GEM Research in the U.S. 299 
2. Within the water-contact family, the positions of curves 2 and 4 relative to each 
other are not meaningful, since they can be substantially changed or reversed by assigning a 
different nozzle-width ratio G/h to the water curtain and/or a different length/beam ratio //b 
to the air curtain with skegs. 
3. Between the two families, absolute performance comparisons are meaningless for the 
same reasons. However, the significant and valid point that the water-contact concepts are 
superior at low speeds, while the no-water-contact concepts are superior at high speeds, is 
very well illustrated. The exact degree of superiority, and speed range to which it applies, 
depends very heavily on the respective parasite drag coefficients. The values of Cp; and 
Cp, w used in Fig. 12 were selected arbitrarily, for illustrative purposes only. 
DISCUSSION 
R. L. Weigel (University of California) 
I do not want this to be construed as a criticism of Mr. Chaplin’s paper: the subject he 
was given was extremely broad to cover. However, there is one important aspect that he did 
not touch upon and that is the wave resistance of this type of vehicle moving over the water 
surface. I have chosen this one aspect because I think it of general interest to naval archi- 
tects and furthermore, as this meeting is dedicated to Sir Thomas Havelock, I think it is 
interesting that the technique used in predicting the wave resistance is due to Sir Thomas 
Havelock. Rather than considering a true ship, he considered a pressure disturbance moving 
over the surface of the water, and this is precisely the problem of the ground effect machine. 
Sir Thomas Havelock’s main advance is that he considered a pressure area rather than a 
pressure point, and he formulated his problem in such a manner that numerical results could 
be calculated from the equations rather than just looking at the integrals and so forth. Fur- 
thermore, he even went so far as to calculate the resistance in almost the exact form that is 
needed to obtain the information for designing a ground effect machine. Sir Thomas Havelock 
considered a pressure disturbance something like that shown in Fig. D1 where pressure is 
measured vertically and the radius of the disturbance is measured along the abscissa, with 
axial symmetry assumed. Now, he chose one particular shape; however, tests that we have 
made have indicated that we can have quite irregular shapes such as a double hump shape 
and the resistance is practically the same as one obtains from this shape that Havelock 
assumed.* 
Havelock’st numerical results are shown in Fig. D2. Very often in naval architecture 
we are dealing with the deep-water aspects and a Froude number based upon the length of 
the ship. If we deal in inland waterways we talk about a Froude number based upon the 
water depth.. It turns out for ground effect machines we must consider both of these simul- 
taneously. So we have a parameter which is the diameter of the pressure disturbance 
divided by the water depth. This is a dimensionless resistance which is a specific weight 
of water (pg) times the resistance, and this is the wave resistance (R) divided by 27 times 
the diameter of the ground effect machine times the maximum pressure squared (that exists 
* R.L. Wiegel, C.M. Snyder, and J.B. Williams, “Water Gravity Waves Generated by a Moving Low 
Pressure Area,” Trans. Amer. Geophys. Union 39 (No. 2):224-236 (Apr.. 1958). 
+ T.H. Havelock, “The Effect of Shallow Water on Wave Resistance,” Proc. Roy. Soc. (London) 
A100 (No. A705):409-505 (Feb. 1, 1922). 
