THE THEORY OF SAILING
The earliest sailing craft carried a square sail and sailed only before the wind. In any other direction they used sweeps or paddles, or simply waited until the wind was in a favourable direction. The first development of fore and aft rig, with the ability to sail against the wind, was seen in the Arab dhow, a type of rig still basically unchanged to this day. The modern fore and aft rig, with the leading edge of the sail secured to a mast or stay, was a much later development.
As the knowledge and understanding of sailing developed, so the design of ships gradually changed. Keels were deepened to create greater lateral resistance, while the high poops and forecastles were reduced in order to offer less wind resistance. Although it was realised that ships could sail against the wind, it was not until the development of the airplane, and the consequent study of aerodynamics, that the reasons for this were fully understood. It had been discovered that it was not merely pressure under the wings that lifted an airplane into the air, but also the low pressure area created above the wings by flowing more quickly over the curved surface. It was soon realised that the same principle applied to sails of modern sailing vessels.
The air accelerates over the arc of the sail, causing a reduction in pressure on the upper side, thus pressure on the underside of the sail is greater than the pressure on the upper side. The natural tendency of different pressures to try to equalize creates a push or lift on the underside of the sail. Since the sail is attached to a body resting in a fluid mass, that body is moved through the mass by the lift on the sail, in other words the vessel is moved through the water.
When a single sail is used the air flow over the sail is not even, turbulence and eddies being formed on the leeward side of the sail of the sail which tends to reduce the venturi effect and consequent lift.
If a second overlapping sail is added much of this turbulence is removed, thus increasing the lift created by the mainsail.
When sailing to windward, only a small proportion of the force of the wind is used in driving the vessel forward. Most of the effort is wasted trying to push the vessel sideways or heeling it over against the righting moment of the ballast. This can be illustrated by the use of a parallelogram of forces which divides the lift of the sail into a forward and leeward component. The sketch below shows a vessel sailing close to the wind and reaching off the wind, with lines A-B indicating the direction and force of the forward lift. By completing the parallelogram it will be seen that A-C is proportional to the leeward drive in A-B, while A-D is proportional to the forward drive. As the point of sailing changes, so the proportions of the forces change, this explains why a vessel sails faster on a reach then when close to the wind.