Page 149 contkol and signal building 2613 



The conditions between the hydrophone and the shore will affect cable-laying 

 operations to an important degree. A beach and a sea bottom of smooth sand, free 

 from rocks, are most desirable; rocky kelp-covered ledges are most unfavorable. Land- 

 ing of cable through the surf is difficult and in general can be done only when there is 

 little swell. In some cases it may be possible to avoid delay in establishing a shore 

 station by selecting a hydrophone site near a cove or small bay that affords a protected 

 landing. 



2613. Cable 



Two types of cable are used in installing shore-station hydrophones. On rocky 

 bottom, in areas of kelp, and through the surf, a heavy-duty cable is required. The 

 specifications for this cable are as follows: 



Cable, armored, '/i-inch center of 19-wire tinned aircraft strand, insulated with ?^2-inch ignition cable compound of at least 33 per- 

 cent pure rubber, covered with double braid and armored with 14 wires of No. 14, B.W.G. double-galvanized wire. 



On smooth bottom in an area where it will not be chafed, a light-duty cable of 

 the following specifications is used: 



Cable, }^-inch center of 19-wire tinned aircraft strand, insulated with f62-inch ignition cable compound of at least 33 percent pure 

 rubber. 



This cable is very strong but it is so light that it will not bury itself in sand. It may be 

 used between the hydrophone and the outer line of breakers. Where the bottom is 

 sandy and the sea smooth with rarely any breakers, it has been used for the entire 

 installation from hydrophone to shore with fairly good results. As single-conductor 

 cable is used in most shore-station installations, a ground must be provided at both 

 ends. At the offshore end, the metal case of the hydrophone in contact with the water 

 has been found satisfactory; and at the shore end, a connection to a metal rod driven 

 in the ground will usually suffice. 



Hydrophone cable should be tested for leaks or shorts before use. An approved 

 method of testing is to submerge the entire reel of cable in sea water, except one end 

 used for the measurements. The other end of the cable, if it is submerged, must be 

 made watertight. The resistance between the conductor cable and sea water may be 

 measured with a 150-volt megger. For a 3,000-foot length of cable, the leakage resis- 

 tance should be not less than 10,000 ohms. 



To locate a leak in a cable, it may be run through sea water so that only about 10 

 feet is submerged at a time, the leakage resistance being measured continuously mean- 

 while. A sudden deflection of the pointer of the megger will indicate that the cable is 

 leaking where it enters the water. 



All splices in a hydrophone cable must be soldered, mainly for strength, and each 

 splice should be covered with several layers of Para-rubber tape which, in turn, is 

 covered with several layers of friction tape. 



A magnetic unit of low impedance, in which other favorable characteristics have 

 not been sacrificed, should be used, if obtainable, because it is less affected by leakage 

 than other types are. 



a. Laying cable. — If the selected hydrophone site is in an inadequately surveyed 

 area, a hasty survey of the vicinity should be made on a special boat sheet. Several 

 short sounding lines in the vicinity of the proposed hydrophone anchorage and several 

 sounding" lines between there and the beach should suffice. The best spot for anchoring 

 the hydroyaone can then be selected, the exact depth will be known, and the 

 approximate lengths of different types of cable required may be determined. 



