Boats & Zinc Anodes: Important Answers For Beginners

What are zinc anodes, and what is their purpose on a boat?

This article will go over zinc anodes, what they are, what their purpose is on a boat, and several other boating questions that you may not have even known you had.

Without further ado, let’s dive into those boating questions!

What Are Zinc Anodes?

Zinc anodes are essentially a metal attachment to the boat’s underwater part, often on a propeller shaft or the propeller.

Basically, a zinc anode is designed to be the thing on your boat that corrodes instead of any steel or other metal components of your boat – like the engine and propeller.  

Why would you want the metal to do this?

You want to prevent a process called Galvanic Corrosion in your engine and drivetrain.

What is Galvanic Corrosion?

When two different metals physically connect in seawater, an electrical current flows between them like a battery.

For this current to exist, one of the metals has to give up pieces of itself.

The reason zinc has to give up pieces of itself is because electrons are created as the metal dissolves, generating the current between the two metals.

This deterioration of metals is what’s known as Galvanic Corrosion.

Let’s go over why you put zinc anodes on boats.

What is the Purpose of a Zinc Anode on a Boat?

As we mentioned, zinc anodes aid in keeping the important metals on your boat from corroding.

Your boat will typically have at least two metals built within it, but zinc is a third metal added for it to be intentionally sacrificed. Basically, the zinc will break itself down instead of the other two boat metals.

Zinc anodes are the unsung hero of protecting your boat’s metal and mechanical equipment.

Since your boat will often be on the water, having a zinc anode is necessary to keep your boat’s engine and propeller lasting a long time.

What Are the Different Types of Anodes?

Now that you know what zinc anodes are, it’s important to have a working knowledge of the other types of anodes you can put on a boat since all boats are not created equal.

Knowing what kind of metals are on your boat will help you know what kind of anode is appropriate for your boat and protect your boat.

Let’s go through the different kinds of anodes so you have an idea of what anode would be a good fit for your boat:

Zinc:

The first type of metal used for an anode is zinc, which we have covered.

This is the most common since it is the easiest to attain.

Zinc works pretty well on most types of boats and will prevent corrosion of your boat’s metal parts in saltwater quite well.

However, zinc is also a fairly heavy metal and has been shown to not be entirely beneficial to the environment.

Aluminum:

The second anode material is aluminum.

Aluminum has recently started to replace zinc as the more commonly used anode on boats used in freshwater.  Aluminum is a light-weight metal and is versatile in more than just saltwater.

In addition to being a more versatile anode, aluminum also lasts longer than zinc.

Research has shown that aluminum will last 50% longer in saltwater than will zinc.

Magnesium:

Magnesium is the third anode.

Magnesium is the most active anode, meaning that it has the shortest lifespan of the three anodes.

However, magnesium has been shown to be very non-toxic and harmful to the environment and aquatic sea life.

Which Anode Should I Choose?

The type of anode you choose will depend on which type of water your boat will be in.

There are three different types of water for anodes: saltwater, freshwater, and brackish water.

Let’s go over which anode is the most ideal for each water type:

Saltwater:

In saltwater, aluminum and zinc will be great choices.

Aluminum will last a fairly long time in saltwater and will provide your boat with excellent protection.

Zinc won’t last quite as long but is much easier to attain, so it may be the more practical anode to use in saltwater.

Brackish Water:

In brackish water, aluminum anodes will be the ideal anode to use.

Aluminum doesn’t corrode nearly as fast as magnesium and will protect your boat against brackish water much better than zinc.

Freshwater:

In freshwater, magnesium is your best anode to use for boat protection.

The material is very conductive in freshwater, so magnesium is ideally suited to protect your boat in freshwater.

If you are more likely to be driving your boat in saltwater, zinc, or aluminum will be your ideal anode choice to ensure your boat’s metals are properly protected.

How Many Zinc Anodes Do You Need?

This number won’t be the same for every boat operator and driver, as different boats come in different sizes and materials, such as steel vs. aluminum hulls.

To find out how many zinc anodes you need for your boat, you will need to know your boat’s size, particularly its weight.

Then you need to calculate the amount of hull surface area that will be touching water and the density and energy content.

This is a formula that was pulled from www.boatzincs.com, which illustrates the complexity of this formula.

Anode Weight (lbs) = [(Wetted Surface Area) x (Current Density) x (Immersion)]
                                          [(Energy Content) x (1000 mA/Amp)]

If you are a beginner, it is probably better to contact your manufacturer or provider, who can walk you through the process or calculate it for you.

Can You “Over Zinc” a Boat?

You can “over zinc” your boat if it is made of steel or aluminum, and it will lead to significant damage if left on your boat over a long period of time.

If your boat is made of fiberglass or has material made of fiberglass, then over zincing won’t be as big of an issue.

However, if your boat is made of more traditional metal material, then over zincing is something you’ll want to keep an eye out for.

Can Zinc Rust in Water?

Zinc can “rust” in water, but it is really corroding to a white color while the other metals do not rust or corrode.

This is why it’s an ideal anode for protecting your boat’s exterior and all of the other metals on your boat.

Zinc truly acts as a silent protector for your boat.

When Should You Replace Zinc on a Boat?

Zinc won’t last forever, and you will need to replace it regularly.

When exactly do you need to replace zinc to optimize your boat’s protection?

Since the anode will provide more protection when it has more mass, you’ll need to monitor your zinc anodes’ weight.

When the zinc anode has dropped in about half of its original weight, it’s time to replace the zinc with new anodes.  If it breaks apart when you try to loosen it, it needs to be replaced.

Ideally, you can expect a zinc anode to last about a year if the boat is kept in the water year-round.

This will be impacted by various factors, including driving time, water conditions, and much more.

Do Zinc Anodes Work in Freshwater?

In the battle of which anode works best in different water, it was concluded that magnesium was the winner of the best anode for freshwater.

Unfortunately, zinc is far less useful in freshwater than in saltwater.

Due to the low-current conditions in freshwater, zinc will be unable to provide adequate protection of your boat’s metals.  

However, if you find yourself driving your boat in freshwater, magnesium is the best choice.

How Long Do Boat Anodes Last?

This is not an exact answer as the number will be affected by several differing factors:

  1. We determined that zinc should last on your boat about a year or more.
  2. If your boat anodes don’t last at least a year, you may need to get new or heavier anodes.
  3. Ideally, boat anodes last as long as they take to get down to about half of their weight.
  4. Whenever your anode is this size, you’ll want to replace the anode as promptly as you can.

A few things that affect how long your anodes last include the type of water in which you drive, the type of boat you have, and the frequency at which you drive your boat.

Do I Need an Anode on My Boat?

Even if you drive a small boat, protection from corrosion is essential to maintaining a long lifespan.

Using an anode is crucial, and your boat should have anodes, no matter what kind they are.

As long as they are best suited for your boat and the water type, anodes should definitely be on your boat or your engine if you have an outboard.

References:

About Corrosion –  nautechmarine.co.nz

Anode Weight Calculation Formula

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