Anodes for Hull and Shaft

Hull / shaft anodes

- Protect your boat with sacrificial anodes -

What are sacrificial anodes?

Sacrificial anodes are metals or alloys that have a more anodic, i.e. less noble, potential than steel and are used against corrosion. The less noble material of the anode "sacrifices" itself and dissolves, thus protecting the hull and the parts of the boat that are susceptible to corrosion. 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.

Where should anodes be mounted?

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.

How often must anodes be changed?

Typically, anodes have to be replaced once per season. In harsh water conditions, a more frequent replacement schedule might be necessary. 

Which anode for which material and which water?

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 or bronze, 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 aluminium 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.

Can sacrificial anodes lose their effect?

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.

Amount of anodes (in kg) required per 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