Deep-Diving Submarine Hydrodynamics 329 
The statements that have been made concerning the compressibility of thin-hull sub- 
marines apply with equal verity to vehicles like the bathyscaphes that use light-density 
fluids for flotation. Although the pressure hull of the Trieste may also be less compres- 
sible than sea water, the fact that the Trieste derives 90 percent of its buoyancy from light- 
density gasoline makes its overall compressibility almost as large as that of gasoline. Since 
gasoline compresses at a rate about twice that of water, the bathyscaphes, like the thin- 
hulled shallow-depth submarines, are unstable in depth. 
There is yet another effect of compressibility with the Aluminaut hull in addition to the 
effect on net buoyancy at depth. The compressibility of the silicone fluid in the stern cap- 
sule of the Aluminaut introduces a significant shift forward in the longitudinal position of 
the center of buoyancy between near surface operation and operation at depth. This fact in 
conjunction with personnel movements on board was used to help determine the capacity of 
the trim tanks that are installed inside the pressure hull. 
Because of the contraction of the pressure hull with depth, allowance has had to be 
made with attachment of all internal and external mountings to make sure that neither they 
are damaged nor the hull is locally restrained. Calculations indicate that at the test depth 
of 17,000, the radial displacement of the Aluminaut pressure hull is about 0.172 inch. 
MANEUVERING IN 80TH PLANES 
Most of the oceanographers who would be the potential users of the Aluminaut empha- 
sized to the designers the necessity for precise control in both the horizontal and vertical 
planes. Because of the low speed of the Aluminaut, it was apparent that the desired degree 
of control could not be achieved by control surfaces alone as it is with higher speed sub- 
marines. lor that reason two techniques are employed that are not usually used with sub- 
marines in addition to several other more conventional techniques. One is a propeller to 
provide thrust in the vertical direction and the other is a swiveling main propulsion pro- 
peller to permit directing the thrust in the horizontal plane. These are described in the 
subsequent sections. 
Control in the Vertical Plane 
To permit positive control in the vertical plane at all speeds a four-foot-diameter pro- 
peller driven by a 5-horsepower motor is mounted on top of a small superstructure about 15 
percent of the length forward of amidship as shown in Figs. 1 and 2. This superstructure, 
which is also oil-filled, houses the motor, which is reversible and has a stepped speed con- 
trol. With 5 horsepower the propeller can develop about 400 pounds of thrust as shown 
subsequently in Fig. 13. This thrust can be directed either upward or downward by revers- 
ing the motor. Since the magnitude of the thrust is not dependent on the speed of advance 
of the boat as it would be if it were developed by control surfaces, it should provide posi- 
tive vertical depth control at any speed of advance. 
In addition to the vertical propeller, two additional systems are provided for control in 
the vertical plane. The most effective of these, particularly in the speed range of the 
Aluminaut, is a conventional hydrostatic trim system that can be used to control pitch angle. 
The system as presently designed employs a pump driven by a 1-1/2-horsepower motor that 
can transfer 1,000 pounds of water between the two trim tanks shown in Fig. 1 in a period 
of two minutes. The moment produced of 26,500 foot-pounds is sufficient to trim the 
