Hallo Martijn
It's all there on the forum in 2014. I wrote about it at some length, partly because I'm a bit of a dramatist I suppose, but also because I learned a lot from the experience and thought I'd pass it on (one of the useful things about a forum I suppose).
My Trouper is 12 ft and very stable, but I managed to capsize her! I was on my own in a F4-5 (one reef down). Before I could right her she turtled. Initially the dagger board was sticking up helpfully, but as I reached for it, it slid down into the hull. Even if I'd dived under the boat (I'm not a super swimmer) and got it up (it would have been up) into the DB case, there was nothing to keep it there.
The most interesting part of it all was that I was about 200 yards from the shore. I waved, people waved back!! It was 45 minutes before it occurred to someone to dial 999. I got mild hypothermia, but no damage to me or the boat (recovered by RNLI).
My modification was simple - a series of deep notches in the after side of the (wooden) DB, and a stout taut shock cord loop round the after end of the DB case, with an extra loose loop to make it easy to adjust it. It's also made adjustments during sailing much better. I'm sure the DB couldn't slide back into the upturned boat now.