Baffled by Bonding and Corrosion

By: Frank Lanier

QuestionTo bond or not. What is your opinion relative to connecting all underwater fittings to the ship’s bonding? When dealing with wood electrolysis, is it appropriate to saturate the damaged wood with epoxy after vinegar treatment? The damage does not appear severe enough to replace the wood.

AnswerBonding of through-hulls is a contentious topic among boat builders, surveyors, boat owners, etc. Bonding all underwater fittings on a wooden boat was promoted as a must-do at one point in the past, but today (based on my experience) people in the know when it comes to wooden boats don’t recommend it.

Navigation and Vessel Inspection Circular (NVIC) 7-95 (the Coast Guard inspector’s bible when it comes to wooden boats) does not require it and discusses potential problems associated with the practice. I’m not a fan of it, but if the choice is made to install a bonding system, the big thing is keeping all the connections tight and free from corrosion, or you’ll be introducing even more trouble into the mix.

Over-zincing is probably one of the most common causes of wood damage in such cases. Zincs provide important protection when dissimilar metals are present, but too many zincs (the less noble metal) only reverses the process — you want a proper balance between the two. Zincs are sacrificial and should waste away, meaning it’s better to have a small zinc that does its job than a big one that lasts forever. If the ones on your boat don’t waste away, you probably have too many.

In regard to dealing with the wood damage mentioned, epoxy saturation may be an option (depending on the condition of the wood), but the cause of the damage should be verified (via a corrosion survey) and corrected before any repairs are implemented. A corrosion survey allows you to see if any problems currently exist, while providing a baseline for future reference.

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