Why can a sailboat sail against the wind? Basic information from sail theory Now let's look at the work of a sail on a yacht
Winds that blow westward in the South Pacific. That is why our route was designed so that the sailing yacht "Juliet" moves from east to west, that is, so that the wind blows in the back.
However, if you look at our route, you will notice that often, for example, when moving from south to north from Samoa to Tokelau, we had to move perpendicular to the wind. And sometimes the direction of the wind changed altogether and had to go against the wind.
Juliet's route
What to do in this case?
Sailing ships have long been able to sail against the wind. The classic Yakov Perelman wrote about this for a long time well and simply in his Second book from the Entertaining Physics cycle. I am citing this piece here verbatim with pictures.
"Sailing against the wind
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 go directly against the wind under sails, but you can only move at an acute angle to the direction of the wind. But this angle is small - about a quarter of the right angle - and it seems, perhaps, equally incomprehensible: whether to swim directly against the wind or at an angle of 22 ° to it.
In reality, 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 the figure below; line AB represents the 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 the force R applied to the middle of the sail. We decompose this force into two: the force Q, perpendicular to the sail, and the force P, directed along it (see the figure above, on the right). The last force pushes the sail nowhere, since the friction of the wind against the canvas is negligible. The force Q remains, which 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 represent the keel line of the vessel.
How can you sail against the wind.
The wind is blowing at an acute angle to this line in the direction indicated by the row of 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 decomposition of forces in the figure. 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: the force R, perpendicular to the keel, and the force S, directed forward, along the keel line of the vessel. Since the movement of the ship in the direction of R encounters strong water resistance (the keel in sailing ships becomes very deep), the force of R is almost completely balanced by the water resistance. There is only one force S, which, as you can see, is directed forward and, therefore, moves the ship at an angle, as if towards the wind. [The force S can be shown to be greatest when the plane of the sail halves 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, this movement of the ship is called "tacking" in the narrow sense of the word. "
Let's now consider all the possible wind directions relative to the boat's course.
The diagram of the ship's course relative to the wind, that is, the angle between the direction of the wind and the vector from stern to bow (heading). 
When the wind blows in the face (leventik), the sails dangle from side to side and it is impossible to move with the sail. Of course, you can always lower the sails and turn on the motor, but this has nothing to do with sailing.
When the wind is blowing right in the back (forewind, tailwind), the dispersed air molecules put pressure on the sail from one side and the boat moves. In this case, the vessel can only move slower than the wind speed. The analogy of cycling in the wind works here - the wind blows in the back and it is easier to pedal.
When moving against the wind (beydewind), the sail moves not because of the pressure of air molecules on the sail from the rear, as in the case of fordewind, but because of the lift that is created due to different air velocities from both sides along the sail. At the same time, due to the keel, the boat moves not in the direction perpendicular to the boat's course, but only forward. That is, the sail in this case is not an umbrella, as in the case of a side haul, but an airplane wing.
During our crossings, we mainly walked backstays and gulfwinds at an average speed of 7-8 knots with a wind speed of 15 knots. Sometimes we went against the wind, Gulfwind and Beydewind. And when the wind died down, they turned on the motor.
In general, a boat with a sail going against the wind is not a miracle, but a reality.
The most interesting thing is that boats can walk not only against the wind, but even faster than the wind. This happens when the boat is backstaying, creating "its own wind".
Just as important as the drag of the hull is the thrust produced by the sails. To get a clearer idea of how sails work, 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 (fordewind heading) and with a headwind (headwind heading). We found out that the force acting on the sails can be decomposed into the force that causes the yacht to roll and drift into the wind, the drift force and the thrust 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 visualize the operation of the 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 form behind the sail, pressure forces appear on the windward side of it, and rarefaction forces on the leeward side. Their resulting R is roughly perpendicular to the plane of the sail. For a correct understanding of the work of the sail, it is convenient to represent it in the form of the resultant of two components of forces: X-directed parallel to the air flow (wind) and Y-perpendicular to it.
Force X parallel to the air flow is called 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 she who, on sharp courses, creates thrust in the direction of movement of the yacht.
