It is THE most visible part of a sail boat when it is fully opened and at times come in decorative and attractive colors and shades. Their shapes vary as per the type of sail boat that you might be sailing in or looking at, but their purpose remains the same - it is any type of a surface used to generate thrust by being placed in a wind. They are the 'coolest' non-mechanical propulsion system for a boat and also one of the oldest.
Sails propel the boat in one of two ways. When the boat is going in the direction of the wind (i.e. downwind - see Points of sail), the sails may be set merely to trap the air as it flows by. Sails acting in this way are aerodynamically stalled. In stronger winds, turbulence created behind stalled sails can lead to aerodynamic instability, which in turn can manifest as increased downwind rolling of the boat. Spinnakers and square-rigged sails are often trimmed so that their upper edges become leading edges and they operate as airfoils again, but with airflow directed more or less vertically downwards. This mode of trim also provides the boat with some actual lift and may reduce both wetted area and the risk of 'digging in' to waves.
The other way sails propel the boat occurs when the boat is traveling across or into the wind. In these situations, the sails propel the boat by redirecting the wind coming in from the side towards the rear. In accordance with the law of conservation of momentum, air is redirected backwards, making the boat go forward. This driving force is called lift although it acts largely horizontally.
Modern sails can be classified into three main categories: Mainsail, Headsail, and Spinnaker or downwind sail (also termed Kite) and at the same time the construction techniques have been heavily modernised thanks to Computer-Aided Design-ing as compared to various age-old methods.
There are some sails that are customized as per the special requirements for a particualr boat or for some kind of wind-condition.
