Use aluminum anodes in salt, brackish or fresh water, zinc anodes only in salt water, and magnesium anodes only in fresh water. Aluminum alloy anodes protect well across all three and last longer than zinc, which is why many outboard makers now fit them from the factory. Zinc stops working in fresh water because it builds an oxide coating, and magnesium is too active for salt water. Replace any anode when it is about half worn away, and never paint one.
What an outboard anode does
An outboard’s gearcase and midsection are mostly aluminum. In water, metals connected together form a battery, and the less noble metal corrodes first. A sacrificial anode is made of a metal that is less noble than the engine, so it corrodes instead of the gearcase, trim rams and bracket. When the anodes are used up, the engine itself starts to corrode.
Zinc vs aluminum vs magnesium
| Anode | Salt water | Brackish | Fresh water |
|---|---|---|---|
| Aluminum alloy | Yes | Yes | Yes |
| Zinc | Yes | Limited | No; coats over and stops protecting |
| Magnesium | No; too active | No | Yes |
Do not mix anode metals on the same boat. Choose one type for the water you use most, and match the rest of the boat to it.
Where the anodes are
- The trim tab or fin under the anti-ventilation plate, which on many outboards is itself an anode.
- Blocks or plates on the gearcase, transom bracket and trim rams.
- Inside the cooling passages of some engines, which need checking during service.
When to replace them
Check anodes at every service and before storage. Replace them when about half the material is gone. An anode that is not wearing at all is a warning sign too: it may be painted, coated with oxide (a zinc in fresh water) or not electrically connected. Clean the mounting surface to bare metal before fitting a new one, and torque the screws so it makes good contact.
The trim tab anode and steering pull
The fin-shaped trim tab anode also offsets steering torque. If the boat pulls to one side at speed, loosen the tab and move its trailing edge a small amount, then test again. Many manuals say to move the trailing edge toward the side the boat pulls to; follow the direction in your engine’s manual, and change one small step at a time.
Never paint an anode
Paint insulates the anode so it cannot work. When painting a gearcase or bracket, mask the anodes and their mounting points.
Related
See outboard motor maintenance, how to winterize an outboard, milky lower unit oil and outboard water pump impellers. To compare engines, browse outboard motors.
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