A complete hydraulic circuit requires a high-pressure pump, a 
motor, filters, connecting hoses, and valves for control. Although present 
filters, connecting hoses, and valves should prove adequate if made of corrosion- 
resistant materials, investigations at NCEL (Daly, 1969) and NSRDC (Robbins, 
Schneider, and Mehnert, 1968) have indicated that typical commercial hydrau- 
lic pumps and motors will not perform satisfactorily when using seawater as a 
hydraulic fluid. The early and drastic failures of these commercial units (Daly, 
1969) indicate that pumps and motors using seawater as a hydraulic fluid must 
be carefully designed to compensate for the low viscosity, poor lubricating 
properties, and corrosive nature of seawater. Proposed uses for this system, 
such as supplying power to protable diver’s tools, also require that the motor 
design be compact and lightweight and the system pressure be high enough to 
eliminate large flow requirements. The seawater can be filtered to remove 
organic materials and sand. However, other treatments such as the addition 
of lubricants, corrosion inhibitors, or viscosity increasing agents should be 
avoided because the motor would be used in an open-cycle circuit where the 
additives would contaminate and be lost in the ocean. Since currently success- 
ful lubricating additives have to be present in percentages in excess of 5%, the 
cost of using these additives would be excessive (Anonymous, 1967a). In 
addition, the use of additives would introduce the complication of a mecha- 
nism for the measuring and mixing of the additive and the logistics problem 
of keeping supplies of the additives available at the work site, which might be 
deep in the ocean. 
The ratio of the absolute viscosity of seawater (approximately 1 
centipoise) to the viscosity of typical hydraulic oils varies from 0.05 to 0.01 
depending on temperature (Marks, 1958). The low viscosity of seawater 
directly affects the performance of pumps and motors by allowing an increase 
in internal leakages thereby reducing volumetric efficiency. The various high- 
pressure pump and motor designs are not affected by this low viscosity to 
the same degree. Units with rotating gears or sliding vanes are relatively 
simple and compact, but subject to leakage past the gears or rotors. Since 
the internal leakage is approximately inversely proportional to the viscosity 
of the working fluid (Hadekel, 1951), the use of seawater as a working fluid 
will cause a large increase in internal leakage. To counteract this effect, the 
internal clearances of seawater pumps and motors will have to be reduced, 
perhaps to as little as 0.0001 inch per inch. These small clearances will 
impose more stringent requirements on the selection of materials as regards 
thermal expansion and rates of wear. Sliding vane units also have inherent 
friction at the point of contact between the vanes and supporting slots and 
are generally limited to applications with moderate operating pressures. 
Current commercial models of piston-type pumps are particularly useful 
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