Intermittent Knot Meter Paddle Wheel Sensor

[Al - s/v Persephone]

I just returned from a 5 day cruise up the coast and back. It was delightful except for one thing - every leg had me removing water from my normally dry bilge; accumulated water that gushed in each of the many times I pulled my Raymarine ST60 knotmeter's AirMar sensor to make it spin again, so my wind machine would be able to compute true wind.There was absolutely no seaweed or any other crud visible on the wheel or its shaft, but vigorously spinning the wheel a few times would restore function.Is there some intermittent failure mode with these things?Can some non-visible crud build up around the tiny shaft inside its bearing (a plastic drill hole)? If so, can I use acetone, or some other solvent to dissolve it and keep it clean for longer periods without disolving the plastic housing?My last boat had exactly the same kind of sensor in an ST40, but except for a few seaweed clogs, I had no such trouble. I gather that mostinstrument manufactirers buy these AirMar sensors, not just Raymarine, so I hope someone has had (and fixed) a similar problem.Thanks.

 

[Bill Roosa]

Try cleaning the housing

While the paddle wheel has to be clean the space that it turns in has to be also. The clearance between the paddle wheel and where it moves when not in the water stream is quite close. Spinning the wheel probably removes some of this but not enough to prevent it growing back in a matter of hours.Look at the housing as you rotate the wheel. You cannot see the entire housing as the wheel paddle is in the way but as you rotate the wheel you can get access to the entire housing. Also the clearance between the side of the wheel and the housing is close and the entire thing need to be cleaned to get long lasting results. I find a tooth brush an excellent tool for this job.This whole job can be reduced if you coat the wheel and housing with a very thin coat of anti-fouling.

 

[Al - s/v Persephone]

What kind of anti-fouling?

Thanks, Bill.This makes sense, as one symptom is more sticking on one tack than the other. The clearance between the side surface of the paddle wheel's hub and the side of the plastic housing seems closer on one side than the other, which would fit the scenario.What had me flummoxed is the fact that all last year (1450 nm) I had no problem, except for one seaweed incident. Now, sailing in the same (cold) waters for only 400nm thus far this season, I'm deluged with failures.Maybe the year-old ferrous metal wheel/hub has oxidized and the slight layer of rust is sufficiently rough to provide a lot of little growing niches for the critters?I'll thoroughly scrape/brush/emery-cloth both sides of the hub, as well as the plastic surface. Perhaps a little acetone on the plastic might be called for?Pardon my ignorance, but what kind of "anti-fouling?" Bottom paint? How about winch grease? It keeps seawater off winch gears. Do the plankton/whatever grow in that?

 

[Alan]

The paddle wheel

I use the Air Mar self sealing knot log. When the paddle wheel is removed, a small flap closes inside the housing keeping the water flow to a trickle. I remove the log and plug it with the sealing plug whenever the boat is left. This prevents any build up of any little nasties on the log and precludes the need for bottom paint on it. The change over takes less than 10 seconds and produces very little water.

 

[Al - s/v Persephone]

Self-sealing

Interesting, Alan.Do you have a URL to any info about it on the 'net? Is it compatible with the ST60 instrument head?I imagine I'd have to replace the through-hull that holds the present paddle wheel sensor.It sounds like something to try next winter, when Persephone is back in the Land of the Dead (boat yard).

 

[chuckwayne]

self sealing head

Al, are you sure you don't already have it? our H356 with the st-60 sender already has the self sealing flap

 

[JC on Bainbridge]

Transducer paint by MDR

I use an anti-fouling transducer paint by MDR that I purchased at West Marine many years ago. It is in a small white plastic bottle, and has a brush in the cap. Every time I haul the boat out, I paint the transducer for the depth sounder, and all the little flaps in the paddle wheel.

 

[Bill Roosa]

I ALWAYS use

The cheapest bottom paint on the hull and just take a artist paint brush to the thru-hull innards and transducers.

 

[Al - s/v Persephone]

No longer intermittent

Mystery solved.The transducer is now totally dead.It appears that there may have been an intermittent connection inside the unit. (I had wiggled the cable that runs from the top of the transducer to the instrument head, ruling out connection problems/broken wires there.)It was probably from the delicate wires in the sensing coil that the ferrous paddles stimulate to generate the pulses as they pass by.I'll just buy a new one (Raymarine E26031, formerly M78712). A little splicing, some heat-shrink tubing, and I should be reading water speed (and, most importantly, true wind speed/angle) again.However, the gusher that comes in the through-hull when swapping the plug for the transducer is certainly not the small amount that supposedly results from a properly functioning flap.That issue will have to wait until Persephone's on the hard.Thanks, Guys for all the (as usual) thoughtful and helpful inputs.