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Before diving into sail performance, there are two short but important points to consider:
1. Determine which wind affects the sails.
2. Explain specific nautical terminology associated with wind courses.

True and pennant wind in yachting.

The wind that acts on a moving vessel and everything on it is different from that which acts on any stationary object.
The wind itself, as an atmospheric phenomenon blowing relative to land or water, we call the true wind.
In yachting, the wind relative to the yacht in motion is called the apparent wind and is the sum of the true wind and the oncoming air flow caused by the movement of the vessel.
Pennant wind always blows at a sharper angle to the boat than true wind.
The apparent wind speed can be higher (if the true wind is head-on or side), or less than the true one (if it is from the following directions).

Directions relative to the wind.

In the wind means from the side from which the wind blows.
Under the wind - on the side where the wind blows.
These terms, as well as derivatives from them, such as "windward", "leeward", are used very widely, and not only in yachting.
When these terms are applied to a ship, it is also customary to talk about the windward and leeward sides.
If the wind blows from the starboard side of the yacht, then this side is called upwind, left side - leeward respectively.
Port and starboard tack are two terms directly related to the previous ones: if the wind blows to the starboard side of the vessel, then they say that it goes starboard tack, if to the port it is left.
In English nautical terminology, what is associated with starboard and port is different from the usual Right and Left. About the starboard and everything related to it, they say Starboard, about the left - Port.

Wind courses.

Headings over wind vary depending on the angle between the direction of the apparent wind and the direction of the boat. They can be divided into spicy and full.

Beidewind - A sharp course relative to the wind. when the wind is blowing at an angle of less than 80 °. It can be steep (up to 50 °) and full (from 50 to 80 °).
Full headings with respect to the wind are courses when the wind blows at an angle of 90 ° or more to the direction of the boat.
These courses include:
Gulfwind - the wind blows at an angle of 80 to 100 °.
Backstay - the wind blows at an angle of 100 to 150 ° (steep backstay) and from 150 to 170 ° (full backstay).
Fordewind - the wind blows aft at an angle of over 170 °.
Leventic - the wind is strictly head-on or close to such. Since a sailing vessel cannot move against such a wind, it is often called not a course, but a position relative to the wind.

Wind-related maneuvers.

When a yacht under sail changes its course so that the angle between the wind and the direction of travel decreases, it is said that the ship is given... In other words, landing means going at a sharper angle to the wind.
If the opposite process occurs, i.e. the yacht changes course towards an increase in the angle between it and the wind, the ship rolls away .
Let's clarify that the terms ("lead" and "roll away" are used when the boat changes course relative to the wind within the same tack.
If the ship changes tack, then (and only then!) Such a maneuver in yachting is called a turn.
There are two different ways to change tack and therefore two turns: overstag and fordewind .
An overstag is a turn against the wind. The boat is driven, the bow of the boat crosses the wind line, at some point the boat goes through the leventic position, and then lays down on another tack.
Yachting when turning fordewind occurs in the opposite way: the ship rolls away, the stern crosses the wind line, the sails are transferred to the other side, the yacht lies on another tack. Most often it is a turn from one full course to another.

Sail operation when yachting.

One of the primary concerns for the sailor when working with sails is to orient the sail at the optimum angle with respect to the wind in order to best propel forward. To do this, you need to understand how the sail interacts with the wind.
The work of the sail is in many ways similar to the work of an airplane wing and occurs according to the laws of aerodynamics. For especially curious yachtsmen, you can learn more about the aerodynamics of a sail as a wing in a series of articles:. But it is better to do this after reading this article, gradually moving from easy to more complex material. Although, to whom am I saying this? Real yachtsmen are not afraid of difficulties. And you can do everything exactly the opposite.

The main difference between a sail and an airplane wing is that for the appearance of aerodynamic force on the sail, a certain nonzero angle between it and the wind is needed, this angle is called the angle of attack. The wing of the aircraft has an asymmetrical profile and can work normally at zero angle of attack, the sail does not.
In the process of the wind flowing around the sail, an aerodynamic force arises, which ultimately propels the yacht forward.
Consider the operation of the sail in yachting at different courses relative to the wind. First, for simplicity, let's imagine that the mast with one sail is dug into the ground and we can direct the wind at different angles to the sail.

