I don’t get how this is physically possible if the direction you want to go is literally the opposite direction of the way the wind is blowing. How can it ‘push’ them the opposite way it is going?
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Fundamentally it’s useful to realize that what a boat really does is extract energy from the difference in the movement of the air and water and then use that energy to move itself.
The sail acting on the air and the hull and keel acting on the water force each to move more like the other.
This is why crazy stuff like the directly into the wind cart is possible.
It’s also why if the wind and the water are moving in the same direction at the same speed the boat can’t do anything, it just flows along with everything else.
And where you run into people saying very counter intuitive things like “a fast boat makes it’s own wind”
From a physical perspective, a sailboat extracts energy by slowing down air, and this energy can be used with clever maneuvers to move even against the direction of the wind.
They can’t go _directly_ into the wind, but they can go about about 30° off on either side. If you need to travel in the exact direction the wind is blowing from, you need to “tack”, that is go in a zig zag path, first to the left of the way you want to go, then to the right.
As to the question of why the wind can push sailboats _any_ direction other than the way it’s blowing, it’s complicated. But a full sail is shaped rather like a plane’s wing. Bernoulli’s principle explains what’s going on.
Because your sail is never facing directly at the wind, it’s facing the wind at 45* so that you can capture the force of the wind with the sails but then use steering to apply the force.
Here’s an interesting article from SciAm:
https://www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/
The title is a bit of click bait. It’s lends credence to the contribution of both low pressure (Bernoulli) and mass force (Newton) and notes that the two combined do not fully address the total lifting forces seen in flight.
There’s an addition region of low pressure air that forms just behind the front top area of the wing that neither theory accounts.
They use the sail to push the boat sideways. Then they point the boat at an angle into the wind. Because the boat can’t go sideways, it obsessed goes into the wind.
For an ELI5 I would go with this.
Imagine you have a sheet of plywood and you go outside on a windy day. If you don’t want the wind to rip it out of your hands you make sure an edge is facing the wind not the full sheet. If you put it on your head and tilt that edge upward just a little the sheet will rise up or lift off your head.
If you shift it off your head and hold it off to the side, keeping the edge to the wind but with the same small tilt, it wouldn’t pull you straight downwind but rather to the side.
Imagine you did that while wearing a pair of ice skates. You could lean your body weight to balance the pull from the plywood off to the side and you might get tugged a little bit with the wind to start with but as you gather speed you could use the edge of the skates to better effect and actually cut up toward the wind, albeit at a pretty big angle. It is only possible while you hold the sheet at that particular angle.
Of course if you turned you would have to do it with the wind not against it so you have head downwind for a short distance before edging up toward it again.
Here is it in action but with more efficient windwings rather than a sheet of plywood but the principle is the same.
A sailboat works exactly the same was but instead of a skate edge they use the keel and rudder to help cut toward the wind.
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