In this blog post, we’ll explore the principles behind how a yacht—which lacks a propulsion system like an engine—can use the wind to sail in the desired direction.
If you watch small yachts, you’ll see them moving around at sea without any propulsion system. If the wind were to blow continuously from land toward the sea, the yacht would eventually be blown off course. Yet, strangely enough, we don’t often come across news articles like “Man in his 20s on a drifting yacht dramatically rescued.” If the wind blows from behind, the sail can catch the wind and propel the yacht forward, but if the wind blows from the side, wouldn’t the yacht simply be carried along by the wind and eventually drift? Let’s explore the principle behind how a sailboat—which has no engine and relies solely on sails—can move in the desired direction without being carried away by the wind.
The key principle behind sailing against the wind is the lift generated by the sails. If you look at a yacht from above, you’ll see that it doesn’t face the wind head-on but sails at an angle to the direction the wind is blowing. By adjusting the direction and angle of the sails to match the wind, the sails become taut and function much like the wings of an airplane. A sail is not simply a piece of fabric that catches the wind and is pushed along; it functions like a wing, using the flow of air to generate lift. The forces acting on the sail can be broadly divided into those that propel the yacht forward and those that push it sideways and cause it to heel. The lift generated by the sail does not directly push the yacht straight ahead; instead, depending on the angle of the sail and the direction of the wind, forces act in various directions. If the force pushing the yacht sideways is left unchecked, the yacht will move forward while simultaneously being pushed sideways. Therefore, for the yacht to move forward, the forces generated by the sail must be balanced by the forces from the devices that control the yacht’s movement in the water.
Let’s take a closer look at the principle behind lift. When viewed from above, the air flowing around the sail creates different flow patterns on either side of the sail. Rather than simply blocking the wind like a flat plate, the sail has a specific curvature and angle that alters the flow of air. A pressure difference arises on either side of the sail, and this pressure difference generates lift on the sail. It is often explained that pressure decreases on one side of the sail because the air moves faster there, while pressure increases on the other side; however, it is not accurate to understand the generation of lift simply as “air moving faster on one side so that it reaches the rear at the same time.” In reality, the shape of the sail, its angle to the wind, and the airflow interact to create a pressure distribution on both sides of the sail, resulting in lift. Therefore, a yacht is able to sail against the wind not simply by facing the wind head-on and being pushed along, but by controlling the airflow around the sail to generate lift.
So, how can we reduce the force generated by the sail’s lift that pushes the yacht sideways? To achieve this, structures such as a keel, centerboard, or daggerboard are installed at the bottom of the yacht, depending on the hull type and size. These structures prevent the yacht from sliding sideways in the water. Just as a wide plate encounters significant resistance when moved sideways underwater, a centerboard or keel also resists the yacht’s sideways drift. In particular, the keel on modern yachts is not merely a structure that creates resistance underwater; it uses a wing-like shape to generate lift within the water flow, helping to balance the lateral force generated by the sail. As a result, the yacht can move forward using the force generated by the sails without drifting significantly to the side. Therefore, the interaction between the aerodynamic forces generated by the sails and the hydrodynamic forces generated by the keel or centerboard is a key principle that allows a yacht to sail against the wind.
Yachts generally cannot sail directly into the wind. If the yacht approaches too close to the direction from which the wind is blowing, the sails cannot generate sufficient lift, causing the speed to drop and eventually resulting in the sails flapping. However, depending on the yacht’s design, the shape of the sails, and the wind speed, it is often possible to sail at an angle of about 40 to 45 degrees from the direction of the wind. The exact angle varies depending on the type and design of the yacht, the condition of the sails, and the wind speed. Therefore, rather than sailing directly into the wind, the yacht moves diagonally to the left or right relative to the wind direction and then changes course, proceeding toward its destination in a zigzag pattern. Changing the sailing direction from one side to the other is called “tacking.” Even when the wind is blowing from the side, you can move forward by utilizing lift if you adjust the angle of the sail appropriately. When sailing with the wind at your back, you can set the sails to face the direction of the wind and harness its power directly. However, caution is required when sailing downwind, as the movement of the sails and boom can become more pronounced, and “jibing”—where the sail suddenly flips over during a course change—can occur. As such, a yacht does not simply drift along by having the wind push it from behind; rather, by properly adjusting the sail, keel, centerboard, and rudder, it can sail at various angles to the wind. Ultimately, the secret to a yacht moving in the desired direction without drifting at sea lies not in overcoming the wind, but in properly harnessing the force generated by the wind through the sail and underwater structures, and maintaining balance.
