DYNAMICS AND KINEMATICS OF SUBMARINE CABLE 



11G5 



If the above 35 fathom change occurred at a depth of say three thous- 

 and fathoms, a very sensitive fathometer would be required to detect it. 

 Thus, although complete restraint of cable movement along the bottom 

 is an extreme and unlikely condition, the above examples indicate that 

 long residual suspensions can occur with essentially no manifestation at 

 the ship, especially in deep water. 



VI. CABLE LAYING CONTROL 



6.1 General 



We have seen that the mean cable tension at the ship reflects the 

 amount of slack which is being paid out and how the cable is covering 

 the bottom. However, in most cases this reflection is not sensitive. For 

 example, the tangential drag force Dt varies with V t , the longitudinal 

 velocity of the cable relative to the water. In theory, as (2) shows, one 

 can therefore determine the amount of slack being paid out from ship- 

 board tension measurements. For cable No. 2, we have plotted in Fig. 

 23 the variation of the mean tension at the ship as a function of slack for 

 a ship speed of six knots and a depth of two thousand fathoms. At three 

 per cent slack the tension is 8,240 pounds, while at six per cent slack it 

 is 8,020 pounds, a difference of only 220 pounds. This amount of tension 



9000 



8600 



IT) 



a 



z 



D 

 O 

 Q- 8200 



Z 7800 

 O 



5 7400 



h- 



7000 



3 4 5 6 7 



PER CENT SLACK 



10 



Fig. 23 — Variation of shipboard tension with per cent slack for hiving cable 

 No. 2 at a ship speed of 6-knots in a depth of two thousand fathoms. 



could be easily obscured by the effect of ship motion. Thus, to measure 

 slack accurately by relating it to cable tensions one would have to know 

 the depth and cable parameters very precisely and, in addition, would 

 need a veiy efficient filter to separate out the "noise" tension caused by 

 ship motion. 



Similarly, it has been shown that residual suspensions can occur with 

 essentially no reflection in th(> tension readings at the ship. Hence, al- 

 though tension readings can give a valuable check on how the cable is 



