sail trim

Sailing is the art of controlling a boat with large (usually fabric) foils called sails. 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.

Current use

While there are still some places in Africa and Asia where sail-powered fishing or transport vessels are used these craft become rare as outboard and modified car engines become available even in the poorest and most remote areas.

In most countries people enjoy sailing as a recreational activity. Recreational sailing or yachting can be divided into racing and cruising. Furthermore use of sailboats can be divided into long-distance sailing (also blue-water sailing or offshore sailing) and daysailing.

History

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 B.C. Advances in sailing technology from the Middle Ages onward enabled Arab, Chinese, Indian and 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. From the 15th century onwards, European 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.

The Physics of Sailing

Introduction

A sailing vessel moves forward because of the reaction 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.

Energy capture

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 at the same speed, 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. 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 on some points of sail.

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. This may include, a keel, centerboard, or some other form of underwater foil or even the hull itself (as in catamarans without centreboard or in a traditional proa). 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 the 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 - 2x35 = 290 degrees).

Tacking is essential 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.

Some non-traditional rigs purportedly capture energy from the wind in a different fashion and 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 autogyro 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.

Points of sail

Further information: Points of sail

The points of sail are the most important parts of sail theory to remember. The wind, irons, or no go zone, is about 45° either side of the true wind, for a racing hull and sail plan optimized for upwind work. More commonly and on cruising sailplans, the best angle achievable upwind is 50° to 55° to the true wind. A boat cannot sail directly into the wind; attempting to do so is called luffing. There are 5 main points of sail. In order from the edge of the no go zone to directly downwind they are:

close haul (22° to the apparent wind)
close reach (half way between close hauled and a beam reach)
beam reach (90° to the apparent wind)
broad reach (22.5° away from directly downwind sailing)
running (directly downwind)

The sail trim (and, on smaller boats, centre board/dagger board position) on a boat is relative to the point of sail one is on: on a beam reach sails are half way out, on a run sails are all the way out, and close hauled sails are pulled in very tightly. A large proportion of the skill of sailing is in trimming the sails correctly for direction and strength of the wind.

Beating

A basic rule of sailing is that it is not possible to sail directly into the wind, simply because the angle of attack of the wind on the sail would be zero. This is easily understood by looking at a flag flapping in the wind (zero angle of attack) and generating no thrust. Generally speaking, a modern boat can sail 45 degrees off the wind. When a boat is sailing this close to the wind, it is close-hauled or beating (beating to weather).

Because a boat cannot sail directly into the wind one can only get to an upwind destination by sailing close-hauled with the wind coming from one side, then tacking (turning the boat through the eye of the wind) and sailing with the wind coming from the other side. By this method of zig-zagging into the wind it is possible to reach any destination directly upwind. For a yacht beating upwind to a mark at a distance upwind of one mile, it will cover a distance through the water of over 1.4 miles, if it can tack through an angle of 90 degrees. An old adage describes beating as sailing for twice the distance at half the speed and three times the discom

sail trim

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sail trim

Current use

History

The Physics of Sailing

Introduction

Energy capture

Effects of wind shear

Points of sail

Beating

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