[Rhodes22-list] Bow pulpit Construction Materials and Tubular metalobjects

[Ronald Lipton]

Carbon fiber is excellent in ratio of mass/modulus, important for some
things,
like reducing weight aloft, but not so important for a bow pulpit.  In
addition
CF is directional and it's strength depends of the direction of the lay-up.
For a
mast the loading directions can be reasonably well predicted.  Harder with a
pulpit.  In addition the joints between horizontal and vertical sections
would
have to be wrapped and glued, not welded or joined with a sleeve. Finally CF
is
much more brittle than stanless. When it yields it fractures - when SS
yields it
bends - much safer.

Ron
----- Original Message -----
From: "Kroposki" <kroposki at innova.net>
To: "'The Rhodes 22 mail list'" <rhodes22-list at rhodes22.org>
Sent: Sunday, February 22, 2004 7:23 AM
Subject: [Rhodes22-list] Bow pulpit Construction Materials and Tubular
metalobjects


> 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
> > __________________________________________________
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> >
> >
>
>
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