Sailing employs the wind—acting on sails, wing sails or kites—to propel a craft on the surface of the water (sailing ship, sailboat, raft, windsurfer, or kitesurfer), on ice (iceboat) or on land (land yacht) over a chosen course, which is often part of a larger plan of navigation.
From prehistory until the second half of the 19th century, sailing craft were the primary means of maritime trade and transportation; exploration across the seas and oceans was reliant on sail for anything other than the shortest distances. Naval power in this period relied on sail to varying degrees, depending on the available technology, culminating in the gun-armed sailing warships of the Age of Sail. Sail was slowly replaced by steam as the method of propulsion for ships over the latter part of the 19th century, as steam technology gradually improved through many developmental steps. Steam allowed scheduled services that ran at higher average speeds than sailing vessels. Large improvements in fuel economy allowed steam to progressively outcompete sail, ultimately in all commercial situations, giving ship-owning investors a better return on capital.
In the 21st century, most sailing represents a form of recreation or sport. Recreational sailing or yachting can be divided into racing and cruising. Cruising can include extended offshore and ocean-crossing trips, coastal sailing within sight of land, and daysailing.
Sailing relies on the physics of sails as they derive power from the wind, generating both lift and drag. On a given course, the sails are set to an angle that optimizes the development of wind power, as determined by the apparent wind, which is the wind as sensed from a moving vessel. The forces transmitted via the sails are resisted by forces from the hull, keel, and rudder of a sailing craft, by forces from skate runners of an iceboat, or by forces from wheels of a land sailing craft which are steering the course. This combination of forces means it is possible to sail both upwind and downwind courses. The course with respect to the true wind direction (as would be indicated by a stationary flag) is called a point of sail. Conventional sailing craft cannot derive wind power on a course with a point of sail that is too close into the wind.
History
Sailing was probably independently invented in at least two regions: Island Southeast Asia and the Mediterranean (including the Nile and other Middle Eastern rivers). The Caribbean, which meets the criteria as a "nursery" region for maritime experimentation, with warm waters and mutually intervisible islands, did not develop sailing technology, but adopted it after the arrival of European explorers. The geography of the Nile, with a northward-flowing current and the prevailing wind blowing in the opposite direction, was conducive to early use of sail. The same wind-and-current combination on the Guayaquil River in Ecuador, together with the otherwise unexplained origin of the region's sailing craft, may provide a third site of independent invention.
Throughout history, sailing was a key form of propulsion that allowed for greater mobility than travel over land. This greater mobility increased the capacity for exploration, trade, transport, warfare, and fishing, especially compared with overland options.
Until the significant improvements in land transportation that occurred during the 19th century, if water transport was an option, it was faster, cheaper, and safer than making the same journey by land. This applied equally to sea crossings, coastal voyages, and river and lake travel. Examples of this include the large grain trade in the Mediterranean during the classical period. Cities such as Rome were totally reliant on sailing ships to deliver the large amounts of grain needed. It has been estimated that it cost less for a sailing ship of the Roman Empire to carry grain the length of the Mediterranean than to move the same amount 15 miles by road. Rome consumed about 150,000 tons of Egyptian grain each year over the first three centuries AD.
A similar but more recent trade in coal was from mines situated close to the River Tyne to London – which was already being carried out in the 14th century and grew as the city increased in size. In 1795, 4,395 cargoes of coal were delivered to London. This would have needed a fleet of about 500 sailing colliers (making 8 or 9 trips a year). This quantity had doubled by 1839. (The first steam-powered collier was not launched until 1852, and sailing colliers continued working into the 20th century.)