Ask SAIL: as the Log Turns

Q I’m trying to figure out why the paddle-wheel transducer for my Raymarine ST60 knot log is registering incorrectly...
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Q I’m trying to figure out why the paddle-wheel transducer for my Raymarine ST60 knot log is registering incorrectly. The plug is clean, and the wheel turns freely. Yesterday my speed over the ground on the GPS was 5 to 7 knots, and the speed registered 0 to 0.8. It does this periodically to irritate me, and I’d like to make it stop.
Chuck E. Weiss, Los Angeles, California

DON CASEY 

A. Generally when a log under-registers or shows zero, the problem is with the transducer or its cable. Paddle-wheel transducers are notorious for fouling almost instantaneously. If it works fine sometimes, fouling is likely the problem.

To test the transducer, pull it from its through-hull housing and watch the display as you spin the paddle wheel. It should spin easily and smoothly. Push the axle pin out with a stiff wire to release the paddle wheel, noting which way the paddles face. Polish the axle free of all growth with a Scotchbrite pad. You should also clean the sides of the wheel, as a small bit of debris here can cause the wheel to jam. Finally, give the transducer body and the wheel a thin coat of transducer antifouling paint. I’ve never found this to be very effective, but it does no harm.

If you still have accuracy problems, try resetting the unit to its factory settings. Your manual will show you how to do this. You will have to recalibrate the unit afterward, but resetting can clear up any electronic glitches. If the unit continues to show zero or some ridiculously low value, the transducer may be defective. Turn off the unit and disconnect the screen and the conductor wires from the head. Measure continuity between the two conductors with a multimeter while you turn the paddle wheel. A good transducer will show the circuit opening and closing with each quarter turn of the wheel.

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