[Rhodes22-list] Bow pulpit

[Michael Meltzer]

I thinks it is the same issues, 2 feet long is big, figure one inch depth, 6 inch wide, is asking for it, thier is also the issue
what loads are going to be on it, either a forstay or anchoring will need a brace(using the trailer eye as a chainplate?). Most
likely will get more milage from using a stainless anchor roller hanging forward a few inches.

MJM



----- Original Message ----- 
From: "Rik Sandberg" <sanderico at earthlink.net>
To: "The Rhodes 22 mail list" <rhodes22-list at rhodes22.org>
Sent: Sunday, February 22, 2004 8:59 AM
Subject: Re: [Rhodes22-list] Bow pulpit


> 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
>
> __________________________________________________
> Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
>
>