Zinc Anodes: How to Tell When Yours Are Spent
By Tomas Iglesias, Yacht service diver, South Florida. Reviewed by Amanda McCallister, editor.
Underwater Inspection | Commercial Diving Contractors Alabama Last Tuesday I dropped under a 50-foot Viking in New River, doing a quick zinc check before the owner got hauled. The starboard shaft zinc was a nub, maybe an inch thick of the original three. It looked like a worn out eraser. And that's how it goes down here: if you wait for it to fall apart, you're already buying a new shaft. I've been diving on running gear for fifteen years, Fort Lauderdale to Key West, and I've seen every kind of spent zinc there is. Some people run them down to tin foil. Others replace them every haul-out whether they need it or not. The truth is somewhere in between, and it depends on your slip, your shore power setup, and how clean your bond system is. This is not rocket science. It's looking, feeling, and occasionally measuring. Here's how I tell a boat owner when their zincs are done.
The 50% rule is about surface area, not weight
Most people have heard the rule: replace your zinc when it's half gone. That's correct, but only if you're talking about surface area, not total weight. A zinc corrodes from the outside in, so by the time it's 50% eaten, the remaining material is a lot less effective at protecting the metal around it. Call it 50% volume if you want, but I'm eyeballing it underwater, not doing the math. Ideally you want the zinc to still have a flat face, or at least a recognizable outline. When the edges start to disappear and the middle is pitted, you're reaching the end. If the outline starts to disappear into the shaft, that's a sign. The zinc should still have a crisp edge. Once the edges are gone, the zinc is basically just a lump of wasted metal. For a standard collared shaft zinc on a 1.5-inch shaft, the rule of thumb is maybe 75 to 90 days in Fort Lauderdale's warm salt water. But that depends on the boat's electrical system and how many of your neighbors are leaking current into the water.
How to check a zinc without hauling the boat
You don't need to haul a boat to check your zincs. I do it on a scuba tank, but you can do it from a dock with a mask and a strong flashlight on a calm day. For a shaft zinc, feel the whole collar. Run your fingers over it. If the surface is raised, fluffy, or crumbling, that's normal erosion. If it's smooth and flat, that's also okay, though it might not be eroding at all, which means your bonding system might be broken. Use a set of calipers if you have them. Measure the thickness of the zinc. A new Martyr 1.5-inch shaft zinc is about 2.5 inches thick at the collar, give or take. When it's half that at the thickest point, replace it. On hull and trim tab zincs, look for weld washers or mounting studs. If the metal around the fastener is getting thin, you're done. A simple set of plastic calipers is fine; metal ones can scratch the zinc.
The three kinds of 'spent' and how to spot them
There's good erosion, bad erosion, and no erosion. Good erosion leaves a uniform, sandy gray surface, like a sand castle that's been rained on. Bad erosion is uneven: one side is gone, the other side looks almost new. That means your shaft or prop has a slight imbalance, or you've got stray current coming from one direction. Check your bonding strap. No erosion is the sneaky one. If your zinc looks perfect after six months, something's wrong. The whole point of a zinc is to give up electrons so your stainless shaft and bronze prop don't. If it's not wearing, the circuit is broken. I've pulled brand-new-looking zincs off a boat that was full of old through-hull fittings and a faulty isolation transformer. And I've seen zincs that look like the surface of the moon. That's fine, actually. It's pitting and caverns, but as long as the structure is still there, you're okay.
When to replace by the calendar, not just the diving
In South Florida, I tell owners on shore power to change shaft and hull zincs at least twice a year. If you're in a marina that's crowded and the boats are older with leaking systems, make it every four months. For a boat on a mooring or that's run every week, you might stretch it to six months. But once a year is never enough in this water. Never. Digression: I can't tell you how many times I've seen a boat owner buy the cheap no-name zincs from Amazon, and they look like Swiss cheese after a month. The classic brands—Martyr, Performance Metals, and even Mercury OEM—use a specific alloy (MIL-A-18001K, so you know). Cheap zincs can be softer and erode way too fast, or worse, they don't bond well and the prop takes the hit. Don't cheap out on something that's already the cheapest insurance you can buy. The best thing you can do is schedule your zinc replacement with a haul-out. If you're on the hard, it takes ten minutes to swap out a shaft zinc. Underwater, it takes me the same ten minutes, but you're paying dive time. If you're pulling the boat for bottom paint or anodes, do it then.
Trim tab zincs and hull anodes are different
Trim tab zincs are smaller and they die faster than shaft zincs. On a boat that sits in the slip, I've seen them completely gone in six weeks. The shape matters too: the trim tab zinc is that flat, hockey-puck thing bolted to the back edge of the tab. If the edge starts to curl up like a potato chip, it's done. And the holes for the mounting screws get elongated; check that the zinc hasn't started to spin on the tab. Hull anodes are usually mounted on a bronze plate or directly on the hull if you've got a metal boat. On fiberglass yachts, they're usually wired into the bonding system. If the anode is bolted to a strap, look at the gap between the anode and the washer. If there's any light visible through the bolt hole, you need to replace it. Also, if the anode is cracked, replace it. A crack means it's been over-converted, and it's not doing its job.
Frequently Asked Questions
How often should I replace my boat zincs? It depends on your water, your slip, and your boat's electrical system. In South Florida, check them every 90 days, and plan to replace shaft and hull zincs every four to six months. If you're on a mooring with no shore power, you can stretch it a bit, but never past six months. Can I paint over my zinc anodes? No. Paint stops the zinc from corroding, which is what it's supposed to do. If you paint it, it can't protect your prop and shaft. Same goes for bottom paint that overlaps onto the zinc. Why is my zinc not eroding? That's a red flag, not a good sign. A zinc that looks brand new after months likely means your bonding system is broken, or the zinc is not actually in contact with the metal. Get a diver to check your strap and connections. What size zinc do I need? Measure the shaft diameter in inches. Yacht shaft zincs come in 1-inch, 1.5-inch, and 2-inch collars. For hull anodes, it depends on your boat's metal hull or bonding system. When in doubt, go by the manufacturer's spec.