Why can a sailboat sail against the wind? Basic information from sail theory The operation of a sail in different winds

Courses relative to the wind. Modern yachts and sailing boats are in most cases equipped with oblique sails. Their distinctive feature is that the main part of the sail or all of it is located behind the mast or forestay. Due to the fact that the leading edge of the sail is pulled tightly along the mast (or by itself), the sail flows around the air flow without flushing when it is positioned at a fairly acute angle to the wind. Thanks to this (and with appropriate hull contours), the ship acquires the ability to move at an acute angle to the direction of the wind.

In Fig. 190 shows the position of the sailboat at different courses relative to the wind. An ordinary sailboat cannot sail directly against the wind - the sail in this case does not create a traction force capable of overcoming the resistance of water and air. The best racing yachts in medium winds can sail close-hauled at an angle of 35-40° to the wind direction; Usually this angle is not less than 45°. Therefore, the sailboat is forced to get to a target located directly against the wind. tacking- alternately starboard and port tack. The angle between the ship's courses on one tack and the other is called tacking angle, and the position of the vessel with its bow directly against the wind is leftist. The ability of a ship to tack and move at maximum speed directly into the wind is one of the main qualities of a sailboat.

Courses from close-hauled to halfwind, when the wind blows at 90° to the ship's port, are called sharp; from gulfwind to jibe (the wind blows directly astern) - full. Distinguish steep(course relative to the wind 90-135°) and full(135-180°) backstays, as well as close-hauled (40-60° and 60-80° to the wind, respectively).

Rice. 190. Courses of a sailing ship relative to the wind.

1 - steep close-hauled; 2 - full close-hauled; 3 - gulfwind; 4 - backstay; 5 - jibe; 6 - leftist.

Apparent wind. The air flow that flows around the sails of the yacht does not coincide with the direction true wind(relative to sushi). If the ship is moving, then a counter flow of air appears, the speed of which is equal to the speed of the ship. When there is wind, its direction relative to the ship is deviated in a certain way due to the oncoming air flow; the magnitude of the speed also changes. Thus, the total flow, called apparent wind. Its direction and speed can be obtained by adding the vectors of the true wind and the oncoming flow (Fig. 191).

Rice. 191. Apparent wind at various courses of the yacht relative to the wind.

1 - close-hauled; 2 - gulfwind; 3 - backstay; 4 - jibe.

v- speed of the yacht; v and - true wind speed; v in - apparent wind speed.

It is obvious that on a close-hauled course the apparent wind speed is the greatest, and on a gybe it is the smallest, since in the latter case the speeds of both flows are directed in exactly opposite directions.

The sails on a yacht are always set in the direction of the apparent wind. Note that the speed of the yacht does not grow in direct proportion to the wind speed, but much more slowly. Therefore, when the wind increases, the angle between the direction of the true and apparent wind decreases, and in weak winds, the speed and direction of the apparent wind differs more noticeably from the true one.

Since the forces acting on a sail as on a wing increase in proportion to the square of the speed of the flow, sailboats with minimal resistance to movement may experience a “self-acceleration” phenomenon, in which their speed exceeds the speed of the wind. These types of sailboats include ice yachts - ice boats, hydrofoil yachts, wheeled (beach) yachts and proa - narrow monohull vessels with an outrigger float. Some of these types of vessels have recorded speeds up to three times the wind speed. So, our national iceboat speed record is 140 km/h, and it was set in a wind whose speed did not exceed 50 km/h. In passing, we note that the absolute speed record for sailing on water is significantly lower: it was set in 1981 on a specially built two-masted catamaran “Crossbau-II” and is equal to 67.3 km/h.

Conventional sailing ships, unless they are designed for planing, rarely exceed the displacement speed limit of v = 5.6 √L km/h (see Chapter I).

Forces acting on a sailing ship. There is a fundamental difference between the system of external forces acting on a sailing vessel and a vessel driven by a mechanical engine. On a motorized vessel, the thrust of the propeller - the propeller or water jet - and the force of water resistance to its movement act in the underwater part, located in the center plane and at a small distance from each other vertically.

On a sailboat, the driving force is applied high above the surface of the water and, therefore, above the line of action of the drag force. If the ship moves at an angle to the direction of the wind - close-hauled, then its sails operate according to the principle of an aerodynamic wing, discussed in Chapter II. When air flows around a sail, a vacuum is created on its leeward (convex) side, and increased pressure is created on the windward side. The sum of these pressures can be reduced to the resulting aerodynamic force A(see Fig. 192), directed approximately perpendicular to the chord of the sail profile and applied at the center of sail (CS) high above the water surface.

