Zinc and magnesium anodes protect the drive and propeller against galvanic corrosion. The anodes protect the drive and propeller against galvanic corrosion caused by the current created between the motion of the boat and the water. This current causes corrosion which would damage drive, propeller, rudder and through-hull fittings were it not for the zinc anode. If 50% or more of the anode has been eaten away by corrosion it must be replaced in order to, once more, fulfil its vital function. The anode must not be painted over and when installing it is important that there is good contact between the anode and the surface to which it is connected.