3 ms·

Imagine a sailboat pointed 90° relative to the wind so that the wind is coming right at its side. The sail is curved so that it takes that wind and redirects it

Imagine a sailboat pointed 90° relative to the wind so that the wind is coming right at its side. The sail is curved so that it takes that wind and redirects it towards the rear of the boat, giving it forward thrust. The boat starts moving forward. But, because the wind is perpendicular to the boat, even when its moving the wind is still coming in at the same velocity, so it's still producing thrust. If you can get the boat efficient enough, it can harness enough of that wind energy to reach a velocity along the sailboat's line of motion higher than the velocity of the wind relative to the ground. Note that boats can only do this when sailing at least somewhat offcenter from the wind direction. When sailing directly downwind, the faster the boat goes, the slower the wind is relative to the boat, leeching away thrust.

bloomer 7y ago
https://en.m.wikipedia.org/wiki/Blackbird_(land_yacht) https://en.m.wikipedia.org/wiki/Blackbird_(land_yacht) Was built to demonstrate that you can sail directly downwind faster than the wind as well.
- hinkley 7y ago
  That is not a conventional land yacht, but I'm sure there are land yachts that can go faster than the wind, by exactly the same principle as sailboats. The motive force on that vehicle is a gear and chain between the prop and the wheels. And prop blades are set at an angle to the wind.
mannykannot 7y ago
There is one complication here: as the boat accelerates, the relative wind moves forward. Consider a boat on a beam reach, where the wind over the water is at 90 degrees to its track. If the boat is travelling at wind speed, the apparent wind over the deck is at 45 degrees to the bow. The useful angle of attack for airfoils goes up to about 15 degrees, so let us assume that the sails are set to this. Therefore, the chord of the sail is at 30 degrees to the boat's track, its lift is at 60 degrees to its track, and so half the total lift is in the direction of motion [1]. So long as this exceeds the total drag of the boat, from the water and the air, then it will continue to accelerate. [1] To simplify (and to go faster!) assume a multihull sailboat, hydrofoil, or a dinghy with its crew hiked out so that is not heeling appreciably, as when a boat is heeled, a component of its sails' lift is directed downwards.

bloomer 7y ago

https://en.m.wikipedia.org/wiki/Blackbird_(land_yacht) https://en.m.wikipedia.org/wiki/Blackbird_(land_yacht) Was built to demonstrate that you can sail directly downwind faster than the wind as well.

hinkley 7y ago
That is not a conventional land yacht, but I'm sure there are land yachts that can go faster than the wind, by exactly the same principle as sailboats. The motive force on that vehicle is a gear and chain between the prop and the wheels. And prop blades are set at an angle to the wind.

mannykannot 7y ago

There is one complication here: as the boat accelerates, the relative wind moves forward. Consider a boat on a beam reach, where the wind over the water is at 90 degrees to its track. If the boat is travelling at wind speed, the apparent wind over the deck is at 45 degrees to the bow. The useful angle of attack for airfoils goes up to about 15 degrees, so let us assume that the sails are set to this. Therefore, the chord of the sail is at 30 degrees to the boat's track, its lift is at 60 degrees to its track, and so half the total lift is in the direction of motion [1]. So long as this exceeds the total drag of the boat, from the water and the air, then it will continue to accelerate. [1] To simplify (and to go faster!) assume a multihull sailboat, hydrofoil, or a dinghy with its crew hiked out so that is not heeling appreciably, as when a boat is heeled, a component of its sails' lift is directed downwards.