Spotting Stray Current at a Dock Before It Eats Your Boat
By Tomas Iglesias, Yacht service diver, South Florida. Reviewed by Dana Whitcombe, technical reviewer.
Cathodic Protection & Anodes | Offshore & Subsea Diving June 17, a Tuesday, at Pier Sixty-Six Marina in Fort Lauderdale. A side-tie slip, water and diesel sheen. I was hauling a 53-foot Hatteras for a survey. The owner met me dockside, said the port shaft zinc was 'gone again' — he'd replaced it three weeks earlier. When the slings lifted her, the bottom looked like a bad moonscape: peeling antifouling, clusters of pimples along the chine, and a fresh zinc that was already showing white powder. That wasn't normal galvanic corrosion. That was stray current. It's DC electricity leaving the dock system, entering your underwater metals through the water, and eating its way to ground. It can strip a prop shaft zinc in weeks, blast the end-play to your rudder, and leave you with a leaky stuffing box. The owners blamed the zinc quality. The marina blamed the paint. I blamed the dock. Here's what I look for, in the same order I look, on every job.
Inspect the Zinc Pattern, Not Just the Zinc Mass
Normal sacrificial anodes erode evenly. They get pitted, granular, get held together by a wire. Stray current makes them look like they've been dipped in acid: powdery white, with a radial pattern of craters and a sharp chemical smell if you get close. If you have more than one anode—shaft, hull, rudder—compare them. A single anode that's been zapped will be far more consumed than the others. That asymmetry is the clue. Also check the prop nuts. If the bronze nut is showing copper streaks, the zinc is gone and the prop is now the anode. That's your first sign.
Measure the Potential Difference with a Half-Cell
A decent multimeter and a silver/silver-chloride reference electrode will cost you under $200 and pay for itself. Ground the meter to a clean point on the bonding system, dip the reference over the side, and read the DC volts. A healthy boat sits around -900 to -1100 mV. That's negative. But you're not really looking at the absolute number—you're looking at flapping. If the needle bounces more than 50 mV, there's an alternating current component in the water. That's knock-off from a stray line. To confirm, switch to AC and see if you get any reading above a few tens of millivolts. Zero is normal. If you see any AC, you're contaminating the water surrounding the hull.
Sniff for the Chemical Signs
Saltwater electrolysis makes chlorine, just like a chlorinator cell in a pool. If you lean over the stern of your boat at a dockside and get a faint bleach smell, that's not clean marina water—that's electricity breaking down chloride ions. You'll often see rising bubbles off the shaft or rudder when there's no propulsion. Do this at low tide, when everything's exposed and the current paths are concentrated. And check the shore power cord and pedestal for ozone smell, which is arcing. Both smells mean there's a live wire touching water somewhere nearby.
Interview the Dockmaster About Your Neighbors
Ask the dockmaster: 'Any other boats losing zincs faster than usual?' and listen for hesitation. Boats with isolation transformers are usually insulated, but the older ones with just galvanic isolation can pass DC to the water. If the marina has had a cluster of failures on one dock, that's a fault in the shore power or a boat that's leaking current continuously. Also look at the power pedestal for scorch marks, a tripped breaker that someone flipped back, or a missing cover. A proper marina will have a maintenance log; many don't. If they act surprised, go inspect the adjacent live-aboard.
Run a Bonding Audit on Your Own Boat
Before you blame the dock, verify your boat's bond. Stray current will follow the path of least resistance—if your shaft isn't bonded to the common ground, current can go through the cutlass bearing instead, and you'll see nicks on the shaft. Check your shaft brush is actually touching, that the bonding wire goes all the way to the engine block, and that your shore power's green ground is intact. The quick check: try to measure continuity between the shaft and the engine negative while the boat is out of water. If you get more than 1 ohm, you have a bad bond. That turns any nearby current into a one-way ticket for your running gear.
Frequently Asked Questions
What's the difference between stray current and galvanic corrosion? Galvanic corrosion is the natural voltage difference between two dissimilar metals—bronze, stainless, and aluminum—largely normal and handled by zincs. Stray current is electricity that's supposed to stay in wires but is leaking into the water through a bad connection. It's far more destructive and needs to be found and fixed. Will adding more zincs protect my boat? No, it just hides the problem for a little longer. The current is coming from somewhere, and the zincs are just a cheap sacrificial point. If you keep replacing them, you're feeding the leak. Find the source or install an isolation transformer to break the loop. Do I need a marine electrician for this? You can do the visual and sniff tests yourself, but if you're seeing these signs, a marine electrician with a clamp meter and a decent test box should be on the job. They can trace the exact circuit and make the repair permanently.