Hydrodynamics of High-Speed Hydrofoils 145 
of water makes it possible for the wing area to be proportionately reduced for the hydrofoil 
boat. The total drag and hence overall L/D would be about the same as an airplane for an 
idealized hydrofoil craft in which resistance of appendages and supports is neglected. How- 
ever, the increased density of water makes it possible with the proper “wing” area to obtain 
sufficient dynamic lift for the hydrofoil boat to “fly” at lower speeds than the airplane in 
air. When the hydrofoil craft is then designed to operate at a speed intermediate between 
that of an airplane and a ship, there should theoretically be a reduction in hull or fuselage 
resistance in air which is proportional to the square of the speed. Hence, the initial appeal 
of the hydrofoil boat was the hope that it could fill the speed gap between ships and air- 
planes, with a higher L/D or lower P/WV than the airplane. Unfortunately, the strut system 
required to put the wings in the water adds more to the resistance than is saved by the 
reduced speed. Hence, even the most enthusiastic hydrofoil man does not expect as good 
an L/D for the hydrofoil craft as for the airplane. 
An important difference remains between the hydrofoil boat and the airplane. At its 
lower speed, the hydrofoil craft requires much less power, which should be advantageous. 
But the lower speed is less efficient for propulsion, and the drive system becomes very 
complex. Hence, no gain in payload from reduced power requirements can be expected in 
practice. Insofar as fuel is concerned, for the same specific fuel rate the total fuel con- 
sumed for equal distances is the same or greater. 
Hence, it appears that the goal of the hydrofoil boat designer is simply to equal the 
performance of aircraft, but at reduced speed. The possibility of doing this in terms of L/D 
and gross weight (payload) appears remote. In short, a seaplane can outperform a hydrofoil 
boat, and attain higher speed at the same time. We must recognize therefore that the hydro- 
foil craft is inherently a special purpose vehicle. 
I realize that it is dangerous in this company to appear antihydrofoil, hence, I hasten 
to add that I am enthusiastic about this development. My reason is simply that it is already 
stimulating technica! progress in naval architecture, bringing advanced hydrodynamics, 
better structures, and more efficient power plants into the picture. No matter what happens 
to the hydrofoil itself, naval architecture will never be the same again. 
