Home Equipment The movement of a sailing ship against the wind. How do sailboats manage to sail against the wind? True and apparent winds in yachting

The movement of a sailing ship against the wind. How do sailboats manage to sail against the wind? True and apparent winds in yachting

We continue the series of publications prepared by the interactive popular science blog “I’ll Explain in Two Minutes.” The blog talks about simple and complex things that surround us every day and do not raise any questions until we think about them. For example, there you can find out how spaceships do not miss and do not collide with the ISS when docking.

1. It is impossible to sail strictly against the wind. However, if the wind is blowing from the front, but slightly at an angle, the yacht may well move. In such cases, the ship is said to be sailing on a sharp course.

2. The thrust of a sail is generated by two factors. Firstly, the wind simply presses on the sails. Secondly, the oblique sails installed on most modern yachts, when air flows around them, work like an airplane wing and create a “lifting force”, only it is directed not upward, but forward. Due to aerodynamics, the air on the convex side of the sail moves faster than on the concave side, and the pressure on the outside of the sail is less than on the inside.

3. The total force created by the sail is directed perpendicular to the canvas. According to the rule of adding vectors, it is possible to distinguish the drift force (red arrow) and the traction force (green arrow).

4. On sharp courses, the drift force is great, but it is countered by the shape of the hull, keel and rudder: the yacht cannot go sideways due to water resistance. But it willingly slides forward even with a small traction force.

5. To sail strictly against the wind, the yacht tacks: it turns to the wind first with one side or the other, moving forward in segments - tacks. How long the tacks should be and at what angle to the wind should be － important issues of skipper tactics.

6. There are five main courses of a ship relative to the wind. Thanks to Peter I, Dutch maritime terminology took root in Russia.

7. Leventik－ the wind blows directly at the bow of the ship. You cannot sail this course, but turning to the wind is used to stop the yacht.

8. Closed wind－ the same acute course. When you go close-hauled, the wind blows in your face, so it seems that the yacht is developing a very high speed. In fact, this feeling is deceptive.

9. Gulfwind－ the wind blows perpendicular to the direction of movement.

10. Backstay－ the wind blows from the stern and from the side. This is the fastest course. Fast racing boats sailing backstayed are able to accelerate to speeds exceeding the speed of the wind due to the lifting force of the sail.

11. Fordewind－ the same tailwind blowing from the stern. Contrary to expectations, it is not the fastest course: here the lifting power of the sail is not used, and the theoretical speed limit does not exceed the speed of the wind. An experienced skipper can predict invisible air currents just like an airplane pilot can predict updrafts and downdrafts.

You can view an interactive version of the diagram on the “I’ll Explain in Two Minutes” blog.

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 encounters strong water resistance (keel in sailing ships becomes very deep), then the strength 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 keel and wind directions.]. Usually 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.

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 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). Maximum speed On a jibe course, the wind speed is always less. 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 dive into the abyss of the sea on some underwater vehicle, but still, the majority would prefer 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 lifting force, only if the lifting force of the aircraft’s wing, using the headwind, pushes the plane upward, then a 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. A vacuum forms on the outside. 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 laterally, with 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. A deep forward bend can create a large pressure area, but if the bend is too large or the leading edge is too steep, the air molecules will not 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.

And here are a few more practical advice use of 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°.

The winds that are in the southern part Pacific Ocean blowing in a westerly direction. That is why our route was designed so that sailing yacht"Juliet" move from east to west, that is, with the wind blowing at your back.

However, if you look at our route, you will notice that often, for example when moving from south to north from Samoa to Tokelau, we had to move perpendicular to the wind. And sometimes the direction of the wind changed completely and we had to go against the wind.

Juliet's route

What to do in this case?

Sailing ships have long been able to sail against the wind. The classic Yakov Perelman wrote about this long ago well and simply in his second book from the series “Entertaining Physics”. I present this piece here verbatim with pictures.

"Sailing against the wind

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. We will split this force into two: force Q, perpendicular to the sail, and 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. The force Q remains, 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 line KK represent 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 represents 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 wind pressure on the sail by force Q, which, we know, must be perpendicular to the sail. Let us divide this force into two: force R, perpendicular to the keel, and force S, directed forward along the keel line of the vessel. Since the movement of the ship in the direction R encounters strong resistance from the water (the keel in sailing ships is very deep), the force R is almost completely balanced by the resistance of the water. There remains only one force S, which, as you 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 is greatest when the plane of the sail bisects the angle between the keel and wind directions.]. 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."

Let's now consider all possible wind directions relative to the boat's heading.

Diagram of the ship's courses relative to the wind, that is, the angle between the wind direction and the vector from stern to bow (course).

When the wind blows in your face (leventik), the sails dangle from side to side and it is impossible to move with the sail. Of course, you can always lower the sails and turn on the engine, but this no longer has anything to do with sailing.

When the wind blows directly behind you (jibe, tailwind), the accelerated air molecules put pressure on the sail on one side and the boat moves. In this case, the ship can only move slower than the wind speed. The analogy of riding a bicycle in the wind works here - the wind blows at your back and it is easier to turn the pedals.

When moving against the wind (close-hauled), the sail moves not because of the pressure of air molecules on the sail from behind, as in the case of a jibe, but because of the lifting force that is created due to different air velocities on both sides along the sail. Moreover, because of the keel, the boat does not move in a direction perpendicular to the course of the boat, but only forward. That is, the sail in this case is not an umbrella, as in the case of a close-hauled sail, but an airplane wing.

During our passages we mainly walked by backstays and gulfwinds with average speed at 7-8 knots at a wind speed of 15 knots. Sometimes we sailed against the wind, halfwind and close-hauled. And when the wind died down, they turned on the engine.

In general, a boat with a sail going against the wind is not a miracle, but a reality.

The most interesting thing is that boats can sail not only against the wind, but even faster than the wind. This happens when the boat backstays, creating its own wind.