I have just had the same issue? what type of yacht do you have?

It's an Allied Luders 33, built in 1971. These are the original, glassed in, chainplates. Not sure if you can tell by the photo, but underneath is an 'L' shaped flange intended to prevent it from pulling through the deck. It's welded on the edges, but since they leaked, water gathered in the void between the two pieces of ss and just sat there and corroded them. That one was rotten all the way through.

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don NV5L S/V Aurora