If, with the same drag of the sail X (Fig. 95), the lift increases, for example, to the value Y1, then, as shown in the figure, the resultant of lift and drag will change by R and, accordingly, the thrust T will increase to T1.
Such a construction makes it easy to verify that with an increase in drag X (with the same lifting force), the thrust T decreases.
Thus, there are two ways to increase the traction force, and, consequently, the speed of the course on sharp courses: increasing the lift of the sail and reducing the drag of the sail and yacht.
In modern sailing, the lifting force of the sail is increased by giving it a concave shape with a certain "paunch" (Fig. 96): the size from the mast to the deepest place of the "belly" is usually 0.3-0.4 of the width of the sail, and the depth of the "belly" -about 6-10% of the width. The lifting force of such a sail is 20-25% higher than that of a completely flat sail with almost the same drag. True, a yacht with flat sails goes a little steeper towards the wind. However, with "pot-bellied" sails, the speed of tacking is greater due to the greater thrust.
Rice. 96. Sail Profile
Note that in pot-bellied sails, not only the thrust increases, but also the drift force, which means that the roll and drift of yachts with pot-bellied sails is greater than with relatively flat sails. Therefore, the “pot-belliedness” of the sail is more than 6-7% in strong winds, since an increase in heel and drift leads to a significant increase in the resistance of the hull and a decrease in the efficiency of the sails, which “eat up” the effect of increasing thrust. In light winds, sails with a "belly" of 9-10% pull better, since the roll is small due to the low total wind pressure on the sail.
Any sail with angles of attack more than 15-20 °, that is, when the yacht is heading 40-50 ° to the wind and more, allows you to reduce the lifting force and increase the drag, since significant eddies are formed on the leeward side. And since the main part of the lift is created by a smooth, without vortices, flow around the leeward side of the sail, the destruction of these vortices should have a great effect.
The eddies formed behind the mainsail are destroyed by setting the staysail (Fig. 97). The flow of air entering the gap between the mainsail and the staysail increases its speed (the so-called nozzle effect) and, when the staysail is adjusted correctly, "licks" the vortices from the mainsail.
Rice. 97. The work of the staysail
The soft sail profile is difficult to maintain consistent across different angles of attack. Previously, on dinghies they put through battens, passing through the entire sail - they were made thinner within the "belly" and thicker towards the leech, where the sail is much flatter. Nowadays, through battens are installed mainly on gullies and catamarans, where it is especially important to maintain the profile and rigidity of the sail at low angles of attack, when an ordinary sail is already paddling the luff.
If only the sail is the source of lift, then drag creates whatever is in the air flow around the yacht. Therefore, an improvement in the traction properties of the sail can also be achieved by reducing the drag of the yacht's hull, spars, rigging and crew. For this purpose, various kinds of fairings are used on the spars and rigging.
The drag of the sail depends on its shape. According to the laws of aerodynamics, the narrower and longer the drag of an airplane wing is, the less it is for the same area. That is why the sail (essentially the same wing, but set vertically) is tried to be made high and narrow. This also allows the use of an upstream 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 tight to prevent 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. Until now, we have only talked about the magnitude and direction of the forces applied to the sail. As we will see later, knowing the application points is essential to understanding how sails work.
The wind pressure is unevenly distributed over the sail surface (the front part experiences more pressure), however, to simplify comparative calculations, it is considered that it is evenly distributed. For approximate calculations, the resultant force of wind pressure on the sails is assumed to be applied to one point; it is taken to be the center of gravity of the surface of the sails when they are placed in the center plane of the yacht. This point is called the center of sail (CP).
Let's dwell on the simplest graphical method for determining the position of the CPU (Fig. 98). Draw the sail of the yacht on the right scale. Then, at the intersection of the medians - the lines connecting the vertices of the triangle with the midpoints of the 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 staysail, two parallel lines OA and O1B are drawn through these centers and on them are laid in opposite directions in any, but the same scale, as many linear units as square meters in the triangle; from the center of the mainsail lay the area of the jib, and from the center of the jib - the area of the mainsail. End points A and B are connected by a straight line AB. Another straight line - O1O connects the centers of the triangles. At the intersection of lines A B and O1O there will be a common center.
