Subject: [harryproa] Re: Schooner v. Unarig
From: Mike Crawford
Date: 12/8/2006, 4:51 PM
To: harryproa@yahoogroups.com.au
Reply-to:
harryproa@yahoogroups.com.au


  I believe that boat motion will be transferred to the mast in either case, whether it is stayed or freestanding.  After all, if the boat heels 20 degrees due to wave action, the mast is going to have to heel with it.

  The nice thing about a freestanding mast is that there won't be all sorts of torsional stresses on the boat, nor point loads and compressive stresses on the mast.  This will definitely simplify design.  But issue of moving the mast when the boat moves still remains.

  Fortunately, forces from the ocean on the mast are likely to be far less than wind forces, so designing for the sail area is still probably a safe bet.  Provided, of course, there's a pretty solid margin of safety.  Just as an rock climber can put several g's of force on a climbing rope with even a 1m fall, the dynamic forces which cause a capsize can still be several times the righting moment.

       - Mike



Herb Desson wrote:

JT,

That is a very interesting and long thread. I have only read the
first two pages, but one thing strikes me - that nearly all of the
problems in design relate to what happens to the stays when a big bump
happens because the entire momentum of the boat is transmitted through
the stays via the mast.

On a free standing mast it seems that the only issues are the righting
moment and fatigue. Once the boat goes over there is very little load
and much bigger problems than a broken mast. But you can never have a
problem with the momentum of the boat being transmitted through the
stays to the mast and deck. So the problem is much simpler.

I think we can design for righting moment only (plus fatigue and
construction error) and be safe enough. Any more would just create
more mass than necessary or would ever be used.

Best regards
Herb

--- In harryproa@yahoogroups.com.au, "jjtctaylor" <jtaylor412@...> wrote:
>
> You will have to choose your fudge factors for sail force. It isn't
> just load to capsize..... I am not an expert but a really long
> discussion can be found at:
>
> http://boatdesign.net/forums/showthread.php?t=2293
>
> Whole books have been written on rig planned loads. Even without
> stays..... forces are dynamic so a whole bunch of engineers and
> experts cannot yet agree on how much engineered strength (loads)is
> enough. The sea is dynamic, wind is dynamic and boat motion as
> well. FEA may tell you what may fail first.....but not the
> conditions that caused it. So have to make a bunch of
> assumptions,...thus most if not all on the forum agree.... too many
> variables. Thus make your planned limit then add something for for
> the unknown.
>
> NO matter what... impractical to design for all possibilities, so if
> designed to function adequately well it be lost under some extreme.
> Choose your limit, you won't be right or wrong.
>
> JT
>
>
> --- In harryproa@yahoogroups.com.au, "Herb Desson" <squirebug@>
> wrote:
> >
> > Ah, I take your point. So the relative force on the mast due to
> sail
> > size is largely irrelevant.
> >
> > So we will gain strength due to the shorter masts, but weight will
> be
> > governed by breaking strength. Need to do some more research.
> >
> > Thanks for straightening me out.
> >
> > Best regards
> > Herb
> >
> > --- In harryproa@yahoogroups.com.au, "Rob Denney" <proa@> wrote:
> > >
> > >
> > > ----- Original Message -----
> > > From: Herb Desson
> > > To: harryproa@yahoogroups.com.au
> > > Sent: Friday, December 01, 2006 5:27 PM
> > > Subject: [harryproa] Re: Schooner v. Unarig
> > >
> > >
> > >
> > > I am not quite sure what to make of the smaller sail area, but
> I think
> > > it is clear that in any given weather there will be less force
> on each
> > > mast for the schooner than for the single mast of the sloop.
> > >
> > >
> > >
> > > Agreed, but the design load is still that force required to
> > capsize the boat, so they have to be as strong as the single
> mast.
> > The same applies to the hull reinforcing. Supporting each mast at
> the
> > end of the beam is very easy. However, if only one sail is doing
> the
> > work, the hull has to be strong enough to transmit this load to the
> > other beam, so ends up the same as if the mast was in the middle.
> > >
> > > I look forward to seing the results of your calculations. I
> know FEA
> > > costs money, but would it be possible to include an analysis of
> > > exactly the same sail shape to get comparability? I am not sure
> how
> > > comparable a jibless schooner is to a balestron sloop from a
> weight
> > > point of view. My first thought is that it wouldn't make much
> > > difference, but clearly my first thoughts are not very reliable
> in
> > > these matters.
> > >
> > >
> > >
> > > You ain't kidding about the cost of FEA! I can't afford to get
> > into sail shape anaysis. We engineer the mast based on the
> scenarios
> > it sees. The ballestron rig and the schooner would be pretty close
> to
> > the same weight.
> > >
> > > Best regards
> > > Herb
> > >
> > >
> > > Recent Activity
> > > a.. 2New Members
> > > b.. 7New Photos
> > > Visit Your Group
> > > .
> > >
> > >
> > >
> > >
> > ----------------------------------------------------------
> ----------
> > >
> > >
> > > Internal Virus Database is out-of-date.
> > > Checked by AVG Free Edition.
> > > Version: 7.1.394 / Virus Database: 268.14.6/536 - Release Date:
> > 11/16/2006
> > >
> >
>

__._,_.___
.

__,_._,___