As things are, however, we have to do a lot of guessing. We also have to put great faith in the pronouncements of scientists who assure us that the moving air we call wind changes speed as it gets higher. Apparently, friction between the air and the ground, or water, slows down the lower layers of wind. The difference is perceptible, they tell us, even over the short distance from deck to masthead.
That in turn affects the murky business we know as apparent wind, a combination of the true wind and the forward movement of the boat. On the mainsail, it means that the apparent wind is freer at the head than the foot by about 5 or 8 degrees. That’s why the head of the sail should fall off, with a gradual twist all the way up the leech.
On most boats I’ve sailed, it seems to do this naturally all by itself, though I’ve never tried to estimate the number of degrees of sag at the head to see if it got it right.
It’s unlikely that your mainsail will maintain the same angle all the way up the leech from the clew to the head of its own accord, but if perchance it does, don’t try to make adjustments. It’s too complicated. Take the sail to a sailmaker and ask him or her to build in the twist that should have been there in the first place.
Of course, one angle of twist can’t be correct for all wind speeds. That would be too simple, and nothing is simple on a yacht. There are, indeed times when you don’t want any twist at all. The death roll so familiar to Laser sailors on the run is blamed on the top of the sail twisting to leeward and forming a shape like the blade of a fan that pulls the boat over suddenly to windward.
So ask your sailmaker for a slight amount of twist, a sort of run-of-the-mill kind of twist, nothing excessive — and hope for the best.
Today’s ThoughtIt is folly to complain of the fickleness of the wind.
— Ovid, Heroides
TailpieceThere was a young woman called Hall
Who wore a newspaper dress to a ball.
The dress caught on fire
And burned her entire
Front page, sports section, and all.
(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)