Rice. 192. Forces acting on the hull and sails.

According to the third law of mechanics, during steady motion of a body in a straight line, each force applied to the body (in in this case- to the sails connected to the hull of the yacht through the mast, standing rigging and sheets), must be counteracted by a force equal in magnitude and oppositely directed. On a sailboat this force is the resultant hydrodynamic force H, attached to the underwater part of the hull (Fig. 192). Thus, between the forces A And H there is a known distance - the shoulder, as a result of which a moment of a pair of forces is formed, tending to rotate the ship relative to an axis oriented in a certain way in space.

To simplify the phenomena that arise during the movement of sailing ships, hydro- and aerodynamic forces and their moments are decomposed into components parallel to the main coordinate axes. Guided by Newton's third law, we can write out in pairs all the components of these forces and moments:

In order for the ship to move steadily along its course, each pair of forces and each pair of moments must be equal to each other. For example, the drift force D and drift resistance force R d create a heeling moment M kr, which must be balanced by the restoring torque M in or moment of lateral stability. This moment is formed due to the action of mass forces P and buoyancy of the vessel γ· V, acting on the shoulder l. The same forces form the moment of resistance to trim or the moment of longitudinal stability M l, equal in magnitude and opposing the trimming moment M d. The terms of the latter are the moments of pairs of forces T - R And A v - H v .

Thus, the movement of a sailing ship on an oblique course to the wind is associated with roll and trim, and the lateral force D, in addition to roll, also causes drift - lateral drift, so any sailing ship does not move strictly in the direction of the DP, like a ship with a mechanical engine, but with a small drift angle β. The hull of a sailboat, its keel and rudder become a hydrofoil, onto which an oncoming flow of water flows at an angle of attack equal to the angle of drift. It is this circumstance that determines the formation of a drift resistance force on the keel of the yacht R d, which is a component of the lift force.

Stability of movement and centering of a sailing vessel. Due to heel, the thrust force of the sails T and resistance force R appear to operate in different vertical planes. They form a pair of forces that bring the ship towards the wind - knocking it off the straight course it is following. This is prevented by the moment of the second pair of forces - heeling D and drift resistance forces R d, as well as a small force N on the steering wheel, which must be applied in order to correct the yacht’s movement along the course.

It is obvious that the vessel’s reaction to the action of all these forces depends both on their magnitude and on the ratio of the arms a And b on which they act. With increasing roll, the arm of the drive pair b also increases, and the leverage of the falling pair a depends on the relative position center of sail(CP - points of application of the resulting aerodynamic forces to the sails) and center of lateral resistance(CBS - points of application of the resulting hydrodynamic forces to the yacht hull).

Accurately determining the position of these points is a rather difficult task, especially when you consider that it changes depending on many factors: the ship's course relative to the wind, the cut and tuning of the sails, the list and trim of the yacht, the shape and profile of the keel and rudder, etc.

When designing and re-equipping yachts, they operate with conventional CPs and CBs, considering them located in the centers of gravity of flat figures, which represent sails set in the DP, and the outlines of the underwater part of the DP with a keel, fins and rudder (Fig. 193). The center of gravity of a triangular sail, for example, is located at the intersection of two medians, and the common center of gravity of the two sails is located on a straight line segment connecting the CP of both sails, and divides this segment in inverse proportion to their area. If the sail has a quadrangular shape, then its area is divided diagonally into two triangles and the CP is obtained as the common center of these triangles.

Rice. 193. Determination of the conditional center of sail of a yacht.

The position of the central center can be determined by balancing a template of the underwater profile of the DP, cut out of thin cardboard, on the tip of a needle. When the template is positioned horizontally, the needle will be at the conditional center point. However, this method is more or less applicable for ships with a large area of ​​the underwater part of the DP - for yachts traditional type with a long keel line, ship's boats, etc. On modern yachts ah, the contours of which are designed on the basis of wing theory, the main role in creating the force of resistance to drift is played by the fin keel and the rudder, which is usually installed separately from the keel. The centers of hydrodynamic pressures on their profiles can be found quite accurately. For example, for profiles with a relative thickness δ/ b about 8% this point is at a distance of about 26% of the chord b from the incoming edge.

