When a boat is in the water, its’ underwater metal components start acting like a battery – one metal will steal ions from another metal nearby via the solution (salt water or fresh water). Left on its own, the anodic metal will eventually be destroyed. When this happens to boat components such as trim tabs, propellers, rudders, and lower units, the damage gets expensive fast. This is why your boat’s metals need to be protected with sacrificial anodes. But before we jump into which anodes to use for your boat, let’s get a little education in.

A Little History About Galvanic Corrosion

Luigi Galvani gave us the term “galvanic” when he discovered the principles of battery operation back in the 1800s. When a boat is in the water and you have water separating the two metals, a small electrical current is created. The current is not strong enough to run your TV (hopefully), but it is strong enough to steal the ions from one metal to another in a process called Galvanic Corrosion. This is why we add sacrificial pieces of metal, often referred to as “zincs,” to the metal parts of our boats.

Types of Anodes

Although boaters and sometimes the boating industry itself often refer to all sacrificial anodes simply as “zincs,” there are different types of metals used for different reasons.

  • Zinc. Zinc is usually preferred in salt water. It is great as a sacrificial piece of metal that always sloughs off while keeping fresh parts of the zinc exposed to the water so the anode to remain active.
  • Aluminum. Zinc anodes contain bits of cadmium which has become a topic of environmental debate. This has made the use of aluminum anodes in salt water more popular in recent years.
  • Magnesium. For your fresh water boaters and anglers out there, magnesium has been proven to protect your underwater metal systems better than zinc. Magnesium is used for fresh water only. If you are in brackish water, stick with aluminum.

Now that we know what’s happening under the water line, let’s put these sacrificial pieces of metal to work for us.  How many and how large a piece to use depends on surface area. Usually the total size of all your anodes should equate to 1% of the metal you are protecting.

When and How Often Do I Replace Anodes?

Once installed and doing their job, you may notice that your anodes tend to erode unevenly. Replace your anodes when about half of the metal has been corroded away. As a rule, sacrificial anodes should be replaced annually. If you are having to replace them faster than that, two things may be occurring. They are either the wrong size (too small), or you may be dealing with we refer to as a “hot dock.”

“Hot Docks” or “Hot Marinas”

Unfortunately, it seems to be more common to encounter docks or marinas that have electrical currents running through the water. These hot dock or hot marina situations usually wipe out anodes much more quickly and will require more of your attention. It’s always a good idea to check your boat’s anodes often, at least for a while after moving into a new harbor or marina. This will give you a sense of the speed of corrosion in the water for that location.

The Final “Martyr”

Now that you have protected your boat with not only the correct type, but the correct sized anodes, is there anything else you can do to save your metal bits? Well, enter the Martyr. The Martyr is one extra piece of metal that can be clamped on to your boat and tossed over the side to add to your boat’s existing anode system. Its design also does a great job counteracting stray current from a “hot marina.”

It usually comes disguised as a grouper attached to about 15 feet of cable. Simply clip the anode’s cable to a grounding strap of some kind or to your boat’s outdrive. You will know it’s working after several months, when the anode, which is molded to look like a fish, starts to look like piranhas snacked on it.   

The Takeaway

We just took the mystery away from the subject of sacrificial anodes. Knowing how they work, which size to get and what type of anode metal works best for the waters you’re in, will certainly help you better protect your investment.