He was also a successful ocean racing skipper and an amateur photographer. His seaborne pusruits included racing his own boats very successfully primarily in Southern California , plus he had extensive crewing experience on longer ocean racing yachts. He has authored numerous magazine articles on sailing aerodynamics and sailboat performance. How do sails really create thrust?
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Gentry was the Boeing engineer who first taught sailors aerodynamics. In the Revised Edition, the authors used computer testing to show where the wind speeds up around a sail plan and where it slows down. And why and by how much? Circulation What if there are two flows — rather than one — around an airfoil like a sail, wing, keel, or rudder?
To that end, mathematicians added another type of flow, called circulation, to the normal wind velocity. Circulation is a special mathematical solution where a second flow rotates around the airfoil. The circulation flow is greatest near the foil and progressively less moving away from it.
In the mathematical solution, circulation air speeds are adjusted so that the Kutta condition at the trailing edge, or leech, is satisfied; that is, the calculated airflow speeds and pressures are the same off both sides of the trailing edge. Starting vortex spinning counterclockwise is formed. As the card reaches the center, the Circulation Flow appears These two forces look something like this: The smaller Starting Vortex wheel, which spins counterclockwise, and the bigger Circulation wheel, which spins clockwise.
Back to the bathtub. Trying to prove that linear flow alone produces lift has confused sailors for decades. This gives the speed differential, top to bottom, the pressure differential, and then the lifting force. With the combined efforts of the two flows, airplanes can fly upside down. Similarly, even though a sail is a thin membrane with almost no measurable difference in distance from one side to the other, the combination of the circulating flow and noncirculating flow allows a sailboat to sail to weather.
Above: Photo U. Richard Rose Jr.
Gentry was instrumental for winning and keeping the America’s Cup