Herb, Mike,
I don't quite get everything you're saying, but that's probably due
to my lack of design experience. Houses, garages, and bridges were my
forte when I was a civil engineer, and none of the stresses encountered
would be like those on multihulls.
Regardless, it looks like we're in general agreement. While the mast
still has to be strong, particularly in bending moment, it won't have
to deal with some pretty fierce loads that stays would put on it. It's
particularly nice not to have to deal with compressive forces as the
mast slowly bends out of column due to wind loading. Let it bend!
That eliminates the need for diamond stays, which lightens the load,
reduces windage, eliminates additional compression and point loads, and
so forth.
As with Rob, I've not yet heard of a carbon mast failing on its own.
Then again, I'm the guy who came up with the wrong elongation at
failure for carbon after finding some odd research, so my chiming in on
this is of questionable value. Thanks again to Myriam and Youri for
sending the link to http://www.matweb.
It would be useful if we could find out the exact specs used on the
freestanding carbon masts at Wyliecat ( http://www.wyliecat
- Mike
Herb Desson wrote:
Sorry I wasn't clear. What I meant was that although the motion of
the boat will be transferred to the masts, the mass affecting the
masts will be only that of the mast and sails themselves, instead of
the entire mass of the boat being transferred through the stays. So
the total energy the unstayed masts have to deal with will be a tiny
fraction of the stayed masts.
A boat with a stayed mast is effectively a large complicated spring
system in which all the mass of the boat is eventually transferred to
the mast. But for an unstayed mast there is no spring system that is
tied to the mast. Still plenty of springs, but none (or very few) of
them wind up being transferred to the mast.
Best regards
Herb
--- In harryproa@yahoogrou
>
>
> 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@yahoogrou
> > <mailto:harryproa%
> > 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.
> > <http://boatdesign.
> > >
> > > 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,
> > > 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@yahoogrou
> > <mailto:harryproa%
> > > 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@yahoogrou
> > <mailto:harryproa%
> > > > >
> > > > >
> > > > > ----- Original Message -----
> > > > > From: Herb Desson
> > > > > To: harryproa@yahoogrou
> > <mailto:harryproa%
> > > > > 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
> > > > >
> > > >
> > >
> >
> >
>
Change settings via the Web (Yahoo! ID required)
Change settings via email: Switch delivery to Daily Digest | Switch format to Traditional
Visit Your Group | Yahoo! Groups Terms of Use | Unsubscribe