I agree with Mike about the dynamic loads of a capsizing wind gust
being significantly greaterr than the steady state wind needed for
capsize. The wind has to overcome inertia as well as weight to get
the boat over in a finite time. I have been hit by extreme gusts in
New guinea coming off the hills that were over 40knots. They weren't
blowing long enough to tip over a large multihull, but were still
imparting pretty heavy loads on the rigs.
Robert--- In harryproa@yahoogrou
<jmichael@..
>
>
> 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%
<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.
> > <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,
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@yahoogrou
> > <mailto:harryproa%
<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@yahoogrou
> > <mailto:harryproa%
wrote:
> > > > >
> > > > >
> > > > > ----- 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
> > > > >
> > > > >
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