Galvanic corrosion on a mooring

Jun 9, 2012
40
Beneteau 352 Centerport, NY
We have an OC352 on a mooring on the North Shore of Long Island. The diver who cleans our bottom every 3 weeks is telling me the prop zinc is deteriorating far to rapidly and the prop itself and the shaft log have galvanic corrosion- I even have a zinc "guppy" in the water.

I turn off all systems as well as the batteries when leaving the boat and I've looked everywhere for a stray or chafed wire laying in the bilge.

Has anyone experienced excessive galvanic action like this and more importantly, how do I stop it?

Thanks
 
Mar 20, 2016
594
Beneteau 351 WYC Whitby
Is your keel grounded? It shouldn't be. It should have a lightening arrestor that looks like fuse (gas discharge),but should be open.
It closes on a strike and then must be replaced. you can get them on ebay.Some people mistake them for a fuse and think there blown and replace them with a fuse thus grounding the keel
 
Last edited:
Dec 19, 2006
5,809
Hunter 36 Punta Gorda
Why are are you having the bottom cleaned every 3 weeks Long Island sound,my boat is in Florida and clean the bottom every 4 to 6 weeks.
Nick
 
Jan 30, 2012
1,123
Nor'Sea 27 "Kiwanda" Portland/ Anacortes
If you are on a mooring then it is likely your boat is the culprit - and (although I do not know what your diver considers rapid) the problem is probably not a galvanic event - that is a value of less than 1.2 vdc.

If it is stray current (and it probably is) then just turning things off will not eliminate the threat because there are more than one 24 hour circuit in play on your boat - bilge pumps, radio memory, stereo memory, etc. Also many boats have continuous power to the starter motor, the cable from battery to the alternator, windlass cabling.

Anyway measure your hull potential - another name for the shaft propeller assembly potential. Then measure with the batteries fully disconnected - and if you have more than one disconnect all of them. The arithmetic is a bit confusing depending on whether your overboard probe is zinc or a silver/silver chloride half cell (which is not so reliable salt water anyway.)

Oh - two more things - do insist on sea shield, electro guard, or some other of the few tip top anode brands, and absolutely no magnesium anodes in salt water.

Here is a how to do it diagram.

Charles
 

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Jan 6, 2006
3,049
Beneteau 423 Mt. Sinai, NY
Why are are you having the bottom cleaned every 3 weeks Long Island sound,my boat is in Florida and clean the bottom every 4 to 6 weeks.
Nick
Was thinking same...... unless you are racing. I'm in Mt. Sinai harbor on the Long Island Sound... had it cleaned in beginning of July and mid august... usually 2 times a season. Check bottom paint...
But back to your question, check stray wire in bilge water.
Good luck!
 
Jun 9, 2012
40
Beneteau 352 Centerport, NY
Is your keel grounded? It shouldn't be. It should have a lightening arrestor that looks like fuse (gas discharge),but should be open.
It closes on a strike and then must be replaced. you can get them on ebay.Some people mistake them for a fuse and think there blown and replace them with a fuse thus grounding the keel
Not sure about that. I know there are grounding wires from the rigging but I don't recall seeing anything attached to the keel bolts and nothing that looks like a fuse. Thanks for the tip though- I'll look closer.
 
Jun 9, 2012
40
Beneteau 352 Centerport, NY
Why are are you having the bottom cleaned every 3 weeks Long Island sound,my boat is in Florida and clean the bottom every 4 to 6 weeks.
Nick
Racers have it done every week. I was on a 4 week schedule but had a lot of growth- I envy all of you in cleaner harbors but around here, 3-4 weeks is necessary.
 
Jun 9, 2012
40
Beneteau 352 Centerport, NY
Was thinking same...... unless you are racing. I'm in Mt. Sinai harbor on the Long Island Sound... had it cleaned in beginning of July and mid august... usually 2 times a season. Check bottom paint...
But back to your question, check stray wire in bilge water.
Good luck!
2 coats of good bottom paint this spring- I'm surprised 2x a season works for you. We're in Northport and the guy who cleans bottoms drives a Ferrari!
 
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Jun 9, 2012
40
Beneteau 352 Centerport, NY
If you are on a mooring then it is likely your boat is the culprit - and (although I do not know what your diver considers rapid) the problem is probably not a galvanic event - that is a value of less than 1.2 vdc.

If it is stray current (and it probably is) then just turning things off will not eliminate the threat because there are more than one 24 hour circuit in play on your boat - bilge pumps, radio memory, stereo memory, etc. Also many boats have continuous power to the starter motor, the cable from battery to the alternator, windlass cabling.

Anyway measure your hull potential - another name for the shaft propeller assembly potential. Then measure with the batteries fully disconnected - and if you have more than one disconnect all of them. The arithmetic is a bit confusing depending on whether your overboard probe is zinc or a silver/silver chloride half cell (which is not so reliable salt water anyway.)

Oh - two more things - do insist on sea shield, electro guard, or some other of the few tip top anode brands, and absolutely no magnesium anodes in salt water.

Here is a how to do it diagram.

