Fouling of Knotmeter Impeller with Zebra Mussels

[CapitalistSailor]

I have a Beneteau 311 with the original Raymarine knotmeter. The sensor for this knotmeter is a paddlewheel that sits in a fitting under the boat in front of the keel and slightly to the side. My antifouling paint does a pretty good job, but I don't paint the impeller. A few weeks into the season the impeller stopped working and at haulout I found this growth as shown in the attached picture (this picture shows the hull as it was at haulout, before pressure washing).

I scrubbed the bottom a couple times during the season from above, but I couldn't dive under the boat to clean it due a bad green algae outbreak (cyanobacteria).

I could try to pull the sensor from inside the boat and clean it while the boat is in the water, but last year I couldn't get it to budge. Before launch I will try to get it loosened up, but it would be better if I could do something preventative.

 

[Hayden Watson]

I pull my impeller when it gets fouled. It let in a surprisingly small amount of water as long as you have the plug in-hand to exchange with it. I have been told (though have never tried) that Desitin cream provides anti-fouling properties without build up. You could try that to decrease how often you need to pull the impeller for cleaning.

 

[Kopite]

I’m not far from you and have had same issue in previous years. Now I just paint the impeller with VC17 when I do the hull, and no problems since..doesn’t seem to affect the impeller plastic at all, and VC17 is so thin it doesn’t affect function.

 

[CapitalistSailor]

Hayden and Kopite,

Thank you for you suggestions. I do use VC-17 on my boat so I could try that, or Desitin. How long does the Desitin last?

With VC-17 the solvent used to be so aggressive that I was afraid to touch anything with it other than the bottom, but in the last several years the solvent has been toned down. I was also worried that the weight of the copper would be difficult to distribute evenly on the impeller so that it would turn evenly. I was also concerned that the copper would interfere with the pickup for the knotmeter sensor, because I am guessing that the paddlewheel has one or magnets that the sensor picks up. Do you have a Raymarine paddlewheel type? Different manufacturers might have different plastics.

Follow-up question: I have had trouble calibrating the knotmeter, which could be traced to the fouling but maybe not. If I calibrate the knotmeter to 5 kn, the reading at 2 kn is way off, and vice versa. Have any of the forum readers had the problem with this circa 2000 Raymarine knotmeter? There is only one fudge factor for calibration so I can't set the zero and span/sensitivity separately. I can use the GPS for speed but without a good knotmeter reading the apparent wind readings are miscalculated. (I sail in Lake Erie mostly so I usually don't have to worry about current more than a few tenths, but sometimes the current is significant when the lake level is shifting in response to storm winds from the NE or SW.)

 

[Hayden Watson]

Here is what Practical Sailor has to say. https://www.practical-sailor.com/blog/A-Diaper-Cream-Cure-for-Barnacles-10907-1.html It was suggested that I put in on my prop and I did at the first haul out and it worked for over a year. I forgot to add do it at the last haul out and regret the memory lapse.

 

[Ron20324]

In the Chesapeake Bay we don't have those Zebras, but in the warmer weather we get mud shrimp and some other small critters that can build fast enough from one weekend to the next that the impellor will not rotate until pulled and cleaned. After a weekend cruise I pull the wheel out and put the dummy in till the following week.

Edit to add: I do paint the part that sticks out of the hull, but not the wheel itself. Use the same paint you'd put on the face of the depth transducer.

 

[shemandr]

If you can remember I think it's best to pull the impeller and put the blank in when you are not using the boat. As far as the calibration, it was a challenge on our 356. We could set the speed to anything we wanted through the calibration process. We would try to set it by sailing up current and down current and averaging but on some level it was an arbitrary number. That makes a lot of the derived info from the instruments arbitrary too.
Gotta go - my grill cover is trying to blow away ...

 

[DougM]

I have had the same problem with Zebra mussels on the impeller, but never that bad. The buildup I usually see are mussels no bigger than a match head at the most. I do not paint the impeller.
I would usually discover the problem as I left the dock and the knot meter not reading. To temporarily correct it, I would just go below and tap on the top of the through hull with the handle end of a screwdriver.
After a sail where that had occurred, I would pull the assembly and clean out the paddlewheel with a toothpick. I never worried about too much water coming in. I could switch out the paddlewheel assembly and the dummy plug easily and a dry towel in the area would soak up the water before it even got to the bilge.
Side note: most of the sensors are Airmar brand.

