Swaan and Wahab 



DISCUSSION 



R. F. Lofft 



Admiralty Experimental Works 

 Gasport, England 



This paper represents a useful addition to the published literature on the 

 behaviour of hovercraft over water. It illustrates the difficulty of testing 

 models of such light, high-speed craft in normal ship model tanks. It is gener- 

 ally impossible to fit equipment in the model to measure all the motions of in- 

 terest, and the arrangement adopted by the authors to permit heave, pitch and 

 roll, and to restrain the model in yaw, sway and surge seems a reasonable 

 compromise. 



Turning now to several points of detail: 



(1) The two diagrams at the bottom of Fig. 4 show that, with rigid jets, the 

 righting arm is much greater over water than over land, while with flexible jets, 

 the difference is much less. No reason can be seen for this and it would be in- 

 teresting to have the authors' comments. 



(2) The results of the shallow water tests in Fig. 8 show a marked peak in 

 the resistance curve at 12 ft depth. It is interesting to note that this occurs at 

 the critical speed for this depth, viz. 11.7 knots. The same does not appear to 

 be true for the other two depths tested, at which the critical speeds are 4,7 

 knots for 2 ft depth and 19 knots for 32 ft. 



(3) Figure 10 gives the results of tests in waves with flexible trunks, in 

 which the mean rise height is given as 25-30 inches. This is nearly 10 inches 

 less than the corresponding figures for still water, from Fig. 6. This is some- 

 what surprising, since the wave tests show the mean rise height to be independ- 

 ent of wave length, and one would therefore expect it to be about the same as in 

 still water. 



(4) The authors suggest that the maximum pitch amplitude would be ex- 

 pected to occur in waves of about air cushion length. This is true of normal 

 ship speeds; but at higher speeds, e.g., in model tests of fast planing craft, it 

 has been found that maximum pitching generally occurs in waves of 2 - 2-1/2 

 times model length. This is consistent with the results in Figures 10 and 11 in 

 which the greatest pitch occurred in waves 105 ft long - nearly twice the air 

 cushion length. 



714 



