The choice of propulsion systems for submer- 
sible vehicles depends on their mission. Typical 
uses—scientific studies, site survey and inspection, 
object recovery and light salvage, transport of 
men and equipment, or mobile tool operations 
—generally demand precise maneuverability, nor- 
mally more important than high speed. 
Other challenges to propulsion system design 
include protection from entanglement, minimum 
disturbance of bottom sediments (especially for 
bottom-sitting vehicles), and creation of large 
forces and moments at zero speed. Propulsion 
system selection will involve weight, volume, 
simplicity, efficiency, reliability, maintainability, 
and mechanical endurance. 
The state of development and features of the 
most common propulsion systems are: 
—Screw propulsion. Well defined, with designs 
available for almost any application. Systems have 
utilized conventional propellers, ducted thrusters, 
or rotatable pods. Precise maneuverability in all 
degrees of motion requires no less than three screw 
propellers. 
—Tandem propulsion. \n early development, not 
progressed beyond the analysis and tank-test 
stages. Has promise for highly maneuverable 
vehicles. 
—Cycloidal (vertical axis propeller) propulsion. In 
use for years on tugs and ferries which require high 
thrust at low speeds and directed thrust for 
maneuvering but not three-dimensional control. 
Only a prototype glass submersible presently 
employs cycloidal propulsion. 
—Water jet propulsion. Uses pumps to expel water 
at high velocity for propulsion. Currently in use on 
at least two commercially operated deep submer- 
sibles. One uses rotatable jets for primary thrust; 
the other uses jet thrusters for maneuvering 
control. 
Many submersible functions and habitat opera- 
tions require power transmission by hydraulic 
pumps and actuators. In theory, complete hydrau- 
lic systems placed externally to the hull at ambient 
pressures can be operated at still higher working 
pressures. However, such operations have often 
failed for one or more of the following reasons: 
VI-38 
—Characteristics of hydraulic fluids change at 
extremely high pressures—viscosity may increase 
100 times while operating a system from zero to 
20,000-foot depths. 
—Waxes may form in hydraulic oils and clog the 
lines. 
—Gases may accumulate and block the system. 
The mechanical parts of pumps, actuators, and 
motors normally designed for 3,000 pound per 
square inch (psi) operation in atmospheric systems 
must be redesigned for deep submergence. 
Most underwater propulsion systems and virtu- 
ally all hydraulic and seawater pumps are driven 
by electric motors. Several methods for condition- 
ing motors to resist the operating environment 
have been developed. These include oil-filling, 
encapsulation in pressure-resistant housings, and 
sealing principle parts such as rotors and stators 
with plastic compounds. None is yet completely 
satisfactory. 
Most undersea electric power sources provide 
direct current. Since no power conversion is 
necessary, direct current motors (especially for 
constant-speed applications) promise high effi- 
ciencies. However, problems of commutation and 
brush wear under high pressure or in oil have 
restricted their use. 
Alternating current motors have the advantage 
of being brushless, but require DC-to-AC inverters 
for power conversion and speed control. Although 
modern inverters have no moving parts, they do 
not operate reliably in ambient pressures, and their 
electrical complexity adds weight. 
b. Future Needs Propulsion system reliability 
and efficiency must be improved for advanced 
undersea systems. The tandem propeller concept 
appears feasible within the foreseeable future. 
Reduction of vehicle drag to reduce power con- 
sumption and increase propulsion system perform- 
ance holds limited promise. 
Solution of the DC motor brush problem 
appears imminent. The Navy has been testing 3 
and 17 horsepower DC motors of a unique brush 
concept with favorable results. Larger motors are 
yet to be developed. AC motor inverter-controllers 
are being refined, but reliability and weight im- 
provements, including possible ambient operation, 
should be pursued. 
