Hi:
    I'm new to this forum, and it's rather interesting to me to find
that the hot thread is exactly the same discussion I'm having on another
forum.   The use of XPS foam, the commonly available blue or pink 2
pound foam in foam sandwich construction for boats.    What is striking
to me is how personal this quickly becomes, and how passionately people
will argue against something, not from actual knowledge and experience,
but quite the opposite.  There is an old saying about this, but I won't
repeat it here, as it might  not be taken well.

    One person wrote that he would not consider building a boat where
XPS was used, or buying one, or sailing on one, or even setting foot on
one......

    XPS foam has been widely used in aircraft wing construction for 40
years now.  It has excellent properties for foam sandwich with the
exception of sharp impact, where it will crush and not rebound.......Hit
it with a hammer......       That makes it unsuitable for core in areas
subject to impact.  Hull sides, bottom, or anywhere where it will hit a
dock or another boat.

     The lower density as compared to H 80.... 2 pound versus 5 pound,
and I use the 2 pound number because that is the commonly found foam,
means that you can use thicker foam without additional weight, and with
the same laminate on either side will bear far more load and be stiffer
as a result.   I can see it in coach roofs and decks, bulkheads and
beams, and interior furnishings.  It can ONLY be used with epoxy.

     The challenge is to determine where it is appropriate, and how
best to use it.    At 1/8 the cost of Divinicell H 80 per sheet of the
same thickness, or in reality 31% of the price per pound in the US, it's
well worth considering when one considers it's pedigree in aviation.  
If one is constructing using polyester resin, then one had better think
things through a bit more carefully.  It's worth remembering that a
thicker lower density core will yield a stronger stiffer more desirable
structure which in many situations is desirable.

     It is said that when your only tool is a hammer, a hammer looks
like the solution to every problem.  From "my" supplier, H-80 and H-100
Divinicell are the only core materials worth considering.

    There have been concerns expressed about delamination, and I intend
to play with it a bit, but based on what I've seen with aircraft, I
don't think it's an issue.   I happen to have a copy of Burt Rutan's
"book" Moldless Composite Homebuilt Sandwich Aircraft Construction right
here on my desk, copyright 1978..... I've owned it since about '81.  
Dick Rutan and Jeana Yeager flew Burt's Voyager aircraft non stop
without refueling around the world in December of '86.  Voyager had blue
XPS foam core with carbon fiber over it on it's 110' 8" long wings.

    All that said, I've considered delamination, as so many people seem
to think it to be a potential problem.........  not from actual
experience anybody had with delamination it would seem, but just
because..............

                                      H.W.

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