Rice. 98. Graphical way 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 drag force of the yacht's hull. The lateral resistance force is considered to be applied at the center of lateral resistance (CLS). 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 the blade of a knife. When the model is balanced, push it lightly, then rotate it 90 ° and counterbalance it again. The intersection of these lines gives us the center of lateral resistance.
When the boat is sailing without heel, the CPU should be on the same vertical line with the CLS (figure 99). If the CP lies in front of the CLS (Fig. 99, b), then the drift force, displaced forward relative to the lateral resistance force, turns the bow of the vessel into the wind - the yacht rolls away. If the CPU is behind the CLS, the yacht will turn with its bow to the wind, or be brought (Fig. 99, c).
Rice. 99. Centering the yacht
Excessive wind-throwing and, in particular, rolling away (misalignment) are detrimental to the yacht's progress, as they force the helmsman to work at the rudder all the time in order to maintain straightness of movement, and this increases the resistance of the hull and slows down the boat's speed. In addition, misalignment leads to a deterioration in controllability, and in some cases - to its complete loss.
If we center the yacht as shown in fig. 99, but, that is, the CPU and CLS 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. First, the true location of the CPU and CLS does not coincide with the theoretical one (both centers are shifted forward, but not the same).
Secondly, and this is the main thing, when heel, the sails' thrust force and the hull's longitudinal resistance force are lying in different vertical planes (Fig. 100), it turns out, as it were, a lever that forces the yacht to be driven. The more the heel, the more the boat's inclination to be driven.
To eliminate this cast, the CPU is placed in front of the CLS. The moment of thrust force and longitudinal drag arising with the roll, forcing the yacht to be driven, is compensated by the catching moment of the forces of drift and lateral drag at the front position of the CPU. For good centering, the CPU has to be placed in front of the CLS at a distance equal to 10-18% of the yacht's waterline length. The less stable the yacht and the higher the CPU is raised above the CLS, the more it needs to be moved into the bow.
In order for the yacht to have a good course, it must be centered, that is, the CPU and CLS must be placed in such a position in which the vessel on the side-hauled course in a weak wind was completely balanced by the sails, in other words, it was stable on the course with the rudder thrown or fixed in the steering wheel (allowed a slight tendency to roll away in very weak winds), and in stronger winds it had a tendency to be led. Every helmsman must be able to correctly center the yacht. On most yachts, the tendency to fly increases when the hind sails are moved and the front sails are lowered. If the front sails are moved and the rear sails are over-etched, the ship will roll away. With an increase in the "pot-belliedness" of the mainsail, as well as poorly standing sails, the yacht tends to be driven to a greater extent.
Rice. 100. Influence of roll on bringing the yacht to the wind
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 go directly against the wind under sails, but you can only move at an acute angle to the direction of the wind. But this angle is small - about a quarter of the right angle - and it seems, perhaps, equally incomprehensible: whether to swim directly against the wind or at an angle of 22 ° to it.
In reality, 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 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 the force R applied to the middle of the sail. We decompose this force into two: the force Q perpendicular to the sail, and the force P directed along it (see the figure above, right). The last force pushes the sail nowhere, since the friction of the wind against the canvas is negligible. Remains strong Q that 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 line QC depicts the keel line of the vessel.
How can you sail against the wind.
The wind is blowing at an acute angle to this line in the direction indicated by the row 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 decomposition of forces in the figure. We represent the pressure of the wind on the sail by force Q which we know must be perpendicular to the sail. We will decompose this force into two: the force R perpendicular to the keel, and the force S forward along the keel line of the vessel. Since the movement of the vessel in the direction R meets strong water resistance (the keel in sailing ships becomes 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 proved that the strength S is greatest when the plane of the sail halves 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, this movement of the ship is called "tacking" in the narrow sense of the word.
