3 ms·

It's kind of counter-intuitive that it's possible to sail against the wind.

by 3rd3 12y ago

davewasthere 12y ago
Or that a boat can sail faster than the wind, say on a broad reach. But it's what makes sailing fun...
muyuu 12y ago
Yep, that's why early sail designs used to be square and looked to maximise sail area vs mast and boom (or frame). Ships can advance into the wind because of: - drag reduction (hull shape, keep, rudder) : sideways force translates to a diagonal forward movement - most importantly, lift: more than pushing the sail, the air pulls the sail sliding from the outter side of it (which is why a lot of the cloth doesn't actually suffer much forces in the middle of the sail which is what initial intuition would tell - by far the most tension happens in the corners). If push is stronger than lift, turbulence on the other side of the sail slows down your boat rather than speeding it up. "Tell tales" stuck in strategic points of the sails allow to quickly tell if the flow is correct, and therefore the trimming is good. Boat speed, flapping and shape also show these forces. http://i.imgur.com/hZlb34r.gif http://i.imgur.com/hZlb34r.gif http://i.imgur.com/uypQGJJ.jpg http://i.imgur.com/uypQGJJ.jpg http://i.imgur.com/5tJLUgF.jpg http://i.imgur.com/5tJLUgF.jpg I don't think there's sail shape or lift in the simulation, but I suspect that the drag is rather low or zero, which allows for some gains into the wind (but real boats do better).
- imakesnowflakes 12y ago
  >I don't think there's sail shape or lift in the simulation.. No. It is a very rough simulation (I am not sure if I can even call that). It only takes into account the force of the wind 'hitting' on the sail and the 'drag reduction'. I am not a physicist or have experience with sailing. I am just a freelance programmer (currently out of work) from India, which means I even haven't seen a real life sail boat. I made this using the stuff I got from Wikipedia and a bit of intuition. So I guess it shows. But I have a question regarding the boat traveling faster than the wind. Isn't it due to the fact that the lift is a constant force, regardless of the speed at which the boat is traveling? But a boat that is using the force of wind hitting the sails, can only sail as fast as the wind, because at that point, there is no wind hitting the sails.
  - Anderkent 12y ago
    Boats can travel faster than the wind because (ignoring surface resistance), they make their own 'wind' thanks to their speed. That wind combines with the true wind in generating the lift on sails even when the boat is travelling at wind speed or higher. ~~This can't happen if the boat is going straight downhill, because the apparent wind would be zero (as the boat is moving with the wind).~~ [ETA: that's not actually true in the general case. You can't have a sailboat move downwind faster than the wind, but if you have a gear mechanism and a turbine you can do it.] But when tacking downhill, the movement of air around sails still generates lift.
  - adrianpike 12y ago
    No, the lift isn't constant. As your speed increases, assuming you're not going dead downwind (very slow) so does your apparent wind speed. http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind
  - muyuu 12y ago
    Hey, the script is cool. A more realistic simulation would probably very quite complex. You can still shape the boat a bit more like a sailboat to make it more intuitive (making it pointy just in the bow end = front). http://www.croatiacharter.me/images/download/1204484832-boat_from_above.jpg http://www.croatiacharter.me/images/download/1204484832-boat... As for the boat traveling faster than the wind, it has more to do with sailing angles than lift proper. The boat can get lift and push from "different air" as it slides perpendicularly with the direction of the wind. It's more of a constant force that produces acceleration until it's cancelled out by forces of resistance. Typically the fastest points of sail of a boat (varies with hull shape, sails and their configuration) are about 45%-75% from the wind direction. http://i.imgur.com/61OGxxx.jpg http://i.imgur.com/61OGxxx.jpg In polar diagrams 0% is into the wind, so 180% is dead downwind and 120% is about 60% away from dead downwind which is aprox the fastest direction for the boat described by that polar diagram.
pmontra 12y ago
The latest America's Cup catamarans sailed faster than the wind both upwind and downwind and that's even more counter intuitive. You probably saw them in the news. Explanation at http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind tl,dr: it's not the real wind that matters but the apparent wind (the self made one you feel when riding a bike) so if build up enough speed you'll always be sailing upwind and build up even more power. This is an even faster boat, build for speed records http://www.youtube.com/watch?v=wnjyusAgk8I http://www.youtube.com/watch?v=wnjyusAgk8I
bdunbar 12y ago
A reason why - I'm told - it's easier to teach kids to sail than adults. Kids don't question a counter-intuitive: just teach them 'point the boat close to the wind, do this to the sail, and go.'

