INTRODUCTION TO SONAR 



thermometer into the bucket so that at least 

 3 inches of the bulb end is immersed in sea 

 water. Stir the thermometer with a circular 

 motion for 20 to 30 seconds and then read it 

 with the stem still immersed in the water. 

 Stir it once or twice more and check the reading. 



The bucket sample must be taken while the 

 BT is being towed. Make the temperature read- 

 ing as soon as possible after the sample is on 

 deck. If the sample is allowed to stand for more 

 than 45 seconds, the temperature reading no 

 longer is valid. It also is of importance to stir 

 the thermometer to bring it to temperature 

 more rapidly and accurately. 



Step 7 — Lower the BT 



Move the winch lever to the neutral position 

 and allow the wire to pay out freely. Success in 

 reaching the maximum desired depth depends 

 chiefly on two factors: (1) having the winch 

 drtun and towing block bearings well lubricated 

 to minimize friction; and (2) getting the BT 

 down below the ship's screw wash as soon as 

 possible. With practice, it is possible to raise 

 the BT slightly, after the surface towing is 

 completed, skip it off the crest of a wave so 

 that it swings forward, and then lower it rapidly. 

 This method enables the BT to plunge into the 

 water, and its momentum will carry it more 

 rapidly past the turbulence of the wash, enabling 

 it to reach a greater depth. This technique is 

 especially useful with the 900-foot instrument. 

 It takes practice, but, to the experienced operator, 

 this method is easy to use and is more effective 

 than the diving lug assembly attached to some 

 of the old models. 



When the ship is making more than 12 knots, 

 there usually is enough drag on the wire while 

 the BT is diving to ensure that it will not slacken 

 and backlash. At lower speeds, and during heavy 

 rolling, the wire may slack between the winch 

 and the towing block. Slack in the wire may 

 cause backlash on the winch drum or a kink at 

 the towing block. The operator should provide 

 himself with a round stick about 15 inches long 

 for the purpose of gently slowing the drum when 

 excessive slack appears. Do not apply too much 

 pressure to the drum with the stick, because 

 once the diving motion of the BT is arrested, it 

 will not dive farther, regardless of the amount 

 of wire payed out. Do not touch the wire with 

 your hands when the drum is in motion; you 

 may be injured seriously. 



Step 8 — Stop at Proper Depth 



To reach a given depth, the amount of cable 

 to be payed out depends on the speed of the ship, 

 the type of BT, and whether the nose sleeve is 

 attached. Table 5-2 provides a rough estimate 

 of speeds at which full depth may be expected 

 to be reached when using 1000 feet of wire. 



The observer should take data on the length 

 of wire payed out and the actual depths recorded 

 by the BT. He then should plot a graph showing 

 counter reading against depth for various ship 

 speeds. 



When the counter indicates that the proper 

 length of wire is payed out, or when the last 

 layer of wire on the drum is reached, the brake 

 should be applied smoothly, allowing the drum 

 to stop without a sudden jerk. An excessive 

 jerk will part the wire. Now the BT will swim 

 back up to near the surface far astern. Check 

 to see that the wire leads properly for hauling 

 in. If it does not lead from the towing block to 

 the center of the drum, adjust the boom guys 

 until it does. 



Step 9 — Haul in the BT 



Move the hand lever smartly from the brake 

 position to the hoist position. Do not pause while 

 going through neutral, otherwise more wire will 

 pay out. Guide the wire back and forth in even 

 layers on the drum, using the stick. The end of 

 an old swab handle will do. Do not use a metal 

 guide. If kelp or seaweed fouls the wire, ease 

 on the brake and clear the wire with a boathook. 

 Haul in at full speed until the BT is close astern 

 but still a safe distance from the ship's screws. 



Step 10 — Bring BT Aboard 



When only about 100 feet of wire is out, the 

 BT should be readily visible at the surface. 

 As the wire is hauled in, the BT will reach a 

 position nearly under the boom where it will 

 begin to porpoise, breaking clear of the surface 

 and swinging forward as the ship rolls or as 

 wave crests pass. This action is the most 

 critical point in the operation. To bring the 

 BT alongside and raise it without too much 

 swing requires practice. If the BT is brought 

 in too fast, it may skip or swing forward of 

 the boom, perhaps hitting the side of the ship 

 or swinging completely over the boom. If the 

 BT skips or swings forward of the boom, it 

 is advisable to shift at once to neutral and 

 allow the BT to sink freely until it passes 



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