I was pondering the effect of closing the gap between the sail and
the deck to improve efficiency and whether it is worth the extra
hassle. There is supposed to be quite a large improvement( according
theoretically to Tom speer recently on proa file and practically to
the bewoulf site and sailboard sailors) so I thought it was probably
worth the effort of setting up the deck of the lw hull and the rig to
accomodate this. Unfortunately this would make it harder to offset
the mast 10 degrees for the self righting. On scheme I considered was
to have a battened extension below the boom that was pulled over to
the hull by a string to the sheet, thus sealing the gap.I am assuming
the extra bouyancy of the wingmasts makes it easier to avoid going
right over, and therefore wouldn't need quite such a large offset
Robert
PS
I figure the shorter masts of the schooner rigs reduces some of the
whipping that can occur from boat movement in waves. Makes up to some
extent for the single rigs extra height and clearer air. I was
wondering if the height of the schooner rig should be set as the
equivalent in capsizing moment to the single mast for given
conditions rather than equivalent area. This would make them a little
taller and heavier but with less loss of light wind performance ---
In harryproa@yahoogrou
>
> 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^.
>
> 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@yahoogrou
> >
> > 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
> > and the square of the radius
> > ( http://en.wikipedia
> >
> > 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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