Anodes for Outboards, Alloy Hulls and Trailers

Our earlier guide on anodes was about Volvo Penta sterndrives, because that is where the questions come from. But the same electrochemistry is quietly eating outboards, plate aluminium hulls and trailers all along the coast, and on those the anodes are smaller, better hidden and more often forgotten.

A short reminder of what an anode is for

Put two different metals in salt water and connect them electrically, and you have made a battery. Current flows, and one of the two metals dissolves to feed it. On a boat that means the aluminium gearcase, the stainless propeller and the bronze fittings are all wired together through the engine and the hull, sitting in an electrolyte, and something is going to corrode. The anode is a lump of metal chosen because it is more willing to corrode than anything else on the boat, so it goes first. When it is gone, the boat is next. The full explanation, and the zinc-aluminium-magnesium question, is in our guide to which anode a Volvo Penta needs; the short version for South Australian salt water is that aluminium or zinc anodes are correct and magnesium, which is for fresh water, is not.

Where the anodes hide on an outboard

Most owners can find one. Most outboards have three or four.

• The trim tab. The small fin under the anti-ventilation plate, just behind the propeller. It is an anode. It is also a steering-torque adjuster, which is why it has an angle setting — note the angle before you take an old one off, and set the new one the same.
• Gearcase anodes. A block bolted to the leg below the anti-ventilation plate on one or both sides, or a ring on the propeller shaft between the propeller and the gearcase on some Yamaha and Suzuki models.
• Transom bracket anodes. One or two bars on the mounting bracket. They protect the bracket and the trim rams and they are the ones most often painted over or missing entirely on a second-hand engine.
• Internal powerhead anodes. Small plugs in the cooling jackets of the block on many four-strokes. Out of sight, and the ones that catch out owners who thought the outside ones were all there was. The service schedule lists them.

The parts for each make are in our Yamaha, Suzuki and Evinrude and Johnson ranges; ring with the engine's serial number if you are not sure which anodes it carries.

Plate aluminium hulls

A plate alloy boat is a large aluminium anode with an outboard bolted to it, unless it has its own protection. The hull anodes are bolted to the transom, usually near the waterline on either side of the engine, and they are protecting the whole hull — every weld, every fitting, the keel strips, the transom itself. On a boat that is moored rather than trailered, they are working continuously and want checking every few months. On a trailer boat they last longer but they are not optional. A plate hull that has been in the water without anodes will tell you with pitting along the chines and around the stainless fittings, and by then the damage is done.

Two rules for hull anodes. They must be in bare metal contact with the hull — an anode bolted through antifoul or paint is electrically disconnected and protecting nothing, so mask the mounting face when you paint. And the hull must not have stray current going through it from a wiring fault; a boat that eats anodes unusually fast has an electrical problem, not an anode problem.

Trailers

A galvanised trailer does not carry anodes because the galvanising is one: a coating of zinc over the steel that corrodes sacrificially, protecting the steel underneath until it is used up. That is why a chip in the galvanising rusts more slowly than bare steel would, and also why a trailer that lives in salt water is on a clock. Rinse it after every launch, touch up chips with a cold galvanising paint from our paints and coatings, and pay attention to the springs, the U-bolts and the inside of the cross-members where the water sits. When the zinc is gone, it is gone.

When to replace

The rule is the same everywhere: replace an anode when it is half gone, not when it has disappeared. A half-consumed anode is still working; a wholly consumed one has not been working for some time, and neither has anything it was supposed to protect. Never paint an anode, never fit one over paint, and when you fit a new one wire-brush the mounting face back to bright metal first. The whole job on an outboard is fifteen minutes with a socket set, once a season, and it is the cheapest insurance on the boat.

If the anodes on your engine or hull are eroding faster than a season, or if you have found pitting and want to know how far it has gone, ring 0412 532 984 and bring the boat in. Finding the stray current is a workshop job, and it is a far better outcome than a new gearcase.