WIND DRIVING FORCE
The NASA website has published very interesting materials about various factors influencing the formation of lift by an aircraft wing. There are also interactive graphical models that demonstrate that lift can also be generated by a symmetrical wing due to flow deflection.
The sail, being at an angle to the air flow, deflects it (Fig. 1d). Coming through the “upper”, leeward side of the sail, the air flow travels a longer path and, in accordance with the principle of flow continuity, moves faster than from the windward, “lower” side. The result is that the pressure on the leeward side of the sail is less than on the windward side.
When sailing on a jibe, when the sail is set perpendicular to the direction of the wind, the degree of increase in pressure on the windward side is greater than the degree of decrease in pressure on the leeward side, in other words, the wind pushes the yacht more than it pulls. As the yacht turns sharper into the wind, this ratio will change. Thus, if the wind is blowing perpendicular to the yacht's course, increasing the pressure on the sail on the windward side has less effect on speed than decreasing the pressure on the leeward side. In other words, the sail pulls the yacht more than it pushes.
The movement of the yacht occurs due to the fact that the wind interacts with the sail. Analysis of this interaction leads to unexpected results for many beginners. It turns out that the maximum speed is achieved not at all when the wind blows directly from behind, and the wish for a “fair wind” carries a completely unexpected meaning.
Both the sail and the keel, when interacting with the flow of air or water, respectively, create lift, therefore, to optimize their operation, wing theory can be applied.
WIND DRIVING FORCE
The air flow has kinetic energy and, interacting with the sails, is capable of moving the yacht. The work of both the sail and the airplane wing is described by Bernoulli's law, according to which an increase in flow speed leads to a decrease in pressure. When moving in the air, the wing divides the flow. Part of it goes around the wing from above, part from below. An airplane wing is designed so that the air flow over the top of the wing moves faster than the air flow under the bottom of the wing. The result is that the pressure above the wing is much lower than below. The pressure difference is the lifting force of the wing (Fig. 1a). Thanks to its complex shape, the wing is capable of generating lift even when cutting through a flow that moves parallel to the plane of the wing.
The sail can move the yacht only if it is at a certain angle to the flow and deflects it. It remains debatable how much of the lift is due to the Bernoulli effect and how much is the result of flow deflection. According to classical wing theory, lift arises solely as a result of the difference in flow velocities above and below an asymmetrical wing. At the same time, it is well known that a symmetrical wing is capable of creating lift if installed at a certain angle to the flow (Fig. 1b). In both cases, the angle between the line connecting the front and rear points of the wing and the direction of the flow is called the angle of attack.
Lift increases with increasing angle of attack, but this relationship only works at small values of this angle. As soon as the angle of attack exceeds a certain critical level and the flow stalls, numerous vortices are formed on the upper surface of the wing, and the lift force decreases sharply (Fig. 1c).
Yachtsmen know that gybe is not the fastest course. If the wind of the same strength blows at an angle of 90 degrees to the heading, the yacht moves much faster. On a jibe course, the force with which the wind presses on the sail depends on the speed of the yacht. With maximum force, the wind presses on the sail of a yacht standing motionless (Fig. 2a). As speed increases, the pressure on the sail drops and becomes minimal when the yacht reaches maximum speed (Fig. 2b). The maximum speed on a gybe course is always less than the wind speed. There are several reasons for this: firstly, friction; during any movement, some part of the energy is spent on overcoming various forces that impede movement. But the main thing is that the force with which the wind presses on the sail is proportional to the square of the speed of the apparent wind, and the speed of the apparent wind on a gybe course is equal to the difference between the speed of the true wind and the speed of the yacht.
With a gulfwind course (at 90º to the wind), sailing yachts are able to move faster than the wind. In this article, we will not discuss the features of the apparent wind; we will only note that on a gulfwind course, the force with which the wind presses on the sails depends to a lesser extent on the speed of the yacht (Fig. 2c).
The main factor that prevents an increase in speed is friction. Therefore, sailboats with little resistance to movement are able to reach speeds much higher than the speed of the wind, but not on a gybe course. For example, a boat, due to the fact that skates have negligible sliding resistance, is capable of accelerating to a speed of 150 km/h with a wind speed of 50 km/h or even less.
The Physics of Sailing Explained: An Introduction
ISBN 1574091700, 9781574091700
I think that many of us would take the chance to plunge into the abyss of the sea on some underwater vehicle, but still, most would prefer a sea voyage on a sailboat. When there were no planes or trains, there were only sailboats. Without them the world was not what it was.
