Even a log raft with a goatskin tied to a pole will sail before the wind.  To sail in other directions, a boat must be designed and rigged so that the force of the wind moves it across the wind or into the wind, as well as moving it with the wind. 

Controlling Direction.  A boat with no means of control will travel straight downwind (in the direction toward which the wind is blowing).  It will do this no matter which direction its bow or stern is pointing.  It may even go sideways.  Using a rudder is the first step in controlling a boat.  With a rudder, the bow of the boat can be pointed in the desired direction. 

But a rudder is not enough to control a boat.  A boat must also have something to keep it from sliding sideways when moving across the wind.  This is done with a keel, centerboard, bilge boards, or leeboards.  Boats with keels can sail only in water deeper than the keel.  Boats with centerboards and bilge boards can sail in shallower water, because the centerboard or bilge boards can be raised or lowered as needed.  Leeboards are a simple way of changing a canoe or rowboat into a sailboat.  They can be swung out of the water when not needed. 

With a rudder for steering and a keel or centerboard to prevent sideward movement, a sailboat can travel in many directions.  The bow of a sailboat is usually sharply pointed, so it can cut through the water easily. 

Why a Boat Sails.  A sail has curved panels sewn into it so it will be shaped like the wing of an airplane when the wind fills it out.  The side of the sail to leeward (away from the wind) corresponds to the top of an airplane wing.  The action of the wind blowing across this curved surface creates a lift similar to the force that enables an airplane to stay in the air.  In a sailboat, this lifting force becomes a pull away from the sail and toward the bow of the boat.  At the same time, the wind also exerts a push against the other side of the sail.  In this way, the action of the wind on the sail combines in two ways to force the boat forward.  These forces make it possible to sail a boat in almost any direction, except directly into the wind and up to 45 degrees on each side of the wind direction. 

Basic Sailing Maneuvers.  There are three basic sailing maneuvers: (1) sailing into the wind, (2) sailing across the wind, and (3) sailing with the wind.