However, the hull of the yacht in a certain way influences the nature of the flow around the keel and rudder, and this influence varies depending on the roll, trim and speed of the vessel. In most cases, on sharp courses into the wind, the true center of gravity moves forward with respect to the center of pressure determined for the keel and rudder as for isolated profiles. Due to the uncertainty in calculating the position of the CP and the central center, when developing a design for sailing ships, designers place the CP at a certain distance a- ahead - ahead of the Central Bank. The amount of advance is determined statistically, from comparison with well-proven yachts that have underwater contours, stability and sailing equipment. The lead is usually set as a percentage of the length of the vessel at the waterline and is 15-18% for a vessel equipped with a Bermuda sloop. L. The less stability of the yacht, the more roll it will receive under the influence of the wind and the greater the advance of the CPU in front of the central steering system is necessary.

Precise adjustment of the relative position of the CP and CB is possible when testing the yacht while underway. If the ship tends to fall into the wind, especially in medium and fresh winds, then this is a major alignment defect. The fact is that the keel deflects the flow of water flowing from it closer to the vessel's DP. Therefore, if the rudder is straight, then its profile operates with a noticeably lower angle of attack than the keel. If, in order to compensate for the tendency of the yacht to sink, the rudder has to be shifted to the wind, then the lifting force generated on it turns out to be directed in the leeward direction - in the same direction as the drift force D on sails. Consequently, the ship will have increased drift.

Another thing is the easy tendency of the yacht to be driven. The rudder, shifted 3-4° to the leeward side, operates with the same or slightly greater angle of attack as the keel, and effectively participates in resistance to drift. Lateral force H, which occurs on the rudder, causes a significant shift of the overall central nervous system towards the stern while simultaneously reducing the drift angle. However, if in order to keep the yacht on a close-hauled course you have to constantly shift the rudder to the leeward side at an angle greater than 2-3°, it is necessary to move the CPU forward or move the central steering system back, which is more difficult.

On a built yacht, you can move the CPU forward by tilting the mast forward, moving it forward (if the step design allows), shortening the mainsail along the luff, and increasing the area of ​​the main jib. To move the central steering wheel backwards, you need to install a fin in front of the steering wheel or increase the size of the steering wheel blade.

To eliminate the yacht's tendency to sink, it is necessary to apply opposite measures: move the CPU back or move the central center forward.

The role of aerodynamic force components in the creation of thrust and drift. The modern theory of the operation of an oblique sail is based on the provisions of the aerodynamics of the wing, the elements of which were discussed in Chapter II. When an air flow flows around a sail set at an angle of attack α to the apparent wind, an aerodynamic force is created on it A, which can be represented in the form of two components: lift Y, directed perpendicular to the air flow (apparent wind), and drag X- force projections A on the direction of air flow. These forces are used when considering the characteristics of the sail and sailing equipment in general.

At the same time force A can be represented in the form of two other components: traction force T, directed along the axis of motion of the yacht, and the drift force perpendicular to it D. Let us recall that the direction of movement of the sailboat (or path) differs from its course by the value of the drift angle β, however, in further analysis this angle can be neglected.

If on a close-hauled course it is possible to increase the lifting force on the sail to the value Y 1, and the frontal resistance remains unchanged, then the forces Y 1 and X, added according to the rule of vector addition, form a new aerodynamic force A 1 (Fig. 194, A). Considering its new components T 1 and D 1, it can be noted that in this case, with an increase in lift, both the thrust force and the drift force increase.

Rice. 194. The role of lift and drag in creating driving force.

With a similar construction, one can be convinced that with an increase in drag on a close-hauled course, the thrust force decreases and the drift force increases. Thus, when sailing close-hauled, the lifting force of the sail plays a decisive role in creating sail thrust; drag should be minimal.

Note that on a close-hauled course the apparent wind has the highest speed, so both components of the aerodynamic force Y And X are quite large.

On a Gulfwind course (Fig. 194, b) lift is the traction force, and drag is the drift force. An increase in the drag of the sail does not affect the amount of traction force: only the drift force increases. However, since the apparent wind speed in the gulfwind is reduced compared to the close-hauled wind, drift affects the ship's performance to a lesser extent.

