I have an O'day 17 foot daysailer I got at a government auction about 16 years ago that
had the same problem. Turns out the FRONT of the centerboard trunk was cracked, what had happened was the boats were rented to the sailors on base and used for sailing classes, and were run aground a lot.
The solution was to tie a line to the stern of the boat and to drag them off with no thought given to the centerboard that stuck in the sand. The centerboard would be pulled forward on its pivot and jammed into the BOTTOM of the FRONT of the centerboard trunk cracking the fiberglass at the BOTTOM of the trunk.
You will most likely need to work from the bottom of the boat to fix it, but it is very do-able.
Keep us posted.
Tom
s/v GAIA