The race is on – the physics of sailing and the America’s Cup yacht race
As the 23-meter (75-foot) [America.sup.3] sailboat speeds through the water, navigator Annie Nelson spots a dark patch of choppy waves off to the right. A gust of wind is headed straight for the boat. Other members of the crew prepare for the gust, pulling in lines to tighten the sails and keep the boat speeding on course.
Every second counts. The 16 sailors aboard [America.sup.3], sailing off the coast of San Diego, California, are competing for the right to represent the U.S. in the grandest sailing race of them all: the America’s Cup.
The U.S. has lost the Cup only once – to Australia in 1983. Not bad for a sporting event that began 144 years ago, when American sailors dared to challenge Britain’s supremacy at sea. Now open to challengers from around the world, the competition is fierce. And this year, the [America.sup.3] team will be the first-ever all-female crew to participate.
They hope to win. But only one American boat can compete in the final series of races, scheduled to begin May 6, 1995. So since January, the women have been racing against two other U.S. teams to prove that they are this country’s fastest.
Being the fastest doesn’t necessarily depend on being the strongest. America’s Cup sailboats are hightech marvels designed to “fly” through wind and water – provided that the sailors aboard know how to take advantage of certain principles of physics.
When wind hits the boat, explains navigator Nelson, “we use instruments to see where [the wind] is actually coming from and the strength of it.” The crew then adjusts the sails to take maximum advantage of that wind’s force, or push.
Sailing downwind, with the wind at your back – which America’s Cup competitors will do for three of the race’s six “legs” (see course diagram, p. 15) – is pretty straightforward, Nelson explains. You simply arrange a large sail (called a spinnaker) perpendicular (at a right angle) to the length of the boat. Wind from behind fills the sail like a giant parachute and pushes the boat forward (see photo, above).
During these downwind legs, the huge, billowing sails can be “hard to control,” says Alison Hamilton, who analyzes computer data of the boat’s performance for the [America.sup.3] team. But the real physics challenge of the America’s Cup, she says, comes during the upwind – into the wind – legs of the race.
INTO THE WIND
A sailboat can’t sail directly into the wind, Hamilton explains, because wind from straight ahead would push the boat backward – not exactly the way to win a race.
So instead of taking a straight-on approach, sailboats travel a zigzag path toward the upwind point, or “mark,” on the course. This method, called “tacking,” is a little like inline skating up a steep hill – zigzagging back and forth across the road to get to the top (see diagram, right).
On each tack, sailors align their sails from the front to the back of the boat, instead of perpendicular to it. The key is to get the sails to form a smooth curve from front to back. The front edge of the curved sail cuts through, or splits, the oncoming windstream.
THE PRESSURE IS ON
Air moving over the bulging, or convex, side of the sail is like water squeezed by a bulging boulder into a narrower stream, says physicist and sailor Donald Drost. That squeezing speeds up the airflow on that side of the sail.
That’s important, says [America.sup.3] design director Jerome Milgram, because “when the flow of air is faster, its pressure decreases.” That’s a rule of physics called Bernoulli’s principle. Result: The fast-moving air on the bulging side of the sail creates a low-pressure zone there.
In contrast, air on the inside of the curved sail, the concave side, isn’t squished or speeded up. Its pressure remains relatively high. In an effort to equalize the pressure, air in the high-pressure zone pushes the sail toward the lowpressure zone. That force moves the boat forward.
The goal is to zigzag toward the mark as fast as possible, without losing time with each change of direction. And that takes incredible teamwork.
“Ready about!” calls out Dawn Riley, who is steering the boat. This is the signal that she’s about to turn from a “zig” to a “zag.” “Hard alee!” she yells as she turns the boat hard into the wind. The crew snaps into action, letting the sails go free. The enormous sails flap from one side of the boat to the other as Riley turns past the mark toward the opposite tack. Pulleys groan loudly as the “grinders,” the strongest crew members, pull the sails into their new position, once again achieving the perfect curve.
“We try to communicate [.with each other] through the feel of the boat,” says navigator Nelson. “We have days where we try to run silent and people have to be aware of the boat. Everyone feels it and everyone does her job, unspoken.”
To watch the women’s team and sailing physics in action, check your local sports section or watch ESPN for coverage of the America’s Cup.
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