Backstay on course (Fig. 194, V) the sail operates at high angles of attack, at which the lifting force is significantly less than the drag. If you increase the drag, the thrust and drift force will also increase. As the lifting force increases, the thrust increases and the drift force decreases (Fig. 194, G). Consequently, on the backstay course, an increase in both lift and (or) drag increases thrust.

During a gybe course, the angle of attack of the sail is close to 90°, so the lifting force on the sail is zero, and the drag is directed along the axis of motion of the vessel and is the traction force. The drift force is zero. Therefore, on a gybe course, to increase the thrust of the sails, it is advisable to increase their drag. On racing yachts this is done by setting additional sails - a spinnaker and a blooper, which have a large area and a poorly streamlined shape. Note that on a gybe course, the yacht's sails are affected by the apparent wind of minimum speed, which causes relatively moderate forces on the sails.

Drift resistance. As shown above, the force of drift depends on the yacht's course relative to the wind. When sailing close-hauled, it is approximately three times the thrust force T, moving the ship forward; on gulfwind both forces are approximately equal; on a steep backstay, the sail thrust turns out to be 2-3 times greater than the drift force, and on a pure gybe there is no drift force at all. Consequently, in order for a sailboat to successfully move forward on courses from close-hauled to gulfwind (at an angle of 40-90° to the wind), it must have sufficient lateral resistance to drift, much greater than the resistance of the water to the movement of the yacht along the course.

The function of creating resistance to drift on modern sailing ships is mainly performed by fin keels or centerboards and rudders. The mechanics of the generation of lift on a wing with a symmetrical profile, such as keels, centerboards and rudders, was discussed in Chapter II (see page 67). Note that the drift angle of modern yachts - the angle of attack of the keel or centerboard profile - rarely exceeds 5°, therefore, when designing a keel or centerboard, it is necessary to select its optimal dimensions, shape and cross-sectional profile in order to obtain maximum lifting force with minimal drag. at low angles of attack.

Tests of aerodynamic symmetrical airfoils have shown that thicker airfoils (with a larger cross-sectional thickness ratio t to his chord b) provide greater lifting force than thin ones. However, at low speeds such profiles have higher drag. Optimal results on sailing yachts can be achieved with keel thickness t/b= 0.09÷0.12, since the lifting force on such profiles depends little on the speed of the vessel.

The maximum thickness of the profile should be located at a distance of 30 to 40% of the chord from the leading edge of the keel profile. The NACA 664‑0 profile also has good qualities with a maximum thickness located at a distance of 50% of the chord from the nose (Fig. 195).

Rice. 195. Profiled keel-fin of a yacht.

For lightweight racing dinghies capable of planing and reaching high speeds, centerboards and rudders with a thinner profile are used ( t/b= 0.044÷0.05) and geometric elongation (deepening ratio d to the middle chord b Wed) to 4.

The elongation of the keels of modern keel yachts ranges from 1 to 3, the rudders - up to 4. Most often, the keel has the form of a trapezoid with an inclined leading edge, and the angle of inclination has a certain effect on the amount of lift and drag of the keel. When extending the keel around λ = 0.6, an inclination of the leading edge of up to 50° can be allowed; at λ = 1 - about 20°; for λ > 1.5, a keel with a vertical leading edge is optimal.

The total area of ​​the keel and rudder to effectively counteract drift is usually taken to be from 1/25 to 1/17 of the area of ​​the main sails.

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.

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.

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 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.

The effect of wind on a ship is determined by its direction and strength, the shape and size of the ship's sail area, the location of the center of sail, the values ​​of draft, roll and trim.

The action of wind within the heading angles of 0-110° causes a loss of speed, and at large heading angles and wind strength not exceeding 3-4 points - a slight increase in speed.

The action of wind within 30-120° is accompanied by drift and wind roll.

A moving ship is affected by a relative (apparent) wind, which is related to the true one in the following relationships (Fig. 7.1)(2):

Where Vi is the true wind speed, m/s;

VK-apparent wind speed, m/s;

V0 - ship speed, m/s;

βo-ship drift angle, degrees.

Yk - apparent wind angle;

Yi is the true wind angle.

The specific wind pressure on the ship in kgf/m is calculated using the formula

Where W is wind speed, m/s.

Rice. 7.1. Relationship between true and apparent wind

Rice. 7.2. Heeling moment effect

So, during a hurricane, when the wind speed reaches 40-50 m/s, the wind load reaches 130-200 kgf/m2.

The total wind pressure on the ship is determined from the expression P = pΩ, where is the sail area of ​​the ship.

