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Old 25-09-2010, 10:37   #1
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Log Impeller

Hi, I am having problems trying to fault find on the boat's log/speed impeller. I have B&G instruments which seem to be working ok. When I take the impeller out and spin it nothing registers on instruments. How do I check if its the impeller, connections or instruments which are at fault?Any help will be greatly appreciated.

Peter
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Old 25-09-2010, 11:57   #2
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I think, but am not sure, that the paddle wheel type of transducers involve a magnet and coil that generate pulses for the electronics to count to measure speed over the water. It would like take an oscilloscope on the end of the cable to display the pulses. I would put the question to B&G if nobody here has better information. You probably do not have an O-scope and B&G may have a simple test with an ohmmeter that could determine if the coil is intact. I assume you have checked that the transducer is connected to the processor box and it's not a lose wire issue.
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Old 25-09-2010, 12:13   #3
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You don't need an O-scope for testing, a simple multimeter will do, but need to know which system you have. Very old style transducer's used a coil and the cable was 2-conductor. Newer ones are hall-effect devices which are powered and use a multiconductor cable.

Eric
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Old 25-09-2010, 12:26   #4
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There is a way of testing using using a transistor radio (never hear this term anymore) and touching wires together. Unfortunately the instructions are on the boat and I'm home today.
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Old 26-09-2010, 08:00   #5
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Goto ➥ Testing Knot-Log Speed Instrument

cruisersforum.com/forums/f13/testing-knot-log-speed-instrument-15440.html
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Old 26-09-2010, 09:18   #6
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Old-school mechanical logs, whether impeller or paddlewheel, are simply not worth the effort ... any cheap GPS will tell you your true speed & distance covered whereas a mechanical log will lie because it has no way of allowing for currents ... I was sailing into the Bay Of Fundy against the outflow, my Datamarine log was showing a speed of 6kt when my GPS told me 1kt ...
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Old 26-09-2010, 10:36   #7
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Isn't it nice to have both so you know that the current was 5 knots on the bow?
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Old 26-09-2010, 11:35   #8
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Does a knotmeter- difference really show current, or is one screwy? My knotmeter rarely agrees with my GPS, and that is sailing in a lake without currents....
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Old 26-09-2010, 13:07   #9
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The paddle wheel instruments can be quite inaccurate. Growth on the hull can affect the flow over the device, and growth can affect the ability of it to turn at the proper rate. If they are clean and calibrated, the difference between their indicated speed over the water, and the GPS indicated speed over the ground, will reasonably indicate current relative to your course.
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Old 31-12-2010, 17:55   #10
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Quote:
Originally Posted by sanctuary1
Hi, I am having problems trying to fault find on the boat's log/speed impeller. I have B&G instruments which seem to be working ok. When I take the impeller out and spin it nothing registers on instruments. How do I check if its the impeller, connections or instruments which are at fault?Any help will be greatly appreciated.

Peter
It's quite common on some models for the magnet to fall out of the impeller, most often during cleaning or washing down with a high-pressure hose. In this respect, the paddle transducer can be 'overspun' when they're hit with the jet of high-pressure cleaner and for the magnet to become dislodged as a result.

Both hall effect an the older inductance coil type transducers both rely on a magnet passing by to register each 'tick' - if you're confident that the electronics are good then check to see if the magnet is still present on the paddle: if not, then you've found your problem.

St.elsewhere is correct in stating that a regularly cleaned and anti-fouled (recommend simple Vaseline) paddle when read in conjunction with the GPS's SOG will give a good indication of net current but only relative to the direction of travel of the vessel - motion directly into a head current or away from a following current will give best results in this case. Of course, a currently directly on the beam will give a disparity between log and SOG due to the 'hypotenuse' the vessel is traveling compared to the 'adjacent' the log is measuring.

I now see that this is an older post - hope you found the root cause of the issue!
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Old 01-01-2011, 09:44   #11
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It's quite common on some models for the magnet to fall out of the impeller,
That is not the case with B&G paddlewheels. There is a magnet in each tooth of the wheel and they are completely embedded inside the wheel.

Eric
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Old 01-01-2011, 10:59   #12
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That is not the case with B&G paddlewheels. There is a magnet in each tooth of the wheel and they are completely embedded inside the wheel.

Eric
Then, as long as the paddle is clean and spins - and the wiring/connectors are good - then, based on a coil inductor, an analogue meter should show pulses if the coil is intact and not compromised. Not sure if B&G is hall-based or not.

Agonizing when relatively simple systems fail to offer up a reason for malfunction - replacement can feel like defeat but it gets the issue resolved.
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Old 01-01-2011, 13:09   #13
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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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Testing Knot-Log Speed Instrument
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