On Mon, 13 Jun 2005 08:34:20 -0400, "Glenn Ashmore"
wrote:


"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

Glenn I checked out you boatbuilding page, and I do remember reading
it a while ago, I liked it, it had personality.

I have done some futher consideration. You know all the cloth dodgers
I have seen, once they are setup they are never taken down (winter
storage may be an exception), which I also know from my own
experience.

The cruising logs I have read many have opted for a "hard dodger" or
commented that they might have prefered one. I have seen a few web
pages about building one...and well they can look OK or not visually
pleasing.

Glenn are you planning on a hard dodger or a dog house on your
construction or??????

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

If you want to avoid working with stainless tube a hard dodger or bimini
might be a good alternative. You can make the form with masonite over some
shaped 2x6s. Just paint and wax the masonite and lay the glass over it.
You would need to provide for additional thickness at the edges and possibly
a ridge across the center to add some stiffness.

As I get older a nice warm hard dodger extending back over the cockpit
sounds more and more attractive but Rutu was not designed for that. We will
have a split Bimini hung fore and aft of the traveler arch and possibly a
soft dodger if the boat stays here on the Georgia so I can extend my sailing
season. In the Leewards where we hope to be when we reach the
"post-tuition" stage of life we will just use the Bimini.

--
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

Glenn I checked out you boatbuilding page, and I do remember reading
it a while ago, I liked it, it had personality.

I have done some futher consideration. You know all the cloth dodgers
I have seen, once they are setup they are never taken down (winter
storage may be an exception), which I also know from my own
experience.

The cruising logs I have read many have opted for a "hard dodger" or
commented that they might have prefered one. I have seen a few web
pages about building one...and well they can look OK or not visually
pleasing.

Glenn are you planning on a hard dodger or a dog house on your
construction or??????

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

Mic wrote:
....

The cruising logs I have read many have opted for a "hard dodger" or
commented that they might have prefered one. I have seen a few web
pages about building one...and well they can look OK or not visually
pleasing.


My boat has a "hardtop" which is really a 'glass platform supported by
heavy steel tubes. Its strong enough to walk on, large enough for
three solar panels, and ridged enough for the main traveler. Its
surrounded by canvas, with large windows and flys screens.

The way our cockpit is laid out, this has become an extra room. With
the full canvas surround, its possible to sail in freezing weather;
when the Sun hits the windows, it becomes a greenhouse.

The downside is that we tend to leave too much canvas up. Those who
sail these boats down south often remove all of the canvas for full
ventilation.

Its construction is far beyond my capabilities, especially since
windows and gutters are molded in, and carbon fibre is used to stiffen
it. If I had to do something like this myself, I would have someone
fabricate a strong frame in stainless, and then sew my own canvas
around it.

Here's a picture taken late in October, when the average gust in 18 knots:

http://www.sv-loki.com/Along_the_Way/surround.jpg