On 10 Aug 2006, DSK wrote:

. . . there was a construction foam being used
to replace rotted floor stingers. It was easily
worked by hand carving to match the hull floor
and could be epoxyed to the fiberglass floor
and the floor boards could be screwed in to it.

Sort of, yes. Any foam core is going to need a rigid
skin over it to complete the structure. Molding fiberglass
stringers in place is pretty easy though. BTW "easily
worked" means different things to different people.
Some of this stuff can be carved pretty readily with
an X-acto knife or such. Some of it is better to be ground
to shape & thermo-formed.

This and the related comments below are well and good . . . provided
one actually is referring under-sole stringers the primary function of
which is mostly just to act as a buffer and adhering means between
hull and floor as distinguished from - as is common in many new boat
designs today, especially of the mid-priced production boats
(Catalina, Hunter, Beneteau) whose under-sole (supposedly)
bonded-to-hull grid system is supposed to perform the additional
important function of very substantially reducing if not fully
preventing flexing of the hull - an function that arguably is all the
more important the larger and more massive (particularly if +/- 37')
the boat.

Unfortunately, what is claimed to be fully bonded is not always that
and what appears to be solid grids may be little more than glass or
other filler only seemingly solid but (because, as DSK might also have
noted, easily covered with an apparently but not necessarily
stress-bearing rigid glassed cover).

Who was it who "defined" what is sometimes referred to as "materials
engineering" as something alone the lines of calculating the
density/componnents of materials whose properties we don't fully
understand to accommodate forces we commonly underestimate in
conditions we guess we sometimes can predict?

Suggested reading
http://www.doityourself.com/stry/foamcorepanels

....

There are about ten thousand different foams on the
market. I buy from airplane kit suppliers like
http://www.wicksaircraft.com/
In fact I have had good results using their 'Last-A-Foam'
which is very much cheaper than the high-dollar stuff like
Airex or Divinycell.
For your purpose, I'd suggest a denser foam core than for
a surface such as a deck or hull which is usually in the 4 ~ 6
pounds per cubic foot range (for comparison, water weighs
60 #/cf and wood from about 45 on up). There are many
suppliers a 18# and 25# foam, if you size it similar to wood
and put 3 layers of 16oz cloth as a skin over the foam &
filleted & bonded to the hull, your boat will be lighter and
stronger and the new floor members will never rot.

Fresh Breezes- Doug King