In most underv/ater acoustic and oceanographic measurennents we are 

 faced ■with just such an unconaf or table area of ignorance concerning the 

 stability and precise location of deeply suspended instrunnents. To impose 

 the ideal laboratory frame of reference with fixed and known positions upon 

 a remote deep-sea environment is almost always impossible. However, 

 when we recognize that the deep sea is perpetually in motion with respect 

 to the earth, we see that in most experiments other than seismological it 

 would be rare to require a fixed placement or even the knowledge of 

 absolute placement for deeply suspended instruments. In fact we would 

 know more about the physical situation if we imposed a known movemient 

 that was much larger than unkno'wn and varying sea motion. If we can 

 achieve truly rectilinear movement and measure it using sensitive radio 

 navigation such as Loran C, then in the observational variations recorded 

 by the deep instruments we can account for it and know precisely what the 

 influences of the sea itself have been. 



TOWING DYNAMICS AND PRINCIPLES OF THE PARALLEL TUBE 

 DEVICE 



Consider the problem of towing in a straight line in terms of the pertur- 

 bations in tow path introduced by the rocking motion of the ship. This situ- 

 ation is depicted in Figure 2. A key element that simplifies the complexi- 

 ties of this problem is due to John Ess of our laboratory who made the 

 following observation while towing a few tons at the end of a cable about 

 7000 ft long. Tracking closely an extremely sensitive depth gage, he was 

 able to observe a strong correlation between small movements imparted by 

 rotating the winch and small depth changes in the load at the end of the 

 cable. Within seconds, hauling in 3 or 4 ft of cable would be reflected in a 

 3 or 4 ft decrease in depth. It was clearly as if the shape of the catenary 

 in the cable were essentially invariant over the order of at least 10 seconds. 

 This is not so unreasonable when we consider the spaces involved and the 

 enormity of the drag forces on the long cable which could easily act like a 

 giant sheave. 



Starting from this point we can relate a change in height A , imparted 

 by the ship at the suspension point, to a change in depth Y of the sus - 

 pended object. The angle <}) will be constant for all Y greater than a. 

 thousand feet since it is determined almost entirely by the ratio of hori- 

 zontal drag force on the object to its weight. As long as the deep currents 

 are small, this drag force will be determined solely by the towing speed. 

 The ship motion is then resolved into components perpendicular to and 

 along the suspension cable. In what follows we have to compare the merits 

 of towing between using a dead weight and using parallel tubes. To be fair, 

 the advantage gained with small 6 is cancelled out, using the rather 

 crude approximation y/l for sin 9 



The idea of a possible advantage in using, instead of dead weight, some 

 system that traps a large water mass is shown in Figure 3. To damp a 

 motion imparted at the surface on a cable ■which retains its shape requires 

 that the tension suspending the weight below vary so as to allow the elas - 



58 



