Knot Meter Spinner

[Pgtjs]

I have a near new 350 (commissioned last May) with a factory option ST60 tridata unit. The knotmeter component is based on a spinner on a through hull unit which is located just forward of the keel. I have had no operational problems with such spinners in the past but this is a new design possibly intended to be more robust. I have had spinner blades get crushed by lift out slings but they seem to work without fouling up with marine growth in this area so dont think that is the cause of my problem.

The new design is much narrower and thicker and seems to foul with sea growths in less than a week so that I have to pull the thru hull out and clean it. It has been necessary to pull it at least 8 or 10 times since new and that detracts from the pleasure of boating as you will all agree.

I have painted it as suggested by a marina guy with this concentrated anti fouling stuff (very expensive) but that made no difference - it seems not to want to spin without being manually activated. I have wondered if it is that the spinner unit is not potruding enough and is being shielded by the hull. When I pulled the unit again last weekend it was quite clean and only needed light finger pressure to cause it to work. Another possibly related issue is that I have had to recalibrate the instrument several times and it seems that it is usually 1 to 2 knots slower that it should be. Thereafter it seems to run fine for a week or two then it slows down again.

The spinner seems to work for a few consecutive days when we take the boat away but when I leave it in the marina for a week and come back it reverts to non operating mode. On a few occasions after we have motored out into the channel it suddenly comes to life, on other occasions it stays "asleep". In mid summer there is much algae in the water but at this time it is cold and clear and that seems to make no difference.

Does anyone have any ideas? I have talked to the dealer as it is still under warranty but he has few ideas - I have found him very helpful in the past but this time he is stumped. It is about 25 miles down to his marina so I dont want to take it there and back unless I have to.

Incidentally I have an intergrated GPS and get SOG with it but we often have 4 -6 knot currents and would like to know boat speed not just SOG, especially when sailing.

I would appreciate any feedback, Cheers, Geoff. (C350 #392 - Blaine WA.)

[KenKrawford]

Geoff,

This probably isn't the answer you want, but have you considered removing the transducer when you're not on the boat and insert the plug that comes with the unit? Kind of a drag but it will probably solve your problem.

[william vanwagoner]

I know it is a pain, but I always pull the transducer and insert the plug after each sail. I keep a sponge in the area to mop up the water after the plug is put back in.

I do this primarily because I have heard of boats sinking from a failed knotmeter transducer, and the plug is much more secure. I am also the type that closes all sea cocks when I leave the boat so I am more conservative than many. Of course, this practice makes marine growth a non-issue.

Bill Van Wagoner #229

[LONGHAWK]

I agree totally that it is quite normal and expected to pull the wheel and insert the plug any and every time the boat will sit for more than two or three days.
I have been told to never have the boat pulled from the water with the wheel in place... I would consider that to be courting disaster.

Sam
LONGHAWK #173

[Triumph]

I have the same type isses. I finally quite using it after interfacing my chartplotter with the speed instrumnet. I would only use the paddle wheel now if I were racing, and needded to compare speed through the water verses speed over ground. The GPS speed is better information I believe.

[Steve A]

Would someone tell me how to access the transducer? I take it I can remove the spinner from inside to clean it (instead of from underneath?) My spinner has worked only when I go under the boat.

Thanks

[russp]

Hi Steve, assuming you have a Raymarine or similar, the thru hull fitting has a flap valve which (mostly) seals the hole while you swap out the transducer for the plug which should have come with the fitting. You just unscrew the retaining cap inside after putting sponges or a towel around the fitting to soak up the pint or two of water that will come in. And, have the plug very close at hand then using the metal pull ring just work the fitting back and forth a bit and it should just pop out, with a bit of water. Put the plug in, press it down, put the retaining cap back on being careful not to cross thread it, and you are done.

When you put the cleaned transducer back in, note the position of the arrow on the head, must point towards the bow.

BTW, I am in same boat as other guys, too lazy to pull out each time I sail, so just use GPS for speed unless occasionally racing, transducer hasn't been in for several months. Downside is you don't just lose boat speed through the water, you also lose true wind speed and direction.

Regards

[C350ia]

For those of us who did not get the "Blanking plug" for the speedo when they purchased the boat, Amazon has a plug available for the 2" OD Airmar thru-hull tube. However.. if you have the flap-valve that helps shunt the water when the impeller is pulled. You will have to cut the bottom portion of the blanking plug off. I cut mine a 1/4" below the upper o-ring.

The Amazon ordering info is: Airmar 20-550-1 plastic blanking plug for 51mm (2") OD non valve thru-hull tubes.

Cost: $21.24 includes shipping.

Seawing
Hull# 309