The magnitude of the heeling moment Mkr (Fig. 7.2) in kgf m for the case of steady motion and the action of wind pressure force P perpendicular to the ship's DP is determined from the expression

Where zn is the ordinate of the center of sail, m;

T - average draft of the ship, m.

Rough seas have the most significant effect on a ship. It is accompanied by the action of significant dynamic loads on the hull and the rolling of the ship. When sailing in rough seas, the resistance of the ship's hull increases and the conditions for the joint operation of the propellers, hull and main engines worsen.

Rice. 7.3. Wave elements

As a result, the speed decreases, the load on the main engines increases, fuel consumption increases and the ship's cruising range decreases. The shape and size of the waves are characterized by the following elements (Fig. 7.3):

Wave height h - vertical distance from the top to the bottom of the wave;

Wavelength λ is the horizontal distance between two adjacent crests or troughs;

Wave period t - the period of time during which the wave travels a distance equal to its length (3);

Wave speed C is the distance traveled by the wave per unit time.

Based on their origin, waves are divided into wind, tidal, anemobaric, earthquake (tsunami) and ship waves. The most common are wind waves. There are three types of waves: wind, swell and mixed. Wind waves are developing, they are under the direct influence of the wind, in contrast to swell, which is an inertial wave, or a wave caused by a storm wind blowing in a remote area. The wind wave profile is not symmetrical. Its leeward slope is steeper than its windward one. At the tops of wind waves, ridges are formed, the tops of which collapse under the influence of the wind, forming foam (lambs), and are torn off in strong winds. The direction of the wind and the direction of wind waves in the open sea, as a rule, coincide or differ by 30-40°. The size of wind waves depends on the wind speed and duration of its influence, the length of the path of wind flows over the water surface and the depth of the given area (Table 7.1).

TABLE 7.1. MAXIMUM VALUES OF WAVE ELEMENTS FOR THE DEEP SEA (Н/Λ > 1/2)

The most intense wave growth is observed at the C/W ratio< 0,4-0,5. Дальнейшее увеличение этого отношения сопровождается уменьшением роста волн. По­этому волны опасны не в момент наибольшего ветра, а при последующем его ослаблении.

For approximate calculations of the average wave height of steady ocean waves, the following formulas are used:

With winds up to 5 points

When the wind is over 5 points

Where B is the wind force in points on the Beaufort scale (§ 23.3).

In conditions of developed waves, there is interference of individual waves (up to 2% of the total number or more), which reach maximum development and exceed the average wave height by two to three times. Such waves are especially dangerous.

The superposition of one wave system on another occurs most intensely when the wind direction changes, there is a frequent alternation of storm winds, and before the front of tropical cyclones (4).

The energy of waves of developed waves is exceptionally high. For a ship drifting, the dynamic effect of waves can be determined from the expression p=0.1 τ² where τ is the true period of the wave, s.

Thus, for wave periods of about 6-10 s, the P value can reach impressive values ​​(3.6-10 t/m²).

When a ship moves against a wave, the dynamic effect of the waves will increase in proportion to the square of the ship's speed, expressed in meters per second.

The wavelength in meters, speed in meters per second and period in seconds are related to each other by the following relationships:

A practically moving ship encounters not the true, but the relative (apparent) wave period τ", which is determined from the expression

Where a is the heading angle of the wave crest front, measured along any side.

Plus refers to the case of movement against the wave, minus - along the wave.

When changing course, the ship is positioned relative to the reduced wavelength λ":

The nature of the ship's rolling has a complex relationship between the wave elements (h, λ, τ and C) and the ship elements (L, D, T1,2 and δ).

The safety of a ship in terms of stability is determined not only by its design and load distribution, but also by its course and speed. In conditions of developed waves, the shape of the existing waterline continuously changes. Accordingly, the shape of the immersed part of the hull, the shape stability arms and the restoring moments change.

The stay of the ship at the bottom of the wave is accompanied by an increase in righting moments. Staying a ship (especially for a long time) on the crest of a wave is dangerous and can lead to capsizing. The most dangerous is resonant rolling, at which the period of the ship's own oscillations T1,2 is equal to the visible (observed) period of the wave?" The nature of the onboard resonant rolling is shown in Fig. 7.4. As follows from the figure, the phenomenon of resonance is observed at a ratio of 0.7< T1 /τ" < 1,3

Resonant rocking is especially dangerous when the ship is positioned with the lag facing the wave.
When a ship follows a course against the wave, losses in speed increase significantly, exposing the ends and sudden surges in speed occur. Wave impacts at the bottom of the bow (slamming phenomenon) can lead to deformation of the hull and tearing of individual mechanisms and devices from the foundations.

