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Salty Sea Dog

Stray Current in Marina

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Hi All

Any help appreciated on a subject I am struggling to get my head around please... !

I am trying to trouble shoot some minor corrosion on my propeller.

I am being told it must be stray current that has caused it.

I am being told stray current can come from -
- my boat
- another boat
- or the shore power

Is this correct ?



8 Replies

  • Yes, that's exactly right.

    * Power or sailboat?
    * Salt or Fresh water?
    * Connected to shore power continuously?
    * Do you have a galvanic isolator?
    * If you have galvanic isolator, is it still protecting you?
    * Any seacocks connected to ground?
    * Placing a VOM between DC ground in the boat and the water in the bilge, what does it read?
    * Placing a VOM between AC ground in the boat and the water in the bilge, what does it read?
    Using Zinc anywhere on the boat, or zinc fish?

    These are questions you need to find answers too before you contact an expert, or start researching yourself.

    From past experience, if your not extremely experienced with shore power systems, you will drive yourself crazy taking measurements as you will find evidence of stray currents but not know whats normal vs. not normal vs. dangerous in a marina.

  • Thanks Dan

    Here goes -
    - sail
    - salt
    - yes
    - yes - and yes - I have tested it
    - all seacocks are isolated - not connected to ground
    - I have no water in bilge - bone dry
    - again, no water in bilge - bone dry
    - Zinc on saildrive and propeller

    The yacht is only 3 years old - so the logic is the new thinking that no seacocks should be bonded. The bilge is dry and has never had a drip of water in it.

    Take your point about driving myself crazy - not sure what to do.


  • Are your zincs corroding as they should and being replaced? How often?

    Are they normal zincs, or made out of something else to be long lasting?

    Armed with this information are you going to go a professional or figure this out yourself?

  • Hi Dan

    Thanks for reply.

    Bit more to the story... Which might help.

    3 year old boat....
    - year 1 - lifted boat - all zincs nicely eaten - propeller looked like brand new
    - year 2 - lifted boat - all zincs nicely eaten - propeller looked like brand new
    - year 3 - lifted boat - zincs on propeller looked brand new and propeller shows corrosion

    The sail drive zincs, every year, eaten nicely, sail drive looks brand new.

    Year 1 and 2 I used anodes from one company - and I am 100% they are zinc.
    Year 3 I used anodes from another company - who claimed they would last longer but also claimed they are zinc.

    Since lifting the boat I have done a bit of Investingating on the anodes that came out of the water looking brand new - and I am 99% sure they have a significant lead content.

    Do you think this would explain my corrosion in year 3 ???

    Regards Tim

  • I very much want to say "ding ding ding, case closed", but I don't know what you mean by "all zincs nicely eaten" ... e.g. are they almost gone?

    1. If by nicely eaten, the portion of the zinc electrically in contact with the metal has 25+% of its life left, then you simply need to return to the original zinc formulation. Maybe check with other owners of your boat to learn their experiences to see if you are normal in year 1 and 2.

    2. If those year 1 and 2 zinc's are nearly entirely eaten away then they would have reduced protection for your boat, and you will need to learn when during the season that protection is ended to replace the zinc more often and/or be alert to a stray voltage problem.

    Some things I would consider next,
    a. Join a group of owners of your boat, and find out from their experiences how long the normal zincs last, and if stray current issues have been found what the causes where.

    b. You see your prop damaged, I would be concerned about your stern drive having unseen damage. Find out from someone in the know what to look for, there should be specific places you can check for damage when out of the water.

    c. Next season inspect the zinc's frequently to learn how long they are lasting and decide if you simply need to replace them mid season or call a professional.

    d. If you were me, you would buy a probe and do some testing, but trust me ... you would be better off using a professional then purchasing the expensive probe.

    I hope the damage isn't that grave. Here is a little something on corrosion and choosing anodes

  • Hi Dan

    Thank you so much for the full reply - very useful - especially the articles.

    I too think the anodes are this issue - but the propeller manufacturer is saying (not surprisingly) that is not the anodes and it must be stray current. So while I am pretty sure (regardless of what they say) the issue is the anodes (and probably solved) - I also want to rule out stray current etc...

    I have been told it could be 1 of or a multiple of 3 things -
    - my boat
    - another boat
    - or the shore power

    Would you mind sanity checking my logic -

    My boat -
    - no skin fitting are bonded - so I cant be leaking current through them
    - the sail drive is electronically isolated from the engine - so it cant be that
    - the rudder is electronically isolated - so no current can leak through that
    - the keel IS bonded to the earth rail
    - so it COULD be leaking through the keel IF there is a bad earth of stray current

    Other boat -
    - how would I ever know !!?!

    Shore power -
    - I have a galvanic isolator
    - I have tested it and it is working just fine
    - so it cant be shore power

    So in summary - if the anodes were not the problem - it can only be -
    - a bad earth / stray current on my boat being leaked through the keel
    - or, another boat

    Is my logic good ?

    Kind regards


  • I cannot validate your logic, and propose you look back at a, b, c above. Nothing like interacting with other owners of your boat. Check Google groups and yahoo groups first.

    By the way, you have a lead keel maybe? You're mast has a thick ground wire connecting it to mast, maybe? Are you sure your sail drive isolated?