1 power source of sailing boat
Most people often think that sailing boats are driven by the wind. In fact, the wind force acts on the sail in two forms, as shown in Figure 1 and Figure 2. The biggest power source of sailboats is the so-called Bernoulli effect.
We know that when the air flows rapidly, the object blocking it in front will be impacted by the air, and the pressure generated by this impact is called dynamic pressure. When sailing with the wind as shown in figure 1, it is the dynamic pressure of the air on the sail that pushes the sailing boat forward. According to Bernoulli's principle that "the flow rate increases and the pressure decreases", when the air flows in one direction, the force acting on the side will be relatively reduced. That is to say, the greater the gas velocity, the greater the dynamic pressure and the smaller the static pressure. The smaller the flow velocity, the smaller the dynamic pressure and the larger the static pressure. In this way, the place with low gas velocity will produce a lateral pressure on the place with high gas velocity, which is called static pressure. When sailing against the wind, as shown in Figure 2, the ship is propelled by the static pressure of the wind.
The static pressure on the sail is mainly because the sail has an arc like a wing. If we compare the cross section of the sail with the cross section of the wing, we can see their similarities. As shown in Figure 3, when the airflow passes through the sail or the wing, the airflow above the wing and in front of the sail has to go a longer distance to meet the airflow below the wing and behind the sail, and the velocity is accelerated, which makes the airflow before and after the sail different from the airflow at the top and bottom of the wing. The pressure at low speed is greater than the static pressure at high speed. This pressure difference causes the wing to generate upward lift and the sail to gain forward power, as shown in Figure 4. We might as well call it an elevator.
Let's see how the static pressure on the sail pushes the ship forward. As shown in fig. 5, the static pressure FT on the sail can't be used to push the ship forward, but the component FR of FT along the bow direction is really used to push the ship forward, and the value of FR is smaller than the component FH that makes the ship move laterally. Although the lateral force is large, it is rare to see the ship move sideways in actual driving. However, the speed of the ship is quite fast. Advanced sailboats and windsurfing can reach the speed of 30-40 kilometers per hour at the earliest. Besides the thrust generated by the sails, the streamlined bottom is also the reason. The transverse cross-sectional area of the underwater part of the ship is much larger than the longitudinal cross-sectional area. Although the thrust FR is smaller than the lateral force FH, the resistance of the ship moving forward in the water is much smaller than that of the ship moving laterally. Therefore, the effect of FR pushing the ship forward is quite remarkable.
2 Restrictions and benefits of navigation routes
Figure 6
Sailing boats can run in the wind driven by dynamic pressure or in the headwind driven by static pressure. However, the sailing course is not completely unlimited, and it is impossible to produce effective propulsion in the range of plus or minus 45 degrees, as shown in area A in Figure 6. But too downwind is not good, because the Bernoulli effect disappears at this time. The ship is driven 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 greater the relative velocity, the greater the dynamic pressure. However, driven by dynamic pressure, the forward speed of the ship will gradually increase, and the relative speed between the wind and the ship will decrease, so the dynamic pressure of the wind on the sail will decrease, the ship speed will slow down again, and it will enter an unstable state, as shown in area C in the figure. Therefore, dynamic pressure is not the power source for sailing ships to continue to be efficient. Only area B is the best sailing direction in the picture. At this time, the sailing direction of the ship forms a certain angle with the wind direction, and the ship can obtain sustained and stable driving force under static pressure, so that the ship can obtain a higher sailing speed.
If the ship is sailing against the wind, the sailing direction of the ship should form an angle with the wind direction, so the Z-shaped route must be adopted. As shown in fig. 7.
Control and navigation of sailing boats
Because there is a certain distance between the center of sail under the action of wind and the center of hull side under the action of water resistance, FH force makes the ship transverse, although it is not significant, but it has a significant effect on tilting the ship in the downwind direction. As shown in Figure 8, this requires athletes to adjust the center of gravity of the ship with their own weight at any time to maintain the balance of the ship (usually called "line pressing"). Because the magnitude of wind force will change at any time, the role of lateral force will also change. Therefore, it is necessary to change strings flexibly at any time, which is an important operating skill for athletes.
Thrust FR pushes the ship forward, and it also has the function of tilting the ship forward. Although it is much smaller than the lateral force FH, it will also stall the ship. Therefore, athletes should always pay attention to the possible pitching and try to keep the balance of the ship by pressing the strings.
Change course, navigation mainly depends on navigation. Windsurfing depends on the change of the position and center of gravity of the sail. Fig. 9 is a schematic diagram of the principle of changing the course of a sailboat through a rudder. When the ship is running, the current gives the rudder a force F perpendicular to the sailing surface. One component of F, F 1, can make the ship rotate, and the other component, F2, can stop the ship from moving forward. Because F2 acts as resistance to the ship, don't push the steering angle too much when turning. Of course, to complete the steering action, in addition to sailing, it is necessary to coordinate the position of the sail with the actions of the crew.
In the steering of windsurfing, when the athlete tilts the movable mast behind the downwind, the head of the board turns to the downwind direction. On the contrary, when the mast tilts in front of the headwind, the head of the board deflects away from the wind. Through the reverse motion of the mast, the center of the sail is moved, so that the sail plate generates a rotating moment, thus prompting it to turn.
As can be seen from the above, sailing and windsurfing have certain technical difficulties, but they can also give you a high sense of accomplishment and excitement. As long as you study hard and practice actively, you will soon experience the fun of riding the wind and waves.