The wind is the only thing that propels the sailing boat, so how can a boat sail against it? Amazingly, the most important force that drives a boat into the wind is suction.
The boat’s sail is like an aircraft’s wing on its side. On the outwardly curved, leeward side the wind has to flow around the sail, creating a powerful suction effect pulling the sail towards it. The same principle applies to an aircraft, which gets lift from the suction on the top of its wings.
The suction effect is produced by the laws of aerodynamics. The air that is diverted around a curved sail becomes compressed so that it can squeeze past. When a moving stream of air is compressed its speed increases – a draught under a door can be surprisingly strong for this reason. And when the wind’s speed increases, a lot of pressure occurs. This is because the faster the air is moving, the fewer molecules there are in any given space.
The area of low pressure on the leeward side of sucks that sail towards it with the twice the force that the same strength of wind can push into it on the windward side.
So the wind forces the boat sideways. However, the keel – or centreboard – of the boat resists the sideways movement. The wind’s force is then converted partly into a forward movement of the boat and partly into a tilt to leeward which the yachtsman has to counteract by leaning out from the other side of the boat. The boat sailing close to the wind is bound to move substantially sideways – an effect called leeway. But the helmsman can compensate when plotting his course.
Boat can sail directly into the wind, but at 12 m yacht can sail only 12-15 degrees off the wind. To go in the direction of wind is coming from, the boat has to zigzag, or make a series of tacks. The closer a boat sails to the wind, the slower its speed will be. The helmsman can go faster by making wider zigzags at a bigger angle to the wind, but then he has to travel farther.
Picture Credit : Google