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A wingsail is an advanced type of wind-powered propulsion for watercraft. It operates in a manner completely different from conventional sails, yet still relies on the wind for propulsion. It is essentially an airfoil, very much like an airplane wing, but is mounted vertically on the deck of a boat in place of a traditional mast and sails. The wingsail uses the same principles that allow airplanes to fly to impart thrust to the boat.
Operating according to principles of physics and aerodynamics in exactly the same way as any other airfoil, a wingsail is shaped much like an airplane wing. The cross-section of an airfoil is roughly teardrop shaped. One long side of the teardrop is flattened. On an airplane, this flattened side would be the bottom of the wing, the rounded side the top, and the point of the teardrop located at the trailing or rear edge of the wing. When air moves over the airfoil, it generates lift.
When used to propel watercraft, a wingsail is mounted vertically on the deck, and is capable of pivoting around a vertical axis that passes through the thickest portion of the airfoil shape. A boat or ship outfitted with a wingsail will appear to have a large airplane wing sticking straight up from its deck. By moving the wingsail on this pivot, sailors can maneuver it so that the wind passing over the airfoil shape provides forward thrust.
Two types of wingsail are currently in use. Hard, or rigid, wingsails are much like airplane wings. Soft sided wingsails use cloth or canvas to cover the airfoil shape and provide the performance of a wingsail, but they can also be adapted, based on conditions, to operate like traditional sails. Soft sided wingsails can also be taken down and stowed, unlike hard wingsails.
A watercraft outfitted with a wingsail has many advantages over traditional sail-rigged vessels, including greater flexibility of movement. It can alter the direction of thrust and move in directions relative to the wind that would be impossible for traditional sailing vessels. Wingsails are more efficient and simpler to operate than traditional sails as well.
In the early 21st century, the technology involved in the manufacture and use of wingsails is still relatively new. While the principles behind their operation have been understood for 200 years or more, they are still relatively uncommon, as designers and manufacturers work to perfect materials and designs. As they become more cost effective and reliable, wingsails, some experts believe, will become more and more common.