Tacking or coming about is a sailing maneuver by which a sailing vessel (which is sailing into the wind) turns its bow through the wind so that the direction from which the wind blows changes from one side to the other. For example, if a vessel is sailing on a starboard tack with the wind blowing from the right side and tacks, it will end up on a port tack with the wind blowing from the left side. See the image at the right; the red arrow indicates the wind direction. In practice, the sails are set at angle of 45° to the wind for conventional sailships and the tacking course is kept as short as possible before a new tack is set in. Rotor ships can tack much closer to the wind, 20 to 30°.
A similar maneuver (termed jibing) is used when sailing before the wind.
Tacking is distinct from jibing, where the ship's stern passes through the wind.
Tacking is sometimes confused with beating to windward, which is a process of beating a course upwind and generally implies (but does not require) actually coming about. In the accompanying figure, the boat is seen to tack three times while beating to windward.
When used without a modifier, the term "tacking" is always synonymous with "coming about"; however, some find it acceptable to say "tack downwind"; i.e., change tack by jibing rather than coming about. Racing sailboats do this because most modern sailboats (especially larger boats with spinnakers and a variety of staysails) sail substantially faster on a broad reach than running dead before the wind. The extra speed gained by zigzagging downwind can more than make up for the extra distance that must be covered. Cruising boats also often tack downwind when the swells are also coming from dead astern (i.e., there is a "following sea"), because of the more stable motion of the hull.
Beating is the procedure by which a ship moves on a zig-zag course to make progress directly in to the wind (upwind). No sailing vessel can move directly upwind but that may be the direction it wants to go. Beating allows the vessel to advance directly upwind.
A ship that is beating will sail as close to the wind as possible; this position is known as close hauled. In general, the closest angle to the wind that a ship can sail is around 35 to 45 degrees. Some modern yachts can sail very near to the wind, while older ships, especially square-rigged ships, were much worse at it.
Thus when a ship is tacking, it is moving both upwind and across the wind. Cross-wind movement is not desired, and may be very much undesirable, if for instance the ship is moving along a narrow channel, or the destination is directly upwind.
Therefore the ship changes tack periodically, reversing the direction of cross-wind movement while continuing the upwind movement. The interval between tacks depends on the lateral space available: in a small navigable channel, tacks may be required every few minutes, while in the open ocean days may pass between tacks, providing that the wind continues to come from the same general direction.
In older vessels that could not sail close to the wind, beating could be a tiresome process that required sailing a total distance several times the distance actually traveled upwind.