Muvthy 



APPENDIX I 



DISTRIBUTION OF SURFACE PRESSURE DURTNG OSCILLATIONS 



Let the pressure distribution on the IFS due to the air cushion 

 be of the basic form 



P s = 



(x.y) 



during "static hovering", i.e. at zero speed ahead. This distribu- 

 tion determines the basic "hull form" of the air cushion. We may 

 assume that the same distribution prevails even during steady trans- 

 lation. The motion of the side hulls through the water will generate 

 an induced wave, but this will only cause a pressure variation in the 

 interior of the fluid leaving conditions on the surface unaltered. This 

 will also be true of the incident waves whose surface pressure is 

 equal to that of the atmosphere, but the action of the waves in altering 

 the cushion pressure due to "wave pumping" should be taken into 

 account. This requires separate treatment. In other words, the sur- 

 face pressure distribution is dictated only by the motion and oscilla- 

 tions of the air cushion and will remain of the basic form if the rela- 

 tive vertical and angular positions of the air gap at the bow and stern 

 are the same as those when the ACV is stationary. 



We are, however, allowing for oscillatory displacements in 

 surge, heave and pitch during uniform translation. The surge displa- 

 cements will not directly affect the pressure distribution, but the 

 displacements in pitch and heave will certainly cause a variation of 

 the design "daylight clearance" along the bow and stern sections of the 

 air cushion and a different distribution of pressure on the water surfa- 

 ce will result. The situation is fully discussed in Appendix II of Refe- 

 rence 1 where the result for the revised pressure distribution is deri- 

 ved in the form 



p g (x,y) = Pq (x,y) + [x + h G 0+o(0)] 



'dP Q (x, y) 



W, (x' , y' ) 



+ z 



dx' 



+ 



Xr X 



(6' 



i[ 



+ h Q d + (0 ) 



]•[■ 



2 



dx 



(x,y) 



dx 



df 



+ 



^x'.y') 



ax' 



+ . . 



X = X 



S 2 i ] (x'.y') 



170 



