What does the anode do?
When different metals are in water and in contact with each other, a weak current flows. The least noble metal corrodes first. A sacrificial anode is a piece of less noble metal that is allowed to corrode instead of the drive, propeller, shaft and engine parts.
The anode is a consumable that should be checked every year and replaced in good time. It only protects if it has good electrical contact with the metal it's mounted on 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.
Do you move the boat between different waters, or are you unsure what your home port counts as? Ask a boatyard or engine dealer in the area.
When should the anode be replaced?
- When about half the anode is used up.
- When it's so eroded that it's come loose.
- When it hasn't worn at all. It may have lost contact with the metal it's meant to protect, so unscrew it and check the contact surfaces.
Do it at haul-out or during spring fitting-out, while the boat is ashore.
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 to get good contact.
- Fit the new anode, with new screws if needed. Tighten firmly.
- Never paint the anode. Paint, dirt and coatings insulate it, and then it stops protecting.
Where the anodes are and which ones apply to your engine is in the engine manufacturer's manual.
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, so it's a common choice. A local yard knows what is usually used in the area.
How often should the anode be replaced?
Check it every year and replace it when about half has been used up.
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 it may be the wrong metal for the water. Unscrew it, clean the contact surface and check which anode type it is.
Sources
- Anoder till båtmotorer, Biltema biltema.fi
- Vad är en anod?, Watski watski.se
- Anoder – skydda mot korrosion, SeaSea seasea.se