So, the center plate is a problem.
Clearly the best thing to do is to remove it and cut another. However, no-one seems to know how to do this. I have consulted various oracles in England and they have been strangely evasive. The problem lies with the location of the center plate pivot bolt. The plate has to swing up and down around a pivot, which is, it seems, hidden behind layers of fibre glass.
We take off the wooden center plate surround on the keel. This is not easy.
Taking it off does not really advance me in the search for center board bolt but it does give an injection of romanticism.
In all my readings of voyages to the tropics, I have always been struck by the danger of the Teredo worm. Said worm burrowed into the wood of boats and basically ate them. This was a major problem. https://en.wikipedia.org/wiki/Teredo_navalis
Anyway having totally exposed the centre plate, I decide that we will repair rather than replace.