The movement of a sailing yacht downwind is actually determined by the simple pressure of the wind on her sail, pushing the vessel forward. However, as wind tunnel research has shown, sailing upwind exposes the sail to a more complex set of forces.

When ram 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, a region of increased pressure is formed on the back surface of the sail, and a region of reduced pressure is formed on the front surface. The pressure difference on the two sides of the sail creates a pulling (pushing) force that moves the yacht forward at an angle to the wind.

A sailing yacht, located approximately at right angles to the wind (in nautical terminology, a yacht is on a tack), moves quickly forward. The sail is subjected to pulling and lateral forces. If a sailboat is sailing at an acute angle to the wind, her speed slows down due to a decrease in towing force and an increase in lateral force. The more the sail is turned aft, the slower the yacht moves forward, in particular due to the large lateral force.

A sailboat 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 port side (1), they say that the yacht is on the left tack, if to starboard (2) - the 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 her sail, as shown in the figure below to the left. To minimize deviation from a straight line, the boat moves by changing course from starboard to port tack 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 rinses for some time, i.e. is inactive (middle figure below the text). The yacht enters the so-called dead zone, losing speed until the wind blows the sail again from the opposite side.

Wind courses. Modern yachts and sailboats 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 stay. Due to the fact that the leading edge of the sail is taut along the mast (or itself), the sail flows around the air flow without rinsing when it is located at a fairly sharp angle to the wind. Thanks to this (and with the appropriate contours of the hull), the ship acquires the ability to move at an acute angle to the direction of the wind.

On fig. 190 shows the position of the sailboat at various courses with respect to the wind. An ordinary sailboat cannot go directly against the wind - in this case, the sail does not create a traction force that can overcome the resistance of water and air. The best racing yachts in medium wind can sail at an angle of 35-40° to the direction of the wind; usually this angle is not less than 45°. Therefore, to a target located directly against the wind, the sailboat is forced to get to tacking- alternately right and left tack. The angle between the ship's courses on either tack is called tack angle, and the position of the ship with its bow directly into the wind - leventik. The ability of a vessel to tack and move at maximum speed in a direction directly into the wind is one of the main qualities of a sailboat.

Courses from close-hauled to gulfwind when the wind is at 90° to the ship's DP are called sharp; from gulfwind to gybe (wind blowing straight into the stern) - complete. Distinguish steep(course over wind 90-135°) and full(135-180°) backstay, as well as hauled wind (respectively 40-60° and 60-80° to the wind).

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

1 - steep sidewind; 2 - full haul; 3 - gulfwind; 4 - backstay; 5 - jibe; 6 - leventik.

Pennant wind. The flow of air that flows around the sails of the yacht does not match the direction true wind(relative to land). If the ship is moving, then a counter flow of air appears, the speed of which is equal to the speed of the ship. In the presence of wind, its direction relative to the vessel is deviated in a certain way due to the oncoming air flow; 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. Apparent wind at different courses of the yacht relative to the wind.

1 - badewind; 2 - gulfwind; 3 - backstay; 4 - jibe.

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

Obviously, on the hauled course, the speed of the pennant wind has the greatest value, and on the gybe - the smallest, since in the latter case the speeds of both streams are directed in opposite directions.

The sails on the yacht are always set, focusing on the direction of the pennant wind. Note that the speed of the yacht does not increase 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 differs more noticeably from the true one.

Since the forces acting on the sail as on a wing grow in proportion to the square of the speed of the flow around, sailboats with minimal resistance to movement may experience “self-acceleration”, in which their speed exceeds the wind speed. These types of sailboats include ice yachts - iceboats, hydrofoil yachts, wheeled (beach) yachts and proa - narrow single-hull vessels with an outrigger float. Some of these types of vessels have recorded speeds up to three times the wind speed. Thus, our national speed record on a buoy is 140 km/h, and it was set in a wind speed that did not exceed 50 km/h. In passing, we note that the absolute speed record under sail 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.

Ordinary sailing vessels, if not designed for planing, in rare cases exceed the speed limit for displacement navigation, equal to 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 ship and a ship driven by a mechanical engine. On a motor vessel, the thrust of the propeller - a propeller or a water jet - and the force of water resistance to its movement act in the underwater part, located in the diametrical plane and at a small 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 badewind, then its sails work on the principle of an aerodynamic wing, discussed in Chapter II. When the sail flows around the sail with a stream of air, a vacuum is created on its leeward (convex) side, and an 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 in the center of the sail (CPU) 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 a body in a straight line, each force applied to the body (in this case, to the sails connected to the yacht's hull through the mast, standing rigging and sheets), must be counteracted by an equal and oppositely directed force. On a sailboat, this force is the resultant hydrodynamic force H applied 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 vessel about an axis oriented in a certain way in space.

To simplify the phenomena that occur during movement 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:

A	- aerodynamic resultant force;
T	- the force of the sails pulling the ship forward:
D	- heeling force or drift force;
A v	- vertical (trim on the nose) force;
P	- mass force (displacement) of the vessel;
M d	- trimming moment;
M kr	- heeling moment;
M P	- the moment leading to the wind;
H	- hydrodynamic resulting force;
R	- the force of water resistance to the movement of the vessel;
R d	- side force or drift resistance force;
H v	- vertical hydrodynamic force;
γ· V	- buoyancy force;
M l	- moment of resistance to trim;
M in	- restoring moment;
M at	- a humbling moment.

