How does a sail work?
I'm going to talk through how a sail works. The lens I always use to look at the world is by asking: why is this useful? Therefore, why should you even care how a sail works? I would never teach this concept to a beginner class. For most people it's not a useful lesson in your first month of sailing. It doesn't give you a quick and dirty rule to follow. The quick and dirty rule for sails being trim it just in from luffing. You can use the strange abbreviation JIFL. That means let it out until your sail luffs or becomes a flag. Then you pull it in until that sail stops luffing, no more. One important thing to remember is that this rule never stops applying. Never. Never. I use it always!
One reason to know how a sail works is because then we know why we JIFL our sail. It emphasizes the rule because we now understand why we do it. Understanding how a sail works also helps us think about the other forces on a sailboat. This helps us see why a centerboard is necessary or why moving crew weight might be useful. It can give you a more complete picture of sailing and what makes a sailboat move efficiently through the water.
A sail can work in two basic ways. I would say that the majority of people that I meet think that a sail is pushed by the wind. A sail can absolutely work in this way. However, it's brutally inefficient. Modern sails work as an airplane wing. Here comes the physics. It's going to be basic so don't worry too much.
When wind hits an object it cannot go through, it must go around. With a sail the wind goes around an outward or an inward curve. The wind across the outward curve is forced to travel faster than the wind on the inward curve. This creates high pressure on the inward and low on the outer curve. There is always flow from high to low. The resulting force on our sail is no exception. Our driving force ends up being at right angles to the sail. This driving force from the sail is referred to as the centre of effort.
Now you know this it changes nothing. JIFL your sail! What it does help me understand is that the luff (the edge of the sail by the mast) needs to be about in line with where the wind is blowing from. If this isn't the case I won't get max flow on both sides of the sail. Instead I'll get a push only or my sail with luff (flap like a flag). To get max. flow on both sides of my sail I JIFL which then gives my sail the max. driving force possible! Genius! Simple right?