Of the total tension supplied, 2/3 goes into plowing and 1/3 is used 

 to overcome other drag forces. The supplied power for plowing, on the 

 average, is 36 hp. While towing is highly inefficient from a power 

 standpoint, it is not meaningful to compare other propulsion system 

 efficiencies with that for towing, because the power is presumably 

 already available on the support ship. 



Towing is the simplest propulsion system resulting in the least 

 complicated machine control system and requiring no electrical power 

 transmission. Towing has a number of drawbacks, however. 



1 . Ship of opportunity navigation systems are not sufficiently 

 accurate to allow burying previously laid cables without picking up the 

 cable and leading it back to the machine. Burying cables while laying 

 requires the use of a cable -laying ship. 



2. Cable-laying ships generally operate at 5 to 8 knots, and plowing- 

 while-laying averages about 1/2 knot due to ship thrust limitations. 



This mismatch makes the laying operation inefficient, and makes the 

 laying /burying operation more vulnerable to adverse weather conditions. 



3. Supplying propulsion from a surface ship forces the burying 

 machine to move at ship speed, regardless of bottom conditions. Thus, 

 quick response ship control is required to avoid breaking the tow cable 

 or damaging the burying machine. 



4. Handling three cables from the support ship — umbilical, tow 

 cable, communication (buried) cable - presents a substantial entanglement 

 problem. 



5. A ship of opportunity has no fine control over the path of the 

 machine — the machine must simply follow the ship. 



6. Supplying large towing forces at slow speeds makes ship control 

 difficult. Sideways thrust control is essential. Utilizing force reduction 

 techniques will reduce the severity of some of these problems, but the 

 ship /machine control problem will remain. 



Track/Wheel Propulsion. Tracked and wheeled vehicle mobility has been 

 developed extensively for all types of terrain except ocean bottoms. The 

 only vehicle of consequence to traffic the seafloor is the RUM vehicle, 

 a converted land-based tracked vehicle. At that, RUM was not called upon 

 to provide large drawbar forces, and its weight on the seafloor is often 

 controlled from the surface. Underwater bulldozers developed in Japan have 

 been reported in trade journals recently, but no performance information 

 has been available. 



The basic mechanism for determining the drawbar force, or tractive 

 effort, developed by a tracked or wheeled vehicle is based upon Coulomb's 

 theory: 



F = W tan (f) + A c 



23 



