JB wrote:
It was mentioned to me that 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.

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

.... Unfortunately, my
friend didn't know the name of the foam. If anyone has heard of this
foam ,its properties, and where to get I would appreciate the
information. Or any information on replacing stingers on a 19' Oday
Rhodes.


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