Knot meter difficulties

[sailorbob]

The knot meter on Second Wind has been crapping out lately. It will run fine for a while and then read 0.0 for a while (where "while" might be a couple of hours, or 10 minutes). I've tried tacking, backing up, etc., with the thought that maybe something is stuck in the impeller, but it doesn't seem to affect it, and it will come back on its own. We haul in a few weeks, so I will see what the impeller looks like then, but I'm thinking the problem will not be apparent even then. Has anyone experienced this?

Bob
Second Wind -hull 301

[R.B.]

I have seen this randomness with all of the ST60 instruments. Reseat the Seatalk cables behind the instruments and see if that fixes it. It did for me. You can pull the speed sensor while in the water. You should have a plug that fits in the hole for the sensor when the sensor is removed. Only a small amount of water comes in if you are quick enough with the plug ... or you can wait until lift out.

[sailorbob]

Thanks for the suggestion, Ralph. I can try reseating the connectors this weekend. How did you get at the back of the instruments?

The knot meter display is reading "0.0" not blank, or dashes. Is that what you experienced as well?

Bob

[R.B.]

Come to think of it Bob it may have been flashing dashes. Not 100% sure now.

[sailorbob]

The boat comes out of the water October 9, so we can check to see if the knot meter impeller is fouled at that time. I'm not keen on pulling the transducer while still in the water, so it will have to keep until then. My display definitely reads "0.0" not dashes or blank, so it behaves like it must be fouled. We'll see.

Thanks for your input.

Bob

[RCattell]

Bob,

It's not a big deal to pull out the transducer for the knot log while the boat is in the water.
That is what the blank plug is for. Your boat should have the blank plug. The fitting has a little flapper door in it so the amount of water that comes in is minimal, just put the plug in after pulling the transducer out. Have a sponge and small bucket ready,the bilge pump would keep up if you left the plug out.

We pull the transducer after every sail here in Florida, otherwise we get barnacles on it. Once I have the transducer out, I clean and lubricate it. We put it back in before we go sailing the next time.

[william vanwagoner]

I also pull the transducer out after every sail. A fair amount of water comes in but a sponge and bucket takes care of it. I have been doing this for years without any problem. Not only does the transducer not foul, I think it is safer as there is less chance of water leaking through the blank plug.

Destiny #229

[sailorbob]

Thanks, folks, for your encouragement re pulling the knot meter transducer. I assume the transducer is not simply force fit. Is there some sort of threaded ring? Does it require tools? Once the plug is inserted in the transducer's place, is that locked in place somehow as well? I am not at the boat now.

Thanks.

Bob
Second Wind #301

[KenKrawford]

Bob, the transducer has a threaded collar. No tools necessary but it may require a bit of "persuasion" if it's been there a while. Make sure you have the plug hady before removing the transducer. The same threaded collar that holds the transducer is used to secure the plug. The in rush of water is a bit disconcerting but should only last a second or two until you seat the plug.

[jking]

My ST60 knot meter was intermittent, I checked all connections, etc., after many hours of looking for the problem I found a reference on the internet that if the temperature sensor is not working properly it causes the speed indicator to become a repeater instrument and display dashes. There are two fixes, new transducer (which I ordered from West Marine) ST800/P120 or Airmar 20-399-01 or put a resistor across the temperature sensor wires so a fixed temperature is displayed (10K to 20K) and the speed will work again but without temperature.

Jan King
#303

From Source Raymarine.com

My instrument is not reporting the correct speed or temperature
Testing the Speed Sensor:

Verify that the transducer’s paddle wheel is free of marine growth and rotates freely on its shaft
Power down the instrument
Disconnect the GREEN and SHIELD transducer leads from the instrument display or instrument pod.
Connect a multimeter to the GREEN and SHIELD transducer leads and configure the multimeter to measure continuity or resistance.
Slowly rotate the transducer paddle wheel by hand. Note that as the paddlewheel is rotated, the multimeter should alternate between an open circuit and a closed circuit with each quarter turn. As such one complete revolution of the paddle wheel should open and close the circuit four times.

Should the multimeter respond as indicated above, then the speed sensor element of the transducer would be deemed functional. If not, then the transducer has failed and should be replaced.

Testing the Temperature Sensor:

The temperature sensor employs a thermistor as its sensing element. The impedance through a thermistor will change as the thermister temperature changes. Increases in temperature will cause a decrease in impedance through the thermister. Correspondingly, decreases in temperature will cause an increase in impedance through the thermister.

Example: If the thermister is immersed in water is 77 degrees F, the multimeter will report an impedance of approximately 10,000 Ohms. If the thermister is immersed in water is 68 degrees F, the multimeter will report an impedance of approximately 12,495 Ohms.

Other examples:

81 degrees F = 9,164 Ohms
77 degrees F = 10,000 Ohms
68 degrees F = 12,495 Ohms
61 degrees F = 15,004 Ohms
57 degrees F = 16,470 Ohms
54 degrees F = 18,098 Ohms
50 degrees F = 19,911 Ohms

It is not possible to directly test the thermistor. Testing is performed indirectly by applying a known resistance to the BROWN and WHITE terminals of the instrument display or instrument pod and noting the temperature reported by the instrument display.

Testing is performed as follows:
Power down the instrument
Disconnect the BROWN and WHITE transducer leads from the instrument display or pod.
Install a 10,000 Ohm resistor across the BROWN and WHITE terminals of the instrument display or instrument pod
Power on the instrument
The display should report a temperature of approximately 77 degrees F.

Should the display respond as indicated above, then the thermistor has failed and should be replaced. If not, then the instrument display or instrument pod has failed and should be serviced by Raymarine's Factory Repair Center.