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.
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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 “lifting force”, only it is directed not upward, but forward. Due to the peculiarities of aerodynamics, the air from the convex side of the sail moves faster than from the concave side, and the pressure from outside There are fewer sails than with the inner one.
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.
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.
Before looking at how a sail works, there are two short but important points to consider:
1. Determine what kind of wind affects the sails.
2.Talk about specific marine terminology associated with courses relative to the wind.
True and apparent winds in yachting.
The wind that acts on a moving ship and everything on it is different from the one that acts on any stationary object.
Actually, the wind as an atmospheric phenomenon blowing relative to land or water is what we call true wind.
In yachting, the wind relative to a moving yacht is called apparent wind and is the sum of the true wind and the oncoming air flow caused by the movement of the vessel.
The apparent wind always blows at a sharper angle to the boat than the true wind.
The apparent wind speed can be greater (if the true wind is headwind or sidewind), or less than the true wind (if it is from a tailwind).
Directions relative to the wind.
In the wind means from the direction from which the wind blows.
Downwind- from the direction 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 customary to also talk about the windward and leeward sides.
If the wind blows from the starboard side of the yacht, then this side is called windward, 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 ship, then they say that it is sailing on the starboard tack, if it is on the left, then on the left.
In English nautical terminology, what is associated with starboard and port is different from the usual Right and Left. They say Starboard about the starboard side and everything related to it, and Port about the left side.
Courses relative to the wind.
Courses relative to the wind vary depending on the angle between the direction of the apparent wind and the direction the vessel is moving. They can be divided into acute and full.
Close-hauled is a sharp course relative to the wind. when the wind blows at an angle of less than 80°. There can be a steep close-hauled wind (up to 50°) or a full close-hauled wind (from 50 to 80°).
Full courses relative to the wind are courses when the wind blows at an angle of 90° or more to the direction the yacht is moving.
These courses include:
Gulfwind - the wind blows at an angle of 80 to 100°.
Backstay - the wind blows at an angle from 100 to 150° (steep backstay) and from 150 to 170° (full backstay).
Fordewind - the wind blows astern at an angle of more than 170°.
Leftist - the wind is strictly headwind or close to it. Since a sailing ship cannot move against such a wind, it is more often called not a course, but a position relative to the wind.
Maneuvers relative to the wind.
When a yacht under sail changes its course so that the angle between the wind and the direction of motion decreases, then the ship is said to be is given. In other words, to flatten means to go at a sharper angle to the wind.
If the reverse process occurs, i.e. the yacht changes course towards increasing the angle between it and the wind, the vessel falls away .
Let us clarify that the terms (“lead” and “fall”) 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, accordingly, two turns: tack And jibe .
A tack is a turn into the wind. The vessel is driven, the bow of the boat crosses the wind line, at some point the vessel passes through the left-hand position, after which it lies on the other tack.
Yachting when jibes occurs in the opposite way: the ship falls away, the stern crosses the wind line, the sails are transferred to the other side, the yacht lies on a different tack. Most often this is a turn from one full course to another.
Sail operation during yachting.
One of the main challenges for a sailor when working with sails is to orient the sail at the optimal angle relative to the wind to best propel the sail forward. To do this, you need to understand how the sail interacts with the wind.
The work of a sail is in many ways similar to the work of an airplane wing and occurs according to the laws of aerodynamics. For particularly curious yachtsmen, you can learn more about the aerodynamics of a sail as a wing in a series of articles:. But it’s better to do this after reading this article, gradually moving from easy to more complex material. Although, who am I telling this to? Real yachtsmen are not afraid of difficulties. And you can do everything exactly the opposite.
The main difference between a sail and an aircraft wing is that for an aerodynamic force to appear on the sail, a certain non-zero angle is needed between it and the wind; this angle is called the angle of attack. The airplane wing has an asymmetrical profile and can operate normally at zero angle of attack, but the sail does not.
As the wind flows around the sail, an aerodynamic force arises, which ultimately moves the yacht forward.
Let's consider the operation of a sail in yachting at different courses relative to the wind. First, for simplicity, let's imagine that a mast with one sail is dug into the ground and we can direct the wind at different angles to the sail.
Angle of attack 0°. The wind blows along the sail, the sail flutters like a flag. There is no aerodynamic force on the sail, there is only drag force.
Angle of attack 7°. An 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.
Angle of attack 90°. Compared to the previous case, the aerodynamic force did not change significantly either in magnitude or direction.
Thus, we see that the aerodynamic force is always directed perpendicular to the sail and its magnitude practically does not change in the angle range 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 fairly 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 potbellied and does not have a very large aspect ratio, then the aerodynamic force on it can be maximum at an angle of attack of about 25-30°.
Now let's look at how a sail works on a yacht.
For simplicity, let's imagine that there is only one sail on the yacht. Let it be a grotto.
First, it’s worth looking at how the yacht + sail system behaves when moving on the sharpest courses relative to the wind, since this usually raises the most questions.
Let’s say the yacht is affected by wind at an angle of 30-35° to the hull. By orienting the sail on course at an angle of approximately 20° to the wind, we obtain a sufficient aerodynamic force A on it.
Since this force acts at right angles to the sail, we see that it pulls the yacht strongly to the side. By decomposing the force A into two components, you can see that the forward thrust force T is several times less than the force pushing the boat sideways (D, drift force).
What causes the yacht to move forward in this case?
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 movement forward. This is facilitated by the keel (or centreboard), rudder and the very shape of the hull.
However, lateral resistance occurs when there is something to resist, i.e., for it to start working, some sideways displacement of the body, the so-called wind drift, is required.
This displacement naturally occurs under the action of the lateral component of the aerodynamic force, and as a response, a lateral drag force S immediately arises, directed in the opposite direction. 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, 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 centreline (diameter plane, or DP, is a smart term for the bow-stern line). There is a constant shift of the yacht to the wind, moving as if a little sideways.
It is known that when yachting on a close-hauled course under average weather conditions, the wind drift as the angle between the DP and the actual trajectory is approximately 10-15°.
Advance against the wind. Tacking.
Since yachting under sails is not possible strictly against the wind, and you can only move at a certain angle, it would be good to have an idea of how sharply the yacht can move in degrees to the wind. And what, accordingly, is that slow 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 needs to be achieved is located strictly against the wind, then yachting to it will not take place in a straight line, but in a zigzag, first on one tack, then on the other. In this case, on each tack, naturally, you will need to try to sail as sharply as possible into 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 tacking angle shows us how effectively we can move towards the target if it is strictly against the wind. For an angle of 110°, for example, the path to the target increases by 1.75 times compared to moving in a straight line.
Sail operation on other courses relative to the wind
It is obvious that already on a gulfwind course the thrust force T significantly exceeds the drift force D, so the drift and roll will be small.
With the backstay, as we see, not much has changed compared to the gulfwind course. The mainsail is placed in a position almost perpendicular to the DP, and this position is extreme for most yachts; it is technically impossible to deploy it even further.
The position of the mainsail on the gybe 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. Typically, a yacht has two sails - a mainsail and a staysail (headsail). So, on a gybe course, the jib (if it is located on the same side as the mainsail) is in the wind shadow of the mainsail and practically does not work. This is one of several reasons why jibes are not a favorite among boaters.
