - tanh |a(x - a)} x 



tanh{j3(y + b)( - tanhj/3 (y - b)} , (5) 



where p Q is the nominal cushion pressure, and a and b are the 

 nominal half-length and half-beam respectively. The rate of pressure 

 fall-off at the edges is determined by the parameters a and /3 . 

 This function is shown in Fig. 1. As a special case, a , /3 — ► °° is 

 equivalent to a uniform pressure acting on a rectangular area 2ax 2b. 



In practice, of course, the pressure at the edge of an ACV 



38 



