Sails are large pieces of fabric attached to a ship’s mast by ropes and lines. They are designed to catch and transmit the flow of the wind so that the energy of the moving air can propel the ship through the water. The rudder, attached to the stern or rear end of the vessel beneath the water, is used to steer the ship. The force of the wind fills the sails and the lines of the rigging can pull the sails taut or change their shape to pick up wind coming from other directions. A ship is unable to sail directly into the wind, but in the 18th century, a zig-zagging course known as tacking allowed vessels to sail upwind. This was accomplished by hauling the sails in so that the wind flowed over them at an acute angle.
Although small boats and vessels such as canoes could rely on the power of rowing and paddling, sails were used as the main energy source on the large vessels that carried expeditions of exploration to the Pacific Northwest in the 18th century. Strong winds and wear could cause tears, so the sailmakers who repaired and replaced the canvas sails, made from plant fibres such as flax and hemp, were part of every crew. Many hands, (sailors working on deck and in the rigging) were required to move the sheets (ropes) and the sails. Some of the sailors’ main duties were bending (attaching the sails to the yards, the cross-supports on the mast), hoisting (raising), and taking down the sails.
Many sailing ships in the year 1700 used four-cornered, square-shaped sails cut with a flat top, bottom, and sides. As the century progressed, more ships used two, three, or even four sails on each mast. The addition of the topgallant, the uppermost sail, was an 18th century innovation that allowed for greater speed and manoeuvrability. The advantage of having several smaller sails over fewer large sails was that smaller crews could handle the work of moving the heavy fabric of the sailcloth into position. If we were to draw a line down the centre of a ship along the keel, the beam around which the hull is built, the sails of square-rigged vessels would hang at a right angle to that line. By the late 1700s, Royal Navy ships and other vessels also hoisted fore-and-aft sails that hung parallel to the keel. The spanker was the furthest aft, to the rear of all the other sails, on the mizzenmast. The gaff sail could be raised as the topsail above the spanker.
There was little in the way of technical scientific understanding of how sails utilized wind (which moves what are actually air particles). Although treatises were published on stability and other aspects of marine architecture from the 16th century onwards, it was not until the 19th century that wind forces were examined in detail. Many people believed that the wind was concentrated on a single point in the middle of the sail, but we know today that everything, even the lines of the rigging, picks up wind energy. The successful development of sail was achieved mostly by trial and error based on extensive practical knowledge and experience.