Wind courses. Modern yachts and sailboats in most cases are 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 headstock. Because the leading edge of the sail is tightly stretched along the mast (or by itself), the sail is flown around without flushing when it is positioned at a fairly acute angle to the wind. Due to this (and with appropriate hull contours), the vessel 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 various headings with respect to the wind. An ordinary sailboat cannot go straight against the wind - the sail in this case does not create a thrust force capable of overcoming the resistance of water and air. The best racing yachts in average winds can sail sidewind at an angle of 35-40 ° to the wind direction; usually this angle is not less than 45 °. Therefore, to the target located directly against the wind, the sailboat is forced to get to tacking- alternately starboard and port tack. The angle between the course of the ship on either tack is called tack angle, and the position of the ship with its bow straight upwind - leventic... The ability of the vessel to maneuver and to move at maximum speed in the direction directly upwind is one of the main qualities of a sailboat.
The courses from steep sidewind to gulfwind, when the wind blows at 90 ° to the ship's DP, are called sharp; from gulfwind to fordewind (wind blows directly aft) - complete... Distinguish steep(course relative to wind 90-135 °) and full(135-180 °) backstays, as well as the sidewind (respectively 40-60 ° and 60-80 ° to the wind).
Rice. 190. Sailing ship's course relative to the wind.
1 - steep sidewind; 2 - full beydewind; 3 - gulfwind; 4 - backstay; 5 - fordewind; 6 - leventic.
Pennant wind. The air flow that flows around the yacht's sails is not in line with the direction true wind(regarding land). If the ship has a course, then a counter air flow appears, the speed of which is equal to the speed of the ship. In the presence of wind, its direction relative to the vessel due to the oncoming air flow is deflected in a certain way; the magnitude of the speed also changes. Thus, the total flow, called pennant wind... Its direction and speed can be obtained by adding the vectors of the true wind and the oncoming flow (Fig. 191).
Rice. 191. Pennant wind at various yacht heading relative to the wind.
1 - beydewind; 2 - gulfwind; 3 - backstay; 4 - fordewind.
v- the speed of the yacht; v and - true wind speed; v c is the apparent wind speed.
Obviously, on the beydewind course, the apparent wind speed has the highest value, and on the fordewind course - the smallest, since in the latter case the speeds of both streams are directed in opposite directions.
Sails on a yacht are always set in the direction of the apparent wind. Note that the yacht's speed 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 a weak wind, the speed and direction of the apparent wind more noticeably differs from the true one.
Since the forces acting on the sail as on the wing grow in proportion to the square of the speed of the flowing stream, in sailboats with minimal resistance to movement, the phenomenon of "self-acceleration" is possible, in which their speed exceeds the wind speed. These types of sailboats include ice yachts - buers, hydrofoils, wheel (beach) yachts and proa - narrow monohull vessels with an outrigger float. Some of these types of vessels have recorded speeds three times the wind speed. So, our national speed record on a gouge is 140 km / h, and it was set when the wind speed did not exceed 50 km / h. Along the way, we note that the absolute speed record under sail on water is significantly lower: it was set in 1981 on a specially built two-mast catamaran "Crossbau-II" and is equal to 67.3 km / h.
Conventional sailing vessels, if not 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 motor ship, the thrust of the propeller - a propeller or a water cannon - and the force of water resistance to its movement act in the underwater part, being located in the diametrical plane and at an insignificant vertical distance from each other.
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 is moving at an angle to the direction of the wind - in a close-hauled direction, then its sails work according to the principle of an aerodynamic wing, discussed in Chapter II. When the air flow around the sail, a vacuum is created on its leeward (convex) side, and increased pressure 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 sail profile chord and applied at the center of sail (CW) high above the water surface.
Rice. 192. Forces acting on the hull and sails.
