Transport Technology – Sailboats
Sailing ships, boats, and recreational vehicles use wind energy, currents, and muscular strength for propulsion and direction.
After the industrial revolution, sailing ships were replaced by steamers because they were less dependent on the weather, required smaller crews, and could get out to sea more quickly. However, sailing remains a popular recreation.
Physics of sailing
Wind energy supplies the power to move a sailing craft. Wind currents can hit from directly behind or from the sides, splitting into two currents that flow around the sail. The air flowing over the windward side of the sail moves faster than the air flowing over the leeward side, creating more pressure on the leeward side.
This is the force that moves the craft. The angle of the sail relative to the wind’s direction determines the drift; if the angle is wrong, the sails will flutter and there is no movement. Using a process called tacking, sailing vessels can move in a direction opposite the wind.
Heeling and stability
Heeling occurs when air currents hit the sides of the sails, causing the ship to tilt along its longitudinal axis. This can lead to capsizing. A boat’s ability to resist heeling is directly dependent on its stability. In keeled ships, a large weight is used to counteract heeling. The undersides of these ships make up 30 to 50 percent of the overall weight.
When the angle of tilt is increased, the keel generates a torque opposite to the tilt and keeps the ship upright. Small sailboats do not have a keel. Instead, they rely on a centerboard and wide-type hull for stability.
These boats can easily roll back to an upright position. A small boat’s crew can also contribute to stability by moving over to one side of the boat to help balance it.
Wave and muscular power
Windsurfing and kite surfing are two popular non-motorized water sports that use wind power. The board and rider are propelled forward by wind caught in the sails or kites.
The surfer uses his body weight and muscular power to determine the direction of movement, to ride waves, or perform tricks like aerial jumps and somersaults.
HYDRODYNAMICS
During motion in the water, a boat’s hull produces bow and wake waves. A ship cannot move faster than the bow and wake waves. The maximum speed a boat can achieve is called the speed of the hull; the shape of the hull determines this speed. Longer ships create longer waves, which disperse rapidly, allowing the boat to reach higher speeds.
Long, flat hulls with wide sterns produce lift forces. Caused by the dynamics of the heel and bow waves, these forces allow the boat to skim over the water surface, a process called planning. When planning, the boat can move faster than the hull speed because it is no longer being slowed down by wave friction.
Instead, it is riding just on top of the surface, where there is far less friction.