When following a wave, the ship is less susceptible to wave impacts. However, following it along the wave at a speed close to the wave speed VK = (0.6--1.4) C (the ship “rided” the wave) leads to a sharp loss lateral stability due to a change in the shape and area of ​​the acting waterline, and this leads to the emergence of a gyroscopic moment acting in the plane of the waterline and significantly worsening the controllability of the ship.

Rice. 7.4. Resonant pitching

The most dangerous navigation of a small ship is in favorable seas, when λ=L of the ship, and VK=C.

Universal pitching diagram Yu.V. Remeza

The universal rolling diagram determines the dependence of the observed wave elements on changes in the elements of the ship's motion.

The diagram is calculated using the formula

Where V is the speed of the ship, knots.

The diagram determines the relationship between X and V sin a for various values ​​of m. It is constructed relative to the prevailing wave system, which can be identified at any sea level and has the most significant effect on the ship's motion (§ 23.4). The universal diagram can only be used in areas with sufficiently large depths (more than 0.4X waves).

The use of a universal pitching diagram allows you to solve the following main problems:
- determine the course and speed at which the ship can get into a position of resonant pitching (pitching and side);

Determine the wavelength in the sailing area;

Determine the course sectors and speed ranges at which the ship will experience strong rolling, close to resonant;

Determine the courses and speeds at which the ship will be in the most dangerous state of reduced lateral stability;

Determine the courses and speeds at which the ship will experience the “slamming” phenomenon.

(1) Further increase in wind is accompanied by wind waves, which reduce the speed of the ship.
(2) The coordinates of the true wind are related to the earth, and the apparent wind to the ship.
(3) In practice, the movement of water particles in wind waves occurs in orbits close in shape to a circle or ellipse. Only the wave profile moves.
(4) The nature of wave formation and its connection with wind elements are discussed in detail in the oceanography course.

I think that many of us would take the chance to dive into the abyss of the sea on some kind of underwater vehicle, but still, most 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 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 lifting force, which scientists call lateral force.

An attentive observer could not help but notice that no matter which way the wind blows sailing yacht always moves where the captain needs 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 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 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 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. 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°.

Apparent wind

Let's try to understand due to what forces, and on the basis of what principles, the movement of a sailing ship occurs under the influence of the wind. Let's consider only oblique sails, as they are the most common at present. The Bermuda-type oblique sail rig is the main rig of most modern single-mast and two-mast vessels. All sport and cruising single-mast yachts are also armed with a Bermuda sloop.

This rig provides maximum opportunities for choosing a course relative to the direction of the wind and requires a significantly smaller crew to control the sails and does not require such a high level of training as in the case of using direct sailing rigs.

A remarkable feature of an oblique sail is its ability to create traction on courses up to 30-40 degrees to the wind direction.

It must be taken into account that the sailing vessel is moving relative to the apparent or apparent wind, and not relative to the true or meteorological wind.

When any object moves in the air, a flow of incoming air arises, the speed of which is determined by the speed of the object. Accordingly, even in the complete absence of wind (calm), the observer on the ship will feel a wind equal to the speed of the ship - a heading wind, which will be equal in magnitude to the speed of the ship, and in the direction opposite to the direction of movement of the ship. Thus, a sailing ship, when moving, experiences the action of two air flows:

The action of a flow caused by the presence of a true wind;

The action of the flow caused by the movement of the vessel - directional wind.

To determine the resulting air flow felt by an observer located on a moving object, it is necessary to perform a vector addition of the flows. The resulting vector will be the speed and direction of the felt or apparent wind, which is called the apparent wind. This wind will be considered as the wind acting on the sails of the ship as it moves (Fig. 1).

This wind is the only wind with which the sails interact, and its decomposition into true wind and directional wind is the result of an analysis of the original air flows.

