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Replacing Your Boat's Water Heater Anode

Often overlooked, the anode in your boat's hot-water heater requires periodic replacement and should be on your annual checklist.

Galley sink with running water

Most water heaters corrode from the inside out. The first indication you have a problem may be that the tank starts to leak water into the bilge. If that happens, there's often nothing that can be done aside from replacing the water heater. But a little preventive maintenance will go a long way to protecting your tank — and wallet — from the hassle and unexpected expense of premature replacement.

There are dissimilar metals inside the tank, including the heating coils and electric element, so these may waste away due to galvanic action, unless they're protected with a magnesium anode. The anode inside the water tank works in a similar way to those on the outside; they waste away protecting metal parts of the boat, prop, sterndrive, and so forth. Except in this case, it's the inside of the water tank that they're protecting.


A good time to check and replace the water heater anode is at winter layup when you'll most likely want to drain the system.

Changing out the anode is a fairly simple DIY job and well worth the effort. I inspect mine each winter when I lay up the boat and replace it if it's more than half wasted. Removing the old anode is often the most difficult aspect of the job, especially if it's never been removed. But once the anode has been replaced, your patience will be rewarded with a water tank that will last many years. Also, as an extra benefit, if your hot water has an odor, changing the anode may help.

Replacing the anode is a fairly straightforward task, but the level of difficulty is directly proportional to the accessibility of the tank. Not all water tanks look like mine, which is a Raritan tank, but the technique will be similar. Here's how to go about it.

Technical Support

Degree Of Difficulty: Easy to moderate

Tools and Materials:
  • Pipe wrench
  • Appropriately sized wrenches
  • New magnesium anode
  • Teflon tape

Time: About 2 hours.

Cost: $40

Tuning off circuit to hot water tank

1. Turn off the circuit breaker to the hot-water tank heating element, then open the hot-water faucet at the galley or head sink to drain water from the tank. Keep the water running until it turns cold, ensuring that you won't be scalded by hot water as you disconnect the necessary pipework.

Pressure relief valve

2. After the water runs cold, turn off the breaker to the freshwater pump and open both the hot and cold faucets to further relieve pressure. As an extra safety measure, pull up the small tab on the pressure-relief valve, staying clear of its outlet. (Inspect this valve after the job to be sure it isn't dribbling.) If the tank has a drain, open it; allow water to drain. If no drain is installed, loosen the cold-water intake to allow water to drain to a level below the hot-water outlet where the anode is normally installed.

Removing plumbing connections

3. Remove the plumbing connections attached to the hot-water outlet on the tank. My boat has copper connections, but alternatives may be flexible hoses or PEX type pipework.

Using pipe wrench to unscrew hot water outlet

4. With plumbing connections out of the way, carefully use a pipe wrench to unscrew the hot water outlet from the tank. The anode is probably connected to this. It's quite likely that this fitting will be tight and stiff to remove.

New vs. old anode

5. No prizes for guessing which is the new anode. The old one was completely wasted away. I was working on a 12-gallon tank, and just to give you some idea, that new anode is 16 inches long!

Wrapping teflon tape around threads od anode

6. Reinstallation is a reverse of the procedure outlined. Wrap Teflon tape around the threads of the anode before screwing it into the water heater.

Opening water supply valve

7. Once everything is back together, turn on the pump or open the water supply valve if that's what's appropriate for your installation. Carefully check for leaks, and don't turn on the electricity to the water heater until it's full of water. Verify this by running the water until there's a steady stream coming from the hot-water faucet.

No Anode, No Problem

Some water heaters don't have an anode fitted as standard, but for less than $20 you can add one. Depending on the heater, remove the drain tap and replace it with a pencil anode on the front of the heater, which can be purchased from an online retailer or chandlery. It's a smart modification, as it will prolong the life of the heater by many years. Best of all you'll have to remove it anyway each winter to drain the tank when you winterize the water system. If it's more than one half eroded, it needs to be replaced.

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Mark Corke

Contributing Editor, BoatUS Magazine

A marine surveyor and holder of RYA Yachtmaster Ocean certification, BoatUS Magazine contributing editor Mark Corke is one of our DIY gurus, creating easy-to-follow how-to articles and videos. Mark has built five boats himself (both power and sail), has been an experienced editor at several top boating magazines (including former associate editor of BoatUS Magazine), worked for the BBC, written four DIY books, skippered two round-the-world yachts, and holds the Guinness World Record for the fastest there-and-back crossing of the English Channel — in a kayak! He and his wife have a Grand Banks 32.