Lineavized Potential Flow Theory for ACVs in a Seaway 



REPLY TO DISCUSSION 



T. K. S. Murthy 



Portsmouth Polytechnic 

 Portsmouth^ U. K. 



Dr. Doctors is quite right. I missed mentioning these things 

 in my hurried talk. I would refer you to page 167 where I have clearly 

 stressed that we have gone into some detail to study the hydrodyna- 

 mic, including hydrostatic, effects on the motions of the ACV. It is 

 only a short paragraph and I will read it : 



"It is assumed for this purpose that the aerodynamic effects are known 

 including, in particular, the stiffness and damping of the peripheral- 

 jet or plenum type of cushion. It may be thought that such effects as 

 that due to "wave pumping" should have been taken into account. . . " 



Of course, that includes the question of fan characteristics 

 as well. These are assumed to be known and they can be fed into the 

 equations. I am only assuming that the ACV is moving under the ac- 

 tion of a constant thrust. I am assuming that the pressure distribu- 

 tion is known. The expressions involve the pitch and heave stiffness 

 of the air cushion itself, which are assumed to be known ; they are 

 the functions fi and £2 given in the text of the paper, which take into 

 account the air gap of the cushion and the stiffness and damping of the 

 air cushion due to variations in the gap. 



The third point is concerned with stability. In my work I am 

 not concerned with the actual cushion ; I only want to know what the 

 pressure distribution is. People think, rather naively, that the cushion 

 is uniform. It is very easy to construct a uniform cushion within a 

 rectangular boundary but one can see very easily that this type of 

 cushion will be unsuitable for high speed motion because if you trans- 

 late it into shipping terms a uniform rectangular cushion will have a 

 hull form in the shape of a vertical cylinder with a vertical bow and 

 stern, and no naval architect would use this type of hull form for a 

 fast bearing planing vessel. 



It is also made clear in the expressions that the distribution 

 of pressure, particularly the diffusion of the pressure - what Dr. 

 Doctors has been calling "smoothing" at the boundaries - is very 



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