What is stray current corrosion in a marina?

Stray current corrosion is the least conspicuous expensive loss in a marina. It happens when current leaks into the water through the shore power system, frequently through a fault on an entirely different boat or in the pontoon wiring. Your boat then becomes the sacrificial anode for the neighbourhood. Shaft, propeller, saildrive, rudder gear, seacocks and keel bolts are affected, precisely the parts you cannot reach afloat.

There is one telltale sign: the sacrificial anodes are gone after a season, sometimes after a few weeks. Treat that as normal wear and you lose the window in which the damage can still be stopped. The second sign shows at lift-out, when the propeller or bronze fittings look pink or porous. That is dezincification, and it is almost always electrolytic.

The insurance position is uncomfortable, because corrosion is a gradual process, and gradual processes are regularly excluded in property policies. The argument then turns on whether it was wear or a sudden external influence. What helps you is documentation: photograph the state of the anodes at every lift-out, note the date, and report unusual anode consumption to the marina at once. A galvanic isolator or an isolating transformer costs a fraction of a new shaft.

Worth remembering

Anodes that vanish in one season are not wear. They are a warning, and usually about somebody else’s wiring.