Sailing dead downwind faster than the wind. Impossible, but they did it.
kimballlivingston.com
kimballlivingston.com
Initial claim of impossibility: http://scienceblogs.com/goodmath/2008/12/windpowered_perpetu...
Recognition of truth ;-): http://scienceblogs.com/goodmath/2008/12/the_real_bozo_attem...
One thing wasn't in it that I found cleared this up a lot, though: with a normal sail, wind that gets caught still moves forward relative to the ground, just slower. With this thing, it winds up going backwards. That supplies the extra needed momentum.
Moral of the story: I don't fail completely at fluid dynamics.
It took me a long time to persuade myself that it wasn't a hoax, and when I submitted it here I was half expecting someone to point out some flaw.
Sailboats do not sail faster than the wind when sailing dead downwind. High-performance sailboats absolutely do sail faster than the wind when sailing upwind or on a reach. This is a result of the apparent wind velocity (the velocity measured on the boat) increasing as the boat accelerates into the wind. This apparent wind is what powers the sails -- more apparent wind, higher boat speed. Terry Tao's explanation is excellent: http://terrytao.wordpress.com/2009/03/23/sailing-into-the-wi...
A recent real-world example is the boat that won the America's Cup this time around, Oracle's 90-foot trimaran BOR 90. BOR 90 handily sails several times faster than the wind upwind. Watch chase boats try and keep in a mere 10 or 12 knots of wind as the boat is going 32 knots: http://en.wikipedia.org/wiki/USA_17_(yacht)
Downwind is a totally different story. As you approach dead-downwind you accelerate with the wind until your boat speed reaches the speed of the wind and your apparent wind approaches zero.
As for sailors knowing about the effect, I can assure you it is just as counter-intuitive to us as anyone else. Maybe more so since we spend a lot of time observing how boats behave downwind. I've raced in San Francisco for more than a decade and had this discussion with maybe a dozen racing sailors in the last few months. Things got heated every time. No intuition to be found.
[Edit: corrected sloppy about dead-downwind vs just downwind. As commenters have noted, sailboats will happily go faster than the wind at angles below 90 degrees, but not at 180 degrees.]
http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind
They have a couple of vector diagrams here, which seem to indicate downwind as faster:
http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind#Ve...
If so, you'd increase speed as you went downwind up to a certain point, and then it would decrease again as you near dead downwind.
So this result shouldn't really be that unexpected. Sailboats can't sail directly downwind faster than the wind, but they can travel downwind faster than the wind.
(and its follow-up videos if you liked it)
Really cool. Here's my best explanation of how they achieve a stable speed faster than the wind; this isn't explained that well in the article. (Disclaimer: I'm not exactly a trained physicist/aerodynamics expert.)
They've built a lightweight vehicle with a huge propeller directly linked to the drive shaft ("directly linked" is important here!) With the vehicle at rest, the propeller catches wind and begins driving the shaft; the car itself also catches some wind. Obviously, the car speeds up.
Once the whole contraption is at or near wind speed, the wind obviously stops driving the propeller. It's clear that the vehicle can maintain this speed just by using itself as a sail.
Now, note that the wheels, directly linked to the propeller, are now driving the propeller. Running through stationary air (since the vehicle runs at wind speed, the air is stationary relative to the propeller), the propeller begins accelerating the vehicle!
Of course, as the whole contraption accelerates beyond wind speed, air and ground friction increases, etc. But all of these effects are gradual, and since we have already seen that it can, in fact, accelerate while traveling at wind speed, it will continue accelerating for a while beyond that, eventually reaching some maximum speed above the actual wind speed. Apparently, this is ~2.8x the wind speed for this particular vehicle, which is an impressive engineering feat.
The article says that this offers opportunities for power generation, but I'm not seeing those. Still, very cool.
When it first starts moving, the propeller moves the opposite direction from what you would expect - so that the propeller is actually pushing back against the wind from t=0.
I don't believe the wheels are providing any energy into the process, but I could be wrong. I think it is the shape of the propeller that causes the effect. The article also said it could most likely work on water as well.
The wind energy applications of this propeller design are intriguing.
I'm sure you can think of other ways to store the wind's energy and use it for a speed boost; the interesting thing is that this vehicle can consistently outrun the wind.
Also note that my explanation does not require the vehicle to be on the ground instead of in the water - think "water wheel".
With respect to "tacking": you'll find that Chu-Caroll's article, which smanek linked to, provides the same explanation as I did (skip the initial part where he explains his error and start reading after "let's look at a new diagram"), and mentions the similarity to "tacking" (start at "Once it's moving, what's going on can be described in a bunch of different ways.") However, note that not even the "perfect" sail that Terence Tao assumes in the article linked above achieves more than 2x wind speed; clearly, there is something about this real-world vehicle that makes it move faster than even theoretical (fixed-)sail-powered cars. I think my comment above is a plausible explanation for how this can happen.