Sailboats with straight sails brought Europeans to America. Their stable decks and capacious holds carried men and supplies to build the New World. But these ancient ships also had their limitations. They walked slowly and almost in the same direction with the wind. A lot has changed since then. Today they use completely different principles for controlling the power of wind and waves. So if you want to ride a modern one, you’ll have to learn some physics.
Modern sailing is not just moving with the wind, it is something that acts on the sail and makes it fly like a wing. And this invisible “something” is called lift, which scientists call lateral force.
An attentive observer could not help but notice that no matter which way the wind blows, the sailing yacht always moves where the captain wants it - even when the wind is headwind. What is the secret of such an amazing combination of stubbornness and obedience.
Many people don’t even realize that a sail is a wing, and the principle of operation of a wing and a sail is the same. It is based on lift only if the lift of the wing aircraft, using the headwind, pushes the plane upward, then the vertically positioned sail directs the sailboat forward. To explain this from a scientific point of view, it is necessary to go back to the basics - how a sail works.
Look at the simulated process that shows how air acts on the plane of the sail. Here you can see that the air flows under the model, which have a greater bend, bend to go around it. In this case, the flow has to speed up a little. As a result, an area of low pressure appears - this generates lift. The low pressure on the underside pulls the sail down.
In other words, an area of high pressure tries to move toward an area of low pressure, putting pressure on the sail. A pressure difference arises, which generates lift. Due to the shape of the sail, the wind speed on the inside windward side is lower than on the leeward side. On outside a vacuum is formed. Air is literally sucked into the sail, which pushes the sailing yacht forward.
In fact, this principle is quite simple to understand; just take a closer look at any sailing ship. The trick here is that the sail, no matter how it is positioned, transfers wind energy to the ship, and even if visually it seems that the sail should slow down the yacht, the center of application of forces is closer to the bow of the sailboat, and the force of the wind ensures forward motion.
But this is a theory, but in practice everything is a little different. In fact, a sailing yacht cannot sail against the wind - it moves at a certain angle to it, the so-called tacks.
A sailboat moves due to the balance of forces. The sails act like wings. Most of the lift they produce is directed to the side, and only a small amount forward. However, the secret to this wonderful phenomenon is the so-called “invisible” sail, which is located under the bottom of the yacht. This is a keel or, in nautical language, a centerboard. The lift of the centerboard also produces lift, which is also directed mainly to the side. The keel resists heel and the opposing force acting on the sail.
In addition to the lifting force, a roll also occurs - a phenomenon harmful to forward movement and dangerous for the crew of the ship. But that’s why the crew exists on the yacht, to serve as a living counterweight to the inexorable laws of physics.
In a modern sailboat, both the keel and the sail work together to propel the sailboat forward. But as any novice sailor will confirm, in practice everything is much more complicated than in theory. An experienced sailor knows that the slightest change in the bend of the sail makes it possible to obtain more lift and control its direction. By changing the bend of the sail, a skilled sailor controls the size and location of the area that produces lift. With a deep forward bend you can create large area pressure, but if the bend is too great or the leading edge of the air molecules is too steep, the flow around them will no longer follow the bend. In other words, if the object has sharp corners, the particles of the flow cannot make a turn - the momentum of the movement is too strong, this phenomenon is called “separated flow”. The result of this effect is that the sail will “sweep”, losing the wind.
Here are some more practical tips for using wind energy. Optimal heading into the wind (racing close-hauled wind). Sailors call it “sailing against the wind.” The apparent wind, which has a speed of 17 knots, is noticeably faster than the true wind that creates the wave system. The difference in their directions is 12°. Course to apparent wind - 33°, to true wind- 45°.
It is difficult to imagine how sailing ships can go “against the wind” - or, as sailors say, go “close-hauled”. True, a sailor will tell you that you cannot sail directly against the wind, but you can only move at an acute angle to the direction of the wind. But this angle is small - about a quarter of a right angle - and it seems, perhaps, equally incomprehensible: whether to sail directly against the wind or at an angle to it of 22°.
In reality, however, this is not indifferent, and we will now explain how it is possible to move towards it at a slight angle by the force of the wind. First, let's look at how the wind generally acts on the sail, that is, where it pushes the sail when it blows on it. You probably think that the wind always pushes the sail in the direction it blows. But this is not so: wherever the wind blows, it pushes the sail perpendicular to the plane of the sail. Indeed: let the wind blow in the direction indicated by the arrows in the figure below; line AB denotes a sail.
The wind always pushes the sail at right angles to its plane.
