[Rhodes22-list] Bow pulpit Construction Materials and Tubular metal objects

[Kroposki]

Roger:
	You lost me.  Are you saying that a 1" stainless steel rod is
stronger than a 1" fiberglass rod of the same shape :-). 

	If that follows, then would a stainless steel rod also be safer?
Would the margin of safety be statistically significant?  What are
breaking points in pounds (or whatever the metric measurement is)?  What
is the statistical difference? 
       Yesterday, on Lake Hartwell, a guy was attempting to launch his
MacGregor 26, which he bought on Friday.  In changing launch sites, he
was moving his boat with the mast up.  The mast hit a limb and
completely bent over.  Can we say with reasonable certainty that he
cannot straighten his tubular aluminum mast?  Can we now say that he has
a metal pretzel?
                        Ed K


-----Original Message-----
From: rhodes22-list-bounces at rhodes22.org
[mailto:rhodes22-list-bounces at rhodes22.org] On Behalf Of Roger Pihlaja
Sent: Sunday, February 22, 2004 5:15 AM
To: bestpestcontrol at earthlink.net; The Rhodes 22 mail list
Subject: Re: [Rhodes22-list] Bow pulpit

Barney,

The material properties of FRP composites (fiber reinforced polyester or
"fiberglass") do not lend themselves to building long "skinny"
structures
like bow pulpits.  For example, Young's Modulus for 316 stainless steel
is
about 28,000,000 psi while Young's Modulus for a typical FRP composite
is
only about 6,500,000 psi in the axial direction (oriented along the
fibers)
and 1,800,000 psi in the transverse direction (oriented at 90 deg to the
fibers).  Since the bow pulpit is too "stupid" to know which way the
fibers
should have been oriented during lay-up for any given load situation,
such
structures must be designed using the lower value for Young's Modulus.
Young's Modulus is a measure of the inherent stiffness of a material.
Therefore, FRP composites are only 1.8E6 / 28E6 = 0.0643 X as stiff as
316
SS.  In order to have an FRP composite bow pulpit that was acceptable
stiff,
the legs and railing would have to be so thick that there would be an
excessive amount of windage up on the bow.

In addition, FRP composites are not ductile like metals.  In other
words,
when an FRP composite is stressed beyond its yield point, there is some
internal damage.  Some of the glass fibers break and the chemical bond
between the polymer matrix and some of the glass fibers fails in shear.
This damage is cumulative and irreparable.  The next time the damaged
FRP
composite is loaded, it yields at a lower value of stress than before &
more
internal damage occurs.  However, since the damage is internal to the
composite structure, it may not be visible on the surface.  This
cumulative
damage can progress to the point where the bow pulpit might fail when
someone merely leaned on it.  In contrast, when a metal is stressed
beyond
its yield point, it can deform and bend a lot before failure.  After the
deformation, the metal is said to be strain hardened & the yield stress
is
actually greater than before the incident.  This is why it's nearly
impossible to straighten a piece of tubing back to its original shape
after
bending.  In a collision situation, the 316 SS bow pulpit might come
away
bent.  But, as long as there were no visible cracks and the mounts were
not
pulled out of the foredeck, the bow pulpit would still be safe to lean
on.
With an FRP composite bow pulpit, you wouldn't know unless you ran an
ultrasound nondestructive test on it.

Bottom line - for long "skinny" structures like bow pulpits that have to
withstand shock loading & have a safety function, FRP composites bad -
metals good.

Hope this helps.

Roger Pihlaja
S/V Dynamic Equilibrium

----- Original Message -----
From: <bestpestcontrol at earthlink.net>
To: <rhodes22-list at rhodes22.org>
Sent: Sunday, February 22, 2004 12:32 AM
Subject: [Rhodes22-list] Bow pulpit


> from Barney-- Has anyone seen a fiberglass bow pulpit on a Rhodes 22?
>
>
>
> it looks to extend about 2 ft in front of the bow. Im thinking of
installing one on my rhodes
> __________________________________________________
> Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
>
>


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