"Howard Peer" wrote
Idle curiosity acting here. Suppose...............you filled the PVC (or
copper or whatever) with urethane foam. You know, the foam they sell for
filling voids around windos at Lowes and Home Depot. I think that they
now actually sell it in a couple of different formulations. Anyway, the
stuff is a closed cell foam. I think it is pretty tough in compression
and does not add much weight.

Just wondering.

Not to get to far into the physics of materials but bending stress is the
primary force acting on a Bimini frame. As a tube is put under stress the
top will go under compression and the bottom tension. The sides and
anything inside will be under varying amount of tension and compression that
reduces to zero at the center or "neutral axis".

The ability of a beam to resist bending is determined by the strength of the
material, the shape and the distance of the furthest fiber to the "neutral
Axis". The strength is expressed as the "Modulus of Elasticity" or how much
a material will deform (strain) for a given amount of force (stress). The
shape is expressed as the "Moment of Inertia" which is more or less the
amount of material and its average distance from the neutral axis. (Greatly
over simplified)

Some examples of Modulus of elasticity a
Stainless Steel 30,000,000
Hard copper 16,000,000
ABS 331,000 (about 1/100 of stainless)

Examples of Moment of Inertia:
1/16" wall 1" tube .(stainless) .0203133
1/8" wall 1" tube (ABS) .0335558
Solid rod .0490874

Given that all are 1" in diameter the maximum distance to the neutral axis
is the same .5" so an ABS pipe will be about 1/50 as stiff as a stainless
tube. Notice that the Moment of a solid rod is only 45% greater than a 1/8"
wall pipe. Filling a tube with a material with good compressive strength
will prevent the tube from crimping but will do very little to keep it from
bending.

Also the corners must be considered. The large radius corners of a
stainless frame spread the bending load over the whole arc while a typical
ABS plumbing sweep Ell will concentrate the stress at the ends of the tube.
This is especially bad for copper as the ends will be annealed from the heat
of soldering.

Considering the number of mangled Bimini frames you will find in any marine
salage yard, an ABS frame would never survive even the slightest accidental
knock.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com