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, a segment of it, which the sailboat passes when it moves against the wind. 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.
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 or collide with the ISS during 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 vector addition, 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 on 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. It is impossible to sail this way, 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.]. 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.
No less important than the resistance of the hull is the traction force developed by the sails. To more clearly imagine the work of sails, let's get acquainted with the basic concepts of sail theory.
We have already talked about the main forces acting on the sails of a yacht sailing with a tailwind (jibed course) and a headwind (behind wind course). We found out that the force acting on the sails can be decomposed into the force that causes the yacht to roll and drift downwind, the drift force and the traction force (see Fig. 2 and 3).
Now let's see how the total force of wind pressure on the sails is determined and what the thrust and drift forces depend on.
To imagine the operation of a sail on sharp courses, it is convenient to first consider a flat sail (Fig. 94), which experiences wind pressure at a certain angle of attack. In this case, vortices are formed behind the sail, pressure forces arise on the windward side, and rarefaction forces arise on the leeward side. Their resulting R is directed approximately perpendicular to the plane of the sail. To properly understand the operation of a sail, it is convenient to imagine it as the resultant of two component forces: X-directed parallel to the air flow (wind) and Y-directed perpendicular to it.
The force X directed parallel to the air flow is called the drag force; It is created, in addition to the sail, also by the hull, rigging, spars and crew of the yacht.
The force Y directed perpendicular to the air flow is called lift in aerodynamics. It is this that creates thrust in the direction of movement of the yacht on sharp courses.
If, with the same drag of the sail X (Fig. 95), the lift force increases, for example, to the value Y1, then, as shown in the figure, the resultant of the lift force and drag will change by R and, accordingly, the thrust force T will increase to T1.
Such a construction makes it easy to verify that with an increase in drag X (at the same lift force), the thrust T decreases.
Thus, there are two ways to increase the traction force, and therefore the speed on sharp courses: increasing the lifting force of the sail and reducing the drag of the sail and the yacht.
In modern sailing, the lifting force of the sail is increased by giving it a concave shape with some “belliness” (Fig. 96): the size from the mast to the most deep place The "belly" is usually 0.3-0.4 of the sail's width, and the depth of the "belly" is about 6-10% of the width. The lifting force of such a sail is 20-25% greater than that of a completely flat sail with almost the same drag. True, a yacht with flat sails sails a little steeper into the wind. However, with potbellied sails, the speed of progress into the tack is greater due to the greater thrust.
Rice. 96. Sail profile
Note that with potbellied sails, not only the thrust increases, but also the drift force, which means that the roll and drift of yachts with potbellied sails is greater than with relatively flat ones. Therefore, a sail “bulge” of more than 6-7% in strong winds is unprofitable, since an increase in heel and drift leads to a significant increase in hull resistance and a decrease in the efficiency of the sails, which “eat up” the effect of increasing thrust. In weak winds, sails with a “belly” of 9-10% pull better, since due to the low total wind pressure on the sail, the heel is small.
Any sail at angles of attack greater than 15-20°, that is, when the yacht is heading 40-50° to the wind or more, can reduce lift and increase drag, since significant turbulence is formed on the leeward side. And since the main part of the lifting force is created by a smooth, turbulent-free flow around the leeward side of the sail, the destruction of these vortices should have a great effect.
The turbulence that forms behind the mainsail is destroyed by setting the jib (Fig. 97). The air flow entering the gap between the mainsail and the jib increases its speed (the so-called nozzle effect) and, when the jib is adjusted correctly, “licks” the vortices from the mainsail.
Rice. 97. Jib work
The profile of a soft sail is difficult to maintain constant at different angles of attack. Previously, dinghies had through battens running through the entire sail - they were made thinner within the “belly” and thicker towards the luff, where the sail is much flatter. Nowadays, through battens are installed mainly on ice boats and catamarans, where it is especially important to maintain the profile and rigidity of the sail at low angles of attack, when a regular sail is already lashing along the luff.
If the source of lift is only the sail, then drag is created by everything that ends up in the air flow flowing around the yacht. Therefore, improving the traction properties of the sail can also be achieved by reducing the drag of the yacht's hull, mast, rigging and crew. For this purpose, various types of fairings are used on the spar and rigging.
The amount of drag on a sail depends on its shape. According to the laws of aerodynamics, the drag of an aircraft wing is lower, the narrower and longer it is for the same area. That is why they try to make the sail (essentially the same wing, but placed vertically) high and narrow. This also allows you to use the upper wind.
The drag of a sail depends to a very large extent on the condition of its leading edge. The luffs of all sails should be covered tightly to prevent the possibility of vibration.