The angle of attack is 0 °. The wind blows along the sail, the sail flaps like a flag. There is no aerodynamic force on the sail, there is only drag force.
Angle of attack 7 °. Aerodynamic force begins to appear. It is directed perpendicular to the sail and is still small in size.
The angle of attack is about 20 °. The aerodynamic force has reached its maximum value and is directed perpendicular to the sail.
The angle of attack is 90 °. In relation to the previous case, the aerodynamic force did not significantly change either in magnitude or in direction.
Thus, we see that the aerodynamic force is always directed perpendicular to the sail and its magnitude practically does not change in the range of angles from 20 to 90 °.
Angles of attack greater than 90 ° do not make sense to consider, since the sails on a yacht are usually not set at such angles relative to the wind.

The above dependences of the aerodynamic force on the angle of attack are largely simplified and averaged.
In fact, these properties vary markedly depending on the shape of the sail. For example, a long, narrow and rather flat mainsail of racing yachts will have a maximum aerodynamic force at an angle of attack of about 15 °, at higher angles the force will be slightly less. If the sail is more bellied and does not have a very large elongation, then the aerodynamic force on it can be maximum at an angle of attack of about 25-30 °.

Now let's look at the work of the sail on a yacht.

For simplicity, let's imagine that there is only one sail on a yacht. Let it be a grotto.
First, it is worth looking at how the yacht + sail system behaves when moving in the sharpest courses relative to the wind, as this usually raises the most questions.

Let's say the wind acts on a yacht at an angle of 30-35 ° to the hull. Orienting the sail on a course at an angle of approximately 20 ° to the wind, we will obtain on it a sufficient aerodynamic force A.
Since this force acts at right angles to the sail, we see that it pulls the yacht strongly to the side. Having decomposed the force A into two components, we can see that the forward thrust T is several times less than the force pushing the boat sideways (D, the force of drift).
How, then, does the yacht move forward?
The fact is that the design of the underwater part of the hull is such that the resistance of the hull to movement to the side (the so-called lateral resistance) is also several times greater than the resistance to forward movement. This is facilitated by the keel (or centerboard), rudder and the very shape of the hull.
However, lateral resistance arises when there is something to resist, that is, in order for it to start working, a certain displacement of the body to the side is necessary, the so-called wind drift.

This displacement naturally arises under the action of the lateral component of the aerodynamic force, and as a response, the lateral resistance force S, directed in the opposite direction, immediately arises. As a rule, they balance each other at a drift angle of about 10-15 °.
So, it is obvious that the lateral component of the aerodynamic force, which is most pronounced on sharp courses relative to the wind, causes two undesirable phenomena: wind drift and roll.

Wind drift means that the yacht's trajectory does not coincide with its centerline (centerline, or DP, is a smart term for the bow-stern line). There is a constant shift of the yacht into the wind, the movement seems to be a little sideways.
It is known that when yachting on a sidewind course under average weather conditions, the wind drift as the angle between the DP and the real trajectory is approximately 10-15 °.

Progress against the wind. Tacking.

Since yachting under sails is impossible strictly against the wind, but you can only move at a certain angle, it would be good to have an idea of \u200b\u200bhow sharply the yacht can move to the wind in degrees. And what, accordingly, is that non-moving sector of courses relative to the wind, in which movement against the wind is impossible.
Experience shows that a regular cruising yacht (not a racing yacht) can effectively sail at an angle of 50-55 ° to the true wind.

Thus, if the goal that must be achieved is strictly against the wind, then yachting to it will not occur in a straight line, but in a zigzag with one tack, then another. In this case, on each tack, naturally, you will need to try to go as sharply as possible to the wind. This process is called tacking.

The angle between the trajectories of yachts on two adjacent tacks when tacking is called tacking. Obviously, with a sharpness of movement to the wind of 50-55 °, the tacking angle will be 100-110 °.

The magnitude of the tack angle shows us how effectively we can move towards the target if it is located strictly against the wind. For an angle of 110 °, for example, the path to the target is increased by 1.75 times compared to moving in a straight line.

Sail operation on different wind courses

Obviously, already on the Gulfwind course, the thrust T significantly exceeds the drift force D, so that the drift and roll will be small.

With the backstay, as we can see, in comparison with the Gulfwind course, not much has changed. The mainsail is placed in a position almost perpendicular to the DP, and this position is for most yachts, it is technically impossible to deploy it even further.

The position of the mainsail on the fordewind course is no different from the position on the backstay course.
Here, for simplicity, when considering the physics of the process in yachting, we take into account only one sail - the mainsail. Usually there are two sails on the yacht - the mainsail and the staysail (head sail). So, on the course fordewind the staysail (if it is located on the same side as the mainsail) is in the wind shadow from the mainsail and practically does not work. This is one of several reasons why the fordewind course is disliked by sailors.

