Generally speaking, sailboats are believed to be propelled by the wind. In fact, the power of the wind acts on the sail in two forms, and the biggest source of power for sailboats is the so-called "Bernoulli effect".

We know that when air flows rapidly, objects blocking it from the front will be impacted by the air, and the pressure generated by this impact is called dynamic pressure. When the sailboat is sailing along the wind as shown in the left figure in Figure 2, it is the dynamic pressure of the air on the sail that drives the sailboat forward. According to the Bernoulli principle of "increasing flow velocity and decreasing pressure", when air flows in one direction, its lateral force will relatively decrease. That is to say, the higher the gas flow speed, the greater the dynamic pressure and the lower the static pressure. The smaller the flow velocity, the smaller the dynamic pressure and the greater the static pressure. In this way, areas with low gas flow rates and high convective velocities will generate a lateral pressure, which is called static pressure. When sailing against the wind, as shown in the right figure in Figure 2, the ship is propelled forward by the static pressure of the wind.

The generation of static pressure on a sail is mainly due to its curved shape resembling a wing. Let's compare the cross-section of the sail with the cross-section of the wing, and we can see their commonalities. As shown in the left figure in Figure 3, when air flows through the sail or wing, the airflow above and in front of the sail needs to travel a longer distance to meet the airflow below and behind the sail, thus accelerating the flow velocity, resulting in different flow velocities in front and behind the sail, as well as on the top and bottom of the wing. The pressure at the slow flow rate is stronger than the static pressure at the fast flow rate. This pressure difference generates upward lift on the wing and also gives the sail forward power, as shown in the right figure of Figure 3. It can also be referred to as "lift" here.

Let's take a look at how the static pressure on the sail propels the ship forward. As shown in Figure 4, the static pressure FT exerted on the sail cannot be entirely used to propel the ship forward. What is truly used to propel the ship forward is the component force FR along the bow direction of FT, which is smaller than the component force FH that causes the ship to move laterally. Although the lateral force is relatively large, it is rare to see the ship move laterally during actual driving. However, the ship's forward speed is quite large. Advanced sailboats and windsurfing, the fastest speed, can reach 30 to 40 km per hour, resulting in such forward speed. In addition to the thrust generated by the sail, another important factor is the streamline of the bottom of the ship. The transverse section area of the submerged part of the ship is far larger than the longitudinal section area. Although the thrust FR is smaller than the transverse force FH, the resistance of the ship when moving forward in the water is much smaller than the resistance of the ship's lateral movement. So, the effect of FR pushing the ship forward is quite significant.

Heading Restriction Benefits

Figure 5

Sailing boats can be driven either by dynamic pressure to sail with the wind or by static pressure to sail against the wind. However, the sailing direction of the sailboat is not completely unrestricted, and it cannot generate effective propulsion within approximately 45 degrees of wind and headwind, as shown in Zone A in Figure 5. But too smooth a wind is not very good, because at this point the Bernoulli effect disappears. The ship is propelled by the dynamic pressure of the wind on the sail, and the magnitude of the dynamic pressure depends on the relative speed of the wind on the sail. The higher the relative speed, the greater the dynamic pressure. However, under the propulsion of dynamic pressure, the forward speed of the ship gradually increases, and the relative speed between the wind and the ship will decrease. Therefore, the dynamic pressure of the wind on the sail will decrease, and the ship's speed will slow down again, and it will enter an unstable state, as shown in Zone C in Figure 5. So dynamic pressure is not a sustainable and efficient source of power for sailboats. Only Zone B in Figure 5 is the best sailing direction, where the ship's sailing direction is at a certain angle to the wind direction. Under static pressure, the ship can obtain a continuous and stable driving force, enabling it to achieve a relatively high sailing speed. If a ship wants to sail against the wind, its direction of navigation should be at an angle to the wind direction, so a Z-shaped route must be adopted.

Control and steering

Sailboat

Due to the distance between the center point of the sail subjected to wind force and the center of the ship's side subjected to water resistance, although the force of FH does not significantly cause the ship to move laterally, its effect on tilting the ship downwind is quite significant. This requires athletes to adjust the center of gravity of the boat with their own weight at any time to maintain the balance of the boat (often referred to as "pressure string").

Due to the varying magnitude of wind force, the effect of lateral force also varies. So string pressing requires flexible changes at any time, which is an important operational skill for athletes.

The thrust FR also has a function of causing the ship to tilt forward while pushing the ship forward. Although it is much smaller than the lateral force FH that causes the ship to tilt, it can also cause the ship to stall. Therefore, athletes should always pay attention to possible longitudinal inclination and try to maintain the ship's balance by pressing the strings.

Change course, sailboats mainly rely on the rudder. The sailboard relies on the position of the sail and the transformation of the center of gravity. When the ship is moving, the water flow gives the rudder a vertical force F, one component of which F1 can cause the ship to rotate, and the other component F2 blocks the ship from moving forward. Due to the resistance effect of F2 on the ship, the rudder angle should generally not be pushed too much when turning. Of course, to complete the steering action, in addition to the rudder, it also needs to be coordinated with the position of the sail and the movement of the crew.

The turning of the windsurfing, when the athlete flips the movable mast towards the rear of the wind, the bow of the board turns towards the wind. On the contrary, if the mast is tilted towards the front of the wind, the bow of the board deflects away from the wind. By flipping the mast and moving the sail center, the sail generates a rotational torque, thereby causing it to turn.

sail against the wind

Sailing boats advance against the wind on a zigzag path, with an angle greater than zero between the wind direction and the direction of the ship's travel. The vertical component of the wind blowing towards the sail is further decomposed into two components - the component F1 along the direction of the ship's travel and the component F2 perpendicular to the direction of the ship's travel. Due to the high resistance f2 of the water perpendicular to the direction of the ship's travel (ship shape, Stokes principle), F2 is balanced with f2, preventing the ship from turning sideways. The resistance of the water in the direction of travel is very small, F1 is slightly greater than F1, at least equal to F1, and the ship accelerates or advances at a constant speed in this direction. Of course, in order not to deviate too far from the target, at a certain moment, the heading needs to be changed so that the wind blows from the other side. The principle is the same as above. So we need to take the zigzag path to take advantage of the headwind without deviating from the direction. Sailors should still use this principle to compete.