Behaviour of a Ground Effect Machine 



Mr. Swaan's paper presents results obtained in regular seas. We have 

 found tests in such an environment of only limited usefulness and it is now our 

 practice to run all hovercraft tests whether for ourselves or customers in ir- 

 regular seas having energy spectra based on a Darbyshire formulation modified 

 to make it represent the coastal and local conditions with which we have to deal 

 in our full scale trials. 



We have found good agreement between the derived amplitude response 

 operators and the accelerations obtained from irregular sea tests and regular 

 sea tests up to sea conditions associated with a wind of Beaufort force 5 but be- 

 yond this, I feel that irregular sea testing is essential because of the non- 

 linearity of response. 



A further point which is perhaps worth mentioning is that we have used mo- 

 tion extinction curves similar to those given in Mr. Swaan's paper, together with 

 the static characteristics and a fairly simple wave impact theory as inputs to an 

 analogue computer. In this work we have found that we can predict the model 

 motion and accelerations and the effect of pitch control systems, steering sys- 

 tem failures, etc., to a very acceptable degree of accuracy at least in regular 

 waves and irregular waves up to a Beaufort force 5 wind. 



(This work has been published by my colleague, Mr. J. Stafford, as a paper 

 read before the British Society of Instrument Technology in June, 1963 and ap- 

 pears in their transactions). 



The Wageningen data shows that the value of v/viL at which the hump pitch- 

 ing attitude occurs decreases as water depth decreases. The way in which it 

 does this agrees precisely with our own findings and the way in which the cor- 

 responding resistance varies is also generally similar. 



In connection with the presentation of the data, I would like to see values 

 given for the momentum drag. This would facilitate analysis and comparison 

 with other data. 



In closing I would like to make a further small criticism. It is a pity the 

 test points have not been shown in Figs. 6 to 9. I feel the curves have been 

 over-smoothed and that there should be almost a discontinuity for example in 

 the heave curves at Froude numbers between 0.3 and 0.6 depending on water 

 depth. 



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