Story and photography by Capt. Vince Daniello
Damaging and dangerous stray current can come from a chafed bilge-pump wire or a wet terminal strip near bilge water, electrical equipment leaking power into a grounded case, improper electrical work on your boat or on the dock, or innumerable other sources.
To check for stray current, with the boat in the water and the shore power disconnected, take the bonding wire off one of your through-hull fittings so it is totally disconnected from the bonding system. With your multimeter, measure the amperage that passes between the bonding-system wire and the through-hull. Anything approaching one milliamp is too high. Turn off circuit breakers one by one to look for and correct a specific source. Also, try checking for stray current again with the shore power connected. If none of this resolves the problem, it’s time to call a marine electrician.