According to the third law of mechanics, with a steady motion of the body in a straight line, each force applied to the body (in this case, to the sails connected to the hull of the yacht through the mast, standing rigging and sheets) must be counteracted by an equal in magnitude and oppositely directed force. On a sailboat, this force is the resulting hydrodynamic force H attached to the underwater part of the hull (Fig. 192). So between the forces A and H there is a known distance - a 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, you can write out in pairs all the components of these forces and moments:
| A | - aerodynamic resultant force; |
| T | - the thrust force of the sails, propelling the ship forward: |
| D | - heeling force or drift force; |
| A v | - vertical (nose trimming) force; |
| P | - the force of mass (displacement) of the vessel; |
| M d | - trimming moment; |
| M cr | - heeling moment; |
| M P | - the moment leading to the wind; |
| H | - hydrodynamic resultant force; |
| R | - the force of water resistance to the movement of the vessel; |
| R d | - lateral force or force of resistance to drift; |
| H v | - vertical hydrodynamic force; |
| γ· V | - buoyancy force; |
| M l | - moment of resistance to trim; |
| M v | - restoring moment; |
| M at | - a blowing moment. |
In order for the ship to steadily follow the course, each pair of forces and each pair of moments must be equal to each other. For example, the strength of the drift D and the force of resistance to drift R d create a heeling moment M cr, which must be balanced by the restoring moment M at or moment of lateral stability. This moment is formed due to the action of the forces of mass P and the buoyancy of the vessel γ V acting on the shoulder l... The same forces form a moment of resistance to trim or a moment of longitudinal stability M l equal in magnitude and counteracting the trim moment M The terms of the latter are the moments of the pairs of forces T - R and A v - H v .
Thus, the movement of a sailing vessel obliquely towards the wind is associated with heel and trim, and the lateral force D, in addition to roll, also causes drift - lateral drift, so any sailing vessel does not move strictly in the direction of the DP, like a vessel with a mechanical engine, but with a small drift angle β. The hull of the sailboat, its keel and rudder become a hydrofoil, on which the oncoming stream of water runs 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 the roll, the pulling force of the sails T and the force of resistance R turn out to be acting in different vertical planes. They form a pair of forces that propel the ship into 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 the forces of resistance to drift R d, as well as a small force N on the rudder, which must be applied in order to correct the yacht's heading.
Obviously, the ship's reaction to the action of all these forces depends both on their magnitude and on the ratio of the shoulders a and b on which they act. With increasing roll, the shoulder of the leading pair b also increases, and the size of the shoulder of the bearing 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(CLS - points of application of the resulting hydrodynamic forces to the hull of the yacht).
Determining the exact position of these points is a rather difficult task, especially when you consider that it changes depending on many factors: the course of the vessel relative to the wind, the cut and setting of the sails, the roll and trim of the yacht, the shape and profile of the keel and rudder, etc.
When designing and re-equipping yachts, they operate with conditional CP and CLS, considering them located in the centers of gravity of flat figures, which represent the sails set in the DP, and the outlines of the underwater part of the DP with the 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 two sails is located on a line segment connecting the CPUs 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 by the diagonal into two triangles and the CPU is obtained as the common center of these triangles.
Rice. 193. Determination of the conventional sailing center of the yacht.
The position of the CLS can be determined by balancing the template of the underwater profile of the DP, cut from thin cardboard at the point of the needle. When the template is horizontal, the needle will be at the point of the conditional CLS. However, this method is more or less applicable for vessels with a large area of the underwater part of the DP - for yachts of the traditional type with a long keel line, ship's boats, etc. The fin keel and the rudder, usually installed separately from the keel, play a drift. 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 leading edge.
However, the yacht's hull has a certain effect on the flow of the keel and rudder, and this effect varies depending on the roll, trim and speed of the vessel. In most cases, on sharp courses to the wind, the true CLS moves forward with respect to the center of pressure defined for the keel and rudder as for isolated profiles. Due to the uncertainty in the calculation of the position of the CPU and CLS, the designers, when developing the project of sailing ships, have the CPU at a certain distance a- ahead - ahead of the central banking system. The amount of lead is determined statistically, from comparison with well-proven yachts, which have underwater contours, stability and sailing equipment close to the project. 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 lower the stability of the yacht, the more heel it will receive under the influence of the wind and the more it is necessary to advance the CPU in front of the CLS.
Accurate adjustment of the relative position of the CPU and CLS is possible when testing the yacht on the move. If the vessel tends to sink into the wind, especially in a medium and fresh wind, then this is a large centering defect. The fact is that the keel deflects the flow of water flowing from it closer to the ship's DP. Therefore, if the rudder is straight, then its profile works with a noticeably smaller angle of attack than the keel. If, to compensate for the yacht's tendency to roll away, the rudder has to be shifted to the wind, then the lifting force generated on it turns out to be directed to the leeward side - in the same direction as the drift force D on sails. Consequently, the vessel will have an increased drift.
