Is it the apparent wind created by forward momentum? Is it possible to go downwind faster than the wind speed?
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As another reply mentions, ice skating is an excellent analogy.
The keel/centerboard is like the skate planted on the ice. The sail is like a skate that constantly pushes sideways.
Minimise drag and maximize strength and sailfoil boats can attain nearly limitless (at least ridiculous) speeds.
The best analogy I’ve heard for this is ice skating. When you are ice skating and trying to accelerate, you push the skate at an outward angle with every stride – and you will end up going faster than the speed at which you are pushing out against the ice.
Because the wind is not pushing against a sail that is perpendicular to it (with the wind coming from behind the boat) – if it were you would only be able to go as fast as the wind. Instead, the sail is angled (e.g. at say 30 degrees off perpendicular, with the wind coming from the side of the boat) so that you effectively create a horizontal and vertical component to the push which adds up to a speed greater than the wind itself
[Here](https://youtu.be/u5InZ6iknUM?t=382) is an explanation that made it click for me
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The ice skating analogy makes a lot of sense. But to me that’s more like how a propeller moves a motor boat with stored energy. A sailboat passes energy between the wind and water.
Another way to think about it (at the risk of being more confusing): the boat is capturing energy from the difference in momentum between the wind and the water. Imagine the water and wind flowing in the same direction, at the same speed. From the boats frame of reference that is equivalent to being in still water with no wind. No sailing going to happen. Only a motor boat with stored energy could move.
Think of a windmill facing upwind. The windmill is on little wheels connected to the windmill drive shaft by a chain or belt. You can imagine that you could drive the windmill slowly upwind. From the frame of reference of the windmill that’s the same as going downwind faster than the wind.
I used to play with bars of soap in the bath tub when I was a kid. If you put your hand down on a wet bar of soap at a slight angle and pressed, it would pop out like a rocket and slide around the tub. That true even though nothing about you was ever moving very fast. You could imagine a point where pushing down at an angle makes the bar pop out faster than you could push the bar from behind.
Similar dealio with the wind.
If you think that’s cool, check out this application of the same principles. An RC *glider* doing 548mph by dipping in and out of the wind current coming over a mountain ridge.
There’s even an idea for doing something similar for spacecraft using the solar wind.
Ultimately, they extract energy from the difference in speed between the air and the water, and how fast you go is determined by how fast you can extract that energy, balanced against drag. Sailboats can do this most effectively moving sideways relative to the wind. Basically the wind hits you from the side, and you direct it backwards, you’re pushed sideways away from the wind(which is balanced by the keel), and also forward. If the wind is pushing you with more force than is needed to maintain speed, you can accelerate, sort of like going faster than you’re pedaling when in a high gear on a bicycle.
You can outrun the wind going downwind, but not with a simple sail(because that would stop extracting energy once you reach the wind speed): https://en.wikipedia.org/wiki/Blackbird_(wind-powered_vehicle) The way that makes sense to me is that the gearing shifts the frame of reference such that the wind sees the turbine not moving downwind, so it can still push against it.
Sailing is not about being pushed by the wind, it’s about exploiting the difference in velocity between the air and the water.
The sail and keel are a wedge between air and water. As the air pushes, it “squirts” the vessel in a direction in much the same way something slippery squirts out of your hand. The speed of that ejection can be higher than the speed of your fingers.
Yes, it is possible to go downwind faster than the wind, as Veritasium [demonstrated on land here](https://youtu.be/jyQwgBAaBag)
The thing that made it click for me was when sails were compared to airfoils. The wind moving over the airfoil creates a lift force and it’s that lift that is being exploited in addition to the wind’s direct push.
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