Sailboats do not have engines, and therefore they rely solely on the wind to propel them. This means that the sails are typically much wider than the hull so that they can effectively harness the power of the wind.

While sailboats can reach high speeds when sailing downwind, they face a different challenge when sailing into the wind.

It is commonly believed that sailboats can only move in the direction of the wind, or downwind. However, with the use of a spinnaker, a sailboat can also sail into the wind (upwind). Before we can understand how to sail upwind, we need to have a basic understanding of how sails work.

The front edge of a sail is called the leading edge, while the rear edge is called the trailing edge. The imaginary line that runs horizontally between the leading and trailing edges is known as the chord.

The curvature of the sail is called the draft, and the distance between the chord and the point of the maximum draft is called the chord depth. The side of the sail that is filled with air to form a concave bend is called the windward side, while the side that forms a raised shape is called the leeward side.

The sailboat moves in the windward direction due to the force generated by each side of the sail. The positive force (thrust) on the windward side and the negative force (pull) on the leeward side combine to form a net force that propels the boat forward. However, it's worth noting that the pull is typically stronger than the thrust.

In 1738, scientist Daniel Bernoulli discovered that the speed of airflow increases in proportion to the surrounding free airflow, resulting in a decrease in pressure that can make the airflow even faster. This phenomenon occurs on the leeward side of the sail, where the air flows faster and creates a low-pressure area behind the sail.

So why does the airspeed up?

Air, like water, is fluid. When the wind hits the sail, some of it attaches to the bulge and pulls the sail upwards. For the "unattached" air to pass through the sail, the sail must be bent out of the air stream that is not affected by the sail. However, this free-flowing air tends to keep its straight flow, which impedes sailing.

The curved sail and the free airflow together form a narrow channel through which the initial airflow must pass. As the air cannot compress itself, it must accelerate to squeeze through the narrow channel. This is why the air velocity increases on the raised side of the sail.

Once this happens, Bernoulli's theory comes into effect. The increased airflow in the narrow channel is faster than the surrounding air, causing the pressure to drop in the area where the airflow is accelerating, creating a chain reaction.

As the new airflow approaches the sail's edge, it separates and flows more towards the leeward side, as the airflow is attracted to the low-pressure region and repelled by the high-pressure region. Even larger chunks of air must now squeeze faster into the narrow channel formed by the raised sail surface and the free airflow, which leads to a further decrease in air pressure.

This process continues until the maximum low-pressure area is formed on the leeward side, and the maximum speed for the available wind conditions is reached. It's worth noting that the airflow only increases after it reaches the deepest point of the sail's surface (chord depth). Beyond this point, the air separates and decelerates until it reaches the velocity of the surrounding air.