Re: [harryproa] Re: Schooner v. Unarig

G'day,

The big advantage of stayed rigs is the loads on both mast and hull are relatively easy to determine, and highly localised. The trick then is to spread the hull loads over a large area to avoid high stress concentrations. Carbon tow is a superb material for this. Each tow has a breaking load of almost a tonne/ton. We use 20 of them to spread the load over about 600mm all round the mast. To further spread the load there is a bulkhead under the mast and a stringer along the keel and under the deck.

Difficult to know where the rig will fail. The lower part is comparatively massive to provide enough stiffness. I suspect just above the hounds, but it would vary a lot depending on the rig at the time.

Finite element analysis is a good tool, but only if you are looking to minimise everything. On the cruisers we use large safety factors, and overbuild critical areas so standard engineering is fine.

My experience of carbon is that it is pretty good on impact. I have dropped carbon masts off roof racks under circumstances that would have dented alloy. The carbon masts are still standing. My experience is supported by golf club, racquet sports and F1 cars bouncing over kerbs, all of which see severe impacts with surprisingly little damage. Where carbon is at a disadvantage is that impact damage is not visible. It is very rare to hear of a mast failing due to the carbon. It is almost always a rigging failure, or can be traced to a former incident which damaged the mast. I know of no unstayed masts which have failed. If anyone does, please let me know the details.

Carbon is particularly good for unstayed masts as along with low weight and high stiffness, it has the best fatigue properties of any of the common building materials.

regards,

Rob

----- Original Message -----

From: jjtctaylor

To: harryproa@yahoogroups.com.au

Sent: Sunday, December 10, 2006 5:38 PM

Subject: [harryproa] Re: Schooner v. Unarig

Question for Rob,.....

Cause I know hew loves the epoxy.... How is the mast held in place
and what takes the load. Looking at the design 1500lb wind load or
so with a 12-15 ft CE above the hull makes for a lotta torque, not
withstanding any peak loads due to accel or decel. Top ring takes
5/8 of that and bottom ring takes the balance. Is there another
material, balsa, etc that can handle high compressive forces ? How
does that force get distributed over the hull top/bottom.

So basically where will the rig fail if overloaded and how ?
Strength of materials may support the unusual tip over, rather than
an actual failure. Visionarries may have too high a CE and thus
break the mast whereas the lower CE of Elementaries and HarryP's get
a dunking.

Finite Element Ananlysis despite high cost could determine answers on
much is the mast capability. Nice about the unsupported is there is
a clean point load at the mast contact with hull, albeit a fairly
sharp load point.

Tougher questions is how does the carbon fail, which was a discussion
a while back. That still takes expertise for what is failing in the
laminate structure and when is limit reached. All are unanimous that
carbon does not fair well with shock loads, lots of high stress cup
racing rings falling down. Those folks may never tell the public
why...... but has always caused wonder, since us HarryProa fans like
an unsupported rig type, but with a material that may not like it !

Nice to have a broad discussion over over how the rig is actually
supported and where ARE the risks.

JT

--- In harryproa@yahoogroups.com.au, Mike Crawford <jmichael@...>
wrote:
>
> Chris,
>
> You're entirely right. Those loads will definitely be transmitted
> through the boat, and the structure will have to be designed to
handle
> them. Additionally, with a 9m beam, there will obviously be torsion
> when one hull hits a wave before the other does.
>
> There is a proa design out there with a single large beam that
allows
> the hulls to pitch independently, but I personally have doubts about
> trusting such a joint long-term. Besides, a single aka with
independent
> hulls leaves no room for a large deck with massive trampolines.
Why get
> a large multihull if you can't stretch out?
>
> The difference in the unstayed mast would be in the way the
structure
> is loaded. Given a catamaran or trimaran with a stayed mast,
there's a
> lot of stress the structure has to handle. There a large
compressive
> load in the center where the mast is stepped, large point loads on
the
> hulls/amas at the chainplates, loads due to righting moment while
> heeling, and also wave loads.
>
> Compounding the matter is the problem of keeping the structure
> torsionally stiff enough to keep the leeward stays from getting too
> loose as the boat twists. This not only puts more force on the
> forestay, it also allows a jolting point load on a chainplate when
the
> mast snaps back in the opposite direction due to wind/wave action
and/or
> twisting in the structure.
>
> That's a lot of loading to handle while attempting to keep the
boat
> from twisting lengthwise. If there were four symmetric stays,
either
> shrouds parallel with the mast step and fore- and mainstays, or two
> shrouds on a side, there's less torsion to deal with. But when
compared
> to swept shrouds and a forestay, there's a lot of stress induced in
the
> structure while flying a hull.
>
> While the harryproas will still have to keep themselves together
and
> relatively stiff as each hull hits different waves, they won't have
to
> deal with the additional huge wrenching forces induced by stays in
> tension with a mast in compression.
>
> - Mike
>
>
>
> Chris Ostlind wrote:
> >
> > Hi Mike,
> >
> > I understand the part below about the compressive stress on the
boat
> > due to lack of shrouds, but the other two (torsional and point
load
> > stress) kind of escape me with regards to a freestanding mast.
> >
> > Is not the mast point loaded at the mast partner location (the
deck in
> > most cases) once dropped into place and placed under wind load by
the
> > sail(s)?
> >
> > Is not the moment generated through the mast focused into the hull
> > torsionally through the partner location as fulcrum and then,
> > subsequently, to the mast step?
> >
> > Chris
> >
> >
> >
> > ----- Original Message -----
> > From: Mike Crawford <mailto:jmichael@...>
> >
> >
> >
> > 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.
> >
> > _._,___
> >
> >
>

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