
Sailing dead downwind faster than the wind. Impossible, but they did it. - nl
http://kimballlivingston.com/?p=3971
======
smanek
This is a really good explanation I saw a while back of the phenomenon. This
debunker (a PhD @ Google) originally said it was impossible - and then
realized he was wrong.

Initial claim of impossibility:
[http://scienceblogs.com/goodmath/2008/12/windpowered_perpetu...](http://scienceblogs.com/goodmath/2008/12/windpowered_perpetual_motion.php)

Recognition of truth ;-):
[http://scienceblogs.com/goodmath/2008/12/the_real_bozo_attem...](http://scienceblogs.com/goodmath/2008/12/the_real_bozo_attempts_to_aton.php)

~~~
jedbrown
I prefer Terry Tao's explanation
([http://terrytao.wordpress.com/2009/03/23/sailing-into-the-
wi...](http://terrytao.wordpress.com/2009/03/23/sailing-into-the-wind-or-
faster-than-the-wind/)), carry the result over to land by replacing the "keel"
with gears.

~~~
Jun8
As always, Tao's explanation is crystal clear, takes you from the absolute
basic layman level to as deep as you want to go.

------
jbyers
Several comments in this thread say something to the effect of "sailboats do
this all the time" or "sailors have known this for years."

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...](http://terrytao.wordpress.com/2009/03/23/sailing-into-the-wind-or-
faster-than-the-wind/)

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.]

~~~
richardw
Nice links, thanks. I respectfully disagree with the upwind/downwind speeds
though. In my experience the most force on the sailboat/windsurfer (and
resulting speed of the boat) comes from sailing at a broad reach, roughly 45%
downwind. Linked from your USA_17 article:

<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...](http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind#Vector_diagrams_and_formulas)

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.

~~~
jbyers
Very sloppy on my part. Corrected, thanks!

------
imurray
A brilliant video on this issue is _Under the ruler faster than the ruler_ :
<http://www.youtube.com/watch?v=k-trDF8Yldc>

(and its follow-up videos if you liked it)

~~~
baxter
This is excellent, I was very surprised at the result. It's made me rethink
some basic assumptions. Thanks very much! :)

~~~
jules
Which basic assumptions?

------
JoachimSchipper
[EDIT: I started typing this up before smanek's post. It's essentially the
same argument, but I intentionally left out the formulae.]

 _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.

~~~
illumin8
Are you sure that is correct, that the wheels are powering the propeller at
some point? The article explained it completely differently. The article said
that the effect is similar to when a sailboat tacks downwind sharply, they can
briefly accelerate beyond wind speed. The propeller is essentially shaped as
if each side is a sail continuously tacking, so the force from each of the two
"sails" drives the propeller in a circular motion.

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.

~~~
stygianguest
Yes, the wheels drive the propeller directly. Read the articles posted in the
first comment. They will explain it. Very short summary: The prop creates an
air cushion that acts as a virtual sail.

------
pmjordan
This seems really interesting but I found the article almost unbearable to
read - it seems like the author's train of thought directly transcribed. The
project's website is <http://www.fasterthanthewind.org/> though I'm still
looking for a good explanation on how it actually works.

EDIT: do follow the link in smanek's comment. It's good. I _think_ I've
understood it from that explanation.

~~~
arn
[http://www.popsci.com/cars/article/2010-06/wind-powered-
car-...](http://www.popsci.com/cars/article/2010-06/wind-powered-car-looks-
odd-answers-tricky-physics-question)

The comments of this article have a great debate between skeptics who are
particularly insulting/rude who then later admit being wrong.

------
techiferous
Very cool.

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.

~~~
mechanical_fish
Yes, this is why the concept of energy is given such a prominent position in
physics. It is a great tool for predicting what might happen even when the
detailed mechanism is unknown or seemingly impossible.

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".

~~~
jacquesm
> build something the size of a star

It will even contract all by itself _and_ ignite.

------
RiderOfGiraffes
If you like that, you'll like this:

<http://news.ycombinator.com/item?id=1877022>

<http://news.ycombinator.com/item?id=1874846>

~~~
JoachimSchipper
Do you have a filing system, or just a ridiculously good memory? This is
hardly the first time you've provided a list of similar or identical
submissions...

~~~
dwwoelfel
"I remember things like this and knew the search terms. Then
<http://searchyc.com/> is your friend." -RiderOfGiraffes

<http://news.ycombinator.com/item?id=940276>

~~~
RiderOfGiraffes
Exactly. In this case just search for "sail" in the submission title:

<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.

~~~
dwwoelfel
I occasionally find them useful, at a rate of about one per week. I didn't
upvote them in the past, but I will now that I know you care.

"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...](https://chrome.google.com/extensions/detail/mkdhfabjcebcgnpgnhefebefiabhmbfn))
set to highlight all of your comments and submissions.

~~~
RiderOfGiraffes
It's not that I care so much - I don't really care about the karma - it's just
that it's a source of information about the community and my actions within
it. Karma is intended to reinforce "good behavior" and discourage "bad
behavior" and so I pay attention to it.

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.
    