It's kind of counter-intuitive that it's possible to sail against the wind.

by 3rd3 12y ago

It's kind of counter-intuitive that it's possible to sail against the wind.

davewasthere 12y ago

Or that a boat can sail faster than the wind, say on a broad reach. But it's what makes sailing fun...

muyuu 12y ago

Yep, that's why early sail designs used to be square and looked to maximise sail area vs mast and boom (or frame). Ships can advance into the wind because of: - drag reduction (hull shape, keep, rudder) : sideways force translates to a diagonal forward movement - most importantly, lift: more than pushing the sail, the air pulls the sail sliding from the outter side of it (which is why a lot of the cloth doesn't actually suffer much forces in the middle of the sail which is what initial intuition would tell - by far the most tension happens in the corners). If push is stronger than lift, turbulence on the other side of the sail slows down your boat rather than speeding it up. "Tell tales" stuck in strategic points of the sails allow to quickly tell if the flow is correct, and therefore the trimming is good. Boat speed, flapping and shape also show these forces. http://i.imgur.com/hZlb34r.gif http://i.imgur.com/hZlb34r.gif http://i.imgur.com/uypQGJJ.jpg http://i.imgur.com/uypQGJJ.jpg http://i.imgur.com/5tJLUgF.jpg http://i.imgur.com/5tJLUgF.jpg I don't think there's sail shape or lift in the simulation, but I suspect that the drag is rather low or zero, which allows for some gains into the wind (but real boats do better).

imakesnowflakes 12y ago
>I don't think there's sail shape or lift in the simulation.. No. It is a very rough simulation (I am not sure if I can even call that). It only takes into account the force of the wind 'hitting' on the sail and the 'drag reduction'. I am not a physicist or have experience with sailing. I am just a freelance programmer (currently out of work) from India, which means I even haven't seen a real life sail boat. I made this using the stuff I got from Wikipedia and a bit of intuition. So I guess it shows. But I have a question regarding the boat traveling faster than the wind. Isn't it due to the fact that the lift is a constant force, regardless of the speed at which the boat is traveling? But a boat that is using the force of wind hitting the sails, can only sail as fast as the wind, because at that point, there is no wind hitting the sails.
- Anderkent 12y ago
  Boats can travel faster than the wind because (ignoring surface resistance), they make their own 'wind' thanks to their speed. That wind combines with the true wind in generating the lift on sails even when the boat is travelling at wind speed or higher. ~~This can't happen if the boat is going straight downhill, because the apparent wind would be zero (as the boat is moving with the wind).~~ [ETA: that's not actually true in the general case. You can't have a sailboat move downwind faster than the wind, but if you have a gear mechanism and a turbine you can do it.] But when tacking downhill, the movement of air around sails still generates lift.
- adrianpike 12y ago
  No, the lift isn't constant. As your speed increases, assuming you're not going dead downwind (very slow) so does your apparent wind speed. http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind
- muyuu 12y ago
  Hey, the script is cool. A more realistic simulation would probably very quite complex. You can still shape the boat a bit more like a sailboat to make it more intuitive (making it pointy just in the bow end = front). http://www.croatiacharter.me/images/download/1204484832-boat_from_above.jpg http://www.croatiacharter.me/images/download/1204484832-boat... As for the boat traveling faster than the wind, it has more to do with sailing angles than lift proper. The boat can get lift and push from "different air" as it slides perpendicularly with the direction of the wind. It's more of a constant force that produces acceleration until it's cancelled out by forces of resistance. Typically the fastest points of sail of a boat (varies with hull shape, sails and their configuration) are about 45%-75% from the wind direction. http://i.imgur.com/61OGxxx.jpg http://i.imgur.com/61OGxxx.jpg In polar diagrams 0% is into the wind, so 180% is dead downwind and 120% is about 60% away from dead downwind which is aprox the fastest direction for the boat described by that polar diagram.

pmontra 12y ago

The latest America's Cup catamarans sailed faster than the wind both upwind and downwind and that's even more counter intuitive. You probably saw them in the news. Explanation at http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind tl,dr: it's not the real wind that matters but the apparent wind (the self made one you feel when riding a bike) so if build up enough speed you'll always be sailing upwind and build up even more power. This is an even faster boat, build for speed records http://www.youtube.com/watch?v=wnjyusAgk8I http://www.youtube.com/watch?v=wnjyusAgk8I

bdunbar 12y ago

A reason why - I'm told - it's easier to teach kids to sail than adults. Kids don't question a counter-intuitive: just teach them 'point the boat close to the wind, do this to the sail, and go.'