the motion of the vessel, the maximum total vertical motion 



of the fantail of the test vessel was about six feet. Since 



the period of the vertical motion was about six seconds, the 



maximum vertical velocity of the depressor was, say, 



•p X 3 feet, or about 3 feet per second. With a forward 



6 sec ' ^ 



velocity, at 7 knots, of roughly 12 feet per second^ the 

 maximum increase in the angle of attack at the depressor 

 would be about 15 degrees. Since the effective angle of 

 attack of the depressor is approximately 14.8 degrees^ this 

 would correspond to doubling the lift coefficient and hence 

 the tension, which is about what was observed. Conversely, 

 on the downstrokej, the tension should have dropped and it 

 did indeed fall to extremely low values following the appear- 

 ance of a high surge load. 



The foregoing discussion is overly simplified, but 

 does serve to illustrate the problem that attends large motions 

 of the towing vessel. This situation might be alleviated by 

 decreasing the impedance offered to the cable at the tow 

 point. It is possible, of course^, that pendulous motion of 

 the system might thus be excited,, with equally undesirable 

 results, but the fact that the cable experienced no pronounced 

 fore and aft motions during the periods of peak surging tends 

 to discount this possibility. 



If the mechanism discussed above is responsible for the 

 observed surges in tension, increased scope will undoubtedly 



34 



