Maritime Wood

Counter Resistance

Counter Resistance

What’s the correct way to wire an anode to the stern gear and engine?

You need to run a bonding wire from a convenient bolt on the engine block to one of the studs on the anode. The wire should be of low electrical resistance and also resistant to corrosion. A conductor diameter of around 3mm (rated to carry about 25 amps) will do, not because the currents are large, but to minimize resistance. Tinned copper stranded wire is ideal, as it’s far less prone to corrosion than the untinned variety.

Don’t use domestic cable because it’s too brittle and might fail. If you can use the same connection to the engine block which serves as the common earthing point that’s fine, but it doesn’t matter if it’s somewhere else as long as the contact is good. Unfortunately, connection to the engine doesn’t necessarily protect the shaft and prop.

There’s often no (or very poor) conductivity across any flexible shaft couplings and, even if the coupling is metal- to-metal, oily and dirty surfaces may make the resistance unacceptable. Flexible couplings can be bridged across with a wire or woven bonding braid, but a better idea is to make direct contact with the shaft by using special brushes which bear against it as it rotates (See opposite). Alternatively, you can fit a shaft anode outside the hull.

Bonding the P bracket can be awkward. Some builders thoughtfully fit connection points inside the hull. If not, you may have to drill through the GRP gooped around the bracket’s stem and tap a thread into the bronze to receive a small length of stainless steel studding - to which, of course, you can bolt your grounding wire. But be careful: it’s easy to end up with a minor leak, so this is perhaps a job for your local boatyard. The sitting of the anode is also important. To protect the stern gear, the anode should be fairly close to it, to keep the electrical path through the water as short as is practicable.

This is why most anodes are usually located in the after half of the hull. American builders usually bond all through-hull fittings to the anode system, but Europeans seldom do. Westerly have always included the engine cooling water inlet seacock in the circuit, attaching the bonding wire by means of a hose clip. And remember, it’s not just fit and forget. It’s essential to keep all connections sound and free from corrosion. Make an annual check and replace your anode long before it erodes completely away.