---

Someone in Australia with a Dell Quay Dory (much the same as a small Whaler) said that he'd cured the problem this way:

"Simple fix is a 5 inch hole cut through the transom, remove the wet material, squirt in some solvent to dissolve/remove any remaining foam, allow to dry and fill the void with foam pool sticks then repair the hole, done in a day

Expanding foan will not provide a lot of support at all, once crushed it will not fully return to it's original size.

Closed cell foam like the stuff in the pool sticks, these will retain their shape provided that are not compressed for months.

As for what to use to remove the old foam, it will depend on it's chemistry.

Most of them will melt if petrol or thinners is applied, neither will effect your fibreglass, there are lots a vehicles with fibreglass fuel tanks going back as far as the Goggomobil.

Acetone will also get rid of it but prolonged contact with fibreglass is not good, it tends to soften the surface after about 12 hours, But does a great job of residue of the melted foam, however is it expensive, so persevere with the petrol or thinners and will eventually remove the residue.

You can speed things up by mechanically removing most of the old foam with scrapers.

The pool sticks definitely work and have been used for many years for this task without problems"

Sounds a bit too good to be true, but then it's clearly worked for him ! Not sure how you'd remove the foam from further forward unless it completely dissolved into mush and then a pressure washer may fetch out the residue. And this must mean that there are no stringers / compartments in the void - no idea about this offhand, maybe Whalers are different from Dell Quays.