About Sailing
Sailing is the art of controlling a sailing vessel. By changing the rigging, rudder and dagger or centre board, a sailor manages the force of the wind on the sails in order to change the direction and speed of a boat. Mastery of the skill requires experience in varying wind and sea conditions, as well as knowledge concerning sailboats. Today most people enjoy sailing as a recreational activity. Recreational sailing can be further divided into racing, cruising and “daysailing” or dinghy sailing. There are many production sailboats available, and several of these manufactured models have ownership associations, such as the Islander 36 association.
Throughout history sailing has been instrumental in the development of civilization. The earliest representation of a ship under sail appears on an Egyptian vase from about 3500 BC. Advances in sailing technology from the 15th century onward enabled European explorers to make longer voyages into regions with extreme weather and climatic conditions. Improvements were made in the design of sails, masts and rigging, and navigational equipment became more sophisticated. Ships went further north, stayed longer on the Grand Banks and in the Gulf of St. Lawrence, and eventually began to explore the Pacific Northwest and the Western Arctic.
A sailboat sailing ship moves forward because of the reaction to the inertia of moving air on its sails. Since the dawn of history this vital technology has afforded mankind greater mobility and capacity for fishing, trade and warfare. From moving the stones of the great pyramids from Aswan to Giza to allowing man to migrate throughout Polynesia to Nelson’s defeat of the French and Spanish navies at the Battle of Trafalgar, mankind’s history has been intertwined with this seemingly simple technology.
The physics of sailing
The energy that drives a sailboat is harnessed by manipulating the relative movement of wind and water speed: if there is no difference in movement, such as on a calm day or when the wind and water current are moving in the same direction, there is no energy to be extracted and the sailboat will not be able to do anything but drift. Where there is a difference in motion, then there is energy to be extracted at the interface, and the sailboat does this by placing the sail(s) in the air and the hull(s) in the water.
Sails are airfoils that work by using an airflow set up by the wind and the motion of the boat. The combination of the two is the apparent wind, which is the relative velocity of the wind relative to the boat’s motion. The sails generate lift using the air that flows around them. The air flowing at the sail surface is not the true wind.
The sail alone is not sufficient to drive the boat in any desired direction, as a sail by itself would only push a boat in the same direction as the wind. Sailboats overcome this by having another physical object below the water line. These include, a keel, centerboard, or some other form of underwater foil or even the hull itself. Thus, the physical portion of the boat which is below water can be regarded as functioning as a “second sail”. Having two surfaces against the wind and water enables the sailor to travel in almost any direction and to generate an additional source of lift from the water. The flow of water over the underwater hull portions creates a hydrodynamic force. The combination of the aerodynamic force from the sails and the hydrodynamic force from the underwater hull section allows motion in almost any direction, except straight into the wind. This can be likened, in simple terms, to squeezing a wet bar of soap with two hands which causes it to shoot out in a direction perpendicular to both opposing forces. Depending on the efficiency of the rig, the angle of travel relative to the true wind can be as little as 35 degrees to over 80 degrees. This angle is called tacking angle. With a 35 degree tacking angle on either side of the wind, it is possible for a sailboat to sail directly over 290 degrees of the compass (360 – 2×35 = 290 degrees).
When sailing upwind, the sails, when correctly adjusted, will generate aerodynamic lift. When sailing downwind, the sails no longer generate aerodynamic lift and airflow is stalled, with the wind push on the sails giving drag only. As the boat is going downwind, the apparent wind is less than the true wind and this allied to the fact that the sails are not producing aerodynamic lift serves to limit the downwind speed.
When moving, the motion of the boat creates its own apparent wind Apparent wind is what is experienced onboard and is the wind that the boat is actually sailing by. Sailing into the wind causes the apparent wind to be greater than the true wind and the direction of the apparent wind will be forward of the true wind. Some extreme design boats are capable of traveling faster than the true windspeed.
Some non-traditional rigs purportedly capture energy from the wind in a much different fashion are capable of feats that traditional rigs are not, such as sailing directly into the wind. One such example is the wind turbine boat, also called the windmill boat, which uses a large windmill to extract energy from the wind, and a propeller to convert this energy to forward motion of the hull. A similar design, called the autogiro boat, uses a wind turbine without the propellor, and functions in a manner similar to a normal sail.
Effects of wind shear
Wind shear affects sailboats in motion by presenting a different wind speed and direction at different heights along the mast. Wind shear occurs because of the greater friction presented by water surface slowing the flow of air, a difference in true wind creates a different apparent wind at different heights. Sailmakers may introduce sail twist in the design of the sail, where the head of the sail is set at a different angle of attack from the foot of the sail in order to change the lift distribution with height. The effect of wind shear can be factored into the selection of twist in the sail design, but this can be difficult to predict since wind shear may vary widely in different weather conditions. Sailors may also adjust the trim of the sail to account for wind gradient, for example using a boom vang.
