The Little Cart
Wind-powered vehicle claims look like hot air.
By Charles Platt
Can a wind-driven vehicle outrun a tailwind? A Florida tinkerer named
Jack Goodman claims it can, and he put a video on You Tube to prove it. Some
sailing enthusiasts became convinced that he was on to something, while
others were equally sure it was a hoax — and I decided to put it to the test.
HOW BOATS WORK
First, we need a quick primer on seafaring aerodynamics. The oldest boats had rectangular sails
that were simply pushed by the wind. This system
prevailed for about 4,000 years, until the 9th
century A.D., when some inventive mariners found
that a triangular sail enabled boats to move into
a headwind at an angle, and when the sail pivoted
around a mast, they could turn back and forth to
tack a zigzag path directly upwind.
Figure 1 (page 62) shows a sailboat traveling
north, into a wind blowing from the northwest. If
the mariner adjusts his sail at a shallow angle to
the breeze, the air creates a force that can be split
into two components, one pushing the boat ahead,
the other trying to push it sideways. If the boat has
a rudder and a keel to fight the sideways force, it
moves ahead. The sail’s forward curve also acts as
an airfoil, slowing the air in back while forcing it to
move faster around the front. This adds lift to the
The idea of sailing into a partial headwind seems
counterintuitive, as if we’re getting something for
nothing. Perhaps this explains why some boating
folk seemed so willing to believe Jack Goodman’s
surprising claim. And because triangular sails lose
their advantages in a tailwind, modern boats do
better sailing across the wind than dead downwind,
when you might think a boat would go the fastest.
Given how effectively boats travel upwind, and
that they can even exceed wind speed while running
across the wind, it seems unfair that when they go
the same direction as the wind they’ll never overtake it. Thus, when Goodman claimed he had a way
to overcome the frustration of being overtaken by a
tailwind, people were predisposed to listen.
To demonstrate his idea, he built a three-wheeled
cart on which he mounted a big, homemade propeller, with its shaft connected to the rear wheels
via a toothed belt. He made a video (makezine.
com/go/jackgoodman) showing this contraption
rolling along, first in response to a tailwind, and
then, amazingly, moving faster than the tailwind, as
demonstrated by a flag hanging in back. Note that
if this happens, the vehicle begins to encounter a
slight headwind relative to its motion.
Goodman claimed that the cart could do this
because its wheels turned the propeller, which
pulled it forward. This was somewhat puzzling
because there was no power to turn the wheels.
Was he just telling whoppers and pranking his
sailing buddies? In the video, it all seemed to work,
leading many people to believe that a guy in a garage
in Florida had come up with something that had
never occurred to anyone else in the history of
boats and aviation, from the ancient Greeks through
Leonardo da Vinci and on up to Northrop Grumman.
Clearly, it was time to build the MAKE version.
THE WIND TEST
Instead of a full-sized replica, I constructed a
tabletop model. It would be easier to test and should perform better, since a size reduction >>