There is a certain sense of logic involved when you think about how a boat can use its sail to travel downwind. It’s getting your mind around the idea of the wind pushing a boat upwind through its sails that poses a challenge to the mind.
It almost seems obvious when you think that a sail that is held perpendicular to the wind will be pushed in the direction of the wind from behind. This very simple principle was even implemented by the ancient Egyptian, Roman and Greek seafarers of the time, as described here: http://www.bbc.co.uk/schools/primaryhistory/ancient_greeks/sea_and_ships/. In order to go faster, they would row, and the same method was used when they chose to travel upwind.
Modern-day sailors are not restricted to the same degree. Although they aren’t able to sail directly upwind, they are able to sail in almost any direction greater than 45 degrees with respect to the wind.
Try to Envision the Following Scenario
There is a boat aiming to sail upwind. The wind comes over the front of the boat out of a direction slightly to the left. Now the wind stretches the sail into a wing shape without moving it, because it is fastened at either end. The sail is taut, cannot move and looks similar to school canopies. The wind is unable to move the sail and therefore changes direction to flow side by side with the sail. Now there is an equal and opposite force on the sail that has been created by the wind that will push the boat forward and towards the right.
The keel is found below the boat and has the purpose of keeping the boat upright and moving it forward. Returning to where we left off, the keel is what causes resistance and stops the boat from travelling sideways. It will move in a forward direction because the wind force on the sail has been cancelled out by the force on the keel.
It is incredible to think that sails have always been used so creatively and that even today companies such as http://signaturestructures.com/school-canopies/ are finding innovative ways to use sails.
Although there are so many factors that can affect how fast a boat travels and in which direction, the basics that have been explained should provide a sufficient understanding of how sails work.a