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THIS ARTICLE WAS IN TODAY'S SAN DIEGO UNION-TRIBUNE. FEB. 8, 2010. GREAT ILLUSTRATIONS, WHICH I'M UNABLE TO COPY AND PASTE.
"FLYING THROUGH THE WATER"
The basic physics of sailing seem obvious: A sail catches the wind. The energy is transferred to the hull. The boat is pushed forward.
Of course, any sailor worth his salt knows it’s not that simple, and sailing the boats of this year’s America’s Cup, scheduled to begin today off the coast of Valencia, Spain, may be something akin to rocket science.
By all reports, both vessels in this year’s 33rd staging of the America’s Cup (racing began in 1851, making it the world’s oldest active sports trophy) are capable of sailing two to three times faster than the wind, so fast in fact that “they make their own wind,” said Bryon Anderson, a physicist at Kent State University and a longtime sailor.
That may sound fantastical, but it’s really just a matter of carefully balancing three conflicting forces to maximum positive effect. First and foremost is the “driving force” created by wind energy captured in a sail. By constantly trimming a sail to maintain the optimum “angle of attack” to the wind’s direction, this force drives a boat forward.
In addition to the driving force, there is a “sideways force” from wind that doesn’t just push the boat forward but also sideways. The sideways force also generates a “heeling force,” which causes the boat to tip or “heel.” The keel counteracts these two forces by deflecting the water passing it and by its weight below the hull, respectively.
Even the most lubberly of landsmen know a boat can’t be sailed directly into the wind or upwind, but sailing downwind, with the wind parallel and pushing from behind, is not the fastest point of sailing, either.
A boat sailing downwind is limited by the wind itself. If a breeze is blowing at 10 knots (one knot equals 1.151 miles per hour) and a boat is making six knots in the same direction, the crew feels a “relative wind” of four knots blowing over the stern. As the boat increases speed, the speed of the relative wind drops toward zero, and the force of the wind upon the sail declines. When boat speed equals true wind speed, the boat has reached maximum velocity, though this is not actually possible since there are always other, slowing forces at work, such as water resistance.
Water wings
Like airplane wings, boat sails exploit a principle of physics first described by the 18th-century mathematician Daniel Bernoulli. In cross-section, aircraft wings taper to a thinner trailing edge. This teardrop-turned-sideways shape causes air passing over the top of the wing to move faster than air passing below. Faster-moving air doesn’t push down as hard on the wing as slower-moving air pushes up. The difference in relative pressures generates the “lift” that permits flight.
The physics of a boat sail are similar, says Anderson. A sail is almost always curved and angled into the wind. Air moving along the “upper” or downwind side of the sail is forced to take a longer path, thus moving faster than air passing along the “lower” or upwind side of the sail. The result is that air on the upwind side of the sail pushes harder.
Sailboats move fastest with the wind coming from “abeam” or the side. When a boat moves perpendicular to the direction of the wind with its sails tipped at an angle, its speed does not decrease the force of the wind upon its sail, and it is able to reach an equilibrium speed determined by the roughly constant force of the wind in the sails and the resistance through the water. If wind force is maximized while resistance is made small, the resulting velocity can be large.
That’s why sail iceboats, which skate across frozen lakes, move so fast. Ice provides very little resistance. Some iceboats, said Anderson, can glide at speeds in excess of 90 miles per hour with a wind abeam blowing at just 30 mph.
Because of resistance created by moving through water, most sailboats cannot come close to moving at three times the wind speed. This resistance comes both from deflecting the water and from “shearing” it. Water along the hull attaches itself to the hull and moves with the boat. However, water several feet away from the boat is not moving with the vessel, which means that water between these points is being sheared. This shearing involves the breaking of bonds between water molecules, called van der Waals forces. Shearing costs energy and provides resistance.
The defending Alinghi boat, based out of Switzerland, and the challenging USA, previously known as BOR-90 and based out of San Francisco, are able to largely lift themselves out of the water, greatly reducing this resistance, which makes them, like iceboats, more likely to reach sailing speeds three times the prevailing wind speed.
“The bonds aren’t strong or powerful,” said Anderson, “but they are very many, and breaking them costs energy, which creates resistive force.”
As the boat moves faster, this resistance grows stronger.
The only way to significantly reduce this resistive force is to minimize the amount of hull surface in contact with water. The hulls of both Alinghi and USA are very narrow and feature wave-piercing bows that tend to slice through waves rather than ride over them, since forward-energy is lost in the up-and-down motion of pitching.
More to the point, neither boat at speed is likely to have more than one of its hulls in the water at one time.
Everything about Alinghi and USA is designed and built for maximum speed.
“Both boats are as lightweight as possible. That’s why you see so much emphasis on materials like carbon fiber,” said Kim Hampton, editor ofYachtPals.com, an international social network for sailors. “Anything that affects speed is considered, down to the smallest detail, things like the weight of uniforms or rounding off corners the way they do in Formula One cars.”
While some Cup observers have complained that these latest, computer-designed (and in some ways, driven) boats represent an unwelcome trend toward “push-button sailing,” Hampton said the races will still come down to old-fashioned sailing, to men and the sea.
“Everything about these boats has evolved in some way, but sailing them still means using lines and sails, blocks and tackles. The captain of the first America’s Cup race probably wouldn’t recognize these boats, but if he stepped aboard, he would probably be able to figure out how to sail them.
“Once you understand how the wind works and how it propels the sails, you can sail anything. Boats change, but the physics of sailing them doesn’t.”
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