To guide the cable to the bottom of the 3-foot ditch without exceeding 

 a minimum bend radius of 5 feet, the feedshoe/plow length must be 10 

 feet (Figure A-2). Using the design parameters discussed previously, 

 the total force required to pull the plowshare at a speed of 1 knot is 

 F = 44,000 pound. This formulation for predicting drawbar pull compares 

 favorably with results published for Sea Plow III [37] . 



References 8 through 14 present analytical and experimental results 

 of using plowshare vibration to reduce the drawbar force required to move 

 the plowshare through the soil. In particular [13], it has been shown that 

 for vibratory plowing, the use of a raked, wedge-shaped blade with machined 

 grooves (Figure 3) reduced the average horizontal plowing force in a silty 

 sand by 98 to 99% when the plowshare was vibrated at a frequency of 20 to 

 40 Hertz at an amplitude of 3/8 inch. In addition, vibrating the plowshare 

 aids in achieving and maintaining depth of penetration. In the case above, 

 a 95% reduction in drawbar force gives 



TOT 



= 2,200 lb 



reduced 



which is well within the target requirement of 10,000 pounds. (Note: 

 Other contributions to drawbar force will be discussed later.) This reduc- 

 tion in drawbar force will impact significantly on the support ship power 

 requirements for towing, and ease control problems. 



The power required to vibrate the plowshare is also an important 

 consideration. Appendix A shows that the power required to produce vibra- 

 tions is 14 hp . Water drag and added mass effects on the power required 

 for vibration are negligible. To move the vibrating plowshare at 1 knot: 



Figure 3. Raked plowshare with 

 machined grooves. 



17 



