Outboard Anodes: What They Are & What They Do

So what is an anode on a boat? What do anodes do? Where are the anodes located on an outboard motor?

In short, outboard anodes are metal pieces attached to the outboard’s transom bracket (or under the motor’s ventilation plate) to protect a boat’s engine from corrosion.

The idea of the outboard anode is roughly that it’s more ‘active’ and corrode faster, therefore saving other metal parts immersed in water (e.g. propellers). That’s why outboard anodes are also called “sacrificial anodes”.

The purposes and functions of outboard anodes are sometimes more than protection.

The trim tab, which is also a zinc anode, is also used to compensate for prop walk and prop torque, making the outboard easier to steer.

The first thing you need to understand is galvanic corrosion.

When you submerge two dissimilar metals in an electrolyte (like saltwater), it forms a weak battery where some current flows between the two metals.

The two dissimilar metals come with different spots on the nobility scale:

• Base metal (cathode): the metal that’s less resistant to corrosion
• Noble metal (anode): the metal more resistant to corrosion

Galvanic corrosion occurs when the base metal gives off electrons to the noble metal to make up the currents.

To counteract galvanic corrosion, we need a third metal that’s even less resistant to corrosion. We call it “sacrificial anode”. When we add the anode to the circuit, it will give up its electrons quicker than the other metals to prevent corrosion.

Outboard Anode Simplified: Just imagine that you are the outboard, the water is a lion’s pen at the zoo, and the anodes are a juicy steak.

If we were to drop you into a lion pen without the steak, you’d be dinner. However, with the steak you can safely roam around the pen, with the lion distracted until the steak is gone.

Generally, boat corrosion anode comes in three materials: magnesium aluminum or zinc.

You may wonder: Which is better zinc or aluminum anodes?

In fact, the preferred type of outboard anode is determined by where the boat is used.

Here is a quick list of what type to go for your reference:

That’s to say, if you run in saltwater a lot, you will need to go for outboard engine zinc anodes for the best protection.

As you can see from the chart, different outboard anode materials are used depending on whether it’s in freshwater, brackish water or salt water. That’s for good reasons:

• Zinc is less aggressive, so it’s perfect for saltwater with a slower dissolve rate. However, in freshwater (which is a poorer conductor) it wouldn’t be aggressive enough to ensure that it’s the anode all the time.
• Magnesium anodes are the most aggressive, so they are great in protecting in fresh water, but they would work so well in saltwater that they’d overprotect and dissolve too quickly.
• Aluminum alloy is somewhere in between – more aggressive than zinc, and less aggressive than magnesium. That’s why it’s perfect for backlish water uses.

Since we’ve all understood the importance of outboard anodes, we need to periodically check and replace the anodes to make sure that we are always in effective protection.

So when to replace boat anodes? How often should a sacrificial anode be replaced? How do I know when my anode needs replacing?

As a rule of thumb, it’s recommended to replace the outboard anode when at 1/2 of its original size, which is typically once a year.

When to replace an anode essentially depends on a range of factors, including usage, location, and the anode materials, etc.

So it’s better to check the manual (usually available download on the official website) and see what the manual shows for anode replacement in terms of hours.

For example, ePropulsion requires you to check or replace the outboard anodes every 200 hours or 12 months.

Then here comes your next step: how to change the outboard anode?

The outboard motor anode replacement steps could be different across different brands (Yamaha, Suzuki, Mercury, Honda, etc).

If you want to make a worry-free replacement, always read the manuals and follow maintenance instructions.

Some reputable brands offer outboard anode kits (with worldwide shipping) to make the replacement easy for you, including ePropulsion, Yamaha, Mercury, and Suzuki.

Here are some tips for you to smoothly replace outboard anodes for any outboard motors:

Tip #1. Ensure Good Electrical Contact

There is a common misconception that outboard anodes can be mounted anywhere.

Unfortunately, this is not true!

For an anode to be effective, you need to make sure the anode is in electrical contact with the metal being protected. That’s to say, you need to either mount the outboard anode directly to the metal being protected or connect the two metals with a wire.

Tip #2. Don’t Paint Outboard Anodes

Remember not to paint over sacrificial anodes, otherwise they will become useless. That’s because outboard anodes only work when they are exposed. They need to be in contact with the water.

Tip #3. Mark Orientation Before Replacement

Before you replace the trim tab anode, mark its orientation and alignment on the cavitation plate so that you can easily find the sweet spot again.

Remember that this sacrificial anode also balances your steering. If you install the outboard anode slightly misaligned to its previous position, you’ll feel it in the boat handling and adjust accordingly.

1. Do freshwater boats need anodes?

Yes, the outboard anodes are needed even for fresh water only immersion.

The outboard engine anodes are always a good idea to reduce corrosion of other metals immersed in water, especially if you have a metal boat or batteries aboard.

Saltwater is no doubt more reactive, but we see a lot of corrosion in freshwater as well. If you are always in fresh water, magnesium anode would be a better choice.

2. Do I need an anode on my boat?

Can you run a boat without an anode? As mentioned above, you should always have anodes on your boat for the best protection. Without outboard engine anodes, underwater metals will be at risk of galvanic corrosion.

3. How long do outboard anodes last?

As mentioned earlier, the lifespan of an outboard anode depends on a lot of factors including usage, location (of your boat), and its material. Typically, a boat’s corrosion anode can last for a year. If your outboard motor anode lasts less than a year, you may get one with more weight to increase its longevity.

4. How to clean the boat anode?

In fact, boat anode cleaning is not necessary since the anodes sacrifice ions as they corrode. The build-up on the surface is normal and it does not need to be cleaned off. You just need to make sure the surface is not painted for better efficiency.

5. How to adjust the anode on the outboard?

Outboard anode adjustment is easy. Take a test run and simply adjust the trim tab in the direction it is misaligned. That’s to say, if the steering pulls right, you need to adjust the trim tab to the right, and vice versa.