Size, Type, and Speed of Ships in the Future 43 
and still just clear the waves. Clearly such a large GEM craft would offer many practical 
design difficulties—and much research will be required to perfect its development. 
On the other hand, research in the hydrodynamics of water based aircraft has been 
supported by the U. S. Navy for over 25 years and the success of this concept has been 
demonstrated time and again in the form of numerous prototypes which have been success- 
fully designed. If the large GEM is to be considered for oceanic transport, I submit that a 
very hard and serious reexamination of the water based aircraft be considered for the same 
mission. Obviously the hovering ability of the GEM is not contained within the capabili- 
ties of a specially designed seaplane—but I have not heard the authors contend that this 
was an important consideration in their studies. Certainly, the GEM will find usefulness 
as a special purpose vehicle, but as a general working member of the family of everyday 
transport vehicles, perhaps other concepts will excel. 
H. P. Rader (Vosper Limited) 
The authors did not mention a type of high speed craft in which we are particularly 
interested, i.e., the hard chine planing craft. I think this may be of some interest because 
we have achieved speeds in excess of 50 knots with hard chine planing craft which have 
been in service for some time. I cannot say offhand how the performance curve of our craft 
compares with the performance curve of the other craft shown by the authors, because we 
did not have the papers in time for carrying out such a comparison. May I, therefore, sub- 
mit the following table for your information and consideration. The figures quoted apply to 
a hard chine craft weighing about 100 tons. 
Speed (knots) Horsepower/ton 
40 71 
45 87 
50 105 
55 126 
One more point concerning propellers. In the section on Types of Propellers Dr. van 
Manen states, “For reasonable application of a supercavitating screw with a diameter of 1 
meter, a minimum speed of 54 knots is desirable.” In our opinion this minimum speed is 
much lower, I would say between 40 and 45 knots, depending on the rate of advance or, to 
be more precise, on the local cavitation number at which the blade sections have to work. 
J. D. van Manen 
In reply to Mr. Rader, although I failed to mention the hard chine planing aircraft in my 
paper, I am sure they will be included in subsequent papers. 
Owen H. Oakley 
I agree with Mr. Savitsky that the water based seaplane operating in ground effect is a 
logical configuration to consider for the ram wing GEM concept. The very high L/D values 
which are theoretically obtainable are most intriguing, and I feel confident that this area 
will receive increased attention as the GEM concept develops. 
