[Rhodes22-list] Mast Crane (was Lou Moore)

[Michael Meltzer]

I know you have it, but the seaward 26 also has it to :-)

MJM

----- Original Message ----- 
From: "Roger Pihlaja" <cen09402 at centurytel.net>
To: "The Rhodes 22 mail list" <rhodes22-list at rhodes22.org>
Sent: Thursday, March 18, 2004 5:11 AM
Subject: Re: [Rhodes22-list] Mast Crane (was Lou Moore)


> Peter,
> 
> Michael is talking about Dynamic Equilibrium's masthead.  I have my double
> backstay attachment point moved aft about 6 inches in order to allow the
> double backstays to clear the roach of my fully battened mainsail.
> 
> Mast bend is highly dependent upon how you setup the tension in the standing
> rigging.  I believe there is there is a standing rigging tuning procedure on
> the FAQ page.
> 
> However, if you modify the masthead geometry, like on my boat, the mast
> operates a little bit differently than standard.  The mast can be modeled as
> a column statically loaded by the standing rigging in compression + a
> cantilever beam point loaded at the masthead.  The standard masthead
> geometry loads the upper end of the mast pretty symetrically.  However, when
> you move the double backstay attachment point aft, you are introducing a
> factor called "eccentricity" (e) into the column loading.  Basically, think
> of e as the amount by which the compression force vector is moved off the
> neutral axis of the column.  i.e. when e = 0, the compression load is
> applied right down the neutral axis of the column & you have pure column
> buckling.  As e is increased, the deflection of the column under the load
> behaves like a combination of column buckling + bending of a cantilever beam
> point loaded on the free end.  This cantilever beam bending from the
> eccentricity is in addition to the cantilever beam bending induced by the
> standing rigging.  As far as inducing mast bend, it's the total cantilever
> beam deflection we are interested in because it's reproducible &
> controllable.  Column buckling is sort of an on/off catastrophic kind of
> failure - i.e. virtually no bend up to some critical load & then, BAM!
> buckled mast.
> 
> You might not think that mast bend is possible in a masthead rig.  However,
> you have to think about the behavior of the mast in a 3-D world.  As the
> mast is bent, the height of the masthead off the deck is also reduced.
> Although the forestay length attached to the front of the masthead doesn't
> change, because the masthead drops down as well as moving aft, the mast can
> still assume a bent shape.  This bent shape can be fine tuned by adjusting
> the tension of the 4 lower sidestays, which collectively restrain the mast
> from moving from side-to-side as well as fore-aft at about the midpoint.
> The standard double backstay tension adjuster with the standard masthead
> geometry is also capable of bending the mast.  However, because e is a very
> small value with the standard masthead geometry, the required backstay
> tension to achieve a given amount of mast bend is much higher, which
> compression loads the mast closer to the critical column buckling load.
> 
> Note, this discussion is only applicable for the standard rig.  Never bend
> an IMF mainsail mast.
> 
> Roger Pihlaja
> S/V Dynamic Equilibrium
> 
> 
> ----- Original Message -----
> From: "Peter Thorn" <pthorn at nc.rr.com>
> To: "The Rhodes 22 mail list" <rhodes22-list at rhodes22.org>
> Sent: Thursday, March 18, 2004 3:27 AM
> Subject: [Rhodes22-list] Mast Crane (was Lou Moore)
> 
> 
> > Michael et al,
> >
> > 12" mast crane?  Very interesting idea..  Would the forestay tension boost
> > with that much leverage?  Or just create mastbend above the spreaders?
> Has
> > anyone, in the life of the universe, ever created upper mast bend with a
> std
> > rig?   (You were probably thinking about clearing a big roach).
> >
> > Afterthought- Does a Harken forestay furler have a soft track?  If its
> hard
> > metal, can you somehow belly the Genoa entry for light air?
> >
> > PT
> >
> > PS - Go fasts are fun, but I'd trade them for a favorable shift.
> >
> >
> > > well now, I think I have to switch sides this year, full-batted main
> sail
> > with a 12 inch crane are the way to go, single line
> > > reefing and a soild boom vang, but IMF is more convent.
> > >
> > > MJM
> > >
> >
> > > >
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> 
> 
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