Anodes: What They Do and Why They Matter

If you own a boat, you need to know what anodes are and how they protect your outboard or sterndrive. 

Think of anodes as small shields that protect your boat's engine from deteriorating. The metals on your boat’s motor corrode over time when immersed in water, and because the bulk of an outboard or sterndrive is made of metal, anodes are crucial to an engine's health. 

Why Outboards Corrode

Whenever two dissimilar metals are placed in an electrolyte like salt water, the metal less resistant to corrosion (base metal) starts to give off electrons to the metal more resistant to corrosion (noble metal). This process is known as galvanic corrosion.

Outboard motor corrosion

If there’s an electrical current within the electrolyte — for example, a stray electrical current from the boat’s electrical system or a poorly connected shore power cable — the rate of corrosion increases. This process is known as electrolytic corrosion. Every time you set your boat in the water, the metals in the outboard or sterndrive experience galvanic corrosion and possibly electrolytic corrosion. In other words, your motor is slowly being eaten up, and anodes are designed to prevent that from happening.

Anodes on a Boat Motor

To prevent outboard corrosion, it’s necessary to introduce a base metal that is even less resistant to corrosion than the metals that make up the motor itself. 

Outboard anodes corrosion

To accomplish this, blocks of base metal known as anodes are attached to the outboard or sterndrive. Anodes are typically made of zinc, which is an extremely effective base metal far less resistant to galvanic and electrolytic corrosion than most other metals. The anodes act as sacrificial metal because they give off their electrons and corrode before the other metals in the motor can be affected. Because corrosion attacks the least resistant metal on a boat’s motor, the anodes or zincs are the first line of defense.

Outboard engine anodes

Anodes are consumed first to prevent other parts of the outboard from getting eaten up by corrosion — hence why they’re often referred to as “sacrificial anodes”— giving off their electrons to be depleted before any other metals are targeted. Eventually there will be so little left of an anode that it ceases to be effective, which is why they need to be replaced periodically.

Boat motor sacrificial anodes

Replacing Outboard and Sterndrive Anodes

Outboard anodes are typically found mounted low on the transom bracket or under the motor’s ventilation plate, where they also act as trim tabs that compensate for prop-walk and keep the boat pointing straight.

Boat motor transom anode diagram

Anodes are also mounted into the powerhead on larger outboards and sterndrives. These anodes protect the powerhead, which requires raw water to be pumped through its cooling system. Without these internal anodes, the motor would corrode from within.

Boat engine trim tab anode parts diagram

It’s recommended you regularly inspect all anodes and replace them once they’re worn down in half, or at least once a year. 

NOTE: Inboard/outboard motors with heat exchangers use “pencil” type anodes, which should also be removed and inspected at least once a year and replaced if necessary.

Outboard engine zinc anodes

If you regularly dock or anchor near other boats, check the anodes more often. The reason being that if a nearby boat’s electrical system isn’t quite right, that neighboring boat may be sending electrical currents into the water that can eat your boat’s anodes even faster. Remember, as soon as the zincs or anodes are gone, your motor's other metal components are going to be eaten next, so you need to stay on top of inspecting them and replacing them when they're at about 50 percent.

Boat outboard engine anode protection

To find the right anodes for your outboard or sterndrive here on Boats.net, click on your motor’s make and model and use the exploded parts diagrams provided to pinpoint the anodes you need. 

NOTE: Although the exact location of anodes varies from motor to motor, generally trim tab anodes and gearcase anodes are found in the Lower Casing and Gearcase diagrams, and transom anodes are found in the Transom Bracket and Power Trim diagrams.

 

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