Battling Corrosion Below the Waterline: Essential Tips for Boat Owners

Examining the constant battle our equipment has with the marine environment
Three sport-fishing boats cruising across the open waters.
The battle against corrosion is literally non-stop, no matter what type of vessel you own. Scott Kerrigan / Aqua Paparazzi

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Marine environments are no joke when it comes to maintaining boats. It’s a battle we face every day, but with proper inspection, care and maintenance, corrosion can be kept at bay, if not avoided altogether.
While corrosion control is not the most interesting maintenance topic, it is for this very reason that it often gets overlooked. But overlooking the importance of staying on top of corrosion is a major mistake, because corrosion can, and will, destroy a vessel if left unchecked. Whether it’s the harsh saltwater environment corroding your metals above the waterline or galvanic corrosion eating away at your running gear, you can be assured that real detrimental scenarios are afoot here, and it doesn’t take long to cause irreparable damage.

What is Corrosion?

Scientifically speaking, corrosion is the breaking down or destruction of a material, especially a metal, through chemical reactions. The most common form of corrosion is rusting, which occurs when iron combines with oxygen and water. In boating, corrosion is the breakdown of metals due to chemical reactions that these metals face in a ­saltwater environment, happening when metals are exposed to air that contains moisture and salt. Whenever metals are immersed in salt water, galvanic corrosion occurs rapidly.

Without getting too technical, galvanic corrosion also occurs when dissimilar metals are immersed in water and are electrically connected, which causes one metal to basically eat away at the other. This is why we employ multiple sacrificial anodes (referred to simply as zincs, because that is the most popular alloy used as sacrificial anodes for a vessel in salt water) throughout our boats in locations such as on the transom, shafts, rudders and engines to help combat this corrosion. The metals we want to remain intact “steal” mass from these anodes.

ship metal bonding to prevent salt water corrosion.
Any metal component that has salt water ­running through or around it should be ­bonded. Chris Rabil

Victims of Corrosion

The most common spots on your boat that are susceptible to corrosion are propellers, cylinder heads, nuts and bolts, aluminum outriggers, towers, fuel tanks, terminal strips and battery terminals, wiring leads, steering rams, and basically any other metal components on your boat. The best corrosion protection above the waterline is simply keeping all metals clean and free of salt. If you keep everything clean and/or protected with corrosion inhibitors (e.g., Boeshield T-9, CRC Marine Heavy Duty Corrosion Inhibitor, and Tef-Gel, which is vital for aluminum items that are fastened with stainless steel), then this is fairly easy to maintain and understand.

The lazarettes of many sportboats house crucial equipment such as steering rams, underwater-light drivers, aluminum fuel tanks, water pumps, ice machines and even gyrostabilizers. Keeping your lazarette dry is crucial, but oftentimes, it’s nearly ­impossible. So, spending a good amount of time in your lazarette will not only keep it clean and dry, but it will also allow you to clean and properly coat these items with corrosion inhibitors, where necessary. Also, always be sure your aluminum fuel tank(s) remains clean and well-painted, so if it does come in contact with salt water, the aluminum itself is protected.


Where many suffer is galvanic corrosion. Galvanic corrosion is a common issue on many boats, and I have had to deal with this on the ones that I have run over the years. The best corrosion protection for ­underwater metal fittings is a well-designed and -­maintained bonding system.

The wiring throughout a bonding system should be a minimum of 8 AWG. I usually go with 6 AWG because this can also help as a lightning-­protection system. All metals throughout a boat that have salt water running through or around them—through-hull fittings, sea strainers, shafts, etc.—should be connected to a central bonding bus bar.

A metal rod running through ship systems.
Short bond runs can be used to ­successfully ­daisy-chain the connection to the central bus bar. Chris Rabil

The central bonding bus is usually a ­copper strip that runs the length of the boat where you connect directly with your AWG wire. This helps to keep the wiring runs short and provides lower resistance throughout the system. If the strip is not thick enough to be tapped properly, you can use a through-bolt with a nut to attach the wire’s ring terminal. Don’t use self-tapping screws for this because it will not ensure a proper connection to the bonding strip. As with any connection on your boat, you should coat your connections with a corrosion inhibitor to be sure you get plenty of life out it, especially because many bonding connections are in—or near—the extremely inhospitable bilge environment. Keeping your bilges clean and dry will save you a long list of headaches, even beyond your bonding system.

The bonding system is attached to a ­sacrificial anode, typically on the transom of your boat by a plate zinc. The zinc will create protection for any metals that are immersed in the same body of water. However, this bonding system will not protect those ­metals connected to that body of water via hoses, such as those on engines and generators. For argument’s sake, the water inside engines and generators is not the same as the body of water that surrounds your boat, so be sure to replace the sacrificial anodes that correspond with your engines, generators, and other various mechanical coolers (steering, transmissions).

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Sacrificial anodes that last an ­unusually long time nearly always means that your ­bonding system is not properly connected. You want to see some deterioration of your zincs over a period of time. It’s always a good idea to check your bonding system to make sure that all of the connections are sound, and that the wires have not broken or the ring terminals have not vibrated loose. It is common, however, for ring terminals to eventually show evidence of decay and break free from the wire, and would require them to be cut back to expose fresh wire and crimped onto a new ring, ensuring that your connection is solid. To increase the connections’ longevity, you can use heat-shrink to ensure a near-­waterproof connection, but inspections should never be neglected entirely. Once your bonding system is sorted, a monthly check is all it takes to keep corrosion at bay.

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