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How can I tell when my zincs need to be replaced? As near as I can tell from the receipts I got from the previous owner's daughter they were last replaced five years ago. The boat is still under shrink wrap so the only ones I can see are on the shafts and rudders. They show some pitting but it looks like there's still plenty of metal left on them.

Thanks,

What about the engine zincs?
Some engines are more peculiar than others and may require yearly changes and specific manufacturer’s standards zincs.

Rule of thumb is to replace at 50% original size.

That said...
--you don't want zincs to disintegrate to the point they fall apart...especially in motors and coolers.
--also zinc in fresh water forms a layer that insulates.

I doubt rain would have caused that layer, but if 5 seasons I'd replace as Anodes are CHEAP insurance and nothing on boats is cheap.

What Carl Said.

My zincs get changed every spring.

Every spring regardless of what they look like on the outside. Gives a chance to emery cloth the mounting surface to ensure good contact. We have two on each shaft for safe keeping. Engines should have them somewhere, likely heat exchangers. Rudders should have them or be bonded to a common plate, perhaps on transom.