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

Thu Mar 18 16:31:21 EST 2004

Peter,

With the forestay cable buried inside of the roller furling foil &
impossible to get at with a Loo's Tension gage, I've never measured the
forestay tension.  I have some calculated theoretical values; but, they
would only apply to my particular boat & masthead geometry & wouldn't be of
much use to anyone else.  When you refer to "chord-inches, I presume you
mean the maximum perpendicular distance from the chord to the radius of
curvature?  Assuming my definition of chord-inch agrees with yours, with the
range of adjustments on Dynamic Equilibrium as currently configured, over
the 333-5/8 inch length of the forestay, I can adjust the chord-inch value
from nearly 0 to about 6 inches.  The aluminum extrusions on the headsail
foils won't sag too much more than that without causing bearing and furling
problems.

On my current fully battened mainsail, I went fairly conservative on the
amount of mast bend the mainsail can respond to.  It is currently cut to
respond to only about 2 chord-inches, mostly above the spreaders.  The mast
curvature actually trails off asymptotically to 0 below the spreaders
because of stretch in the lower sides stays and the differential fore/aft
static load I put on the lower side stays.  The forward lower side stays are
adjusted tighter than the rear lower side stays to facilitate mast bending.
I think the amount of mast bend could be increased to about 4 cord-inches
without any trouble.  When the time comes to replace this mainsail, that's
the spec I plan to ask for.  With the current aluminum mast extrusion,
trying for more bend than about 4 cord-inches is asking for trouble in my
engineering judgement.  The aluminum extrusion might experience fatigue
failure (especially in a salt water environment) and the loads on the
forestay & backstays would start to get a little high for the hull structure
and chain plates.

As you probably already know, you bend the mast to straighten the forestay,
thereby reducing headstay sag plus flatten the mainsail shape.  Both actions
depower the rig for heavy air.  Yes, depowering the rig with the backstay
adjuster is much faster than any other method because it takes care of both
sails at the same time.  I have my backstay adjuster setup with a Harken 144
swivel cam base with 150 cam-matic cleat, 082 single bullet block, & 071
stand-up spring mounted on the aft starboard gunnel.  The swiveling arm cam
base & cam-matic cleat on the end of the arm allows me to reach the backstay
adjuster from virtually any helm position.  The backstay adjuster line is
marked with calibration marks for quick, reproducible tension adjustments.
I used the big Harken 144 swivel cam base because this unit is mounted right
by where people come aboard over the transom from the boarding ladder.  The
144 is sufficiently stout that you can stand on the swiveling arm without
damaging it.  I've been sailing with this backstay adjuster setup since
1988.  It's been absolutely bulletproof so far.

Try it - You'll like it a whole bunch!

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 10:44 AM
Subject: Re: [Rhodes22-list] Mast Crane (was Lou Moore)

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