Upwind Sailing
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This is a diagram of the forces exerted on and from a sailboat heading upwind. The overall purpose of the sail is to propel the boat forward when heading into the wind. This is possible by many forces acting upon the boat. One obvious force is the wind blowing across the sail, this creates lift on the sail, pushing the boat to the side that the sail is on. The force counteracting this force is the centerboard or keel which are a fixed (keel) or unfixed (centerboard) board at the bottom of the boat. This and the boat's bottom in the water keep the boat from sliding. The force of the sail pushing on the keel makes it go in the direction it is pointed because the force of the keel and the force of the sail are not perfectly parallel. The speed of the boat will obviously be affected by the speed of the wind the boat is sailing in, but is not the only factor acting on the boat. There are multiple factors that affect the speed of the sailboat which is why perfect sailing is nearly impossible. Some common factors include surface area of the bottom of the boat, sail area, weight of the sailor(s) and direction of the boat.
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