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In sailing terminology an outhaul is a line that pulls back and shapes the foot, or lower edge, of a sailboat's mainsail so as to increase or reduce the sail's tension and degree of curvature, more commonly referred to as the sail's draft or belly, as well as its power and heel. Tightening the outhaul stretches the mainsail's foot farther out along the boom, increasing tension and lending it a flatter aspect as well as reducing power and heel. Conversely, loosening the outhaul reduces the sail's tension, giving it greater curvature, more power and greater heel. The outhaul is part of the system of sheets, generally ropes, known as the running rigging of a fore-and-aft rigged sailboat, i.e. one where the sails run parallel, as opposed to perpendicular, to the sailboat's keel.
Attached to a metal grommet on the mainsail's clew — its lower aft or rear corner — the aft end of the outhaul typically is a combination metal anchor, pin and wire cable on the boom. The wire cable run rounds a block, or pulley, that leads inside the boom and runs forward to emerge as a sheet at the boom's opposite end. There it is secured to a cleat after being run round a winch, if large sails are being used.
When the outhaul is loosened, tension is decreased and draft is increased, which increases the speed as well as heel of the boat. Tightening the outhaul increases tension, reduces draft and reduces power. It acts directly on the lower edge of the mainsail, so tightening and loosening the outhaul also affects the mainsail's camber. The mainsail on a sailboat is essentially an aerofoil that provides lift and propulsion as it redirects and channels wind flow. Camber is the degree of asymmetry between the top and bottom curves of an aerofoil. When the outhaul is tightened or loosened, it effectively reduces or increases the camber of the mainsail and changes its aerodynamic properties.
Wind condition is the primary factor to take into account when one is deciding how much much to tighten or loosen the outhaul. Light wind conditions generally call for a loose outhaul in order to create more belly and greater draft in the sail. High wind conditions generally call for a tight outhaul, which increases tension on the sail's foot, flattening and depowering the sail so as to prevent excessive heeling of the sailboat over to one side or the other. Moderate wind conditions call for a moderately tight outhaul.