My BC20, Seatern, lives at a jetty where it dries to mud. On two occasions in the last 4 years I've had to work the downhaul/uphaul combination back and forth to get the centreboard to drop (the BC20 board is not ballasted except what's needed to stop it floating).
Peter
Found i needed Peter's up-down jiggle manoeuvre to free up my jammed board last weekend. My board would not drop on departing the mooring at high tide.
Thank you Peter!
In fairness to the boat, she had been dried out completely for an afternoon on a sand bank over low tide, during which time i had been clambering about getting a few odd jobs done.
It had jammed up, and was very solid, resisting a straight pull, but luckily with about 4" of free play in the up/down cordage, i managed to get the board to drop.
A technique worth remembering!