At first glance, AC72 sailboats seem to defy the laws of motion. They’re powered by the wind but can travel at speeds even greater than that of the wind propelling them.
How? The crew behind Team New Zealand, competing in this year’s America’s Cup, explains all.
There are two kinds of wind – true wind and apparent wind. The former is what we feel brushing against us when we walk outside on a breezy day while the latter is a dragging force that moving objects experience. True wind pushes while apparent wind drags.
True wind will always push the boat at the same speed that the wind is traveling. By positioning a vessel’s sails at the appropriate angle, true wind can harness the power of apparent wind, something that it would not be able to do if the sails were placed upright and flat.
The AC72’s most striking feature in accomplishing this is its sail, a staggering 131-foot wing sail that towers above the surface of the water. The hull length is 72.2 feet long with a maximum beam of 45.9 feet and is manned by a crew of 11 people.
“The wind is doing two things,” Margot Gerritsen, an engineering professor at Stanford University in California, explained in an interview with KQED Science. “It’s pushing, but there’s also a part of this wind that is dragging. That dragging is done with this force called ‘lift.’”
The lift is defined as the force that is created by the difference in pressure, which is generated by the apparent air meeting the outside sail and the air on the inside of the sail, which is moving at a slower pace.
Steve Collie is an aerodynamics engineer with Team New Zealand, competing in the race. “In physics we always talk about conservation of energy,” he says. “Sure, we can’t create energy. We can’t produce magic that way. But there’s no sort of parallel theorem of conservation of speed.”
The high-octane boat race sees Emirates Team New Zealand and the US’s Oracle Team competing for the America’s Race cup. The course brings the two teams across the west coast of the US, which kicked off on San Francisco Bay September 7th and will run until the 21st for the final race.
Check out Industry Tap’s previous coverage of the world’s fastest sailboats.