Generally speaking, the type of anode depends on the kind of water. The following is a useful guide:

Of course, boats often move between different waters, travelling between salt and fresh water, or being berthed in marinas or behind tidal barriers where the water is enclosed and brackish.

It is important to note that not all anodes are suitable for every type of water. For example, zinc and aluminium anodes are not suited to fresh water. If they are exposed to fresh water for a prolonged period, they become covered in a white oxide crust that effectively seals the anode and stops it from working when the boat returns to salt waters. Zinc anodes suffer a similar problem in brackish water.

Magnesium anodes are not suited to salt water. They become much more active than usual and this accelerated activity shortens the lifespan of the anodes and can leave a build-up of an off-white calcareous deposit that is difficult to remove. Also, magnesium anodes should never be fitted to wooden hulled boats as they can damage the timber due to a process known as electrical chemical decay.

If you are unsure, you can always use our Find My Anode tool to help you find the anode for your vessel.