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Ask SAIL: Paddlewheel Knot Log Problems

I'm trying to figure out why my paddlewheel knot log is registering incorrectly. The plug is clean, and the wheel turns freely. Yesterday my GPS showed my speed over the ground was 5 to 7 knots, and the speedo registered 0 to 0.8 knot. It does this periodically to irritate me.  
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Ben Wilcox of Naples, Florida asks:

I'm trying to figure out why my paddlewheel knot log is registering incorrectly. The plug is clean, and the wheel turns freely. Yesterday my GPS showed my speed over the ground was 5 to 7 knots, and the speedo registered 0 to 0.8 knot. It does this periodically to irritate me.

Don Casey replies: 

Generally when a knot-log display under-registers or reads 0.0, the problem is with the transducer or its cable. If it works fine sometimes, fouling is likely your problem. You can test the transducer out of its through-hull housing by watching the display as you spin the paddlewheel in the air. It should spin easily and smoothly. To thoroughly clean it, push the axle pin out with a stiff wire to release the paddlewheel. Be sure to note which way the paddles face. Polish the axle and the sides of the wheel with a Scotchbrite pad.

If you still have problems, try resetting the unit to the factory settings. Your manual will show you how to do that. You’ll have to recalibrate the unit afterward, but resetting can clear up electronic glitches. If the unit continues to show 0.0 or some ridiculously low value, the transducer may be defective. You can do a rough check by turning off the unit and disconnecting the screen and the green conductors from the head. Use a multimeter to measure continuity between these two conductors while you turn the paddlewheel. A good transducer will show the circuit opening and closing with each quarter turn of the wheel. 

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