Explaining how a boat sails downwind is fairly straight forward, you open the sails as far as possible and the wind pushes the sails, moving the boat along...simple.
Air travelling around the outside of the curve of the sail takes a longer path than the air around the inside of the sail. This longer path means the air travels faster around the curve of the path, causing a lower pressure than the air surrounding it.
Inside of the sail, the air has a shorter path to travel so travels at a much lower speed and has a lot more pressure.
This difference in pressure between the inside of the sail and the outside of the sail causes lift, a force that helps push the boat through the water.
Lift isn't the only force required for a boat to travel upwind, keels/centreboards prevent the boat from being pushed sideways through the water by the wind. The force applied by the water on the keel acts in the opposite direction to the force of the wind, these 2 forces squeeze the boat, pushing it forwards through the water, similar to when you squeeze a bar of soap in your hands.
Why Doesn&apos;t The Boat Drift Sideways?
Well actually boats do drift sideways ever so slightly, however the water pushes against the keel when the boat drifts and it is this force that prevents the boat drastically drifting.