# Down wind faster than the wind

Discussion in 'Physics & Math' started by Steve100, Jan 2, 2009.

1. ### Steve100O͓͍̯̬̯̙͈̟̥̳̩͒̆̿ͬ̑̀̓̿͋ͬ ̙̳ͅ ̫̪̳͔OValued Senior Member

Messages:
2,346
Down wind faster than the wind.

I only heard of this today. It describes a machine that can travel down wind faster than the wind that is powering it.

Here are some videos that got me looking at the idea.

http://www.youtube.com/watch?v=xHsXcHoJu-A a car that uses its wheels to turn a propeller.
http://boingboing.net/2008/12/12/downwind-faster-than-1.html A different device that seems to work on the same principals.

What do you make of this?
Do you beleive it is real?
Can you explain how it works?

I have read a few different articles on such devices, and none (whether they say it will work or not) have convinced me. But I am leaned towards them working.

A google search for "Down wind faster than the wind" or "dwfttw" will bring up a lot of sites addressing these devices.

Last edited: Jan 2, 2009

3. ### phlogisticianBannedBanned

Messages:
10,342
Of course you can travel 'downwind' faster than the wind, ... although you actually travel at an angle, and it the component of the travel in the same direction as the wind, that is faster than it.

Sails are wings, and they make 'lift' by flying across the path of the wind. So you 'fly' across the path of the wind, far faster than the wind is travelling (a large sail can catch a lot of energy) and it's that force and the weight and drag of the boat that determine the speed.

5. ### Steve100O͓͍̯̬̯̙͈̟̥̳̩͒̆̿ͬ̑̀̓̿͋ͬ ̙̳ͅ ̫̪̳͔OValued Senior Member

Messages:
2,346
What about the device in the first video?

7. ### iceauraValued Senior Member

Messages:
30,994
Ice boats exceed wind speed routinely, while "reaching" (which is downwind). But not "directly" down wind.

By local lore, the contemporary human speed record - barring falling off a really high cliff - was once set by an ice boat on Lake Pepin, MN. Back in the 1800s, IIRC, guy got an ice boat up around something like 118mph - this was back when a mile a minute was a goal of the really hot railroad engines.

Last edited: Jan 2, 2009
8. ### Captain KremmenAll aboard, me Hearties!Valued Senior Member

Messages:
12,738
The second video is very interesting.
I recommend people to make the effort to watch it.
What happens with the ruler is not what common sense says should happen.

Last edited: Jan 2, 2009
9. ### Steve100O͓͍̯̬̯̙͈̟̥̳̩͒̆̿ͬ̑̀̓̿͋ͬ ̙̳ͅ ̫̪̳͔OValued Senior Member

Messages:
2,346
I edited the link and it now works.

10. ### Captain KremmenAll aboard, me Hearties!Valued Senior Member

Messages:
12,738
Thanks. Amazing.
This looks similar to the plane on the conveyor belt controversy that drove everyone loopy.

I look forward to when the Mythbusters take it on.

In http://boingboing.net/2008/12/12/downwind-faster-than-1.html
Any ideas why pushing the wheel with the edge of a ruler has a different effect to moving it with your finger.
What occurs looks impossible.

Last edited: Jan 2, 2009
11. ### iceauraValued Senior Member

Messages:
30,994
The fingers aren't pushing in the same direction as the ruler.

12. ### Captain KremmenAll aboard, me Hearties!Valued Senior Member

Messages:
12,738
When the operator moves the large wheel with his fingers, he also moves the smaller wheels with his fingers. This is just to show that the wheels act as cogs.

13. ### John99BannedBanned

Messages:
22,046
the pressure applied by the ruler is forcing the tire away. expulsion.

all that is need is a slight angle.

14. ### Steve100O͓͍̯̬̯̙͈̟̥̳̩͒̆̿ͬ̑̀̓̿͋ͬ ̙̳ͅ ̫̪̳͔OValued Senior Member

Messages:
2,346
On one of his follow up videos he does it with different angles to the same effect.

