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Old 06-10-2008, 09:02   #1
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SL50 knotmeter inop reading 0.0

I've got an SL50 knotmeter that would often read only 0.0 knots, I'd mess about with the wires behond the instrument (its connected thru a filter) and it would start to work (albeit about one knot less than the GPS would indicate on a lake without significant current).

Now its stopped working altogether - just reads 0.0.

I've got no manual or schematic - its connected thru the wind speed indicator (which is also INOP and has been since I bought the boat)

I've got the paddle wheel assy. removed and propped up in the sink so I can run water over it - neat idea eh?

I'm convinced the problems in the wiring behind the unit - I've got a good VOM and know how to use it, but I'm wondering where to start ???
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Old 06-10-2008, 12:34   #2

Join Date: Aug 2003
Boat: Dragonfly 1000 trimaran
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The first thing I'd do is look for bad circuit board connections where the plug mounts inside the unit, most likely caused by physical stress.

A good oscilloscope would be a more useful tool to see the output of the paddlewheel.

Steve B.
(retired electronic tech)
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Old 07-10-2008, 03:42   #3
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Testing Knot-Log Speed Instruments, previously posted at:

Knot Log Tap-Test:

1) With 12VDC Power and Instrument “On”; remove the coax cable from the connector found on the rear of the speed instrument (display).

Bend a paper clip into a U shape and place one end of the paper clip into the center hole of the connector (signal) on the rear of the unit.
With the other end of the paper clip tap on the outside (ground shield) of the same connector (essentially shorting the inside pin to the outside collar) rapidly while watching the display.

If the speed increases as you tap, then the instrument is capable of producing a speed. You should be able to see about a 5 knot speed with rapid tapping.

If not; you will need to send the knot meter in for service.

If this is ok, proceed to the next step.

2) Re-connect the cable to the rear of the knot meter, and locate the Paddle Wheel Speed Sensor in the hull.
Follow the cable about 10 feet back, and disconnect the cable at the junction.

Perform the same tap test at the end of the cable leading to the knot meter, by shorting the center pin to the outside shell.
Have someone observe the reading on the knot meter.

If you can produce a good reading on the knot meter, then the inter-connect cable is ok, and you will need to check the Paddle Wheel.

3) Remove the speed sensor from the thru hull fitting, and insert the dummy plug (if afloat).
Spin the paddlewheel by hand. If the display shows a speed you will need to replace the Paddle Wheel Transducer assembly.

If it does not show a speed, you will need to replace the Instrument.

Knot-Log Voltage Test:

1. Connect a DC Voltmeter to the centre (Signal) and “bare” (ground) wires of the Transducer.
2. With the Instrument “on”, slowly rotate the paddle wheel one paddle at a time.

The Voltage should switch from about “8” volts to near “0”, volts as the magnets in the paddles pass by the body of the transducer.

3. If the Voltage does not change, as the paddle is slowly rotated, replace the transducer.

Paddle Wheel Type Transducers:
A Paddle Wheel type speed sensor consists of a magnetized paddlewheel, wherein the paddlewheel is rotatably supported in a cavity adjacent to a (usually Hall Effect) magnetic sensing device. The sensor generates electrical pulses corresponding to (proportional to paddle wheel speed) paddlewheel speed, as the wheel rotates when the vessel moves through water.

Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"

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