How can sail boats move in the opposite direction of the wind?
In: Physics
To save people talking about engines and currents, I think he means tacking and jibing. I’d help but I only know the words, I have no idea what they mean.
Generally they have motors on them as well that allow them to proceed against the wind.
The other thing you can do depending on where you’re going and the wind strength is called tacking.
This means you change your direction slightly into the wind and basically zig zag your way to your destination using momentum to keep you going.
Sail boats aren’t just pushed along by a tail wind, in fact that is quite inefficient. Instead, the hull, sales and rigging work together to deflect the wind and so gain a LOT more power.
As for moving up wind, it is akin to how you gain speed on ice skates. You angle your feet outwards and push sideways. Your skate doesn’t want to slide sideways though, so the power instead moves it forward to gain the same effect (foot moving outwards). In skating you now push with the other foot the other way. Sail boats can’t do this, they can turn however. By tacking (zigzagging) into the wind, they move against the wind overall, by moving at an angle to it, then reversing.
Interestingly, just like skating, a sail ship can travel far faster than the wind is blowing, or the leg is moving.
The key bit is they must have a keel. You can think of this as a bit like making them run on a rail – they can’t go sideways at all, only forward and back.
Now picture the wind coming exactly sideways-on to the boat. Because it’s on a rail, it can’t move at all (this is a simplification).
Now put a sail on the boat, at an angle to the wind. The wind will get deflected by the sail and end up pushing it diagonally forward. But we’re on a rail, so the diagonally forward becomes straight forward.
This work well enough that you don’t have to be exactly side on – you can steer up into the wind a bit and still have it work well enough to overcome the bit of back-sliding pressure the wind is putting on you.
Now – a keel isn’t exactly being on a rail etc, but this is the general idea. You can’t sail a boat into wind without a keel.
No sail boat can sail *directly* into the wind without a motor of some sort, but they can move generally in the direction of the wind by [tacking](https://upload.wikimedia.org/wikipedia/commons/thumb/2/22/Beating_to_windward.svg/1233px-Beating_to_windward.svg.png)
A sail boat is physically incapable of sailing directly into the wind, but with the right sails it could sail as close as 20 degrees off the direction of the wind, and big ships were 30-45 degrees off the wind. So if the wind is coming from due north then you sail to the north east for an hour, then turn and sail to the north west for an hour. You’ve averaged heading due north, but you’ve had to travel a lot further to pull it off.
No sailboat can sail directly into the wind, but they can sail relatively close to the wind. This is possible because far from just being pushed by the wind, the sail actually acts like an airplane wing – if you look at a picture of a sailboat sailing upwind, you’ll notice that the sail is stretched into a curved shape. This curved shape and its interaction with the wind causes a lift force to be generated by the sail, as shown in [this diagram](https://upload.wikimedia.org/wikipedia/commons/e/eb/Resolution_of_Total_Force_on_sails_into_Lift_and_Drag_and_Forward_and_Lateral_Force.jpg). V_A is the apparent wind (i.e. the wind that the sail “sees”), and F_T is the force generated by the sail. Most of this force is sideways, and so would push the sailboat to the side or push it over. However, sailboats, thanks to their hull shape and especially their deep keels (compared to motorboats) strongly resist this sideways motion, though they do still lean somewhat away from the wind. In the forwards direction, however, their resistance is much, much less, so that the small forward component of the sail force (called F_R in the diagram) can push the boat forwards at a considerable speed – by adding its own speed through the water to the speed of the wind over the water a sailboat can even, under the right circumstances, go faster than the wind is blowing!
No one here is actually answering your question correctly.
Sails do not act like bedsheets that are held up in the wind, where the wind blows the bedsheet out and pushes the bedsheet forward (and thus anything attached to the sheet would go forward too). This effect can happen when the sailboat is moving “downwind” (the wind is somewhere behind the boat).
INSTEAD, when going “upwind” (the wind is coming from somewhere in front of the boat), the sails act more like an airplanes’ wings. Plane wings work by creating areas of low pressure above them and high pressure below them. Because air moves from high pressure to low pressure, the high pressure air “pushes” the wing up and the plane goes up and flies. In order to produce this area of low pressure above the wing, the top of the wing is curved outward, while the bottom is generally flat. The air has to move “faster” over the top of the wing, so it spreads out and creates less (low) pressure than the air under the wing.
If you took a wing, and turned it sideways, it would still produced the low and high pressure areas, but it would move the plane forward or backward instead of up and down. This is what sails do for boats when going upwind. You set your sails at an angle to the wind so that the wind blows on both sides of the sail, and you curve the sail to produce an area of low pressure in front of the sail. Then, the boat is basically pulled or “sucked” forward.
As others have said, you can’t sail directly into the wind, you have to be at an angle to it for this effect to work.
They can’t.
However they can zigzag diagonally to the wind of those two motions added together comprise of course that is against the wind.
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