What do we mean by wind energy?
Before the 20th century renewed energy sources had a crucial role in economy. People used them in every domain of life. Wind energy is the useful energy we gain when we capture the wind. With its power men could transport bulk goods by sea, process crops, mill, and later drain extended areas. The heyday of wind energy use can be put between the 17th - 18th centuries. Unfortunately, from the beginning of the 19th century, due to the Industrial Revolution and the appearance of the steam-engine, the usage and improvement of windmills dropped significantly. The great energy crisis after the Second World War represented a turning point, but the actual breakthrough is considered to be the oil crisis of the 1970’s. Environmental protection also began to materialize at that time. Today the use of wind energy started to gain more and more ground. As it is a global phenomenon, the energy of wind is the world’s fastest growing energy source, and, most important, its use does not involve emission of any polluting substance. Owing to the development of technology, the present wind energy capacity outranges every forecast.
How did they use it in the past?

Sails

In ancient times humans already used equipments with which they could turn the power of wind to their favor. The best-known example of it is the sail.
            Thousands of years ago people built boats of wood, reed and faggots. They stretched animal skin on a wooden frame and that way they could travel on lakes and rivers. Even the ancient Egyptians used sails to ease the work of the oarsmen. The sail was made of papyrus reed or other woven filaments and was stretched on a mast. Its lower corners were fastened, and ropes led to both yard-arms. This way they were able to sail in a bit different angle from the direction of the wind.
Later, when men learned how to use this power, they put out on seas and oceans. The most suitable rigging for these long journeys was found to be the squaresail or crossjack.

What do we mean by cross-jack rigging?

            In the case of cross-jack rigging, the sails are in transversal position, perpendicular to the ship’s longitudinal axis. With their help the power of the wind can be multiplied significantly.

3000 years BC. men already transported the goods by sea. The rigging of a ship with squaresails was directed so as to sail before the wind. The disadvantages of this kind of rigging were that the propelling force of the wind couldn’t be controlled and if the direction of the wind suddenly changed, the ship had to switch over to “manual mode”.
Because sailing before the wind didn’t turn out to be the best way of traveling, people began to search for new methods to find out how they could sail in a different angle from the direction of the wind. The Arab came up with the solution: they turned their sails lengthwise the ship, in this way they were, to some degree, facing the air-flow. The wind bellied the sails out, and streaming along them, it resulted in a force whose one component moved the ship forward. Their discovery led to the appearance of the longitudinal rigging. With this kind of rigging the ship was much easier to maneuver. The sails were placed along the centerline of the ship. This method was first used in the regions of the Mediterranean Sea and the Indian Ocean. A range of different sail shapes appeared: for sailing in the wind’s eye the triangular sail, while for sailing down the wind the quadrangular sail was found to be better. There were some ships using both types of rigging. The triangular Latin sail appeared on the 5th century’s Polynesian sailing canoes, and later, between the 9th - 13th centuries, it spread all over the Mediterranean Sea.

What kinds of forces act on the sail?

Several forces act on the sails and the hulk, which can be break down to components. In the case of the sail the aerodynamic force is the determining one. The lateral force acts perpendicularly to the water’s and air’s moving direction. The direction of the drag force corresponds to direction of the stream. The lateral force acts in the opposite direction to  the wind’s blowing. The water, in its turn, resists to the lateral movement of the ship. The bigger the quotient between the lateral and the drag force, the quicker the ship is, which means that the lateral force must be as strong as it can be compared to the suitable drag force.
The drag force is the result of the body slowing down the corpuscles of the water and air. The lateral force depends on the curvature and thickness of the hulk and sail. These forces occur due to the difference of pressure between the two sides of the sail. This happens because the air and the water don’t stream evenly along the two sides. If the speed of the air or water rises, the pressure decreases. The air streams more quickly on the sail’s bulging (below the wind) side than on the hollow one, therefore the stream here is stronger, and the force acting on the sail points at this side.

What is a sail made of?

The material of the sail has to be strong, stiff, smooth and resistant to bacteria and light. At present, the ideal material serving this purpose is a kind of polyester called dakron. The hemp cordage was replaced by iron and zinc-plated steel cordage, the wooden yard with ones made of oxygenated aluminum-alloy, fiberglass or composite.