Ask SAIL: Zincs Emergency

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Q: I need advice. All of a sudden I’m burning through zincs while at dock. My gut instinct says that there is a stray current somewhere. Is it most likely from outside my boat (dock or nearby boat) or inside my boat? What kinds of checks can I make with my voltmeter and how do I do it?

George Blakeslee, New Haven, CT


If you are plugged into shorepower then most likely this is galvanic corrosion brought on board by the shorepower cord. You need a galvanic isolator or isolation transformer to block this. (If you already have one, it is not working properly.) If you are not plugged in, then this could be stray current corrosion emanating from either your boat or elsewhere. However, I doubt this. Stray current corrosion is agnostic regarding what metals it corrodes. Regardless of metal composition, any metal that is discharging a DC current into the water will corrode. For your zinc to be the metal that was effected, it would have to be the discharge point, as opposed to a through-hull, the propeller or some other underwater metal. Zinc loss is generally associated with galvanic corrosion. There are tests you can carry out to determine what is going on, but these are rather more involved than I can spell out here. They are, however, covered in detail in my book Boatowner’s Mechanical and Electrical Manual.

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