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Galvanic corrosion on a mooring

Discussion in 'Ask A Beneteau Owner' started by KennyBene, Sep 11, 2017. Add this thread to a FAQ

  1. KennyBene

    KennyBene

    Joined Jun 9, 2012
    28 posts, 2 likes
    Beneteau 352
    US Centerport, NY 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
     


  2. Mechone

    Mechone

    Joined Mar 20, 2016
    23 posts, 3 likes
    Beneteau 351
    ca Oshawa,Ontario, Canada 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: Sep 11, 2017
  3. seadaddler

    seadaddler

    Joined Dec 19, 2006
    5,016 posts, 77 likes
    Hunter 36
    US 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
     


  4. Charles Erwin

    Charles Erwin

    Joined Jan 30, 2012
    834 posts, 63 likes
    Nor'Sea 27 - "Kiwanda"
    US 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
     

    Attached Files:



  5. NYSail

    NYSail

    Joined Jan 6, 2006
    1,446 posts, 31 likes
    Beneteau 423
    US Mt. Sinai, 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!
     


  6. KennyBene

    KennyBene

    Joined Jun 9, 2012
    28 posts, 2 likes
    Beneteau 352
    US Centerport, NY Centerport, NY
    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.
     


  7. KennyBene

    KennyBene

    Joined Jun 9, 2012
    28 posts, 2 likes
    Beneteau 352
    US Centerport, NY Centerport, NY
    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.
     


  8. KennyBene

    KennyBene

    Joined Jun 9, 2012
    28 posts, 2 likes
    Beneteau 352
    US Centerport, NY Centerport, NY
    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!
     


    NYSail likes this.
  9. KennyBene

    KennyBene

    Joined Jun 9, 2012
    28 posts, 2 likes
    Beneteau 352
    US Centerport, NY Centerport, NY
    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.
     


  10. Charles Erwin

    Charles Erwin

    Joined Jan 30, 2012
    834 posts, 63 likes
    Nor'Sea 27 - "Kiwanda"
    US 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
     


  11. Maine Sail

    Maine Sail Moderator

    Joined Feb 6, 1998
    10,313 posts, 296 likes
    Canadian Sailcraft 36T
    US 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.
     


  12. KennyBene

    KennyBene

    Joined Jun 9, 2012
    28 posts, 2 likes
    Beneteau 352
    US Centerport, NY 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
     



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