Galvanic Corrosion at the Dock: Why Your Neighbor's Boat Eats Yours
By Tomas Iglesias, Yacht service diver, South Florida. Reviewed by Amanda McCallister, editor.
Underwater Inspection | Commercial Diving Contractors Alabama Last week I pulled a 46-foot Silverton's prop at Bahia Mar. The wheel was so far gone I could see casting sand in the bronze, and the shaft felt like a washboard. Forty feet away, a shiny new Princess had zincs so white they looked store-bought. That is galvanic corrosion at the dock, and it ain't the water. Saltwater is just the electrolyte, like the acid in a battery. The real damage happens when your boat and your neighbor's boat share a common ground path—and every marina berth with shore power creates one. I've been diving props and hulls out of Fort Lauderdale for 15 years, from Pier 66 to the New River. Here's what I've learned about the ghost voltage that eats your metal, and how to stop it.
The Galvanic Series Is a Cheat Sheet
If you drop a chunk of aluminum and a chunk of bronze into the bay, they'll just sit there. Hook a wire between them, and the aluminum starts dissolving. That's the galvanic series—the list of metals from anode (active) to cathode (noble). Any two different metals, in an electrolyte, with a wire connecting them, make a battery. The anode gives up its electrons and corrodes. On a boat, you've got bronze props, stainless shafts, aluminum outdrives, and copper bonding wires. They're all wired together through the bonding system—that system is your friend as long as the anodes are the most active metal wearing out. That's why you put zincs or aluminum on there. They're more active than the bronze and stainless, so they take the hit. But the bonding system also connects your boat to the dock's shore power ground. And that's where the neighbor comes in.
Why Your Neighbor's Boat Eats Yours
Let's say your boat is bonded well and your zincs are doing their job. Your hull sits in the water at a nice negative potential, around -1.0 volt on a silver/silver chloride reference electrode. Now, let's say the guy next door has a leaky battery charger or a degraded AC shore cord. His boat might be at -0.7 volts, or even positive. The difference in potential wants to balance out. Since both boats have their grounds connected through the marina's shore power system, the current flows through that ground wire—not necessarily through water. But if the marina ground's corroded or the plug's loose, the current takes the next best path: through the water. And on its way, it eats your anodes, and sometimes your hull. I've seen a 2-volt difference across two boats chew a set of trim tabs off in a month. Call it stray current corrosion or just the neighbor's junk. The answer's the same: isolate your boat from the dock's ground.
Stop the Ghost Voltage
There are two solid fixes. A galvanic isolator goes in series with the shore power ground wire and stacks zinc-oxide diodes that block low-voltage DC while still passing a high-voltage AC fault. That's your $150–$300 insurance policy. Big names like Charles Marine and ProMariner make them. Just remember a galvanic isolator needs to be monitored—there's a test button on the good ones. The other option is an isolation transformer, where the dock power runs through a transformer that physically separates your boat's AC circuit from the dock's ground. That blocks all DC path and is the bulletproof fix. Companies like Victron and Northern Lights make them, but they're heavier and pricier. Depends on the plant. The cheapest step? Walk your shore power cords and see if they get warm, or check voltage drop with a multimeter. And, for crying out loud, check your zincs. A 1-pound zinc on a 40-foot boat will last six months if you're not sharing current. If it's gone by the end of the month, you're getting eaten by something else.
What a Diver Sees (and You Can Too)
I always carry a digital multimeter and a silver/silver chloride reference cell. Even a cheap $20 voltmeter works. You touch the reference electrode to the water, then connect the meter's lead to your boat's bonding system. A healthy boat will read between -1.0 and -1.1 volts. If you read less negative than -0.9, you're lacking anode protection. If you read positive, you've got serious trouble. Also, listen for buzzes, hums, or hot-metal smells in the bilge. That's a stray current to a through-hull. I once found a 12-volt bilge pump leaking current to a fitting, and that ate a propeller shaft in a year. I've also seen the opposite: a marina with bad shore-power distribution made every boat in the row lose its zincs. Zinc is the saltwater standard. Martyr and MercAnode make good ones. Aluminum anodes work too, but they get a chalky white film in salt and need a little more attention. Fix your shore power cord with a SmartPlug if it's cracked or warm. And ask your marina to check their distribution boxes for corrosion and loose neutrals—you'd be surprised how often that's the real root cause.
Frequently Asked Questions
What is galvanic corrosion in a marina? It's when different metals on boats are connected by seawater and the shore power ground, creating a battery. The less noble metal corrodes. Your neighbor's boat can speed up that process if it has a different electrical potential. How do I know if my boat is affected? Check your zincs: if they're gone within weeks, you're likely getting stray current. Measure hull potential with a reference electrode—healthy is around -1.0 to -1.1 volts. Also look for warm shore power cords or voltage differences between your ground and the dock. What's the difference between a galvanic isolator and an isolation transformer? A galvanic isolator uses diodes to block low-voltage DC in the ground wire while passing high-voltage AC faults. An isolation transformer physically separates your boat's AC from the dock's AC, completely breaking any DC path. Transformer is heavier and more expensive. Can I use aluminum anodes instead of zinc in saltwater? Yes, aluminum anodes work and are more eco-friendly, but they can form a chalky film in salt. Zinc is the traditional standard. In brackish water, aluminum or even magnesium may be better. Depends on your local water conditions.