GEM Research in the U.S. 301 
speed of only 65 knots, which is low, but if I chose higher it would have been off these 
curves) the resistance will be lower than what I show here. If it is moving over water deep 
enough so that D/d = 0 (where d is the water depth), then V/y gD/2 = 1.6, and pgR/2nD pmax 
= 0.085. In very deep water it turns out that the horsepower that is utilized in wave resist- 
ance is only 12-1/2 hp, for a base pressure of 5 pounds per square foot, which is practically 
negligible, as we are talking about a craft with a 40,000- to 50,000-hp motor. It is almost 
unbelievable. The horsepower loss is dependent upon the square of the base pressure, so if 
you get up to the more modern concepts, which have a higher base pressure loading, and you 
go up to say 30 pounds per square foot the horsepower goes up to 440, and even this is only 
of the order of 1 percent. It is becoming appreciable but is still relatively small. In operat- 
ing the GEM, it will move from deep water into shallow water as it is coming ashore and 
here is where we have to be careful. We can hit some of these “hump” speeds and for one 
combination which I picked at semirandom (I chose a water depth of 100 feet and a forward 
speed of 55 knots) the wave resistance can rise to over 1500 to 1600 hp. In general when we 
are operating over deep water at high speeds, the wave resistance will be negligible. The 
only time it will tend to become important is when one is bringing it in over relatively shal- 
low water. The stopping of one of these vehicles is a major problem. Consequently this 
high resistance that one gets as one comes into shallow water probably will turn out to be 
very useful because I think that maybe for slowing a GEM we can make use of this shallow 
wave resistance. 
In practice the peak pressure will be higher than the average base pressure on a GEM, 
and the power necessary to overcome wave resistance will be higher than the values men- 
tioned, perhaps by a factor of almost two. 
L. W.. Rosenthal (Folland Aircraft Limited) 
I would like to thank Mr. Chaplin for the very pleasant paper he presented, but I am 
going to be so ungracious as to suggest that it had the wrong title! I think perhaps it might 
well have been called “A Simplified Method of Comparing the Performance of Ground Effect 
Machines.” During the whole of the Symposium, I have hoped that the hydrodynamicists 
would have discussed the position of the ground effect machine in the light of the conclu- 
sions that Mr. Chaplin had arrived at. When Mr. Oakley spoke, he did in fact suggest that 
attention had been given to the position of the ground effect machine, with the limitations 
that we know, in the marine transport pattern. Unfortunately, he did not develop this point. 
Mr. Van Manen, discussing the Crewe Eggington hypothetical project, quietly but firmly took 
it to pieces on the structure weight grounds, but he gave no constructive comment on what 
he would suggest instead. Mr. Newton followed this up by saying that the structural problem 
concerned with a large ground effect machine in a seaway needed investigation, but here 
again he gave no lead at all. Mr. Chaplin broadly touches on the place of the ground effect 
machine but his comments are based on a general parameter and not necessarily on the 
specialized uses to which we might be able to put the vehicle at the moment. 
What I would like to suggest, if this is not ungracious again, is that possibly the hydro- 
dynamicist and the naval architect are dragging their feet on this particular problem, or is it 
that no conclusions have yet been reached and we are looking for something or some informa- 
tion which does not yet exist. 
Another point to which Mr. Chaplin made no reference in his assessment of the small 
machines, and this could hardly have been considered a secondary one, is the means of 
obtaining pitch and roll stability and the penalties associated therewith. I would like to 
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