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Jul 16, 2023

Boat anode protection: How much do I need to prevent corrosion?

PBO reader Arthur Gilbert has a question about boat anode protection. PBO's resident expert Colin Brown explains all... PBO reader Arthur Gilbert asks: “Is it necessary, and if so where, to fit anodes

PBO reader Arthur Gilbert has a question about boat anode protection. PBO's resident expert Colin Brown explains all...

PBO reader Arthur Gilbert asks: “Is it necessary, and if so where, to fit anodes on my recently acquired Leisure 20 GRP yacht? It has twin cast iron keels with encapsulated bolts but there are no signs that boat anodes have ever been fitted.

“It will be powered by a new 10hp Honda engine in the cockpit well. There is some stainless steel at the rudder, and only one metal skin fitting at the bows which could be replaced by a composite one if it might be a problem. The boat will be on a drying berth at Porthmadog.”

PBO corrosion expert Colin Brown replies: “The simple answer is that no hull anodes would be required as long as you don’t currently have any corrosion problems. The cast iron keels may get some surface rust but are unlikely ever to lose any mass.

“Keeping the rust at bay by chipping and painting will go on forever, particularly if you are on a drying berth. Some people fit anodes to their keel but you’d need anodes on both sides of both keels and get limited benefit from all the effort.

Article continues below…

Q: I was wondering about maybe going down the Coppercoat route for my 32ft steel Wylo II. Is copper antifouling…

Galvanic corrosion is a phenomenon that affects the majority of boats. If you have two dissimilar metals, such as stainless…

“Your outboard motor should have its own boat anode and you should change that when it gets to 50% wasting which might take years if the outboard is kept tilted out of the water when not in use.

“Stainless steel on a rudder or skeg might be worth protecting with a boat anode. It depends on how big the fittings are and whether there is space for an anode on the rudder or any way of electrically connecting a hull anode.

“I frequently see stainless steel brackets and bolts on rudders with no cathodic protection. Both 316 and A4 stainless steel should give reasonable service under water subject to annual inspection and replacement when required.

“The skin fitting, valve and hose tail should be reasonably corrosion resistant on their own. Cheap brass fittings might only last a few years before suffering dezincification. The loss of zinc weakens the metal and gives a distinct pink colour.

“More resistant copper alloys should last for many years without dezincification. You should check the whole anode assembly regularly.

“I test fittings by hand by giving them a good hard push and pull. This test should only be done when the boat is out of the water just in case the fitting fails.”

This feature appeared in the February 2022 edition of Practical Boat Owner. For more articles like this, including DIY, money-saving advice, great boat projects, expert tips and ways to improve your boat’s performance, take out a magazine subscription to Britain’s best-selling boating magazine.

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