It is necessary to mention one more very important circumstance - the so-called centering of the sails.
It is known from mechanics that any force is determined by its magnitude, direction and point of application. So far we have only talked about the magnitude and direction of the forces applied to the sail. As we will see later, knowledge of the application points is of great importance for understanding the operation of sails.
Wind pressure is distributed unevenly over the surface of the sail (its front part experiences more pressure), however, to simplify comparative calculations, it is assumed that it is distributed evenly. For approximate calculations, the resultant force of wind pressure on the sails is assumed to be applied to one point; the center of gravity of the surface of the sails is taken as it when they are placed in the center plane of the yacht. This point is called the center of sail (CS).
Let's focus on the simplest graphical method for determining the position of the CPU (Fig. 98). Draw the sail area of the yacht on the required scale. Then, at the intersection of medians - lines connecting the vertices of the triangle with the midpoints of opposite sides - the center of each sail is found. Having thus obtained in the drawing the centers O and O1 of the two triangles that make up the mainsail and the jib, draw two parallel lines OA and O1B through these centers and lay on them in opposite directions in any but the same scale as many linear units as square meters in the triangle; the area of the jib is plotted from the center of the mainsail, and the area of the mainsail is plotted from the center of the jib. End points A and B are connected by straight line AB. Another straight line - O1O connects the centers of the triangles. At the intersection of straight lines A B and O1O there will be a common center.
Rice. 98. Graphical method of finding the center of sail
As we have already said, the drift force (we will consider it applied in the center of the sail) is counteracted by the lateral resistance force of the yacht’s hull. The lateral resistance force is considered to be applied at the center of lateral resistance (CLR). The center of lateral resistance is the center of gravity of the projection of the underwater part of the yacht onto the center plane.
The center of lateral resistance can be found by cutting out the outline of the underwater part of the yacht from thick paper and placing this model on a knife blade. When the model is balanced, lightly press it, then rotate it 90° and balance again. The intersection of these lines gives us the center of lateral resistance.
When the yacht sails without heeling, the CP should lie on the same vertical straight line with the CB (Fig. 99). If the CP lies in front of the central station (Fig. 99, b), then the drift force, shifted forward relative to the force of lateral resistance, turns the bow of the vessel into the wind - the yacht falls away. If the CPU is behind the central station, the yacht will turn its bow to the wind, or be driven (Fig. 99, c).
Rice. 99. Yacht alignment
Both excessive adjustment to the wind, and especially stalling (improper centering) are harmful to the sailing of the yacht, as they force the helmsman to constantly work the helm to maintain straightness, and this increases hull resistance and reduces the speed of the vessel. In addition, incorrect alignment leads to deterioration in controllability, and in some cases, to its complete loss.
If we center the yacht as shown in Fig. 99, and, that is, the CPU and the central control system will be on the same vertical, then the ship will be driven very strongly and it will become very difficult to control it. What's the matter? There are two main reasons here. Firstly, the true location of the CPU and central nervous system does not coincide with the theoretical one (both centers are shifted forward, but not equally).
Secondly, and this is the main thing, when heeling, the traction force of the sails and the longitudinal resistance force of the hull turn out to lie in different vertical planes (Fig. 100), it turns out like a lever that forces the yacht to be driven. The greater the roll, the more prone the vessel is to pitch.
To eliminate such adduction, the CP is placed in front of the central nervous system. The moment of traction and longitudinal resistance that arises with the roll, forcing the yacht to be driven, is compensated by the trapping moment of the drift forces and lateral resistance when the CP is located at the front. For good centering, the CP must be placed in front of the CB at a distance equal to 10-18% of the length of the yacht along the waterline. The less stable the yacht is and the higher the CPU is raised above the central station, the more it needs to be moved to the bow.
In order for the yacht to have a good move, it must be centered, that is, put the CP and CB in a position in which the vessel on a close-hauled course in a light wind was completely balanced by the sails, in other words, it was stable on the course with the rudder thrown or fixed in the DP (allowed slight tendency to float in very light winds), and in stronger winds had a tendency to float. Every helmsman must be able to center the yacht correctly. On most yachts, the tendency to roll increases if the rear sails are overhauled and the front sails are loose. If the front sails are overhauled and the rear sails are damaged, the ship will sink. With an increase in the “potbelliness” of the mainsail, it is also bad standing sails the yacht tends to be driven to a greater extent.
Rice. 100. The influence of heel on bringing the yacht into the wind
I think that many of us would take the chance to dive 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 square 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 provides forward motion.
But this is a theory, but in practice everything is a little different. In fact, a sailing yacht cannot go 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 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 there is a crew 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°.
