A point on this subject that may help.
Before the foam block was put in the aft tank it used to be all under the side decks, as in it filled the gap under the sidedeck from aft tank to chainplates.
It was contained by a GRP cover.
It was then found to be to much bouyancy at the widest point of the boat when capsized and made the boat float high in the water.
So the amount was reduced and put in the aft tank,so keeping the same total but distributed about the boat with better result.