32 J. D. van Manen 
0,70 
250 0,60. 
a 
ic) 
— 
200 0.50 
0.40 
@ S.C. propellers TMB 3-50 
@Wide blade prop. Gawn 3-110 
@ Wageningen B 4-40 
i @® “Screw + nozzle’’- prop. 
1 Bp 10 100 
—— S.C. prop. Conventional screw propellers “Screw + nozzle"’- prop. 
Fig. 4. Optimum relationship of B, and 6 for various types of propellers 
Further investigations will have to indicate whether increasing the rpm of the ducted 
screw or of the supercavitating screw, or whether going to an entirely unknown field, ducted 
supercavitating propellers, is preferable. 
GEMS AND HYDROFOIL BOATS 
With the aid of Figs. 1 and 5(a) it is possible now to make estimates of size and shaft 
power for the design of high-speed craft such as GEMs (hovercraft) and hydrofoil boats. 
Let us for the moment assume a reasonable and realistic power coefficient P/V,A = 
0.20. Then it follows from Fig. 5(a) that for an 80-knot hydrofoil boat with a l-meter- 
diameter screw the maximum installed shaft power would be in the order of 4,000 shp, 
whence it follows that for a single screw craft a displacement of about 40 tons could be 
propelled. Therefore, for 80-knot speeds, hydrofoil boats will thus be limited, even with 
the installation of more screws, to cargo carrying capacities of from 40 to 120 tons. 
