Can a wind-driven car travel downwind faster than the wind?

[Janot]

This puzzles me. A car driven solely by wind is driving downwind, and appears to be travelling actually much faster than the wind which is driving it.

Is this a hoax, or can it be explained?

By the way, I haven't posted much recently, I type very slowly.

[Tony Williams]

Racing yachts do this, I believe, so I don't see why cars couldn't.

[chaggle]

Racing yachts do this, I believe, so I don't see why cars couldn't.

Not directly downwind. On a broad reach (nearly downwind) as the yacht gathers speed the apparent wind moves forward and increases and the yacht is able to sail in this faster than the actual wind. For this reason on a dead downwind leg of a course, yachts will perform two broad reaches with a gybe in the middle.

I'll stick my neck out and say that it is not possible to use windpower to move faster than the wind dead downwind.

Incidentally there are two threads about this on jref - one has 737 posts and the other has 3231

[Tony Williams]

Not directly downwind. On a broad reach (nearly downwind) as the yacht gathers speed the apparent wind moves forward and increases and the yacht is able to sail in this faster than the actual wind. For this reason on a dead downwind leg of a course, yachts will perform two broad reaches with a gybe in the middle.

I'll stick my neck out and say that it is not possible to use windpower to move faster than the wind dead downwind.

I accept the correction - I just recall reading about a yacht which could sail "faster than the wind" many years ago, but your explanation makes sense.

[chaggle]

In answer to the OP - I suppose it could hinge on the meaning of "downwind". If it means that the actual wind is coming from a general direction of behind the vehicle then the "broad reach" scenario is included and speeds greater than actual wind speed are possible.

If downwind means dead downwind - I say it is not possible.

ETA a "downwind leg" of a sailing race course is generally taken to be one where the start point of the leg is upwind of the end point - it doesn't have to be a dead downwind leg so the "general direction" definition is correct in that particular case.

[skbuncks]

The short answer is yes, it can. So long as it's a wind powered prop cart, which it is.

The long answer, and it is a very long answer, is over at JREF

A thread that began at JREF back on 11/9/09, Downwind faster than the wind may finally be approaching it's finale after hundreds of pages of debate.

When the discussion began on JREF, much of it centered on whether or not a small prop cart a member-spork-built that could advance up a treadmill was proof that under the right conditions it could go directly downwind faster than the wind. According to his Galilean transformations, all the vector forces were the same. So counter intuitive was this claim, that even one Nobel winning physicist, Dan Kammen, stated he believed it would violate conservation of energy.

As was announced in the original thread last October, spork found a sponsor-Joby Energy-and has now built and tested a drivable wind cart. Since then Google has also become a sponsor, and in late March spork took it to the North American Land Sailing Association (NALSA) meet at Ivanpah Lake in the California desert. They made 3 different runs, one of them self starting, which by their GPS calculations achieved 2.5x wind speed. NALSA official Bob Dill-former holder of the land yacht speed record-observed the test, and they have agreed to certify the cart's performance at their next meet coming later this spring.

Below are links to videos of the 3 trial runs of the sporkmobile. Notice the direction of the streamers attached to a movable bar, and how it out-distances some dust from the truck used to run along with the cart for tracking it's speed and making the videos.

First run: http://www.vimeo.com/10476453
Second run: http://www.vimeo.com/10477373
Last run: http://www.vimeo.com/10476216

For those wishing to learn more about the genesis of this ongoing debate, below are the 3 threads started on various tangents it of it posted here on JREF. Below them are the 7 parts posted on RDF. For a detailed description of the cart's construction including more videos and lot of pictures, there is a link to spork's blog in my sig, along with one to TalkRational where most of the current battle still rages. In the immortal words of Papa John, "What a long, strange trip it's been."

JREF Threads:
http://forums.randi.org/showthread.php?t=128483
http://forums.randi.org/showthread.php?t=131646
http://forums.randi.org/showthread.php?t=130705

RDF Threads:
http://forum.richarddawkins.net/viewtopic.php?t=73123
http://forum.richarddawkins.net/viewtopic.php?t=74154
http://forum.richarddawkins.net/viewtopic.php?t=74988
http://forum.richarddawkins.net/viewtopic.php?t=75643
http://forum.richarddawkins.net/viewtopic.php?t=76400
http://forum.richarddawkins.net/viewtopic.php?t=77451
http://forum.richarddawkins.net/viewtopic.php?t=80402

ETA: You can even build a fully functional LEGO version in under five minutes if you like. The design is somewhere in one of the above links I believe.

[chaggle]

The short answer is yes, it can. So long as it's a wind powered prop cart, which it is.

Is there an equally short explanation?

The long answer, and it is a very long answer, is over at JREF

I just can't look through that lot

[skbuncks]

Is there an equally short explanation?


I just can't look through that lot

Can't help I'm afraid. I'm sure I've read an explanation, which I didn't understand, but I gave up following the various JREF threads not long after my dis non (curse the vino for wiping my mind of the appropriate prefix, ahah, wait think I've got it) inability to understand became apparent.
I would ask over at JREF, however the flaming/misinformation/totally unhelpful demands to just read the threads/trolling such a question would generate is not really worth the bother.

skb

[chaggle]

Well I've done a bit of reading and "downwind" does mean "dead" or "directly" downwind. I don't believe it can be done.

[Mojo]

I suspect that Janot's avatar may be relevant here.

[chaggle]

I suspect that Janot's avatar may be relevant here.

That's the way I'm thinking.

[eyytee]

Well I've done a bit of reading and "downwind" does mean "dead" or "directly" downwind.

Yes, the aim is to go directly downwind, faster than wind, powered only by wind, without using stored energy for propulsion.

I don't believe it can be done.

Well, it has been done. The North American Land Sailing Association has verified a record based on the conditions above. It is 2.8 x true windspeed:
http://www.nalsa.org/DownWind.html

Here an article by one of the NALSA observers:
http://www.boats.com/boat-content/20...ir-the-record/