A Wind-Powered Vehicle that Travels Faster than the Wind?

That's what these "Directly Down Wind Faster Than the Wind" enthusiasts have claimed to have built.   Otherwise known as DDWFTTW (in the hipper circles of geekdom) the video below displays their vehicle in action.    The video has been making rounds in tech blogs, so I figured I might as well weigh in also.   The claim goes something like this:

1) Wind pushes the vehicle forward, spinning the wheels, which...

2) Sends energy to the propeller, pushing the vehicle forward faster, which...

3) Spins the wheels faster, which...

4) Repeat #2

While some might assume they're trying to build a perpetual motion machine (i.e. develop free energy) the explanation is probably fairly simple.   The weathervane in the video indicates the vehicle isn't traveling exactly down wind all the time, but is instead meeting gusts of wind that tack at an angle.   Sailing boats and landsail cars use a similar principle to gain speeds faster than the wind itself.   As long as the wind can place pressure on the sail the boat continues accelerating in the direction its pointed.  

However, these good folk are claiming their vehicle is indeed moving directly downwind 50mph in a 20 mph wind.  This makes all the difference in the world.   However, their camera hardly shows the weathervane.  When it does, we can see the vehicle accelerating in those moments that the weathervane is angled away from the vehicle's path.     I take them at their word that their vehicle traveled downwind, but I imagine that these desert winds are not lab quality.   Gusts of wind occuring at slightly shifting angles might duplicate the effects of tacking while using a large propeller.  

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