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Comment by swiper_lux | Hacker News Reader

"Pilot Glenn Ashby successfully sailed the team's wind-powered craft at a speed of 222.4 kilometres per hour on Sunday — in 22 knots (40.7 kph) of wind."

I am sure someone has already explained this elsewhere on HN, but how is it possible for a wind-powered craft to travel faster than the wind?

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The linked background story [1] in the article has a bit more detail on that, worth checking out. Choice quote:

It might sound paradoxical that a vehicle entirely powered by the wind can travel significantly faster than it.

But, as Dr Karl explained, the principle has been exploited by sailors for centuries.

It involves something called "apparent wind" — and relies on the fact that travelling at an angle to the wind can generate more speed.

For a different take I would recommend the propeller-based vehicles [2] that go faster than the wind without using sailing techniques.

[1]: https://www.abc.net.au/news/2022-07-28/americas-cup-winner-g...

[2]: https://en.wikipedia.org/wiki/Blackbird_(wind-powered_vehicl...

Here are the Veritasium videos on the exact matter

https://www.youtube.com/watch?v=jyQwgBAaBag

https://www.youtube.com/watch?v=yCsgoLc_fzI

They are not sailing downwind in this case - they're sailing across the wind, which is why they are able to travel so much faster.

You can sail downwind faster than the wind: https://www.youtube.com/watch?v=jyQwgBAaBag

Sort of a yes and no thing. Once the apparent wind is forward of the beam you are effectively sailing upwind.

This propeller method is a completely different way of doing something similar, but straight downwind.

Imagine pinching an ice cube. One finger applies force like the sails, the other finger applies force like the keel+rudder, and the cube is the vessel.

This...but the ice "cube" has a slight taper to it, like a wedge. As you might recall from high school or freshman Physics, a slight-taper wedge has a large "mechanical advantage". By squeezing it, you use that in reverse - a slow-but-strong push from your fingers (the wind vs. the keel+rudder) results in a fast-but-weak movement of the ice "cube". (Hence the ultra-streamlined shape of the vehicle, low-rolling-resistance tires, etc.)

Wind powered craft are generally powered by deflecting the wind at an angle, rather than using the wind behind them to travel forwards. It's the same reason that sailboats can move into the wind.

It's very hard for me to explain without diagrams, this wikipedia article explains it pretty well: https://en.wikipedia.org/wiki/High-performance_sailing

Echoing the sibling comment from byhemechi, performance sailing vessels generally do not sail directly downwind (or directly upwind for that matter).

The sails are positioned at an angle to the wind, so that they deflect the incoming air in such a way as to propel the craft forward. It might help to imagine air "coming out the back" of the boat.

The fastest set-up is for the boat to travel roughly perpendicular to the wind, with the sails set at (approximately) 45 degrees. In sailing jargon this is called "reaching".

The speed of the craft is determined by the balance between the force exerted on the sails by the air, and the drag (air resistance) and friction between the boat and the water -- or, in the present case, the ground. In a world without friction or drag the boat would accelerate indefinitely, provided there is some wind to deflect.

Note that although it's not generally how fast boats are sailed, "sailing" a wind-powered craft directly downwind faster than the wind is also possible by, for example, using propellers.

The propeller method was explored in a fascinating series of videos by Veritasium and Xyla Foxlin:

https://www.youtube.com/watch?v=jyQwgBAaBag

https://www.youtube.com/watch?v=yCsgoLc_fzI

https://www.youtube.com/watch?v=VUgajGv4Aok.

Imagine pushing a matchbox car on a table with a ruler, where the ruler is perpendicular to the direction of movement. When the car is on an angle, the car will be moving faster than the ruler (wind). The steeper the angle, the faster the car can go relative to the wind.

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