INTRODUCTION TO SONAR 



operating the brake and clutch, and a drum 

 revolution counter. 



2. Boom: The boom should extend at least 

 8 feet and no more than 10 feet from the side 

 of the ship, and it should be strong enough to 

 withstand a force of 1500 pounds. A typical 

 hoist and a boom are shown in figure 5-5. In 

 the illustration the BT is suspended from the 

 towing block and is ready for lowering. 



3. Towing block: A small counterbalanced 

 wheel that attaches to the end of the boom to 

 allow cable to pay out with minimum friction. 



4. Wire rope: The wire rope is of 3/32-inch 

 diameter, 7x7 stainless steel aircraft control 

 type, and is 3000 feet in length. 



BT ACCESSORIES 



Each BT comes supplied with the following 

 accessories: (1) a grid and slide holder, for 

 viewing the slides, (2) a box of slides, (3) a 

 swivel for attaching the BT to the cable, (4) a 

 pair of tongs for holding the slide to rinse it, 

 and (5) a thermometer for taking bucket tempera- 

 tures. These accessory items are described as 

 this text progresses. 



OPERATION OF THE BT 



Personnel operating the BT at night and 

 during rough sea conditions should wear life- 

 jackets and use safety lines. For each lowering 

 of the BT the succeeding steps must be followed. 



Step 1 — Determine Water Depth 



Determine water depth in order to select the 

 proper BT. 



Step 2— Examine Wire and Connection to BT 



Whenever possible, determine that the wire is 

 hitched to the winch reel in such a manner that 

 it cannot pull loose if all wire should pay off the 

 drum. As an additional precaution, do not pay 

 out the last layer of wire when lowering the 

 BT. The wire should be wound on the drum so 

 that it pays out and reels in at the top of the 

 drum. For survey work, it is recommended 

 that bare wire be used— not plastic-coated wire. 

 If 900-foot-depth BTs are to be used, at least 

 2000 feet of 3/32-inch, 7x7 stainless steel 

 wire should be used. The plastic-coated wire 

 usually comes in 1200-foot lengths and cannot 

 be spliced, hence it is not long enough for use 

 with 9 00 -foot BTs. Run the free end of the wire 



Figure 5-6.- 



71.76 

 -BT cable hitch. 



through the towing block at the end of the boom. 

 This block is of a special counterbalanced design 

 for BT use. 



The type of cable hitch used to connect the 

 BT to the wire differs slightly with various 

 models. The instruction book accompanying each 

 BT shows the method of attachment for that 

 model. One such cable hitch is shown in figure i 

 5-6. If the connection is frayed, rusted, kinked, 

 or doubtful in any way, cut off the faulty part 

 of the wire and make a new connection. Be 

 sure to seize the hitch to prevent its coming 

 free. Check the swivel carefully. On those models 

 that use a Fiege-type swivel connector, make 

 sure the Fiege sleeve is screwed into the socket 

 as tightly as possible. More BTs are lost be- 

 cause of poor connections than from any other 

 cause. 



Step 3 — Check Winch 



The hand lever on the winch serves as both 

 brake and clutch. It has three positions. When 

 it is vertical, the winch is in neutral and the 

 drum can be turned in either direction. When 

 it is pushed outboard to the engaged (or hoist) 

 position, the motor turns the drum and spools 

 on the wire. When the lever is pulled inboard 

 (or toward the operator) to the brake position, 

 the drum is locked and cannot be rotated. 



With the winch lever in neutral, turn on the 

 motor to make sure power is available. The 

 shaft bearings should be kept well lubricated 

 according to the instructions provided with each 

 model winch. The drum should turn freely. 



The winch installation should be such that 

 the wire passes across the top of the drum. 

 The hand lever should move away from the 

 operator to engage the motor, and toward the 

 operator to set the brake. On some earlier 

 model winches, the hand lever operation is 

 just the reverse; that is, the brake is away. 



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