But NEVER pinch when it's REALLY windy kids, I WILL sail through your lee!
Q: How do airplanes fly?
A: They use wings.
Q: So do they have to tie the airplanes to the ground so they don't flow away?
A: No, wings only create lift when there is a flow of air over them.
Q: How do they create lift?
A: The air has to take a longer path over one side vs. the other, and therefore flow faster.
Q: So you create that special wing "Foil" shape for that right?
A: Right, but you can also use a flat shape, angled against the wind. This is called angle of attack.
Q: Huh?
A: Driving down the highway, stick your hand out the window, so that your hand is flat -- parallel to the highway.
Your hand is exactly parallel to the direction of travel. It has no "angle of attack."
As long as you hold your hand completely flat, the air doesn't try to force it up or down. Now, tilt your hand so that the leading edge is slightly higher than the trailing edge. That creates an "angle of attack" because your hand is now pointing a slightly different direction than the direction the wind is coming from.
Do you feel how your hand is being lifted? This illustrates how angle of attack creates lift, even in the absense of foil shape.
Keep increasing the angle of your hand. Two things happen:
1. The "backward" force keeps increasing. As the angle of attack increases, drag increases.
2. For a while, as you increase the angle, up to a certain point. The "up force" increases as you increase the angle but:
3. BEYOND a certain point, the up force stops, and all you have left is the "back force."
When you get to step 3, the "angle of attack" is too great, and the foil (your hand) is STALLED and no longer creates up force.
Q: So what does that have to do with boats?
A: Water flowing over the centerboard/keel and rudder creates lift in the same way. This lift is what allows boats to sail to windward.
Q: So how do I create this "angle of attack"
A: The angle your boat is travelling is slightly different than the angle it is pointing toward the wind. The more you "pinch up" slowing down and increasing the angle of attack vs. the direction you're travelling, the more you increase the angle of attack. That's why pinching works. But go "too far" and just like the air over your hand, the water stalls over the foils and you slip sideways.