Apparent wind is a variable value even when the true wind is stable in speed and direction, since its speed and direction depend on the speed and direction of the vessel's movement. For simplicity of reasoning, let us consider the case in which Fig. 1.

the true wind is directed at right angles to the direction of movement of the vessel and the speed of the true wind is equal to the speed of the vessel (Fig. 2). The figure shows that when moving at an angle of 90 degrees to the true wind, the ship is moving at an angle of 45 degrees to the apparent wind.

true In accordance with the above, you can

wind apparent wind assert that two vessels moving at the same

him and the same course, with the same wind conditions

conditions, but with at different speeds relative to the water they will move at different angles to the apparent wind. A vessel moving at a higher speed will sail sharper into the apparent wind while maintaining the same heading angle relative to the true wind. At the same time, wind indicators will be located on the masts of these ships.

the directional wind is at different angles to the ship's DP, fixing the direction

rice. 2 the apparent wind of each vessel (Fig. 3).

ship 1 ship 2

It can be seen from the figure that a ship traveling at a higher speed moves at a smaller angle to the apparent wind. From this we can conclude that as the speed of the vessel increases, the apparent wind sets in (the angle between the direction of the vessel’s movement and the apparent wind decreases). With a further increase in the speed of the vessel (better lines, less friction, sails work more efficiently, a different design of the vessel's hull), the angle between the direction of the vessel's movement and the apparent wind will become less than the minimum tacking angle (the minimum angle between the direction of the vessel's movement and the apparent wind, at which the possibility of effective sail operation). After this, the vessel, which has a high speed, will be forced to fall off (increase the angle between the direction of the vessel's movement and the direction of the apparent wind) until the minimum tacking angle is restored. This explains the different windward angles (the angle between the direction of the true wind and the direction the ship is moving). At the same time, the speed of approach to the wind (the speed of approach to the point of arrival located in the wind) can be greater for a vessel with a large angle of departure to the wind, but also a higher speed. As an example, consider the speed of a keel yacht, a sports dinghy and a catamaran going into the wind (Fig. 4).

A keel yacht, which has the lowest speed of all these vessels, moves sharper into the wind. Behind it comes a sports dinghy and the sports catamaran, which is least sensitive to the true wind. Each of these ships sails at the same angle to the apparent wind, but at different angles to the true wind. But at the same time, a sports catamaran will have the highest speed when going into the wind. From considering the speed triangle, it becomes clear that it is possible to reduce gusts of wind to true wind (short-term wind acceleration). In a gust, the speed of the true wind increases, but the speed of the ship remains, for some time, the same. The apparent wind moves away and it becomes possible to settle down and restore the tacking angle relative to the apparent wind (Fig. 5)

rice. 4

Keel yacht

dinghy

Catamaran

After some time, the ship's speed will increase, and it will be forced to fall back to its previous course relative to the true wind, maintaining an angle relative to the apparent wind. However, an increase in the speed of the vessel is possible until the speed limit for the movement of the vessel in displacement mode is reached (the speed of the vessel in displacement mode, expressed in knots, cannot exceed the length of the vessel, expressed in meters). Consequently, with a further increase in wind speed, the ship's speed will not increase and the ship's course relative to the true wind may be sharper.

The presence of currents in the area where the vessel is sailing is very important from the point of view of the behavior of the apparent wind. When sailing in a current, the speed of the vessel is vectorially added to the speed of the current. As a result, the absolute speed of the vessel changes and the speed and direction of the apparent wind changes. When moving with a tailwind, the apparent wind enters, and when moving with a countercurrent, it moves away. Consequently, with a tailwind, the tacking angle increases, and with a headwind, it decreases. At the same time, the speed of the yacht going into the wind remains almost unchanged. When the current is directed in the direction or against the direction of the true wind, a change in the speed of the true wind occurs. When the wind and current are unidirectional, the apparent wind enters, and when it is multidirectional, it moves away due to an increase in the speed of the true wind. The interaction of wind and current will change the ship's tacking angles relative to the true wind.

Modern navigation equipment makes it possible to obtain information not only about the direction and strength of the apparent wind, but also about the strength and direction of the true wind, by recalculating the speed triangle (Fig. 1). GPS provides information about the speed and direction of the vessel's movement, and an anemometer provides information about the speed and direction of the apparent wind. By recalculating the speed triangle, the system obtains information about the speed and direction of the true wind.

Understanding the behavior of apparent wind is key to planning a ship's route, given the direction and speed of the true wind and the actual speed of the sailing vessel.

However, for slow-moving ships, the angle between the direction of the true and apparent wind is insignificant and it can be stated, with a certain degree of accuracy, that this angle is within 10-20 degrees.