April 1, 2012

Twisted thinking

I DON'T KNOW ABOUT YOU, but I get skeptical when I read (and quite repeatedly) that when a boat is sailing, the apparent wind direction changes by between 5 and 8 degrees from the bottom of the mast to the top.

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

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.)


doryman said...

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.


Anonymous said...

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?

Justin C said...

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.

Anonymous said...

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.