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Electrolysis question


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Hey everyone, I've been lurking on this sight for a while but have never posted up before. I'm at my wits end with a boat I bought a few months ago and need some advice... so here's my question. I recently bought a 1994 Sylvan aluminum boat with a 115 horse merc on it. I'm having problems with electrolysis on the transom. The boat is used in both fresh and saltwater but so far I've only used it in salt. The old owner had problems too but never mentioned it to me (kinda suspicious I guess). I plugged the minor holes I had but now want to know how to stop it. The old owner painted the transom so I'm going to strip the paint down and use what I know is the correct paint. But if he had electrolysis before he painted it, it must be something else.

I've gone over the whole hull with a multimeter and see no discrepency in ohm readings anywhere on the hull or motor... all were .5 to .6 ohms. There is no trace amps running through the motor, hull, etc with or without the motor running. I have no voltage on the hull with the exception of the post for the trim/tilt motor has about a half volt difference from the hull all the time (even with the battery off). That may be a problem but I'm thinking it's not an electrical issue.

The hull is painted from the manufacturer but it has no zincs on it... the zincs on the motor are chewed up a bit but not too bad. I've talked to Sylvan/smokercraft and they say the zincs on the motor should be enough... but obviously they're not. I'm thinking that I need to put zincs (actually magnesiums from what I've been reading) on the hull to stop the problem.

So my question (finally) is, am I on the right track? Or can anybody lead me in a differenct (and better) direction?

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Is there by any chance a through hull fitting near where you are getting the re-action ?

It's not unusual to need a mounting plate with a zinc for the boat.

There can bot be any bottom paint between the plate and the hull.

Also the wrong bottom paint can lead to a re-action.


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  • 1 month later...

I agree w/ Doug, check the bottom paint first, if a copper based paint was used that needs to come off immediately. If its not the paint my next question would be: Do you keep the boat plugged in? at a marina or private dock? If so it may not be your boat. if another boat is putting voltage in the water the shortest path to ground may be through your boat. If not check for bad wiring, bad connections, old equipment with bad grounds, etc. Another reference is www.proboat.com this is a trade magazine for all the marine industry and you can search past issues for specifics. Also Nigel Calder has very good reference books on this subject.

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Make sure all your electronics and lights are wired directly to the battery with a gauge wire equal to your largest wire, 25 watt VHF radios need a 10 gauge wire to carry the transmit current. Do not use the hull to ground out any electical units like you use in your car where the frame carries your ground circuit. The antifouling paint to use on an aluminum boat uses the term "ablative", check with the dealer. Make sure your zincs are not painted over for a matching appearance. That's the tab on the lower unit next to the propeller.

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