It can be shown that the theoretical mechanical efficiency, e,, of a 
water jet system is the same as that for a propeller and can be expressed as 
C.F "aE (16) 
where AV is the absolute fluid velocity in the jet and V, is the speed of the 
boat or vehicle being propelled. 
Other things being equal, AV/V, should be as small as possible. This 
indicates that the water jet is most efficient on a high-speed boat where V, is 
large. For low speeds, however, V, must be very small. At any given speed, 
the resistance force, F, is determined by the body and the fluid in which it 
moves; therefore, in Equation 15 for AV to be very small, o Q must be very 
large. For this reason, a water jet unit on a slow moving vehicle would require 
a very large jet with a low velocity discharge. The type of pump best suited 
for large flow at small head is the axial flow propeller pump. In effect, a 
propeller is an axial flow pump with the pump casing and jet pipe removed. 
Therefore, for a slow moving underwater vehicle, propeller propulsion would 
be more efficient and lighter than a comparable water jet propulsion system. 
A water jet system can be justified for slow moving craft in applications 
where a propeller would be vulnerable to damage from collision or grounding. 
In the deep ocean, however, there is no reason why propellers could not be 
mounted above the deck or protruding from the sides of the craft where they 
would not come in contact with the bottom. 
On tugs and towboats where high thrust at slow speed is required, 
the Kort nozzle is very effective. This is a nozzle or ring of airfoil section in 
which the propeller rotates. In effect, it provides a cross between a propeller 
and a large discharge area water jet. It has the additional advantage of providing 
a protective ring which may assist in keeping foreign objects such as ropes or 
cables from becoming entangled in the propeller. 
Summary and Future Plans 
It is apparent that probably no one traction method will function on 
all possible ocean bottom materials. On solid or rough surfaces, large tires 
should prove most practical. For most soft bottoms typical of the deep ocean, 
tires will not be functional, and some form of low-ground-pressure track will 
be necessary. Interchangeability of tires and tracks on the same machine may 
prove the best approach. 
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