for mobility and drawbar effectiveness. Since seafloor soils are generally 

 weak, variable, and often times their characteristics unknown, vehicle 

 performance cannot be predicted with any confidence. This, coupled with 

 general lack of experience in seafloor traff icability, makes it impossible 

 to judge whether a burial system propelled with tracks, wheels, or screw- 

 wheels will be successful. Finally, these types of running gear are rela- 

 tively complicated mechanisms that may impact on the reliability of the 

 burial system. 



Thrusters. One of the most common means of propelling underwater 

 craft, thrusters, includes open and shrouded propellers and jet pumps. 

 Propeller theory is well known, although propeller design is iterative 

 in nature. Since a prop uses the water medium to develop thrust (as 

 opposed to soil for traction) , it is relatively straightforward to pre- 

 dict the performance of a propeller driven machine. Propeller efficiency 

 is defined as: 



ehp 

 p shp 



where e = propeller efficiency 

 P 

 ehp = effective horsepower = thrust x velocity 



shp = shaft horsepower 



It can be seen that if the vehicle is stationary, the propeller may be 

 generating large amounts of thrust, but the ''efficiency'' will be zero. 

 Thus, several propeller configurations are considered from the standpoint 

 of thrust and power rather than efficiency. A preliminary analysis on 

 ducted and non-ducted propellers is summarized in Tables 5 and 6. In one 

 case, two 5-1 /2-foot-diameter non-ducted propellers will provide 8,000 

 pounds of thrust at a burial speed of 2.48 ft/sec (1.5 knots), requiring 

 160 shp. In the case where the machine is stalled, two-ducted propellers, 

 4 feet in diameter, can supply 9,400 pounds of thrust at 160 shp. In 

 general, ducted propellers are more ''efficient" (i.e., require less 

 shp for the same output conditions) by 10 to 30% than non-ducted propellers. 

 One major advantage of thruster propulsion is that, by operating at a 

 constant thrust level, the machine speed will vary as a function of the 

 soil resistance. Thus, in tough materials, the machine will proceed more 

 slowly than in soft materials, and overstressing of the excavating means 

 will be eliminated. Also, props may be directly driven from submersible 

 electric motors, or by hydraulics, and may be articulated to provide 

 continuous steering control. 



Cable Traction Propulsion. Pipeline burial systems generally straddle 

 the pipeline, depending on the pipe for guidance, and some use traction 

 drives to pull themselves along the pipeline. The most distinct advantage 

 of a cable burial machine which pulls itself along the cable is that it 

 clearly will follow the cable. The breaking strength of 18,000 pounds for 

 unarmored SD cable is adequate for a system that meets the target force 



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