Forces on sails apart, which generally prevents heeling. Each sailing craft is a system that mobilizes wind force through its sails—supported by spars and rigging—which provide motive power and reactive force from the underbody of a sailboat—including the keel, centerboard, rudder or other underwater foils—or the running gear of an ice boat or land craft, which allows it to be kept on a course. Without the ability to mobilize reactive forces in directions different from the wind direction, a craft would simply be adrift before the wind. Accordingly, motive and heeling forces on sailing craft are either "components of" or "reactions to" the
Forces on sails aerodynamics, the aspect ratio of a sail is the ratio of its length to its breadth (chord). A high aspect ratio indicates a long, narrow sail, whereas a low aspect ratio indicates a short, wide sail. For most sails, the length of the chord is not a constant but varies along the wing, so the aspect ratio "AR" is defined as the square of the sail height "b" divided by the area "A" of the sail planform: Aspect ratio and planform can be used to predict the aerodynamic performance of a sail. For a given sail area, the aspect ratio,