15. ### Captain KremmenAll aboard, me Hearties!Valued Senior Member

Messages:
12,738
So you think it is a trick then?

16. ### rpennerFully WiredValued Senior Member

Messages:
4,833
The best test of such a sail car is that it should continue to move faster than the wind when the air is at rest. It is clear in the classic 1-D setup (parallel wind and movement, frictionless bearing as wheels) it is impossible for a classic sailcar to continue to accelerate faster than the wind.

The two most likely reasons for an honest experimenter giving such results are 1) measurement error or 2) poor experimental design such as the car is actually going down hill which would be difficult to demonstrate was not the case from video.

But this sail car is not a sail car. It's a propeller car. And the propeller is not being driven by the wind, but by the wheels. And this can work, because it is driven by the difference in speed between air and ground, not (as for a sail car) by the wind itself.

http://scienceblogs.com/goodmath/2008/12/the_real_bozo_attempts_to_aton.php

And there will be a Myth-Busters (locally produced!) episode to cover it.

http://boingboing.net/2008/12/03/directly-downwind-fa.html

17. ### Captain KremmenAll aboard, me Hearties!Valued Senior Member

Messages:
12,738
It's the ruler experiment that I can't fathom.
Unless the whole thing is some kind of magician's trick, how can energy directly applied to a wheel to turn it in one direction result in its turning in the opposite direction?
I've looked at the video to see if there is any appreciable angle on the ruler which would cause an unexpected effect, and it does not seem that there is.
http://boingboing.net/2008/12/12/dow...er-than-1.html

18. ### rpennerFully WiredValued Senior Member

Messages:
4,833
You just have to solve the equations of motion for all the pieces simultaneously, as the system of wheels is a rigid system. It's not this-causes-that-which-causes-that type logic. It's a purely linear relationship between the differential speeds of the top and bottom surfaces and the motion of the wheeled system.

19. ### iceauraValued Senior Member

Messages:
30,994
The way I visualize it is to mentally draw a line(s) or path from the contact point of the ruler through the upper wheel to the forward bottom wheel, and imagine the force of the ruler applied along that path. If the imagined path is in segments imagined as rigid sticks, the first dropping straight to the axle of the upper wheel, the second connecting the two axles, and the third from the bottom wheel axle to the ground, that seems to help my intuition.

Then it all starts to come together, for me. The friction of the ground provides the traction that rotates the wheels and causes them to roll along the ruler (that the ruler is moving makes no difference - if the ground were moving instead, the same effect would be seen).

If there were no ground friction, the entire apparatus would slide along with the ruler's horizontal push.

If there were no ground friction, and the axles of the wheels were pinned in place, by inertia even, then the wheels would rotate like gears in the initial intuitive directions relative to the ruler.

Does that help?

Last edited: Jan 5, 2009
20. ### ThinAirDesignsRegistered Member

Messages:
1
This is absolutely correct iceaura. Not only does their speed across the ground exceed that of the wind (by quite a number of multiples), but their steady state downwind VMG (velocity made good) or the downwind component of their angled path also exceeds that of the wind routinely.

Also correct. The chassis of the ice-boat and its sailing rig are forced to take the same exact path -- something that the cart's chassis and it's sails are not.

While the chassis of the cart travels DDW, it's two sails take the same angled path through the air as those of an ice-boat on a broad reach.

It's this 'independent packaging' of the sails and chassis that leave most confounded. Sails *cannot* go steady state directly downwind faster than the wind, but there's no rule stating that a chassis and sail must be constrained to the same path.

JB

21. ### spidergoatpubic dioramaValued Senior Member

Messages:
54,036
I have no comment on the original post, but I did see a working boat design that uses a windmill instead of a sail to power a boat directly into the wind.

22. ### Steve100O͓͍̯̬̯̙͈̟̥̳̩͒̆̿ͬ̑̀̓̿͋ͬ ̙̳ͅ ̫̪̳͔OValued Senior Member

Messages:
2,346
Thanks. Especially rpenner.

23. ### John99BannedBanned

Messages:
22,046
just a guess.