cabin sole / compression post / construction
I decided to buy the boat and fix the 3/8th" of sag under the compression post. The construction was as follows. The bulkhead plywood fits into a slot which runs down the length of the compression post. Screws hold it in place. A 3" lagbolt through the mast collar, cabin top and aluminium plate is screwed into the top of the compression post. The base of the compression post rests on top of the fibreglass cabin sole. The fibreglass rests on a 5/8" marine plywood which is used to construct the raised cabin sole in the head. In order to transfer the compression load to the hull there is a fir wood block underneath the marine plywood. The bottom of the wooden block is taper into a a rough V shape and it rests in the V shape of the hull. The sides of the block are covered in a thin sheet of fibreglass. The bottom of the block is covered with some sort of hard bedding compound.
Problems with the construction which I think contributed to the 3/8" sag (i) the marine plywood compressed a bit (ii)there was a 3/8" void between the fibreglass and the plywood comprising a 1/2" channel, which made the starboard side of the base under the compression post a bit unstable (iii) the hard bedding compound had become brittle and some of it had chipped off.
Solution: In order to expose the construction it was necessary to cut out a rectangle in the fibreglass and then chisel and cut out the marine plywood and finally the wooden block. The V in the hull was made flat by adding fibreglass to create a flat base. A teak block will be installed to support the compression post and this will be finished in fibreglass.