Behaviour of a Ground Effect Machine 



REPLY TO THE DISCUSSION 



W. A. Swaan and R. Wahab 



Netherlands Ship Model Basin 



Wageningen, Netherlands 



It is gratifying to have Mr. Crago's comment on the test results because of 

 his experience both with models and actual hovercrafts. The small size of the 

 model was necessary because of the maximum available carriage speed other- 

 wise it would certainly have been desirable to use a bigger model. The use of 

 regular waves was selected in order to obtain an impression about the transfer 

 functions. Moreover the wave generator in the Seakeeping Laboratory is not 

 suitable for the generation of irregular long crested oblique waves. It is true 

 however that the use of irregular waves is to be preferred in many respects 

 when predictions have to be made for the performance in a given area where the 

 sea conditions are known. Because the air flow for the jet system was not 

 measured the momentum drag can not be determined from the experiments. 

 However Fig. 8 gives the total resistance when flying over concrete. Because 

 of the negligible trim, resistance must be mainly momentum drag. 



In regard to Mr. Lofft's question about the effect of the flexible trunks on 

 the difference between the stability over land and over water it must be re- 

 marked that the cause of this phenomenon can only be found when measurements 

 are taken of both pressure and water surface shape under the vehicle. The en- 

 ergy loss caused by the smooth water surface waves is not only compensated by 

 the resistance of the GEM but also by the air cushion. This can be shown by the 

 fact that no "wave resistance" will be found notwithstanding the visibility of 

 surface waves if the GEM is kept horizontal, provided that the air flow is kept 

 constant. Therefore it is the opinion of the authors that coincidence of maxi- 

 mum resistance with the critical speed is not physically necessary. 



In the conclusions it is mentioned that pitch angles are the largest with 

 waves of about vehicle length or somewhat larger while Mr. Lofft notices that 

 the diagrams show a maximum at about double the air cushion length. However, 

 if the system had no damping, the maximum pitch angles would occur at reso- 

 nance; that is a wave length of 175 ft at the speed of 30 knots in bow seas. 



Because the maximum under these conditions occurs in much shorter waves 

 it is clear that damping is rather large. Therefore the pitch angles are much 

 more determined by the wave moment than by the frequency of encounter. The 

 remark about the effect of wave length or pitch must be considered in this light, 

 although it is admitted that the expression "about air cushion length" was 

 stretched somewhat too far. 



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