[Rhodes22-list] Bow pulpit

[Rik Sandberg]

Roger,

I agree with your assessment for the use of composites for a bow pulpit, or 
for that matter any rail or stanchion on a boat. But, I wonder, could Barney 
be thinking of a bowsprit??? I know that's not what he said, but sometimes 
folks don't yet know the proper term for a particular part. It's possible 
that this is the case here.

My apologies for making wild assumptions if this is not the case. :-)

Rik 

On Sun, Feb 22 2004 04:14 am, Roger Pihlaja wrote:
> 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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-- 
Rik Sandberg
Long Prairie, MN
S/V Happy Little Girls 2
Pacific Seacraft Flicka

All things significant are small and slow