The rule
of thumb, therefore, is that the leech at the head of the sail should lie farther
off the wind than the leech near the clew.
But I have always worked on the assumption that in light to moderate
weather at least, the mainsail works better if the leech has little or no
twist. Only when it blows hard should
you allow the tops of the sails to twist off and spill wind.
So,
between what I've read I should do,
and what I think I should do, there
is evidently a lot of confusion. The gurus say the need for twist comes about
because the wind velocity rises with altitude, where it is less affected by the
friction of rubbing against the water. Thus, if the true wind speed is higher
up aloft, the apparent wind direction up there is less affected by the boat's
forward speed. It is nearer the true wind direction. So the top of the sail does not need to be
sheeted as close to the wind as does the bottom.
They also
say you don't have to worry too much about this in practice, because your
sailmaker knows all about it and has already built the correct twist and camber
into your sail. Well, I'm sorry to say
that I do worry about it because it
just doesn't look right to me, and
I've often wondered if the wind direction really changes enough from bottom to
top in the size of the boats I sail for it to make any difference
whatsoever. Perhaps on a mast 120 feet
tall there might be a measurable difference in wind direction alow and aloft,
but it's hard to believe it would happen on a mast only 30 or 40 feet tall.
If my sailmaker has deliberately built twist
into my sail, and I deliberately try to take it out all the time, what's the
point? I find it hard to rid myself of
the suspicion that this is one of the theoretical aspects of aerodynamics that
experts tuck up their sleeves and bring out on occasion to amaze and astonish
us gullible groundlings. I wonder if these theories actually work out in practice,
and I also wonder if the fact that I'm always trying to get the twist out of
the mainsail accounts for the fact that my trophy drawer is astonishingly
empty, considering my vast potential for winning races.
Today's Thought
It is folly to complain of the fickleness of
the wind.— Ovid, Heroides
Tailpiece
A
small-town vicar was asked to lecture the local young girls’ club on
Christianity and Sex. But because his wife was very strait-laced, he told her
he was going to lecture on sailing.A few days later, the vicar’s wife met one of the girls in the street. The girl said the vicar’s lecture had been very interesting and informative.
“Huh,” the vicar’s wife snorted, “I can’t imagine what he knows about it. He’s only done it two times. The first time he got sick. The second time his hat blew off.”
(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)
4 comments:
John,
When I was racing (a great learning tool) we induced twist in the main when the winds were over 35 knots. But as the main trimmer, my theory is that the top of the sail, because it's area is small and there is so much pressure from the lower portion, naturally it tensions tighter. This is more extreme as a sail stretches with age.
There are many superstitious beliefs among racers passed along as gospel, but I don't believe that in 30-50 feet of atmospheric pressure, there is any appreciable difference in wind direction or velocity.
michael
Should be easy enough to find out. Anchor out somewhere, put wind instruments up on the bow (so they're unobstructed), see how the readings compare to the masthead instruments. Hasn't anyone ever done this?
This bugs me too, and what about the Coriolis effect? If the wind direction at the deck is different from the direction at the top of the mast, isn't that difference dependent on the Coriolis effect? In which case, on one tack the twist might be right, but surely, on the other tack it's worse than all wrong, it's inverted! ... Or do I not understand this either?
Justin C, by the sea.
Check out wikipedia for the coriolis effect, it's not applicable to small scale interactions like this. The boundary effects from a fluid (air) as it gets closer to a surface (the sea or land) are very relevant, the fluid will slow down as it is closer to the surface. In the real world it is very hard to model as it is a mixture of turbulent and laminar flows (more of the former). There is lost of mostly readable discussion of what's going on around a sailboat in Sailing Theory and Practice if you can find a copy to look at.
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