 

[Jackdaw]

Holy crap! I thought we had a zebra mussel issue when ONE lodged in my impeller and jammed it.. that’s crazy.

 

[kappykaplan]

After a weekend cruise I pull the wheel out and put the dummy in till the following week.

 

[ToddS]

So I think after reading this, I'm just an overly cautious boat owner (better than the opposite I guess)... but I've NEVER attempted to remove any thru-hull anything (knotmeter wheel, sonar, or whatever) with the boat in the water... It is probably completely irrational, but I just have this vision of either a firehose-like gush of water coming in, or the "thing" (knotmeter, dummy, etc.) breaking, or cross-threading, or somehow failing me and me with catastrophic results. When you guys say you swap out the paddle-wheel for the dummy, I'm guessing we're talking about 1 or 2 seconds of open thru-hull time for the swap... how much water is gonna get in through that 1.5" or 2" hole in one second... right? Assuming all goes well (it scares me to even say that) swapping them out would seem to be the obvious, and best solution. I'm sure I'm wrong, and you are right, and that between the dummy, an emergency plug (just in case) and a towel, it would be just (gulp!) fine.

 

[Ron20324]

My inflow is maybe a pint, but then I've been doing this for a decade. I do it while laying down. Pull the dummy with your left, have the worker in the right hand. I coat the rubber ring on the dummy and the real one with KY before insertion.

 

[jon hansen]

'scatt' is an antifouling wax. goes on like car wax. might be a product to cure your paddle wheel problems.

 

[Hello Below]

When you guys say you swap out the paddle-wheel for the dummy, I'm guessing we're talking about 1 or 2 seconds of open thru-hull time for the swap... how much water is gonna get in through that 1.5" or 2" hole in one second... right?

That really depends on whether your thru hull housing incorporates a self closing valve or not. If it is still there it will greatly minimize water flow into your boat. So in answer to your question a 1.5" hole at 1' below the water line will allow approximately 1.3 quarts in 1 second. A 2' hole at 1' will allow 1.5 gallons in 2 seconds. Again much less if you have the self closing flapper and the flapper is still there.

http://www.whsyc.org/Flooding/Flooding.html

 

[kappykaplan]

how much water is gonna get in through that 1.5" or 2" hole in one second... right?

With the flapper, at most a half gallon - unless I goon something up and can't get the dummy to seat properly!

 

[Hayden Watson]

A 1-second swap would be really slow actually. With one hand on each plug you pull one out until it is almost to the end and then is it just a swap. usually only takes a fraction of a second. The water is an outflow, not a fire hose. There is very little pressure, although it will spray a bit just as the plug starts to close the hole. If I get a pint I was being sloppy. I do not have flappers on any of my sensors. I usually but an old towel down and it catches everything.
The hardest part is getting my fat old body into a position where I can work with both hands.

 

[helmfam]

Our knot meter will foul within a couple days if we are not sailing actively. I now mostly sail without it. It is a good skill to learn how to change it out with the dummy. A little scary the first time, but no big deal, I can clean up the water that comes in with a small bath towel. Go for it.

 

[CapitalistSailor]

Holy crap! I thought we had a zebra mussel issue when ONE lodged in my impeller and jammed it.. that’s crazy.

Jackdaw, my boat is on the Portage River just off Lake Erie, and we get a lot of farm land runoff, a big problem in Ohio where we have many more pigs and cows than people, and dozens of times more chickens. Also, the river is warmer than the lake which promotes fouling growth. I bought the boat from someone who kept it it Traverse Bay off Lake Michigan, across from you. I think he only put paint on every couple years.

On the positive side, the picture on my original post shows that the VC-17 antifouling paint does a great job on the hull.

 

[CapitalistSailor]

I do have plugs for the knotmeter and depth transducer, so I'll see if I can loosen up the knotmeter plug when things warm up. We still have snow but Lake Erie is thawing so boating season is coming!

 

[Ron20324]

... so I'll see if I can loosen up the knotmeter plug when things warm up. ...!

If you have the boat hauled, take out the depth one, too, just to make sure it is not frozen in place.. Give the face a coat of that special paint for transducers.