Semisubmerged Ships 543 
Because of the high ship speeds and relatively low thrust loading considered, it does 
not appear difficult to obtain a conventional propeller of good efficiency. A single shaft 
should be used to obtain a high propulsive coefficient. There are virtually no limitations 
on propeller diameter (other than construction) and rpm may be selected to give a good effi- 
ciency along with low machinery weight. Submergence depth enters into the cavitation cri- 
terion to some degree, but there appears to be no difficulty in avoiding cavitation up to 45 
or 50 knots. Cavitation is a problem for control surfaces, however. 
The installation of a high-power propulsion plant dictates a minimum weight and space 
allowance. Gas-turbine main engines seem to satisfy these requirements. For naval pur- 
poses the ship must be able to achieve a high speed for short time intervals and must also 
be able to cruise at a considerably reduced speed for long time intervals. A single gas- 
turbine engine does not have the ability to operate efficiently over this considerable power 
range. Hence, to achieve high engine efficiency, multiple engines should be considered. At 
top speed all engines would be running. For this operation, engine life is a prime considera- 
tion. At low speed, fewer engines would be running and the idle ones could be disconnected 
from the reduction gear with a fluid coupling to facilitate maintenance and to eliminate wind- 
age losses. 
The reduction gears to reduce the turbine shaft speed to a practical propeller speed in- 
volve very strict weight and, more important, space limitations. These considerations sug- 
gest an epicyclic gear train with high surface hardness and high K factor. This type of gear 
should have the ability to transmit power efficiently over a large range of powers. Even so, 
the gearing will undoubtedly be the largest single item of machinery weight. 
Another possibility for reduced weight is to design a ducted type of propeller operating 
at very much higher rpm, perhaps in the supercavitating range. This would permit a drastic 
reduction of gear weight. 
Air supply and turbine exhaust involve serious engineering problems to minimize pres- 
sure losses and yet keep the surface-piercing strut to minimum size. In any case, the duct- 
ing will certainly be a big item in machinery weight. 
It is proposed that a compact control station be located at the top of the surface-piercing 
strut for good visibility. Permanent ballast of about 10 percent of displacement might be 
necessary to provide satisfactory static transverse stability. 
Basic trends of semisubmarine characteristics have been studied in a preliminary way 
in connection with work of the Panel on Naval Vehicle Systems, Undersea Warfare Committee, 
National Academy of Sciences-National Research Council. Tentative design features of a 
series of craft of varying size were worked out on the basis of certain assumptions regarding 
technical potentialities [10].. These assumptions will be summarized before presenting the 
results. 
Resistance 
Resistance was estimated from model tests of simple bodies of revolution, with axes 
1-1/4 diameters below the surface. An Ogive strut was selected, the size was established 
by the estimated engine air requirements. A length of 14 percent of ship length was used 
for the series. Strut resistance was calculated from Ref. 11, neglecting the elimination of 
tip losses by the attachment of the strut to a hull. The characteristics that were selected 
were not necessarily optimum. 