EDIT: do follow the link in smanek's comment. It's good. I think I've understood it from that explanation.
The comments of this article have a great debate between skeptics who are particularly insulting/rude who then later admit being wrong.
It seems less paradoxical when you think in terms of energy instead of speed. In other words, thinking about it as (1) drawing energy from the wind and transforming this energy into forward physical motion instead of (2) the wind pushing something faster than itself.
I haven't yet taken the time to understand the details of these explanations, but when I heard about this cart I immediately thought "hmm, the wind is blowing, there's energy all around, so some tricky person can probably figure out how to make use of that energy; hence, it's plausible".
Of course, the flip side of this principle is that physicists can be driven to spend years and years and years trying to extract energy that is obviously there, but that has no known mechanism for getting it out. The classic example is nuclear fusion. Everybody knows it's energetically favorable; nobody knows how to get over or under the activation barrier in any useful way, other than "build something the size of a star".
It will even contract all by itself and ignite.
http://searchyc.com/submissions/sail?sort=by_date
Doing this has often led me to articles of interest I would otherwise not have found, and I believe that cross-referencing like this is potentially of use, a little now, potentially more later.
However, with a karma of nearly 14k but an average of 2.29 it's clear that such cross-referencing rarely gets upvotes. A quick check of my comments shows that:
http://news.ycombinator.com/threads?id=RiderOfGiraffes
Net result is that I'm probably mostly wasting my time. More than once I've considered stopping, more than once my mild obsessive/compulsive tendencies have caused me to carry on. I am again considering stopping and using HN differently. My "contributions" are rarely upvoted much, and that suggests my interests aren't that good a fit for the current population.
"My 'contributions' are rarely upvoted much, and that suggests my interests aren't that good a fit for the current population."
For what it's worth, I have the hacker friends extension (https://chrome.google.com/extensions/detail/mkdhfabjcebcgnpg...) set to highlight all of your comments and submissions.
Not least, as I'm running a closed community I'm paying very close attention to the effect, and to the discussions that occasionally pop up.
> I have the hacker friends extension ... set
> to highlight all of your comments and submissions.
Conversely, http://news.ycombinator.com/item?id=1881992 took far too long (over an hour) to write, was far more informative, and earned 9 upvotes. At one point, http://news.ycombinator.com/item?id=1882453 had more upvotes - I guess because of the pig?
I'm not ashamed of the first comment, but it saddens me to see that the second got so far fewer votes... and this seems to be the case for many other comment(s/ors) as well.
For what it's worth, I know your username and read your comments with interest.
The sailing against the wind is not an intuitive, mostly because there is no simple mathematical formula for the underlying physics. However one can do some convincing models. I have some matlab code which can, probably in the simplest way, demonstrate theoretically the principle. In short, if it is possible do sail down the wind faster the the wind than by having rotating sail (blades) DDWFTTW is possible. See: http://goo.gl/TtEfM
It is clear that the angle of attack of the sails is not at 90 degrees to the wind therefore strictly speaking perhaps this is not sailing dead downwind.
It is well known that sail driven vehicles can greatly exceed actual wind speed due to "apparent wind" effects among others.
The (false) idea is, if a 20km/h wind pushes your back, once you're close to 20km/h, you've got almost zero relative wind, so you shouldn't be able to accelerate further.
Boats, including the hydroptere (which is special because of vertical lifting, but behaves as a normal sailboat in the horizontal plane), can go faster than the wind when then wind comes from the front, not when it comes from the back.
The essential difference is, boats use water resistance sideways, i.e. they tend to behave as if they were on railtracks. The demonstrator in the article exploits rolling resistance, which boats try to minimize.
True, but it may be worth of mentioning that no regular sailboat (i.e. without underwater propulsion) can go directly to upwind (0 degrees). For modern sailboats, the maximum upwind direction is roughly 45 degrees, the sidewind (90 degrees) being the most optimal.
[1] http://terrytao.wordpress.com/2009/03/23/sailing-into-the-wi...
No, the article is about sailing dead downwind faster than wind. No regular sailboat can do that.
Here's wikipedia's take on that: http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind
http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind#Sa...
In order to prove something impossible you have to do two things, a) have a complete mastery over every property and tool that can exist and b) attempt to apply or implement every tool or property to the problem in every sequence.
That's why it's an interesting story.
That said, yes, it's use as a shorthand is misleading and annoying.
A corollary is that if you want to expand your skills, work on impossible problems. A second corollary is that more entrepreneurs should be working on impossible problems, because the small number that are actually impossible is outweighed by the heavy returns for the ones that aren't impossible but scare off competitors because they look that way.
It isn't about transferring "speed" from the wind, it's about transferring energy. Traditional methods, such as sails, didn't have a high enough wind capture/energy output ratio.
By using a propellor they are capturing and using the energy from the wind much more efficient than they would with a sail.
Obviously, that doesn't (and I can't) explain the finer detail, but I don't think it's hard to understand conceptually.