[Rhodes22-list] Bow Pulpit Material Of Construction

[Roger Pihlaja]

> Barney,
>
> I just reread your original post & I may have misunderstood you.  When you
> say "bow pulpit", are you referring to a short platform extending forward
> from the bow, usually with a roller on the end used to stow, launch, &
> retrieve an anchor?  In my original reply, I thought you were referring to
> the metal railing up on the bow.
>
> Assuming you meant an anchor platform, there is no reason why it couldn't
be
> built of FRP composite.  However, just realize that the forces on such an
> anchor platform can be quite large.  Be certain the anchor platform you
> purchase or build is designed and built to withstand the loads it will be
> required to handle.  In addition, be careful about how you attach it to
the
> foredeck.  The anchor platform will need to be thru bolted with big bolts
on
> close center-to-center spacing with big backing plates under the foredeck
to
> distribute the loads.  The foredeck is cored with plywood, so all the thru
> bolt holes will need to be sealed with epoxy to prevent water intrusion
into
> the plywood & subsequent rot.
>
> Alternatively, the anchor platform could be glassed into the hull.  If you
> opt for this fabrication method, be certain your knowledge of FRP
composite
> lay-up design and construction techniques is adequate to provide a
> sufficiently strong joint.
>
> The fore/aft trim of the boat will be affected somewhat, especially if you
> routinely sail with an anchor stowed on the platform.  The Rhodes 22 is
very
> sensitive to fore/aft trim.  The extra weight will contribute to a bow
down
> condition.  A slight bow down trim is usually faster in smooth water.
> However, a bow down trim condition in a heavy sea can leave the boat with
> insufficient reserve floatation at the bow to "punch" thru waves.  In
minor
> form, this can result in a wetter, slower ride and a reduction in the
boat's
> ability to beat to windward.  More extreme cases of bow down trim in waves
> can result in "submarining" the bow into the wave instead of riding up and
> over the wave.  The hydrodynamic forces on the anchor platform and
foredeck
> during such a submarine exercise will be huge & could easily rip the whole
> assembly right off the boat.  Obviously, this could ruin your whole day.
>
> Finally, please realize an anchor platform represents a fairly large
surface
> area for windage at the extreme front end of the boat.  You can expect it
> will introduce a significant amount of lee helm and make the boat more
> susceptable to getting caught in irons when coming about.  It may also
> affect the boat's ability to maneuver in close quarters in windy
situations.
>
> Only you can decide if these potential problems are worth the salty look
and
> convienence of launching, retrieving, and stowing the anchor.
>
> I'm sorry if I misunderstood your question the first time around.
>
> Roger Pihlaja
> S/V Dynamic Equilibrium
>
> ----- Original Message -----
> From: "Roger Pihlaja" <cen09402 at centurytel.net>
> To: <bestpestcontrol at earthlink.net>; "The Rhodes 22 mail list"
> <rhodes22-list at rhodes22.org>
> Sent: Sunday, February 22, 2004 5:14 AM
> 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
> > >
> > >
> >
>