Depends.

If it's an open compartment, consider extruded foam, like the 2" insulation
sheets at Home Depot. Either pink (OCF) or blue (Dow) will work fine, epoxy
them in place.

If a closed compartment, why would you need to glue the blocks?

And if it is closed, will it be really sealed? Watertight for the life of
the boat? If so, use the lightest stuff you can find; just needs to float
for hours/days in the event of hull failure.

Will it be closed, but with some possibility of moisture infiltration? If
so, consider that poured-in-place foam will create an ideal environment for
rot - even a tiny amount of water will be trapped between the foam and wood.

Personally, I prefer to fill compartments loosely with light foam and/or
plastic seltzer bottles, and leave some kind of drain plug to let out the
water that always seems to accumulate. I think the soda bottles are lighter
than foam, and certainly cheaper, but only fill about 80 or 90% of the
volume.

There's a Coast Guard boatbuilder's site, but they don't have any specific
requirements for flotation materials, except that it has to be
gas/oil-resistant if in the bilge...

Sal's Dad

"Donnie Gilliland" wrote in message
...
Hello all,
I'm building a 10' tunnelhull and need to add some floatation (for my own
peace of mind) in the aft section soon. I have a question concerning the
weight and ease of use of Styrofoam blocks vs. a pourable, 2 part type.
It's
a
small boat, so, I could easily cut and form blocks of Styro to fit, but
I'd
have to find some way of gluing into place, or, would I be better off
using
a pourable type? I'm convinced the Styro is much lighter but not as easy
to
make fit snuggly into place.
How about using Great Stuff from Dow, spray it in in small
layers/quantities
to allow for a complete cure before adding more?

Anyone have suggestions? What's easier? What's more worth it? Any pourable
Mfgr's suggestions?
I'm a newby to boat construction but NOT to woodworking.

Thanks in advance
Don