Hmm.
The "P" in GRP is a little less dense than water, but the "G" is MORE
dense than water, probably more than twice as dense. So you would
get very little buoyancy out of fiberglass, if any. For example, if
the plastic has a density of zero, and the glass was half the volume,
you would have no better than neutral buoyancy. But the plastic has
a much higher density than zero -- in fact, I think it is only very
slightly less dense than water -- a bleach bottle completely full of
water doesn't float very high.
On the other hand, the cored areas (deck and transom) WOULD be less
dense than water. I think you would have to remove all the flotation
and actually sink the boat to be able to tell for sure. Probably
older potters are different than newer ones, as well...
-- Kent Crispin "Do good, and you'll be kent@songbird.com lonesome." -- Mark Twain