BLUEMOMENT

I would have thought that there would be a solution to this problem if, as you say, there are a few boats with the keels dropping off.

Is there not an owners site, or somesuch, where this has been discussed and solutions expounded already?

Just a thought.

This bit, which I assume you've seen is from "Classic Boat" -

"The bilge keels are made from 3/8in mild steel plate, and can be a weak spot as they often break off. They are bolted onto longitudinal ply stringers on the hull and rely heavily on the flexibility of the ply FOR WHAT, EXACTLY. The older the ply gets the more brittle it becomes and it will need beefing up." (My red bit)

From the above, it seems to me that the problem is that the ply becomes brittle with age. This also suggests to me that simply replacing the ply will be sufficient.

I'm no boatbuilder, but in my (very) limited experience, if you make something too solid, it simply transfers the stress somewhere else.

Have you tried emailing the original designers to see what they would recommend? here - info@tucker-designs.com

HTH

Angled keels settling in soft deep mud may be the reason why some flexing is advisable. Has been a problem for some Centaurs as well.

This would imply that rigid reinforcing may not be the best solution.

How about glueing in some additional longitudinal stringers either side of the keelbolts. Don't make them as stiff as the originals so that the hull will still be able to flex in the places it was designed to flex - just not so much.

I did something similar in an old Mirror with lengths of DIY shed hard(!)wood where the aim was to stop the alarming cracks coming from the 40 year old plywood hull when I stood on it.

It worked well, unlike your avatar - unless it's a little white square with a red cross.