The balance of wind and water

SUN JOURNAL

Sailing: The forces at work on a graceful craft cutting through the waves are the stuff of a specialized science.

May 27, 2001|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

Sailing seems simple enough.

The wind blows. The moving air bumps into the sail and pushes the boat ahead of it. You are sailing. So what's the big mystery?

What's hidden from the shore birds who watch the pretty sails go by is the physics of sailing.

Sailing is awash in science that most sailors - unless they're designing an America's Cup racing yacht - have no time to contemplate.

"They're too busy pulling on ropes," says John Kimball, a physics professor at the State University of New York at Albany, a sailor on Saratoga Lake and the author of a recent scientific paper on the physics of sailing.

Sailing science generates waves of books, papers and computer analyses, replete with graphs and formulas describing lift forces and Froude numbers, vectors and vortexes, airfoils and the Bernoulli Principle.

Some of it is counter-intuitive. Singer-songwriter Christopher Cross concluded that "the canvas can do miracles." Sailing science explains how.

How, for example, can sailboats sail into the wind? And how is it that some of them, sometimes, can sail faster than the wind?

It turns out that sailing exactly with the wind slows you down. As the boat's forward speed increases, the wind it "feels" goes down. A boat sailing 3 mph with a 10-mph breeze from the stern feels an "apparent" wind of just 7 mph. If it motored up to 10 mph, it would feel no apparent wind, and its sails would go limp.

So, downwind at least, "you can't sail as fast as the wind," says Lt. Cmdr. Warren Mazanec, director of sail training at the U.S. Naval Academy.

And just as the apparent wind is falling, the fluid drag is climbing. Fluid drag is the combined force of all the collisions of water molecules directly on the hull, and of all the water molecules that jam up behind them.

Water molecules pile up ahead of the boat's hull, creating the bow wave. The boat rides in the wave's trough, and the next wave peaks just off the stern.

The longer the boat's hull, the shallower the wave's slope, and the less energy is drained from the boat's sail force to pile it up, says Paul H. Miller, assistant professor of naval architecture at the Naval Academy. The longer the hull, he says, "the faster the boat will go."

Fluid drag increases with the square of the speed, however, so as speed doubles in one direction, the drag quadruples in the opposite. Just as our downwind sailor feels the apparent wind drop, the fluid drag beneath his hull climbs. And when all the forces reach equilibrium, he can sail no faster.

Sailing science is full of this kind of balance, Ross Garrett explains in his book on sailing physics, "The Symmetry of Sailing."

"One can think of a yacht as a sort of analog computer that automatically adjusts its speed so that the water forces and the wind forces are always exactly equal and opposite," he writes.

Fortunately, the combined physics of wind and water also work to help the sailor "beat" upwind. But it took millennia for people to devise boats to do it well.

Even in 1492, ships had little ability to sail to windward - no closer than 80 degrees. (Directly into the wind would be zero degrees; broadside to the wind would be 90 degrees off the wind - called a "beam reach.")

Columbus relied on tropical trade winds from the east to blow him across the Atlantic Ocean to the Caribbean. The Nina and Pinta rode the North Atlantic's westerlies to get back home. (The Santa Maria sank.)

"They used to think you could not sail upwind," says Chris Rowsom, executive director of the 1854 sloop-of-war Constellation. A ship trying it stalls; it is said to be "in irons."

Effective upwind sailing had to await the development of more efficient sails and hulls in the late 18th and 19th centuries.

Seated on the deck of the square-rigged Constellation, Rowsom says, "A ship like this one can sail about 68 degrees off the wind." That means 68 degrees to the right or left of the direction from which the wind is coming. Windward progress required laborious zig-zag maneuvers, called "tacking." Or a lucky wind shift.

Modern racing yachts, he says, have pushed the limits to 30 degrees off the wind. But most pleasure boats can get no closer than 45 degrees. And that's possible only because of the "airfoil" effect.

A modern sail, especially the familiar "fore-and-aft" triangular sail, is crafted with a graceful curve, like an eyelash, from front to back. Its shape mimics - actually it foreshadowed - the curve on the top of an airplane wing.

Such a sail is, in fact, an airfoil - a kind of vertical wing. And just as the flow of air over and under an airplane's wing provides the vertical "lift" needed to get the plane off the ground, so the flow of air past a sail provides horizontal "lift" to propel the boat on an upwind tack.

Wing and sail both work because of the Bernoulli Principle. Named for the 18th-century Swiss physicist who discovered it, it states that the pressure of a fluid - in this case, air - decreases as its speed increases.

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