|Thanks for the info, I did a bit more excavation in that area today and what I am seeing pretty well matches what you have described.
Yes, the plywood has some issues but mostly OK, I am thinking that I can make it good for another season and next winter have the mast removed at haul-out time in the fall and next spring do a complete repair.
Think that water may have entered where the wiring goes through the deck. No deck fittings in that location.
Tom G.Tom Graham, Shelburne VT
"Monk" 27 Mk III