Merely moving from seawater to the fresh or 
brackish water of a river mouth changes displace- 
ment perceptibly. 
Descent from warm surface waters to cold 
bottom waters or transit through several thermal 
or salinity layers places a heavy burden on 
buoyancy control systems. Existing systems re- 
quire constant attention of experienced personnel 
to compensate for changing conditions. 
Buoyancy control is achieved most simply by 
pumping seawater in and out of hard tanks, 
changing the ratio of vehicle weight to displace- 
ment. This method has proven satisfactory to 
2,000 feet, but pumping water against the high 
pressures of greater depths requires expenditure of 
much precious energy. 
When transporting specimens, minerals, or re- 
covered objects to the surface, it will be necessary 
to provide buoyancy at least equal to the wet 
weight of the cargo. Dropping weights may be 
inexpensive, but pumping seawater ballast is more 
desirable because it is reversible. Further, systems 
operating submerged for long periods may not 
have the opportunity to replace dropped weights. 
Trim in undersea vehicles has been controlled 
by shifting ballast, changing the pitch of fins or 
vanes, or applying propulsion. In shifting ballast, 
seawater or mercury is pumped from one region of 
the vehicle to another, working effectively even at 
slow or zero forward speeds. However, the system 
responds slowly, and the ballast, tankage, intercon- 
necting piping, and pumping system add weight, 
volume, and complexity. The vehicle must have 
some relative forward or reverse motion to effect 
trim by the use of lifting surfaces. 
b. Future Needs New fast and completely auto- 
matic buoyancy control must be developed. Chem- 
ical propellants may achieve a more satisfactory 
ratio between the weight of energy storage and the 
change in vehicle buoyancy. The solution may lie 
in designing vehicles so their bulk modulus closely 
matches that of seawater, utilizing materials and 
devices which vary in displacement to compensate 
for changes in pressure and temperature, thus 
providing automatic buoyancy control and mini- 
mal requirements for variable ballast. 
More efficient trim control methods with 
quicker response will be needed as vehicles become 
larger and faster. Automatic trim controls to free 
VI-40 
the vehicle operator for other duties will be 
required. 
4. Conclusions 
External machinery systems and equipment not 
designed for undersea use have proved generally 
inadequate. However, due to the unavailability of 
special subsea commercial equipment, equipment 
designed for other purposes has been used in the 
oceans. Even equipment specially designed for 
submerged application requires extensive improve- 
ment. The state-of-the-art in external machinery 
systems and equipment is summarized as follows: 
Propulsion Systems Problem 
Screw Maneuverability requires 
several units 
Tandem Not tested on a full-scale ve- 
hicle; complex mechanism 
Cycloidal Not tested on a full-scale ve- 
hicle; complex mechanism 
Water Jets Sediment disturbance; low 
efficiency 
New Methods Need to be developed 
Electric Motors Problem 
DC Motors Commutation and brush wear 
AC Motors Inverter/controller weight and 
reliability; flooded opera- 
tion 
Electrical Problem 
Penetrations Weight; reliability of seals and 
insulation; continuity of 
circuit 
Distribution Weight and bulk of oil-filled 
junction boxes; mechanical 
circuit interruption; cable 
reliability 
Control Problem 
Buoyancy Ballast pump rates and 
reliability; automatic con- 
trol; buoyancy generation 
at great pressure 
Trim System weight and speed; 
automatic control 
