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Bruce in Bangkok[_5_]

On Wed, 05 Mar 2008 13:56:36 -0500, wrote:

I am attaching plywood bulkheads to the hull of my boat to provide
support for fuel tanks on a Morgan OI41. Best info I have been able
to assemble says to put foam under the plywood before tabbing them to
hull. Question is: What sort of foam? Thickness of foam? Seems to
md that this application would require something denser than the
standard sheet foam available in home supply stores. Any suggestions?

The foam simply acts to prevent the bulkhead from joining directly to
the hull and forming a "hard place". The bulkhead is held in place by
fiberglass tape glassed in place. As fiberglass cannot be folded
sharply a fillet is used to create a radius that the glass cloth will
form to. You could either form the foam to a radius or use filler to
form a radius.

The explanation for this is that if you attach the bulkhead directly
to the hull you are concentrating any force on an area the width of
the bulkhead. If you separate the bulkhead from the hull and then tape
it into place using at least 8 inch tape (4 inches on the bulkhead and
4 inches on the hull), on both sides of the bulkhead, you have now
spread the load bearing area out to an area 8 inches wide.

Since the foam does nothing but space the bulkhead away from the hull
any foam what will withstand epoxy will work. In fact if you could
simply leave a gap between the bulkhead and the hull and tape it into
place except that it is difficult to keep things aligned.

The so called "tape" overlaps both the hull and the bulkhead by a
minimum of at least 4 inches, and preferably more, and will be rather
substantial. Depending on the bulkhead I would use, say 3 - 4 layers
of 600 - 800 gm. roving, each layer being somewhat narrower then the
previous - say 12", 10", 8", 6". The taping being applied to both
sides of the bulkhead as far as possible.

If you do this you will have built up an attachment that is stronger
then the material the bulkhead is made from so there is no reason to
drill holes and thread pieces of tape through the bulkhead.

If you are in doubt make a test structure. Use two pieces of 6" X 12"
ply and assemble them as a "T" using the above schedule of bonding.
When it is cured try to break it.

By the way. You MUST use epoxy resin and woven glass cloth. polyester
resin and chopped mat will make a much weaker bond.

Bruce-in-Bangkok
(correct email address for reply)