Murthy 



DISCUSSION 



Paul Kaplan 



Oceanics Inc. 



PlainvieWj New-York 3 U.S.A. 



I certainly agree as regards the importance of pressure. 

 This is a craft which by definition is an air-cushion vehicle with 90 

 to 100 per cent of its weight supported by air pressure. A deviation 

 in the equilibrium pressure of 50 per cent, which could easily occur 

 and could be computed for simple "wave pumping" by waves commen- 

 surate with the length of the craft (without such great height that would 

 violate linearity) will show changes of pressure of the order of 50 per 

 cent of the equilibrium pressure. Therefore accelerations of )/2g 

 immediately are possible. In order to account for this you do not just 

 say that the equation contains a damping coefficient and stiffness coef- 

 ficient that represent pressure effects. Pressure is a degree of free- 

 dom which must be added into any equation system. Therefore you 

 have heave, pitch, surge, and pressure is also a state variable ; it 

 couples with the others and it is the most important element. That in- 

 troduces the fan as well as the Plenum geometry and the wave pumping. 

 The end result of this is that if you neglect anything with the air pres- 

 sure in the manner shown here, then motion responses are erroneous. 

 I would suggest that this paper is very useful in many respects for 

 determining hydrodynamic forces, but the title is somewhat erroneous. 

 I would suggest it be changed somewhat and there it would still main- 

 tain utility. The paper should be called "A linearised potential flow 

 theory for the hydrodynamic forces associated with the motions of 

 air-cushion-vehicles in a sea-way". 



Now here are some positive contributions to balance what I 

 have said before. The aspect of linearity is important and the result- 

 ing ability to predict motions in irregular seas even more so. There 

 has been published recently some results for motions in regular waves 

 which include equations which also have this pressure degree of free- 

 dom. The agreement with experiments, while not perfect, is more 

 than adequate. The predictions based on the application of spectral 

 analysis to predict RNS values of accelerations in irregular seas 

 agrees quite well also. Therefore, not only is there an aspect of utility 

 of the theory for getting hydrodynamic forces ; there also is a virtue 

 in linearity, so that is a benefit as well. 



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