What does the anode do?
When different metals are in water and in contact with each other, a weak current forms. The least noble metal corrodes first. A sacrificial anode is a piece of less noble metal that corrodes instead of the drive, propeller, shaft and engine parts.
The anode is a consumable. It should be checked every year and replaced in time.
The protection only works as long as the anode has good electrical contact with the metal it protects and is the right metal for the water.
Choose the metal to suit the water
| Water | Anode |
|---|---|
| Salt water | Zinc (or aluminium) |
| Brackish water | Aluminium |
| Fresh water | Magnesium |
Magnesium is highly reactive and is only used in fresh water. Aluminium works in both salt and brackish water. Zinc is the traditional choice for salt water.
If you move your boat between different waters, or aren't sure what your home harbour counts as, ask a local boatyard or engine dealer.
When should the anode be replaced?
- When about half of the anode has been used up.
- If it is so corroded that it has come loose.
- If it hasn't been consumed at all – it may have lost contact with the metal it is meant to protect. Check the mounting and contact surfaces.
The natural time to check is at haul-out and spring commissioning, when the boat is on land.
How to replace it
- Find out the model of your engine and drive – many anodes are shaped for a specific engine or drive.
- Unscrew the old anode.
- Clean the contact surface down to bare metal so there is good electrical contact.
- Fit the new anode, using new screws if needed. Tighten firmly.
- Never paint the anode. Paint, dirt and fouling insulate it and make it useless.
Always follow the engine manufacturer's manual for where the anodes are located and which ones apply to your engine.
Where are the anodes?
- Sterndrives and outboards: on the anti-cavitation plate, trim tab or drive housing.
- Propeller and shaft: shaft anode or propeller anode.
- Inboards: sometimes in the cooling system or heat exchanger.
- Rudder and hull: rudder or hull anode on some boats.
No time or hard to get at?
Anodes are often replaced by the boatyard at haul-out or during spring service. Ask for a quote and state the boat's home waters and the engine and drive model.
Frequently asked questions
Which anode should I use in the Baltic?
It depends on the salinity where the boat is kept. Aluminium works in both brackish and salt water, which makes it a common choice. If you're unsure, ask a local boatyard.
How often should the anode be replaced?
Check it every year and replace it when about half of it is worn away.
Can you paint the anode?
No. A painted or dirty anode is insulated and gives no protection.
Why hasn't my anode worn away at all?
It may have lost electrical contact with the metal it is meant to protect, or be the wrong metal for the water. Check the mounting and the anode type.
Sources
- Anoder till båtmotorer, Biltema biltema.fi
- Vad är en anod?, Watski watski.se
- Anoder – skydda mot korrosion, SeaSea seasea.se