The trim tab anode does two jobs. It is a sacrificial anode, made of zinc, aluminium or magnesium, which corrodes away instead of the aluminium of the lower unit. Its angle also counteracts the torque of the propeller, which otherwise makes the steering pull to one side at speed.

Replace the anode once about half of it has worn away. Never paint it, as paint stops it working, and make sure it sits on clean, bare metal so it has a good electrical connection to the lower unit. An anode that isn't wearing at all usually isn't connected properly.

If the steering pulls to one side, loosen the anode's bolt and move the back edge of the fin a little towards the side the boat pulls to, then test it again. On many outboards the bolt is reached through a hole in the top of the anti-ventilation plate, under a rubber plug. Before removing an old anode, mark its position so the new one can be fitted at the same angle.