Sacrificial boat anodes are made from different metals and work by being more reactive than the metal they're protecting. They corrode first, sparing your propeller, shaft or hull fittings. Anode purpose and maintenance is often misunderstood or has low awareness and priority for boaters.
Are your boat's sacrificial anodes happy? Five ways to check:
- Anodes show steady wear (wear is a good thing)
- They are the right metal for the water
- They are all firmly attached
- Anodes are free from paint, antifoul and heavy debris
- They all show consistent wear.
Steady wear
Anodes should show wear - that's the sacrificial bit and proves they're doing their job. But when they're more than 50% worn away, they need replacing. Once an anode has been consumed, your boat's underwater metals become the next target for corrosion. An anode that looks untouched after several months probably isn't effective; it may not be making good electrical contact or it could be the wrong type for the water your boat is used in. The best time to inspect anodes is during the boat's annual lift-out for antifouling or servicing, or if the boat is lifted out for maintenance. You can usually get it hoisted out for a quick check too.
Image: a pitted, rough and gently eroding anode is a working anode! A pretty anode is not doing its job.
The right metal
Water affects how well sacrificial anodes work. Freshwater is a poor conductor of electricity and because it's harder for the protective electrical current to flow, you need a more active metal. Magnesium is the most reactive of the common anode materials, so it provides enough electrical potential to protect the boat in freshwater. Saltwater is an excellent conductor of electricity because it contains dissolved salts. The protective current flows much more easily.
For freshwater lakes and canals, choose magnesium.
For brackish water, choose aluminium.
For saltwater, choose zinc.
Using the wrong type means your anodes may offer little protection. Why not use magnesium everywhere? Because in saltwater it's too reactive and can wear away very quickly, needing early replacing.
Many boaters buy a vessel that used to sit on the coast or take advice from boating forums where zinc is synonymous with protection, yet the low conductivity of fresh water requires magnesium.
Firmly attached
A sacrificial anode protects your propeller, shaft or hull by creating a small electrical circuit - it needs a clean, tight connection to do its job with no wiggles or wobbles. If the anode becomes loose, that circuit is weakened or broken, so the anode can't sacrifice itself or provide effective protection. During routine maintenance, make sure all mounting bolts are secure and shaft anodes haven't slipped along the shaft.
Top tip: When replacing an anode, clean the surfaces with emery cloth or a wire brush before fitting. Removing paint, oxidation and marine growth ensures good metal-to-metal contact and helps the anode work as intended, so you don't need to inspect them for a while - hopefully!
Free from paint and antifoul
An anode only works effectively when it's indirect contact with water. Accidentally or not, anodes can end up covered in paint, blacking or antifoul when the hull is treated, and this paint acts as an insulating barrier. Preventing seawater or freshwater touching the anode, the small electrical current that provides corrosion protection can't flow properly. Remember, a painted anode might look aesthetic and in good condition, but this is not a positive indicator. Thick marine growth and debris need a mention too; they don't usually stop an anode working completely, but they can reduce its effectiveness.
Consistent wear
On boats fitted with multiple anodes, it's normal for them to wear at slightly different rates, but they should all gradually reduce in size over time. If one looks almost new while another has nearly disappeared, it could be a warning sign. Before replacing the worn one, it's worth asking an engineer to assess why and address any problems such as stray electrical currents or a loose - or poorly fitted - anode.
Top tip: When your boat is lifted out, take a photo of every anode. Comparing them year after year makes it much easier to spot wear patterns before they become costly problems.
Sacrificial anodes and galvanic corrosion
Galvanic corrosion can occur between moored boats, especially in inland marinas or tight mooring areas where boats are close together and connected to shore power. It results in irreversible damage to your boat’s underwater components - propellers, shafts, through-hull fittings etc. Sacrificial anodes help but may not be enough in a marina. You can improve mitigation with a galvanic corrosion isolator. A plug-in version requires no installation whatsoever.
