Thanks guys, I think I can do this

So what I am thinking is that I cut the hole in the deck over the top of the bulkhead so that I have access to both sides of the bulkhead, so I can I unmount the chainplate, strengthen the bulkhead and remount the chainplate. If cut this in a rectangle or oval, I can laminate a second peice of ply to the underside of the peice I removed which will be glued to the underside of the hull around the cutout. By ' flairing' , I am guessing this means sanding the surrounding area around the repair so that I can blend in the paint?

Does the deck rest on the bulkhead and will they be glued together? Have I interpreted this correctly, or do you suggest I cutout the hull where the chainplate exits the hill?

Excuse my ignorance, this is all new to me!

Thanks again,
Andrew