36 J. D. van Manen 
If we continue this estimate of dimensions of prototypes in the same manner for a hover- 
craft, such as, for example, the 400-ton Crewe and Eggington project, then a power coeffi- 
cient of P/V,A = 0.15 appears to have been chosen by the designers (P = 40,000 hp, V, = 
100 knots, A = 400 tons). 
A rough weight distribution might appear as follows: 
800 passengers plus 80 motor cars 160 tons 
40,000-hp turbine, at 1.5 kg/hp* 60 tons 
Fuel for 24 hours 130 tons 
Hull 50 tons 
With a diameter of 100 meters (air cushion pressure 50 kg/m?) the weight of the struc- 
ture becomes 5 kg/m2. A further study of the lifting mechanisms and possible structural 
forms will be necessary to indicate how much the hull weight has to increase at the expense 
of the pay load. 
If the supposition is made that 50 to 60 hp per ton of displacement are required for 
hovering, then from 20,000 to 24,000 hp will be required to maintain the air cushion. How 
much special provisions such as Weiland’s labyrinth seal can improve this situation remains 
yet to be seen. We will therefore use 20,000 to 16,000 hp to give the 400-ton GEM a 100- 
knot speed. The important question which then pre~>nts itself for the case of a seaborne 
GEM is whether to use air or water propulsion. In Fig. 7 the optimum diameter and effi- 
ciency of supercavitating propellers in water and ducted propellers in air are compared 
against a basis of B,. From this it appears that the diameter of the ducted propeller in air 
is approximately three times as great as that of a comparable supercavitating screw in 
water. The efficiency in air is approximately 40 percent lower than the efficiency in water. 
Proceeding from an assumed propulsive efficiency of 0.75 in water and 0.45 in air, then 
for the available power of 20,000 hp and a speed of 100 knots, the following resistances per 
ton of displacement can be overcome: 
55 kg/ton for water propulsion 
33 kg/ton for air propulsion. 
Whether it will be possible to choose the hull form and the operating height of the GEM 
so that these values can be reached, the future will have to show us. If we consider the 
water resistance negligible at these high speeds and assume the transverse sectional area 
of hovercraft and air cushion to be 600 m2, then a resistance of 55 kg/ton in the case of 
water propulsion implies a drag coefficient of 0.23, which is comparable to that of an 
automobile. 
In view of the large difference in efficiencies for water and air propulsion for a 100- 
knot GEM, water propulsion for a seaborne GEM should be considered. However, from these 
calculations for high speed GEMs one might be inclined to consider displacements, as pre- 
dicted for hydrofoil boats, in the range from 40 to 120 tons to be more attractive. 
Summarizing this introductory paper to this Symposium on High Performance Ships the 
following might be concluded. 
*See Fig. 6. 
