Assuming you're in a 12m Schooner-rigged Harry with 12m masts, a ten
degree cant will increase the effective beam by about 2.1m, yielding a
total beam of 8.85m instead of 6.75m. You would have to watch the
masts in tight quarters, but then, what's another 2m when the boat is
already so beamy? After all, it's a multihull with a wide beam and a
lot of righting moment.
Is the additional width worth it? That's really a matter of what
your priorities are.
Personally, there's nothing less than 10m wide through which I'll
have to travel, in which case the extra width doesn't matter. The
ability to stop a capsize, or potentially self-right, though, is
incredible.
It's fun racing Hobies or Prindles, pushing too far, and then
righting the boat and getting back on your way. At least if you do it
in the summertime in warm waters. Take a 9m to 12m catamaran or
trimaran, though, and it's a different story. There's no getting the
boat back if you're pushing it and the wrong combination of gusts and
waves hits you at the same time. It's not too difficult to lose a
mast, or worse, when that happens, especially if you're in shallow
water.
I sail a 27' catamaran in the waters off of Maine, where the water
rarely goes above 15 degrees celsius in the summer, and I have to say
that racing in big winds or seas over 1m can be a bit frightening. It
would be great to know that I'm not risking thousands of dollars every
time I fly a hull.
Some would say that's the price of going fast. Others would say that
it's silly to push things so far. Both are probably right. But the
point of a safety feature isn't just to make it easier for fools to be
foolish. You can get a freak just and a rogue wave at the same time
even if you're careful.
No righting system is perfect, but the idea that your large multihull
could be self-righted without worrying about compressed-gas systems is
quite amazing. I'm not sure you'll find any other 9m+ multihulls that
can stop a capsize or even pop back down from one.
What can I say? I'm a safety nut.
In any case, you're right: canting the masts will incur the price of
additional beam and a slightly odd appearance.
- Mike
audeojude wrote:
I would think that canting the masts to leward would be fraught with
problems. I can just see the boat hitting bridge pilings, and other
sailboats as it goes by. How far out to the side of the boat would the
top of your mast end up being? You now have effectively increased the
beam of the boat at the masthead by some ammount.
--- In harryproa@yahoogroups.com.au , "Rob Denney" <proa@...> wrote:
>
> G'day,
>
> You are right, the angle is more relevant than the amount. Same
weight is closer to the truth, but still low, I think. We will be
doing some numbers on a schooner rigged 22m Visionarry which we are
starting early next year. I will let you know the results.
>
> Designing a mast to break before the boat capsizes is fraught. It
implies no safety factors and some hard decisions about waves and
payloads. Then you are sailing along with full sail up, a big gust
hits and you fly a hull. Ease one sail and the other mast breaks. I
would prefer to cant both masts to leeward 10 degrees, keep the weight
low in the windward hull and have a chance of self righting from 80
degrees, or more realistically, not be able to capsize this far.
>
>
> regards,
>
> Rob
>
>
> Oops.
>
> There is an error in my calculation.
>
> The weight of each mast is not 1/((2^.5) * (2^.5)) as shown below. It
> should be 1/((2^.25) * (2^.25)). This is because the luff is being
> reduced by 2^.5, which means that the reduction in weight is square
> root (2^.5) = 2^.25.
>
> So with the schooner sail the weight of the luff portion of the
> mast is now 1 / ((2^.25) * (2^.25)) = 1/2^.5 = .707. And for two
> masts it would be 2*.707 = 1.415.
>
> However the force on each mast is still 1/2 of the force on a unarig,
> so we can still reduce (I think) the diameter of each mast by 1/2^.5.
> This gives the total weight as 2/(2^.5)(2^.5) = 1.
>
> Hopefully this is a bit closer to the truth.
>
> Also, I was looking at the same angle of deflection - not the distance
> of deflection, on the assumption that what was relevant was the angle
> at the top of the mast. The link quoted gives that the angle is
> proportional to the square of length and radius, whereas distance of
> deflection is proportional to the cube. Should we be concerned here
> with angle or distance of deflection?
>
> One other point, on a cruising boat after a drama with 1 sail up and
> one down, do we really want to have 1 mast pointing up or 2 masts
> pointing down? If the former, perhaps the breaking strenght of one
> mast should be less than the weight of the boat.
>
> Best regards
> Herb
>
> --- In harryproa@yahoogroups.com.au , "Rob Denney" <proa@> wrote:
> >
> > G'day,
> >
> >
> >
> >
> > Rob I don't mind the schooner rig, in fact on a big boat where a
> mainsheet winch is required, and if the fore boom of an Easy rig is
> too high to easily reach, they have a lot going for them. Horses for
> courses.
> >
> > Herb The angle of deflection of a circular thin wall beam is
> proportional
> > to the square of the length
> > ( http://en.wikipedia.org/wiki/ )Deflection
> > and the square of the radius
> > ( http://en.wikipedia.org/wiki/ ).List_of_moments_ of_inertia
> >
> > R
> >
> > The formula for cantilever deflection is
> >
> > (Load*length cubed)/8*E*I Therefore it is a cube function of
> length, not a square. Halve the length, one eight the deflection.
> >
> > E is the material properties, I is the 2nd moment of area about
> the neutral axis. I= pi*radius cubed *wall thickness. The radius is
> also a cube function, so half the radius, 8 times the deflection.
> >
> >
> > H
> >
> > However, each mast now has only 1/2 as much sail area, so the
force on
> > each mast is 1/2 of what it would be for the single mast.
> >
> >
> >
> > R
> >
> > Yes and no. Each mast has to be strong enough to capsize the
> boat, as it is possible that only one sail would be fully powered up
> in a capsize scenario. This does not make each mast as heavy as a
> single one as to be stiff enough they are already stronger than
> required. However, depending on bury and other variables, it does
> make each mast much more than half the weight of a single one. On a
> harry, the bury will be very similar, although although a higher
> percentage of the overall length, which reduces the sheer loads
> somewhat.
> >
> >
> > H
> >
> > So I am confused. Is a schooner rig really about 2/3 the weight and
> > cost of a unarig (ignoring sails), or have I misunderstood
something?
> >
> > R
> >
> > I would say 3/2 is closer to reality for the weight and cost.
> >
> > regards,
> >
> > Rob
> >
> >
> >
> > Best regards
> > Herb
> >
> >
> >
> >
> >
> >
> >
> --------------------- --------- --------- --------- --------- -
> >
> >
> > 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
> >
>
>
>
>
>
>
>
--------------------- --------- --------- --------- --------- -
>
>
> 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
>
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