Welcome to the SBO forums!
Adding a few pictures to your post would help members make specific suggestions to answer your question. As Johnb mentioned it would be wise to know how the board is lifted prior to cutting into the centerboard trunk. If that cut is done above the waterline and done small enough maybe a round, threaded inspection plate could be positioned in a way to do your repair and allow future access.
Does the weight of the boat preclude lifting the boat off the trailer to lower the keel/centerboard? (and....as the hull is lifted the centerboard might lower and stay on the trailer) There have been several discussions of that activity. You could probably find those discussions by using the "Search" portion at the top bar of the page.
Best Wishes!
Thank you, 31seahorse. I'll try to figure out how to post photos in my posts. Do I just paste them inside the post box?
The boat weights 2900 pounds and we can lift it with hydraulic jacks, but I doubt we can get it high enough to safely get under it to remove the keel. We may build supports out of four by four wooden posts that could support the weight. We plan to place the boat on a modified boat lift which will expose the swing keel once we place it on the lift, but that would be over water several feet deep.
I did find a drawing of the Yankee 24 on the Internet today that shows where the cable is connected, which is right at the top of the swing keel when it is in the down position. In the up position, where the keel is now, that connection ends up right next to the area where the cable goes through the hull next to one of the bulkheads. We may be able to open up that area where the cable goes down through the hull. I'll show the drawing to my son and see what he thinks. The rope appears to be in good shape, but we want to change it to a stainless steel cable and stainless connector at the keel. If that swing pin ever came loose the rope would not be strong enough to prevent the keel from going to the bottom of the lake, where the steel cable could easily support the 250 pound load.
One photo in the Yankee 24 brochure shows the boat on its side in shallow water after being pulled down by placing a rope at the top of the mast and pulling it from the side. I guess that would be one way to get to the swing keel and get it out, assuming we don't swamp the boat in the process.
Here is the link to the Yankee 24 brochure:
http://www.dolphin24.org/images/yankee24bro.pdf