It is difficult to imagine how sailing ships can go "against the wind" - or, in the words of sailors, go "sidewind". True, the sailor will tell you that you cannot sail directly against the wind under sails, but you can only move at an acute angle to the direction of the wind 3. But this angle is small - about a quarter of the right angle - and it seems, perhaps, equally incomprehensible: whether to sail directly against the wind or at an angle to it of 22 °.

3 (It is possible to use only wind energy and move strictly against the wind if the sail is replaced with a wind turbine such as a windmill, which will rotate the ship's propeller. There is even a well-known problem of P. L. Kapitsa about such a seemingly unusual vessel (see also the journal: Katera i Yakhty, 1981, No. 1, p. 25).)

In practice, however, this is not indifferent, and we will now explain how the force of the wind can go towards it at a slight angle. First, consider how the wind acts on the sail in general, i.e. where it pushes the sail when it blows on it. You probably think that the wind always pushes the sail in the direction it is blowing. 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 Fig. 17, line AB depicts a sail.

Since the wind presses evenly on the entire surface of the sail, we replace the force of the wind pressure with the force Rattached to the middle of the sail. We decompose this force into two: the force Qperpendicular to the sail, and the force R directed forward along it. Force R does not push the sail anywhere, as the wind friction against the canvas is negligible. Power remains Qwhich pushes the sail at right angles to it.

Knowing this, we can easily understand how a sailing vessel can go at an acute angle towards the wind. Let the KK line (Fig. 18) represent the keel line of the vessel. The wind blows at an acute angle to this line in the direction indicated by the arrows. Line AB represents the 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 figure 18. behind the decay of forces. We represent the wind pressure on the sail by the force Q, which, we know, must be perpendicular to the sail. We decompose this force into two: force B, 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 of B encounters strong water resistance (the keel in sailing ships becomes very deep), the force B is almost completely balanced by the water resistance. Only the force S remains, which, as you can see, is directed forward and, therefore, moves the ship at an angle, as if towards the wind *. Usually this movement is performed in zigzags, as shown in Figure 19. In the language of seafarers, such movement of the vessel is called "tacking" in the full sense of the word 4.

* (It can be shown that the S-force gains its greatest value when the sail plane halves the angle between the keel and wind directions.)

4 (There are a number of issues in sailing that are interesting from the point of view of a physicist. More information about this sport and some technical problems of sailing can be found, for example, from the books: V. Glovatsky. The Fascinating World of Sails: Essays on the History of Sailing Sports. - M .: Progress, 1979; Proctor J. Sailing. Wind, excitement and current), Leningrad: Gidrometeoizdat, 1981.)

The downwind movement of a sailing yacht is actually determined by the simple wind pressure on its sail, pushing the boat forward. However, wind tunnel studies have 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 \u200b\u200bincreased pressure forms on the rear 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, a yacht is tacking), moves quickly forward. The sail is subjected to pulling and lateral forces. If a sailing yacht is sailing at an acute angle to the wind, her speed will slow down due to a decrease in pulling force and an increase in lateral 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 straight into the wind, but it can propel itself forward in a series of short zigzag moves at an angle to the wind, called tacks. If the wind blows to the port side (1), the yacht is said to be sailing on the port tack, if to the starboard side (2), on the starboard tack. In order to go faster the distance, the yachtsman tries to increase the speed of the yacht to the limit by adjusting the position of her sail, as shown in the figure below on the left. To minimize deviation from a straight line, the yacht moves from starboard 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 line of the wind, it rushes for some time, i.e. is inactive (middle figure below the text). The yacht falls into the so-called dead zone, losing speed until the wind again inflates the sail from the opposite side.

"Fair wind!" - wish all sailors, and it is completely in vain: when the wind blows from the stern, the yacht is not able to develop maximum speed. This scheme was helped by Vadim Zhdan, a professional skipper, racer, organizer and host of yacht regattas. Read the tooltips on the diagram to figure it out.

2. The thrust of the sail is generated by two factors. First, the wind just presses on the sails. Secondly, the oblique sails installed on most modern yachts, when flowing with air, work like an airplane wing, and, 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 generated by the sail is perpendicular to the sail. According to the vector addition rule, it is possible to distinguish the drift force (red arrow) and thrust force (green arrow) in it.

5. To sail strictly against the wind, the yacht maneuvers: turns to the wind with one or the other side, moving forward in segments - tacks. How long the tack should be and at what angle to the wind to go are important questions of skipper's tactics.

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

11. Fordewind - the same tailwind blowing from the stern. Contrary to expectations, not the fastest course: the lift of the sail is not used here, and the theoretical speed limit does not exceed the wind speed. An experienced skipper is able to predict invisible air currents in the same way as