Depleted anodes: when to replace boat zincs and how surveyors write it up

Anodes are there to corrode. They're the cheapest metal on the underwater gear, wired into the dearer metal around them, and they waste away so the propeller, shaft and through-hulls don't. Once an anode is gone, the next cheapest metal starts paying instead.

How a surveyor checks them

Find every anode: on the shaft, the hull, the rudder, any trim tabs, bow thruster and the engine's own heat exchanger anodes. Judge how much is left. Martyr Anodes' rule is replace at 50 percent worn or at least once a year, depending on the water and use.

Check each is tight, with bare metal under it and no paint on it. Check the metal suits the water. Martyr Anodes lists zinc or aluminium for salt water, aluminium for brackish and magnesium for fresh, and warns against mixing metals.

Then look at what the anodes protect. Pink, pitted propeller edges, pitting on the shaft, or corroded through-hull fittings mean protection has failed or isn't enough.

What decides the grade

Whether the protected metal has started to go. Anodes gone with damage on the propeller or fittings is an A, and the finding asks why. Anodes past half wasted, loose or of the wrong metal is a B. Anodes with life left that will be due at the next haul-out are a C.

What replacement involves

Fast wastage points at a fault. See no galvanic isolator for one common cause on boats plugged into shore power.

Photos to take

Each anode where it's fitted, the propeller edges, and any pitted fittings close up. With Axsail, say how much of each anode is left as you take the photo and it goes on the underwater finding.

Writing it up

List each anode by position, how much has wasted, whether it's secure and clean, the metal and the water the boat lives in, and the state of the propeller, shaft and fittings. If they wasted fast or not at all, say so and recommend the check.