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 total aerodynamic force (F) on sails, which is a function of apparent wind velocity (V) and varies with point of sail. The forward driving force (F) component contributes to boat velocity (V), which is, itself, a determinant of apparent wind velocity. Absent lateral reactive forces to F from a keel (in water), a skate runner (on ice) or a wheel (on land), a craft would only be able to move downwind and the sail would not be able to develop lift. At a stable angle of heel (for a sailboat) and a steady speed, aerodynamic and hydrodynamic forces are in