have to be replaced once per season. In harsh water conditions, a more frequent replacement schedule might be necessary. The anodes are mounted along the keel. The electric current
is higher at the stern post due to the propeller being located there. It is advisable to install several anodes in this area. Anodes should also be mounted on the rudder and propeller shaft. If the anodes don't show any signs of dissolving after a while, something might be wrong. Something is always getting dissolved. If the sacrificial anodes
are not attacked by corrosion, another area is probaly affected by corrosion. The purpose of the sacrificial anode is to be dissolved.
For the protection of hulls, engines and outboard motors made from aluminium alloys
, zinc anodes are commonly used in salt- and brackish water. In fresh water conditions, magnesium anodes
are commonly used. For hulls and other underwater parts made from steel, brass
, zinc anodes
may be used. However they tend to be replaced more frequently than aluminium anodes
due to the fact that zinc anodes always contain a small percentage of the toxic heavy metal cadmium. This is also why aluminum anodes are considered to be a lot more environmentally friendly. Moreover, aluminium anodes are more efficient, longer-lasting and recommended for usage in brackish waters or when going back and forth between salt- and freshwater.
Anodes must never be painted or lubricated! This is because they would lose their electrical conductivity with water and thus, they will not be able to properly prevent corrosion. If a boat stays on land for a long period of time, some layers of oxidation (especially in the case of aluminium anodes) might prevent them from functioning properly, as the potential difference might be too low.
Table for calculating the required amount of anodes (in kg), depending on the corresponding unprotected metal surface:
Metal surfaces in m2 / zinc anodes in kg
5 to 10 m2 / 5.5 kg
10 to 20 m2 / 9.5 kg
20 to 30 m2 / 18.0 kg
30 to 40 m2 / 20.0 kg
40 to 50 m2 / 25.0 kg