The easy tendency of the yacht is different. The rudder shifted 3-4 ° to the leeward side works with the same or slightly higher angle of attack as the keel, and effectively participates in resistance to drift. Transverse force H, arising on the rudder, causes a significant displacement of the general CLS to the stern with a simultaneous decrease in the drift angle. However, if in order to keep the yacht on a side-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 CLS back, which is more difficult.
On a built yacht, the CPU can be moved forward by tilting the mast forward, shifting it forward (if the design of the step allows), shortening the mainsail along the luff, increasing the area of the main staysail. To move the CLS back, you need to install a fin in front of the rudder or increase the size of the rudder feather.
To eliminate the yacht's tendency to roll away, it is necessary to take the opposite measures: move the CPU backwards or move the CLS forward.
The role of aerodynamic force components in the creation of thrust and drift. The modern theory of oblique sail operation is based on the provisions of wing aerodynamics, the elements of which were discussed in Chapter II. When the air flow 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 perpendicular to the air flow (apparent wind) and drag X- force projections A on the direction of the air flow. These forces are used when considering the characteristics of the sail and the entire rig in general.
Simultaneously force A can be represented in the form of two other components: traction forces T, directed along the axis of movement of the yacht, and perpendicular to her drift force D... 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 it is possible to increase the lift on the sail to a value of Y 1, and the frontal resistance remains unchanged, then the forces Y 1 and X added according to the vector addition rule form a new aerodynamic force A 1 (fig. 194, a). Considering its new components T 1 and D 1, it can be seen that in this case, with an increase in the lift force, both the thrust force and the drift force increase.
Rice. 194. The role of lift and drag in the creation of driving force.
With a similar construction, one can make sure that with an increase in drag on a side-hauled course, the thrust force decreases, and the drift force increases. Thus, when sailing in close-hauled sailing, the lifting force of the sail plays a decisive role in creating the thrust of the sails; drag should be minimal.
Note that on the beydewind course, the apparent wind has the highest speed; therefore, both components of the aerodynamic force Y and X are quite large.
On the Gulfwind course (Fig. 194, b) the lift force is the thrust force, and the drag is the drift force. The increase in the drag of the sail does not affect the magnitude of the traction force: only the drift force increases. However, since the apparent wind speed on a gulfwind is reduced compared to a sidewind, drift affects the ship's performance to a lesser extent.
On the backstag course (Fig. 194, v) the sail operates at high angles of attack, at which the lift is significantly less than the frontal resistance. If you increase the drag, the thrust and drift force will also increase. With an increase in the lifting force, the thrust increases, and the drift force decreases (Fig. 194, G). Consequently, on a backstay course, an increase in both lift and / or drag increases thrust.
With the forewind heading, the angle of attack of the sail is close to 90 °, so the lift on the sail is zero, and the drag is directed along the axis of movement of the vessel and is the thrust force. The drift force is zero. Therefore, on a forewind course, in order to increase the thrust of the sails, it is desirable 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 are poorly streamlined. Note that on the fordewind course, the yacht's sails are affected by an apparent wind of the minimum speed, which determines relatively moderate forces on the sails.
Drift resistance. As shown above, the strength of the drift depends on the course of the boat relative to the wind. When swimming in a steep sidewind, it is about three times the thrust T moving the ship forward; on Gulfwind, both powers are approximately equal; on a steep backstay, the sail thrust is 2-3 times greater than the drift force, and on a clean fordewind there is no drift force at all. Consequently, in order for a sailboat to successfully move forward on a course from side hauled to gulfwind (at an angle of 40-90 ° to the wind), it must have sufficient lateral drift resistance, much higher than the water resistance to the movement of the yacht along the course.
The function of creating a force of resistance to drift on modern sailing ships is performed mainly by fin keels or centerboards and rudders. The mechanics of the generation of lift on a wing with a symmetrical profile, which are keels, centerboards and rudders, was discussed in Chapter II (see page 67). Note that the magnitude of 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 choose its optimal dimensions, shape and cross-sectional profile in order to obtain maximum lift with minimum drag, namely at low angles of attack.
