Everything You Need To Know About Boat Anodes

In this article, we are going to cover everything you need to know about boat anodes, including:

  1. Boat anodes explained
  2. Where you’ll find boat anodes
  3. Different types of anode material
  4. The do’s and don’ts of boat anodes
  5. Galvanic corrosion

Let’s get into it, shall we?

Boat anodes explained

An anode is a sacrificial material that is added to a boat circuit. Anodes protect metal structures from corroding as they will be eaten away by the electrons prior to the more important items like the propeller, sterndrive and coolers for example.

Metals in water suffer from what you call galvanic corrosion (more on that later). For example, a bronze propeller and a stainless steel shaft are two different types of metal. When you add water around, it creates the possibility of current flow. To avoid the expensive bronze propeller and stainless steel shaft corroding away, a sacrificial anode is introduced. The sacrificial anode must be a suitable metal, and we’ll cover those later in the article.

The sacrificial anode is made of a less noble material, meaning that it will lose the electrons first in the circuit and protect the other material like your propeller and shaft. For this reason, we always bang on about anodes and how important they are. It’s imperative that they are installed correctly and checked regularly. If they aren’t, other components on your boat will be at risk of damage. These other components are also usually a lot more expensive to replace than the anodes themselves.

Volvo active anode system

Where you’ll find boat anodes

You will find anodes in a range of places onboard as well as below your boat, including:

  • Outboards
  • Engine coolers and blocks
  • Sterndrives
  • Rudders
  • Trim tabs
  • Shafts
  • Metal struts in contact with water
  • Exhaust components

All these systems need anodes because they’re in contact with saltwater in some way and need to combat galvanic corrosion. In modern boats, you may have a centralised active system where all the metals are bonded to one large anode on the transom.

Different types of anode material

Most commonly, there are three different types of anode material used in the white boat industry in Australia:

  1. Zinc
  2. Aluminium
  3. Magnesium

In the following table, you’ll find the different types of anode material relative to the water your boat will be stored in. We’ll discuss the different materials in more detail below.

Anodes Material

Zinc

You might hear people often say, “You need to have some Zincs fitted!” but this could actually make the problem worse. Zinc is for saltwater use only, which the majority of the boats we inspect get used on. The problem with using zinc in brackish or freshwater is that it will become too passive and will cover itself in a zinc film and become inactive.

Aluminium

Aluminium is a good choice for general use if you are unsure where most of your boating will be. This is because aluminium is more active than zinc and will still provide protection where the water is not salty.

Magnesium

Magnesium is your best choice for freshwater, but it’s for freshwater use only. If you use magnesium in saltwater, it will corrode rapidly and you’ll leave your boat with zero protection. It is perfect for freshwater as it is less noble than zinc and aluminium. Magnesium also has high conductivity which is needed to combat the high resistivity of freshwater.

The do’s and don’ts of boat anodes

There are some key do’s and don’ts to follow with boat anodes. You may have heard some old timers come up with the brilliant idea of hanging an extra anode off the side of his boat on a bit of fishing line to help combat the issues they are experiencing, but this is a massive don’t.

For the anode to work correctly, it must be connected to the component it is protecting either through direct contact or through a wire connecting the two parts.

Bow Thruster with an incorrectly antifouled anode

Here is a few don’ts of boat anodes:

  • Never paint an anode because it will render it useless.
  • Never paint a component before the anode is installed because it will lose its conductivity.
  • Don’t add additional anodes to the system – an anode system needs to be planned correctly, and if you have too many anodes it will overprotect, and instead of protecting your components, they will wear away.

Some simple do’s you can do include:

  • Make sure there is good contact between the two metals and the surface the anode is being installed to is bare and bright prior to installation.
  • Test the continuity of your wires to make sure they are all operating correctly and have no breaks in the wires.
  • Keep a log of when the anodes were last changed.
  • Replace the anodes at every antifoul and service (typically yearly).
  • If you feel confident doing so, remove the anodes every three months to see how they are performing and replace them before they are completely worn.

A top tip when buying a new boat and taking it to a new location is checking the state of the anodes after the first month. Anodes will deplete at different rates according to what type of water they’re in and how decent the marina’s earthing system is.

Galvanic corrosion to propellor

Galvanic corrosion

We’ve spoken a bit about galvanic corrosion in this article, but what is it? Galvanic corrosion is more commonly known as electrolysis. It’s the corrosion damage that occurs when two different metals are in electrical contact or are connected together in water. The result is a chemical reaction causing the weaker (less noble) metal to corrode at a faster rate.

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