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Old 20-05-2008, 04:03   #1
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Testing Knot-Log Speed Instruments

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 (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.

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.
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Old 21-11-2016, 12:57   #2
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Re: Testing Knot-Log Speed Instrument

Gord: Can you expand a little more on your knot-log voltage test? I have a 1996 transducer that was working until I upgraded the 1996 Autohelm equipment to the newest Raymarine backbone and plotter. The transducers now connect to the backbone with a Raymarine ITC-5. The spinning wheel transducer worked until the upgrade, so I tend to suspect a faulty ITC-5. However, the depth and wind transducers are working fine through it.

My transducer has five wires: red, bare, green, yellow, brown; and I had heard that I could use a 9 volt battery to send a voltage across two of the wires, and check for the switching of 0v to 9v on two other wires, but which wires?
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Old 01-07-2018, 03:21   #3
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Re: Testing Knot-Log Speed Instrument

Sonosailor:
Sorry for the delay - I’ve been off-line for the past 3 weeks.
With so many wires, it sounds like your transducer has more functions than just speed/distance (perhaps water temperature).
I’m afraid I have no idea which wires are which.
Good luck!
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Old 25-04-2021, 02:52   #4
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Re: Testing Knot-Log Speed Instrument

This is an old thread, but it could do with an update.

My Bidata was showing 0 knots. With the sensor out, flicking the paddlewheel produced a reading on the meter. That most likely eliminated the instrument and cable as problematic.

So next I removed the paddlewheel from the sensor probe by gently pushing out the spindle. There was a very small amount of what looked like calcium deposits on the spindle and on the side faces of the paddle wheel hub. I removed these by gently scrubbing with oxalic acid using a blue scourer. Then I put it back in, and Bingo! it worked perfectly.

The moral of the story is that it appears to take only a surprisingly small amount of build-up to stop the wheel from turning, despite it feeling friction-free when I flicked it. So before you give up and spend $$$$, try this. As the kids say, ymmv, but it worked for me.

Cheers, Graeme
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Old 25-04-2021, 04:02   #5
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Re: Testing Knot-Log Speed Instruments

Locker, the recommendation is to take the paddlewheel out of the water if the boat is meant not to move (or any flow under it) for a week or so. That is the reason for the dummy plug delivered with it.
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Old 25-04-2021, 15:42   #6
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Re: Testing Knot-Log Speed Instruments

Quote:
Originally Posted by Quebramar View Post
Locker, the recommendation is to take the paddlewheel out of the water if the boat is meant not to move (or any flow under it) for a week or so. That is the reason for the dummy plug delivered with it.
Thank you. That is exactly what I have been doing these past ten years or so.

The tiny bit of seawater left on it each time it was removed obviously left salts behind as it evaporated, causing the build-up. If you leave it in the water, your problem is more likely to be algal or weedy type growth.
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Old 04-10-2021, 06:48   #7
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Re: Testing Knot-Log Speed Instruments

Hi Gord,

thanks for your reply on my Airmar question in the other thread.

I have a combination cable, which also has a "free" end for NMEA interfacing.

Can you see if we could measure on that free end and which cables we should use?
See attached.

In the manual it says that cutting/splicing the wire voids the warranty, but that is not really an issue on an instrument from 2004.
Is the cable length/type relevant due to voltage drop?Click image for larger version

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Old 07-10-2021, 02:14   #8
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Re: Testing Knot-Log Speed Instruments

Anyone else on my last post here?
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Old 07-10-2021, 02:29   #9
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Re: Testing Knot-Log Speed Instruments

You can cut the cable. There’s no issue I did it on a new install
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