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post #6 of Old 02-10-2002
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Ayrodynamics of Sailing

Ahoy, Zach. (Part 2: How do you position the sail according to where you want to go and direction of wind.) I refer you to my discussion under Learning To Sail, Sail Trim For Beginners, and to my discussions under Learning To Sail, Heeling Paranoia. Also, my discussion under General, to Bob Ohler, regarding Mast Rake. These illuminate in a simple way the fundamental forces acting on a sail boat and their implications.
What we are striving for is to maintain the sail as a wing so that we get the lift we were talking about in part 1 at all times. In order for that to happen, there must be smooth air flow over both surfaces so that Bernoulli effect can operated. The wind must strike the leading edge, the luff, and part, running down both sides with as little turbulence as possible. We put tale-tales on the sail because we can''t see wind, but can see its effects on the tale-tale. When they are streaming back, laying along the sail, we can see that we have smooth air flow. When they flutter, dance, run up the sail vertically, or hang straight down, we know that the air is disturbed. To have smooth flow, the air must stay in contact with the surfaces of the sail. If we tilt the wing too much, the wind will not be able to do this. It will break away from the surface, become turbulent (we want laminar flow), and not flow along the surface in the desired manner. We say that the wing has stalled.
Now, if I face directly into the wind, I will find, try as I may, that I can not adjust the sail to create a wing. I will also find that if the wind is behind me, I can not. Down wind we are just pushed along by the wind striking the sails from behind and are not lifted as we would be if the sail were acting as a wing with lift. Into the wind we will stop and make no progress at all.
Now, if you begin easing the bow from dead into the wind to off the wind, you will find that at about 45 degrees (it is different for different boats), you can adjust the sails so they take up the desired shape and act as a lifting force again. As I continue to turn the bow of the boat away from the wind, I find I will have to continually ease the sail out to maintain that proper angle to the wind (called angle of attack). At 90 degrees to the wind, my hull is at right angles to the wind, and my sail is out over the lee side of the boat. Beyond that point, I can no longer adjust the sail to the proper angle to the wind because I can not move it any farther forward. At this point I enter the push mode and not the lift mode and that holds all the way through dead down wind.
I go into the wind with a series of runs across it, called tacks. I sail at 45 degrees to the wind on one leg then tack roughly 90 degrees and sail back across it the other way. That allows me to keep the sail angled correctly to the wind and it lifts me up to wind. I is slow, hard, sloppy going to wind relative to other points of sail, but I can get there.
As I turn away from the wind, I am not beating into waves and running so directly into the wind. My hull doesn''t heel so much , the ride is easier and I sail faster. Beyond 90 degrees I am being pushed.
Generally speaking, you will have your main sheet cleated on a beat and maintain the angle of your sails to the wind by using your tiller to turn the bow into and away from the wind as your tale-tales indicate. Up in a puff, and down in a lull.
On a reach, because you want to stay on your course, your rhumb line, you will adjust your angle of attack with the sheets, easing in and easing out as you need to to keep the tale-tales streaming. Always trim, whether with sheet or tiller, toward the stalling tale-tale.
A good way to trim sails is to ease them just until the wind starts getting behind the luff. It will break and bend inwards. Then harden your sheet just until the cupping effect, the break in your luff disappears. You will have the right angle of attack and smooth flow over the sail.
Finally, you will normally have at least two sails, the foresail and the main. Always trim the foresail first, then the main. You will have a bi-wing effect. Air is compressed and forced through the slot between the two sails, speeds up even more over the main, increasing lift. This is called slot effect.
The trailing edge of both sails should be as parallel as possible all the way up to ensure as much slot as possible, and your leeches should allow smooth reconnection of the upper and lower wind stream at the trailing edge. Too loose they flap and too tight they trap the wind stream breaking the smooth flow. IT IS SLOT SHAPE AND LEECH TENSION THAT DETERMINE THE DRIVE EFFICIENCY OF YOUR SAILS.
Summary: Ease the foresail just till the luff breaks and then harden slightly till the break disappears. Do the same with the main. Keep this angle of attack on a beat into the wind by steering the boat, and on a reach by easing and hardening the sheets.
Hope this helps. Good luck. Go sailing. There is no wind in books.
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