Tests of symmetrical aerodynamic airfoils have shown that thicker airfoils (with a greater ratio of section thickness t to its chord b) give more lifting force than thin ones. However, at low speeds, such profiles have a higher frontal resistance. Optimum results on sailing yachts can be obtained with keel thicknesses t/b= 0.09 ÷ 0.12, since the lifting force on such profiles depends little on the ship's speed.
The maximum profile thickness should be between 30% and 40% chord from the leading edge of the keel profile. Good qualities are also possessed by the NACA 664-0 profile with a maximum thickness located at a distance of 50% of the chord from the nose (Fig. 195).
Rice. 195. Yacht's profiled fin keel.
| Distance from the nose x, % b | ||||||
| 2,5 | 5 | 10 | 20 | 30 | 40 | |
| Ordinates y, % b | ||||||
| NACA-66; δ = 0.05 | 2,18 | 2,96 | 3,90 | 4,78 | 5,00 | 4,83 |
| 2,00 | 2,60 | 3,50 | 4,20 | 4,40 | 4,26 | |
| - | 3,40 | 5,23 | 8,72 | 10,74 | 11,85 | |
| Profile; relative thickness δ | Distance from the nose x, % b | |||||
| 50 | 60 | 70 | 80 | 90 | 100 | |
| Ordinates y, % b | ||||||
| NACA-66; δ = 0.05 | 4,41 | 3,80 | 3,05 | 2,19 | 1,21 | 0,11 |
| Profile for centerboards; δ = 0.04 | 3,88 | 3,34 | 2,68 | 1,92 | 1,06 | 0,10 |
| Keel of yacht NACA 664-0; δ = 0.12 | 12,00 | 10,94 | 8,35 | 4,99 | 2,59 | 0 |
For light racing sailing dinghies capable of reaching the planing mode and developing high speeds, centerboards and rudders with a thinner profile ( t/b= 0.044 ÷ 0.05) and geometric elongation (deepening ratio d to the middle chord b Wed) until 4.
The elongation of keels of modern keel yachts ranges from 1 to 3, 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 magnitude of the lift and drag of the keel. With an elongation of the keel of about λ = 0.6, an inclination of the leading edge up to 50 ° can be allowed; at λ = 1 - about 20 °; for λ> 1.5, the keel with a vertical leading edge is optimal.
The total area of the keel and rudder for effective resistance to drift is usually taken equal to from 1/25 to 1/17 of the area of the main sails.
Russian poet Mikhail Yurievich Lermontov loved sea and in his works he often mentioned him. He wrote a wonderful poem about the whitening sail, which rushes among the waves in the distant sea. You are probably familiar with Lermontov's poem, because these are the most famous poetic lines about sailing ships. Reading them, one can imagine a raging sea and beautiful ships among its waves. The wind blows the sails. And, thanks to the force of the wind, the ships move forward. But how do sailboats manage to sail against the wind?
In order to answer this, you first have to learn an unfamiliar word. "tack".Halsom is the direction of movement of the vessel relative to the wind. The tack can be left-handed when the wind is blowing from the left, or right-handed when the wind is blowing from the right. It is also important to know the second meaning of the word "tack" - it is a part of the path, or rather, its segment that a sailboat passes when it moves against the wind... Remember?
Now, in order to understand how sailboats manage to sail against the wind, let's deal with the sails. They come in different shapes and sizes on a sailboat - straight and oblique... And everyone is doing their job. When a headwind blows, the ship is steered by means of oblique sails that turn one way or the other.
Following them, the ship turns in one direction or the other. Turns and walks forward. The sailors call this movement - variable tack... Its essence lies in the fact that the wind presses on the oblique sails and blows the ship slightly sideways and forward. The rudder of the sailboat does not allow it to turn completely, and the skilled sailors set the sails in motion in time, changing their position. So, in small zigzags, and moves forward.
Of course, variable tacking is very difficult for the entire sailboat crew. But the sailors are seasoned guys. They are not afraid of difficulties and are very fond of the sea.