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Have you ever wondered how a sailor can sail boats into the wind? It just doesn’t quite seem logical.
Peter Isler can tell you how. Isler is a two-time America’s Cup winner, editor-at-large for Sailing World and co-author of "Sailing for Dummies." He’s been sailing for 43 years. As he explains, a sailboat is actually a lot like a plane.
According to Isler, a sailboat has four basic parts: a hull, an underwater fin, a mast and a sail. We all know that a sail is a piece of fabric that catches the wind and powers the boat. Sailing with the wind makes sense - it’s easy to visualize and understand how it works.
But when a sailor wants to move his craft into the wind, the dynamics get more complex. This brings us to the fourth part of a sailboat: the underwater fin, also called the keel or centerboard.
Hanging underneath the back of the boat is the rudder, which allows for fine-tuned steering of the boat. Also attached to the sailboat’s underside is a second fin, much larger than the rudder, called a keel or centerboard, which runs right down the center of the hull.
The keel serves two purposes. Most of the time, the wind pushing on a sailboat pushes it from its side, from various angles. The keel’s primary purpose is to keep the boat from being pushed sideways from the force of the wind. It’s second purpose is to provide lift, which, in physics terminology, is a force exerted on an airfoil that pushes in a direction perpendicular to the direction of motion.
It works on the same principle as an airplane wing. An airplane wing is curved on its upper surface. Air passing over the wing travels over the curved part of the wing at a higher velocity than it travels over the flat part of the wing. This creates lower pressure over the curved part of the wing and lifts the wing. To put it most plainly, the low pressure created by the wind passing over the curve of the wing creates a vacuum that lifts the wing.
A sailboat uses this same principle when sailing into the wind. The sailor turns his sailboat at about a 45 degree angle into the wind, pulls in the sail and fills it with wind. The wind-filled sail creates an airfoil shape, just like an airplane wing; the wind flowing over the backside of the sail moves faster than the air moving across the front (flat) side of the sail. This creates lift, and pushes the boat sideways and forwards. And this is where the keel's second function comes into play.
Isler says to think of the sail, protruding into the sky, as one wing and the keel, hanging in the water, as the second wing. The water flowing over the backside of the keel goes faster than the water passing over the front side, which results in differing water pressures, and that pulls the boat forward and sideways.
But picture the wind hitting a sailboat’s sail. As the wind hits the sail, it tilts it over in that direction. But the sailboat’s two wings (the sail and the keel) pivot on the ship’s hull. This means that beneath the ship the keel is tilting the opposite direction of the sail, which means the keel’s lift is lifting in the opposite direction of the sail’s lift. The two sideways forces cancel each other out and only the forward force remains.
Most modern sailboats can sail about 45 degrees in a windward direction. The trick is to keep enough wind filled in the sail to keep its airfoil shape. If a sailboat tries to sail directly into the wind, the wind moves straight across the sail and it loses the pocket of wind that gives it its airfoil shape and instead the sail flaps like a flag. Once the sail loses its airfoil shape, it loses its forward and sideways energy.
“A sailor can sail to a point that lies directly into the wind, he just can’t steer straight for it," said Isler. “He must approach it in a zigzag manner, called tacking.”
In steering toward the point that he wants to reach, he comes at it at about a 45 degree angle, then he tacks, or turns his boat about 90 degrees in the other direction, and after traveling in that direction for a ways, he tacks again back to his original angle.
So what about those ancient multi-masted, multi-sail ships sailed by the likes of Columbus and Magellan? Do they work the same way or does having all those sails confound those principles? For that, I asked Jan Miles, captain of the Pride of Baltimore 2, which is a multi-masted, multi-sail ship. The Pride of Baltimore 2 was built in 1988, and Miles has been its captain from day one. The Pride of Baltimore 2 was built using the same plans as privateer vessels built by the Americans for the War of 1812.
Miles explains that the hydrodynamics and aerodynamics of a square-rigged (they use square sails) tall ship are the same as today’s smaller, single-sail boats. But, while the principles may be the same, the practice is a quite a bit different. The multi-masted ships still form and position their sails into an airfoil shape, they still rely on the keel’s counter force, they just don’t get the same results as today’s modern sailboats.
Ancient mariners had a basic working knowledge of how wind powered their ship and how to position their sail and their ship to best take advantage of it, but they didn’t understand the physics of an airfoil and how it works.
Today’s modern boats are built with airfoil technology maximized into their design. Modern sails are cut to form the most efficient airfoil. Same goes for their keels. Ancient sails and keels were not.
“Modern sailboats can sail into the wind at an angle as close as 45 degrees,” Miles says. “The old ships could only sail into the wind at about 60 degrees.”
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