A friend working on an aluminium Saugeen Witch told me they put one on a tide grid. checked her out , then went to the pub. It had 1 inch of sprayfoam in in. When they got back they found that they had forgot to close a thru hull
. The boat was full of water
and still had a foot of freeboard. They took a picture of several of them standing on deck
with the boat sytill loating high.That was Bruce Cope , of Cope Aluminium boats in Parksville, BC.
I remember a picture in a British Yachting magazine a few years back of a foam cored 48 foter , similar to a Maple Leaf 48 which ad been holed , yet refused to sink due too the foam core
in her. They towed her to a yard ,still full of seawater and still floating.
Its possible to make many boats unsinkable if you put an inch of foam over the entire interior
, rather than a few big lumps o ffoam.