I think that in my case it was a combination of a 33 year old boat and (I suspect) some sort of grounding early on in it’s life. I have found evidence of a (bad) repair to internal non-structural bulkheads.
The tell tale for me was that when the boat was out of the water, the keel/hull join weeped water all winter…. Are you seeing this? Might be worth next time it is out of the water, rake out all the old sealant and monitor it. If there is no sign of water, it may be just that the sealant has cracked and got old. Clean up with acetone and seal as per below.
I wasn’t too surprised about needing to attend to keel bolts. It does happen in older boats regardless of design and make. In my case the bolts were fine, and I think what had happened is that some had slightly loosened (maybe as result of the impact) allowing some movement that had cracked the ageing sealant and allowed a weep of water in.
I couldn’t really see how to change the bolts (actually studs) anyway as they are embedded into the casting of the keel but there are enough of them and they were in good condition.
I glued the keel back on with:
qty 20 of 600ml sausages of Sabatack 750XL
1 litre of Primer 9102
2 litres Sabaclean 48
As well as making a flexible waterproof seal, it sticks like buggery and provides an enormous amount of additional strength to the mechanical join. You might want to consider this if you are re-caulking your join.
The worst bit of the job was getting at the keel nuts. I had to remove the engine, water tanks (which were rusted through due to the leak so had to be replaced anyway) and a bit of the internal bulkheads. The rear most bolt: it was easier to cut sideways into the keel to get to the nut, and the front one required removing the main mast and chiselling out a really hard mixture of iron punchings set into resin. It turned out that that was one of the ones that had been leaking – maybe as a result of impact on the forward end of the keel. Some of the nuts were only hand tight…