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

[Roger Pihlaja]

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