[Rhodes22-list] Bow pulpit

[Roger Pihlaja]

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
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