Since the wind presses evenly on the entire surface of the sail, we replace the wind pressure with a force R applied to the middle of the sail. Let's break this force down into two: force Q, perpendicular to the sail, and the force P directed along it (see figure above, right). The last force pushes the sail nowhere, since the friction of the wind on the canvas is insignificant. Strength remains Q, which pushes the sail at right angles to it.
Knowing this, we can easily understand how a sailing ship can sail at an acute angle towards the wind. Let the line QC depicts the keel line of the ship.
How can you sail against the wind?
The wind blows at an acute angle to this line in the direction indicated by a series of arrows. Line AB depicts a sail; it is placed so that its plane bisects the angle between the direction of the keel and the direction of the wind. Follow the distribution of forces in the figure. We represent the force of the wind on the sail Q, which we know should be perpendicular to the sail. Let's break this force down into two: force R, perpendicular to the keel, and the force S, directed forward, along the keel line of the vessel. Since the ship's movement is in the direction R meets strong water resistance (the keel in sailing ships is made very deep), then the force R almost completely balanced by water resistance. Only strength remains S, which, as you can see, is directed forward and, therefore, moves the ship at an angle, as if towards the wind. [It can be proven that the force S receives the greatest value when the plane of the sail bisects the angle between the directions of the keel and the wind.]. Typically this movement is performed in zigzags, as shown in the figure below. In the language of sailors, such a movement of the ship is called “tacking” in the strict sense of the word.
The movement of a sailing yacht in the wind is actually determined by the simple pressure of the wind on its sail, pushing the ship forward. However, wind tunnel research has shown that sailing upwind exposes the sail to a more complex set of forces.
When the incoming air flows around the concave rear surface of the sail, the air speed decreases, while when flowing around the convex front surface of the sail, this speed increases. As a result, an area of high pressure is formed on the back surface of the sail, and a low pressure area on the front surface. The pressure difference on the two sides of the sail creates a pulling (pushing) force that moves the yacht forward at an angle to the wind.
A sailing yacht located approximately at right angles to the wind (in nautical terminology - the yacht is coming tack), moves quickly forward. The sail is subject to pulling and lateral forces. If a sailing yacht sails at an acute angle to the wind, its speed slows down due to a decrease in the pulling force and an increase in the side force. The more the sail is turned towards the stern, the slower the yacht moves forward, in particular due to the large lateral force.
A sailing yacht cannot sail directly into the wind, but it can move forward by making a series of short zigzag movements at an angle to the wind, called tacks. If the wind blows to the left side (1), the yacht is said to be sailing on port tack; if it is blowing to starboard (2), it is said to be sailing on starboard tack. In order to cover the distance faster, the yachtsman tries to increase the speed of the yacht to the limit by adjusting the position of its sail, as shown in the figure below left. To minimize deviation to the side from a straight line, the yacht moves, changing course from starboard tack to port and vice versa. When the yacht changes course, the sail is thrown to the other side, and when its plane coincides with the wind line, it flutters for some time, i.e. is inactive (middle picture below the text). The yacht finds itself in the so-called dead zone, losing speed until the wind again inflates the sail from the opposite direction.
Russian poet Mikhail Yurievich Lermontov loved sea and often mentioned him in his works. He wrote a wonderful poem about the whitening sail, which rushes among the waves in the distant expanses of the sea. You are probably familiar with Lermontov’s poem, because these are the most famous lines of poetry about sailing ships. Reading them, you can imagine a raging sea and beautiful ships among its waves. The wind fills the sails. And, thanks to the power of the wind, the ships move forward. But how do sailboats manage to sail against the wind?
In order to answer this, you will first have to learn an unfamiliar word "tack".Galsom The direction of movement of the ship relative to the wind is called. The tack can be port when the wind is blowing from the left, or starboard when the wind is blowing from the right. It is important to know the second meaning of the word “tack” - this is part of the path, or rather, the segment of it that the sailboat passes when it moves against the wind. Do you remember?
Now, to understand how sailboats manage to sail against the wind, let's look at the sails. They are on a sailboat different forms and sizes - straight and oblique. And everyone does their job. When a headwind blows, the ship is steered using oblique sails, which turn first one way and then the other.
Following them, the ship turns in one direction or another. He turns and walks forward. Sailors call this movement - moving on alternating tacks. Its essence is that the wind presses on the slanting sails and blows the ship slightly sideways and forward. The rudder of a sailboat does not allow it to turn completely, and skilled sailors set the sails in motion in time, changing their position. So, in small zigzags, it moves forward.
Of course, moving on alternating tacks is a very difficult task for the entire crew of a sailboat. But the sailors are seasoned guys. They are not afraid of difficulties and love the sea very much.
