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

[Peter Thorn]

Roger,

Thanks for the detailed explanation.  I am the child of an ME, but not one
myself.  All my engineering is pants seat.  I'll add "eccentric loading" to
my sailing jargon vocabulary.

So, how much tension can you add to your forestay by adjusting your
backstay?  Can the 12" crane actually put more tension on forestay than std
crane can?  We all know a light beam straight entry points much better.

And, will your Harken furler track allow the forestay to sag and power up
(for light air)?

For any boat with sail reduction capacity, mast bending might be a bit
unnecessary - but  non-the-less interesting.  After all, one can bend the
mast probably quicker than any other de-powering technique.  During a race
it wouldn't distract from more important things (like paying attention to
the wind) nearly as much as furling would.  How much mast bend (say by
"cord/inches") can you induce in Dynamic Equilibrium's mast above the
spreaders?

As always, I enjoy your posts.

PT



> 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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