requirement of less than 10,000 pounds. There are two major problems 

 identified for this type of propulsion system. While the traction unit 

 is pulling itself along the cable, it is also ''pushing" it astern. 

 Any slack in the cable will be taken up by the traction unit and will 

 result in buckled cable astern, which will tend not to stay in the trench 

 and may damage the cable. The other problem is one of size. In order to 

 avoid overstressing the cable locally, the traction unit must be approx- 

 imately 35 feet long for SD cable. This requirement will result in a 

 large, unwieldy burial machine. Finally, the traction unit is a relatively 

 complex piece of equipment which may prove unreliable on the ocean bottom 

 where the sediment content of the water may be very high due to the ma- 

 chine's presence. 



Table 5. Thrust for Ducted Propellers at Zero Advance Speed* 



Assumptions 



Torque Q = 2,000 ft -lb = constant 



shp < 100 hp 



v = 



d = 4 ft 



Blade thickness fraction = 0.045 



Mean width ratio = 0.189 



Rake angle = 15° 



Number of blades = 4 



Total blade area 



ttd2/4 



0.55 





Nozzle A 



Nozzle B 



n (rps) 

 shp (hp) 

 p/d 

 T (lb) 



l/d = 0.50 

 s/l = 0.15 

 a = 12.7° 



£/d = 0.83 

 s/X, = 0.15 

 a = 12.7° 



Case 1 



Case 2 



Case 1 



Case 2 



4 



91.3 



1.21 



4,915 



3.5 



79.9 



1.38 



4,453 



4 



91.3 



1.21 



5,325 



3.5 



79.9 



1.38 



4,704 



* See Table 6 for definition of terms. 



26 