Gosh. <blush> Thanks.

~~~
JoachimSchipper
For better or worse, karma has little to do with good.
<http://news.ycombinator.com/item?id=1872693> was near the top, mixed snark
and a (decent) reference, cost maybe 30 seconds to type, and earned 36 upvotes
as of this post.

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.

------
neilxdsouza
Let C be a cylinder positioned at the center of the propeller with the same
radius as the propeller and whose length = the velocity of the wind. Based on
the density of the air and the volume of air contained in the cylinder and
velocity of the wind we can calculate the total momentum available. There are
theorems on how efficiently you can extract this energy - based on propeller
theory. let this number be defined as "eta". if M * V * eta is the momentum
available and the mass of the vehicle is less that M * eta where M * V * eta
is the momentum extracted i.e. m < M * eta - then the velocity of the vehicle
"v" must be greater than V the velocity of the wind. If you notice the size of
the propeller in the picture - it is huge - so that the mass of the wind it
intercepts > than the mass of the vehicle

------
gvb
FWIIW, this is the same car as written up in Wired
[http://www.wired.com/autopia/2010/06/downwind-faster-than-
th...](http://www.wired.com/autopia/2010/06/downwind-faster-than-the-wind/)
and discussed in YC <http://news.ycombinator.com/item?id=1397954> (my
explanation, which I like to think is correct -
<http://news.ycombinator.com/item?id=1398738>).

------
marcinofulus
Nice story,

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>

------
colanderman
I don't see why anyone would think this to be _impossible_. Neglecting
friction, a vehicle can travel at an arbitrarily high speed without requiring
energy input. Proof by gedanken construction: construct a vehicle with an
arbitrarily large collapsible wind turbine. Generate and store electricity
while at rest, for say 1 minute. Collapse & store wind turbine, and use stored
electricity to drive at ludicrous speed for 9 minutes. QED.

------
SeanDav
Like most people my gut reaction was "that's not possible" and in fact I
believe that depending on how you define "sailing dead downwind" it may or may
not be possible.

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.

~~~
InclinedPlane
It is not sailing in any conventional since. The purpose of the vehicle is to
demonstrate whether _any_ wind powered vehicle is capable of traveling
directly downwind faster than the wind. Many people believed, or still
believe, that it's impossible.

------
harscoat
" _These loonies were pursuing a pointless goal, doomed to failure, but there
was genuine merit in the myth and their enthusiasm._ "

------
snippyhollow
It's been long since the sailors out there know that fact. Ships create their
own wind when close hauled. Check out
<http://en.wikipedia.org/wiki/Hydropt%C3%A8re> for a particularly striking
evidence!

~~~
fab13n
The point that made it counter-intuitive, here, is the "under the wind" part.

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.

~~~
Yaggo
> [Boats] can go faster than the wind when then wind comes from the front, not
> when it comes from the back.

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.

------
qjz
Why can't I get the word "deathtrap" out of my mind whenever I see a picture
of this contraption?

------
Jun8
So, the trick is to utilize the three dimensions. Can this be generalized to
_n_ dimensions? How fast can a four dimensional sailboat go upwind?

------
hook
Video: <http://www.youtube.com/watch?v=5CcgmpBGSCI>

------
Devilboy
LAND sailing. I don't think you could do this on water.

~~~
pmjordan
Apparently sailboats do this all the time. Who knew? I can only assume they
somehow use tacking techniques to travel against the (relative) wind. Though
it's not clear to me how they accelerate past the wind speed in the first
place.

~~~
Robin_Message
I've seen an explanation [1] that in water it depends on having _two_
aerofoils — one in the air and one in the water (they should be called
fluidfoils. I guess the one in the water, the keel, is acting as a hydrofoil.)
At any rate, in theory, with two fluidfoils and fluids with any different
velocities you can go at any velocity (i.e. in any direction, at any speed.)

[1] [http://terrytao.wordpress.com/2009/03/23/sailing-into-the-
wi...](http://terrytao.wordpress.com/2009/03/23/sailing-into-the-wind-or-
faster-than-the-wind/)

~~~
snippyhollow
Yes, check this out: <http://en.wikipedia.org/wiki/Hydropt%C3%A8re>

------
maeon3
I've noticed a nasty trend where people use the word "Impossible" as another
word for difficult, time consuming or hard.

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.

~~~
jasonkester
In this case though, nobody was using it to mean "hard". There really were PhD
Physicists arguing at length that it was physically impossible to do this.

That's why it's an interesting story.

~~~
randallsquared
No one ever argued that it was impossible to use wind energy to move faster
than the wind. The real problem here was describing the effect as _sailing
directly downwind_. The "sails" (on the prop) do not go directly downwind, and
there's stuff going on that isn't best described as "sailing". But by
describing it that way, people managed to extract blustery pronouncements and
explanations of why it was impossible (which _sailing_ downwind faster than
the wind _is_ ).

------
sh1mmer
It's obvious to me why this is possible.

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.

~~~
jules
It's not about the efficiency, it's about the relative speeds: the blades of
the propellor are moving relative to the vehicle whereas a sail is stationary.