Charles
Thanks Carles- Admittedly, I don't know much about electrical stuff but your test seems like it will confirm what I pretty much already know- I have a grounding problem. Finding the problem is what's eluding me.
Odd as it sounds, I do not have a bilge float switch and the pump is above the water line. There are lots of added electrical accessories- inverter, radar, etc. but none of that has wiring contacting water inside the boat.
All suggestions are much appreciated.
 
Jan 30, 2012
1,123
Nor'Sea 27 "Kiwanda" Portland/ Anacortes
As to keel bonding theory

Since anode wasting rate has changed - evidently dramatically - keel bonding is not the first suspect.

If that were the case then the wasting rate would not have changed from factory new until now. The reason is that the exposed keel surface area is pretty small. That is - the keel is very much protected by paint - thus the unpainted surface area is way too small to be a factor.

But even if that were so - the recent onset (of accelerated) anode wasting cannot be explained by factory installed keel bonding failure even if the keel were unpainted. So something else is going on.

The central point here is that there is no amount of guesswork - no matter how well indentioned - that can possibly substitute for actual VOM measurement.

Charles
 
Feb 6, 1998
11,667
Canadian Sailcraft 36T Casco Bay, ME
This really can not be trouble shot without the proper tools. It could be as simple as someone using one of the green grounding conductors as a DC negative path back to the battery bank or as complicated as a failed alternator leaking current or a failed electronic bilge switch or micro-cracking in a bilge wire jacket or a moist salt path to a bad termination....

Bad DC stray current corrosion is very fast so what ever it is for a leak it's quite small and will require instruments that can detect very, very low DC current flow. This is really not a job that can be done with visual inspections, it's a job for the correct tools and the time to fully go through the vessel.

If you do not understand marine corrosion it might be best to look up your local ABYC corrosion specialist and have them come find and fix it.
 
Jun 9, 2012
40
Beneteau 352 Centerport, NY
Thanks Maine- I didn't know thee were specialists in corrosion but after looking it up, it happens that the man who did our pre-purchase survey is also an ABYC corrosion specialist- my luck.
BTW, a good friend owns a CS36 that he just restored to almost new- love that boat!
Than you
 
Oct 10, 2017
6
Beneteau 37 West River, MD
I had this same problem and eventually tracked it down to the masthead vhf antenna. It destroyed my prop in a month though, before I found it!

So the mast is grounded to the keel for lightning protection, right... but it turns out that the vhf antenna L-bracket that attaches it to the mast was allowing a small charge to flow into the mast (where the antenna / cable connection passes through the bracket. When the radio was on,that current would then flow through the lightning protection wire, into the keel bolt, into the keel, and through the water to the zinc on the prop... a circuit that quickly ate zincs.

I only could find it using the silver chloride reference anode and systematically testing each on/off circuit. You immediately could see the voltage drop on the multimeter with the vhf antenna plugged in.

The solution (for every boat) is to use a nylon shoulder washer to ensure the antenna is isolated from the L-bracket attaching it to the mast.

Hope that helps.
 
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Mar 20, 2016
594
Beneteau 351 WYC Whitby
Is your keel grounded? It shouldn't be. It should have a lightening arrestor that looks like fuse (gas discharge),but should be open.
It closes on a strike and then must be replaced. you can get them on ebay.Some people mistake them for a fuse and think there blown and replace them with a fuse thus grounding the keel
As I wrote above your mast should not be grounded it should have gas discharge lightening arrestor that looks like a fuse. Follow the ground wire it should go to the block for the arrestor then to the chain plate
 
Jun 9, 2012
40
Beneteau 352 Centerport, NY
excellent suggestions gents- Thank you- I will check both. I had an ABYC corrosion specialist aboard but we didn't have time to go through every circuit or the mast ground arrestor. Looks like I have to take the long way to eliminate this giant PIA problem. Many thanks for the sound advice!
 
Oct 10, 2017
6
Beneteau 37 West River, MD
I had an ABYC corrosion specialist aboard but we didn't have time to go through every circuit or the mast ground arrestor.
I spent two weeks going through every circuit on the boat using a multi meter, turning switches on and off, disconnecting batteries etc and couldn't isolate the problem. You need the silver chloride reference zinc to track this down. Costs $125 here (http://www.boatzincs.com/corrosion-reference-electrode-specs.html) and worth every penny, never would have found the problem otherwise. If you ABYC corrosion specialist didn't have one, fire him.
 
Mar 20, 2016
594
Beneteau 351 WYC Whitby
I actually added a second ground wire from the port chain plate thru another gas discharge arrestor (ebay).
I believe noaa did studies in Florida ,that boats that were grounded were more likely to be hit. With Beneteau system the mast is not grounded however if it is hit it will ground .
 

jviss

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Feb 5, 2004
6,745
Tartan 3800 20 Westport, MA
Everything I've ever read about grounding for lighting protection, for external, metal ballast keels, says to ground the mast to the keel.

Here's the schematic for my boat. No gas discharge device. I have no galvanic corrosion problem, either. I think most boats are grounded this way.