A Complete Guide to Boat Anodes

When it comes to protecting your boat assets, you must ensure you consider every part of it. This includes what the eye can't see, specifically the metal components underwater. Your propellers, shafts, rudders and other submerged parts are all vulnerable to corrosion due to constant exposure to metal ions in the water. 

This guide will help you understand what anodes are on boats, how they work, the types of corrosion and when you should replace anodes. 

A Complete Guide to Boat Anodes

What Are Anodes on a Boat? 

Boat anodes are also called sacrificial blocks or sacrificial anodes because they wear away quicker due to the fast rate of corrosion. They essentially ‘sacrifice’ themselves by corroding faster rather than your boat's metal, with the aim of preserving your metal parts for longer. Failure to replace the anodes may lead to these parts corroding faster, in turn warranting their costly replacement.

Anodes are made of different types of material, like zinc, aluminum or magnesium, and their purpose is to protect your metal parts by absorbing ions that damage your boat's metal during corrosion. While you might think that having more anodes is better, this could be counterproductive for their performance — it's important to place them strategically for optimal effectiveness.

Where Are Anodes Placed on Boats?

Anodes are placed underneath the boat, attached to all metal parts and must remain submerged to perform their function. They are secured by welding them to a metal hull surface or directly on metal components. Or, they are bolted and tied with brackets. Welding them keeps them tightly in place, but they are easier to replace when tied or bolted. 

For anodes to perform optimally, they should stay untouched and free of paint and other solvents when attached to the metal parts or they will lose conductivity. 

How Do Anodes Work?

Iron is evident throughout boats since it is found in metal, making it susceptible to corrosion. Water and air provide the perfect environment for oxidation and with constant exposure to these elements, a boat's metals are subject to falter. Anodes work similarly to electrolysis to bring about a chemical reaction in something that would otherwise not react. In this case, dissolving electrons in anodes protects the metal from rusting.

Their exact function is explained through the types of corrosion boats experience.

What Types of Corrosion Impact Boats?

There are two types of corrosion that impact boats — electrolytic and galvanic corrosion. Although you might predominantly hear about galvanic corrosion, it's important to understand both kinds of corrosion.


When electrolytic corrosion takes place, seawater acts as an electrolyte solvent to remove anode electrons through oxidization, forming a layer across the metal surface. The active metal reacts strongly to the electrons, thereby promoting faster oxidization. The anode electrons that have dissipated across the surface will now start corroding first since it has replaced the top layer of the metal. 

It will take longer to reach the metal surface to cause significant damage to the boat's metal components but the anodes will eventually completely corrode, leaving your metal exposed to further corrosion. 


When multiple ships are docked, the submerged metal parts in one will start conducting ions from the other boats' metal parts. The water in which it is submerged thus acts as the conductor, creating a weak electrical current. When this happens, corrosion occurs faster in the parts that are losing ions, causing substantial damage to those parts. Here, the metal that is less active corrodes faster than the other metal, also called the cathode. 


The absence of anodes will hasten corrosion, which is why they play such an important role in delaying the process. By adding anodes to the submerged metal parts of your boat, they slow down galvanic corrosion and preserve parts like the propeller, rudder and other metal segments. 

Determining Which Anode Alloy Is Best for Your Boat

The term anode might be less familiar than the term zinc in the boating industry, mainly because it was the most common material in which anodes were manufactured for many years. Three predominant types of alloy — zinc, aluminum and magnesium — require specific circumstances to function efficiently and offer the utmost protection. 


Although zinc is preferred, it is not suitable for freshwater and should only be used in saltwater. The reaction in saltwater ensures the anode's gradual corrosion so that new zinc layers are consistently exposed to the water until it is completely corroded. Freshwater would inhibit this reaction and produce a layer of zinc over the anode instead of uncovering the zinc through corrosion. 


Aluminum is more active and therefore more compatible with different water types than zinc and magnesium. Boats that dock in saltwater and brackish water are more suited to use aluminum anodes. Brackish water is basically a mixture of saltwater and freshwater, where they meet at estuaries or dams. Anodes used in these water types perform well and won't work in freshwater.


There is no in-between with magnesium as it can only be used for freshwater applications, the same way zinc can only be used in saltwater. Due to low salt concentration in freshwater, magnesium is less likely to corrode quickly. The inverse applies when used in saltwater, leading to rapid corrosion because of magnesium's low resistance to salt.

When to Replace an Anode

Depending on their size and location on your boat, you would typically allow for at least half of the anode to corrode before replacing it. If you leave it to corrode further, it will no longer offer the same protection for metal parts. For some, replacing it could be as often as once a year. In other cases, it could take two or three years before replacement is necessary. By inspecting your boat regularly, you'll be able to accurately keep track of the rate at which corrosion occurs. 

Additionally, consider the area where you dock your boat. A phenomenon called ‘hot docks’ causes increased electrical currents in the vicinity to corrode anodes alarmingly fast. This is especially evident when there is a large concentration of boats with different metal hulls in close proximity. 

You'll be able to identify a hot dock by checking the state of your anodes at least once a month. Record your findings and check them regularly for any changes. This all forms part of maintaining your boat to keep it in pristine condition and prevent loss due to damage. 

Contact Fawcett Boat Supplies for Your Anode Replacements

Contact Fawcett Boat Supplies for Your Anode Replacements

At Fawcett Boat Supplies, you're in capable hands. We have years of experience in the boating supply industry and go above and beyond to get our valued customers what they need. With parts and services, popular brands and a wide selection of products, we are sure to have what you're looking for. 

Shop online today or leave us a message and we'll get back to you to discuss your requirements. 

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