Recovering The Towfish; Action If The Towfish Fouls - C-Max CM2 User Manual

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CM2 Sidescan Sonar User Manual

5. Operating the Towfish

As soon as the altimeter has locked to the bottom and the image is normal, wind the cable in or out, or adjust

speed, until the towfish flies at an altitude equivalent to 5 -10% of the range limit.

The towfish must be flown high enough to clear any obstacles. Over uneven ground, flying high increases

safety and prevents excessive shadowing. Over flat and featureless ground, flying low enhances the

shadows of any object that may be present. In shallow water it may be necessary to fly lower to stay below

the wake.

In very shallow water it may be better to deploy the towfish alongside, or at the bow, rather than astern, but

keep the towfish away from the propeller!

Depending on speed and depth expect to pay out about 3 metres of tow cable for every metre of towfish

depth (not altitude). Always keep a reserve of at least 3 turns of cable on the winch drum.

See Notes for the Helmsman.

Tow in the range 2.5 to 6kt (speed through the water; this may differ from speed over the ground or GPS-

indicated speed). Low over-the-ground speeds give more scans per metre of travel and a correspondingly

better image resolution. Sometimes, however, a higher speed will give a steadier course.

Caution: the towfish will sink towards the bottom whenever the towing vessel slows or turns.

Recovering the Towfish

Slow to 2 to 4 knots before the towfish hits the wake. Watch for the tape marker warning that the towfish is

close.

Wash the towfish, cable and winch with fresh water immediately after use in seawater. When the tow cable is

disconnected from the towfish, cap the cable connector to keep it clean.

DO NOT EXPOSE THE TRANSDUCERS TO HEAT FROM STRONG SUNLIGHT

If the towfish hits the bottom or other obstruction, the increased towing force will usually break the breakable

washer. The safety lanyard will then apply tension to the rear of the towfish. This will normally cause it to

tumble and to free itself of the obstruction. In the meantime the towing vessel should, of course, slow down

but without going astern into the cable!

Recover the towfish and replace the breakable washer. Inspect the towfish and tow cable for any damage.

Version 3.6 (2008)

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