In order for the ship to steer steadily on 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 cr, which must be balanced by a restoring moment M in or moment of lateral stability. This moment is formed due to the action of mass forces P and ship buoyancy 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 trim moment M e. 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 vessel in an oblique course to the wind is associated with roll and trim, and the lateral force D, in addition to roll, it also causes drift - lateral drift, therefore, 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 a sailboat, its keel and rudder become a hydrofoil, which is attacked by an oncoming flow of water at an angle of attack equal to the drift angle. It is this circumstance that causes 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 ship. Due to the heel, the traction 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 into the wind - knocking it off the straight course that it follows. 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 rudder, which must be applied in order to correct the movement of the yacht on the course.

Obviously, the reaction of the vessel 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 an increase in roll, the shoulder of the driving pair b also increases, and the value of the leverage of the trailing pair a depends on the relative position sail center(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 hull of the yacht).

Precise determination of the position of these points is a rather difficult task, especially considering that it varies depending on many factors: the course of the vessel relative to the wind, the cut and adjustment 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 CPU and CBS, considering them located in the centers of gravity of flat figures, which are sails set in the DP, and the outlines of the underwater part of the DP with a keel, fins and a 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 the segment of the straight line connecting the CPU 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 a diagonal into two triangles and the CPU is obtained as the common center of these triangles.

Rice. 193. Determination of the conditional center of sailing of the yacht.

The position of the CBS can be determined by balancing the template of the underwater profile of the DP, cut out of thin cardboard, on the tip of the needle. When the template is horizontal, the needle will be at the point of the conditional CBS. However, this method is more or less applicable to vessels with a large area of ​​the underwater part of the DP - for traditional-type yachts with a long keel line, ship boats, etc. On modern yachts, the contours of which are designed based on wing theory, the main role in creating the drag force drift is played by a fin keel and a rudder, 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 about 26% of the chord b from the leading edge.

However, the hull of the yacht in a certain way affects 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 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 the CBS, designers, when developing a project for sailing ships, have the CPU at a certain distance a- advancing - ahead of the CBS. The amount of advance is determined statistically, from a comparison with well-established yachts that have close to the project contours of the underwater part, stability and sailing equipment. The advance is usually set as a percentage of the length of the vessel along the waterline and for a vessel equipped with a Bermuda sloop, 15-18% L. The lower the stability of the yacht, the greater the roll it will receive under the influence of the wind and the greater the need for the lead of the CPU in front of the CBS.

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 bear away downwind, especially in 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 DP of the vessel. Therefore, if the rudder is straight, then its profile works with a noticeably smaller angle of attack than the keel. If, in order to compensate for the tendency of the yacht to bear away, the rudder has to be shifted to the wind, then the lift force formed on it turns out to be directed to the leeward side - in the same direction as the drift force D on sails. Consequently, the ship will have increased drift.

Another thing is the light tendency of the yacht to be driven. 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 drift resistance. Shear force H, arising on the rudder, causes a significant shift of the total CLS to the stern while reducing the drift angle. However, if in order to keep the yacht on a badewind course, you have to constantly shift the rudder to the leeward side at an angle greater than 2-3 °, you need to move the CPU forward or move the CLS 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, increasing the area of ​​​​the main staysail. To move the CLS back, you need to install a fin in front of the steering wheel or increase the size of the rudder blade.

To eliminate the tendency of the yacht to bear away, it is necessary to apply the opposite measures: move the CPU back or move the CLS forward.

The role of the components of the aerodynamic force in creating thrust and drift. The modern theory of the work of a slanting sail is based on the provisions of the aerodynamics of the wing, the elements of which were considered in chapter II. When the sail, placed at an angle of attack α to the pennant wind, flows around the sail, an aerodynamic force is created on it A, which can be represented as two components: lifting force Y, directed perpendicular to the air flow (the pennant wind), and drag X- force projections A to the direction of air flow. These forces are used when considering the characteristics of the sail and the entire sailing rig as a whole.

Simultaneously force A can be represented in the form of two other components: thrust force T, directed along the axis of movement of the yacht, and the drift force perpendicular to it D. Recall that the direction of movement of a sailboat (or path) differs from its course by the value of the drift angle β, but this angle can be neglected in further analysis.

If on a badewind course it is possible to increase the lift on the sail to a value 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 lifting force, both the thrust force and the drift force increase.

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

With a similar construction, it can be seen that with an increase in drag on a hauled course, the traction force decreases, and the drift force increases. Thus, when sailing in close wind, the lifting force of the sail plays a decisive role in creating the thrust of the sails; frontal resistance should be minimal.

Note that on the hauled course, the pennant wind has the highest speed, so both components of the aerodynamic force Y and X are large enough.

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

On the backstay course (Fig. 194, in) the sail operates at high angles of attack, at which the lifting force is much less than the drag. If you increase the drag, then 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 the backstay course, an increase in both lift and (or) drag increases traction.

On a jibe, 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 the vessel's movement and is the thrust force. The drift force is zero. Therefore, on a jibe 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 a poorly streamlined shape. It should be noted that on the gybe course, the sails of the yacht are affected by the pennant wind of minimum speed, which causes relatively moderate forces on the sails.

drift resistance. As shown above, the strength of the drift depends on the course of the yacht relative to the wind. When sailing in close-hauled, it is approximately three times the thrust T, moving the ship forward; on a gulfwind both forces are approximately equal; on a steep backstay, the pull of the sail is 2-3 times greater than the drift force, and on a clean gybe, the drift force is absent at all. Therefore, in order for a sailboat to successfully move forward on courses from 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 water to the movement of the yacht along the course.

The function of creating a drift resistance force on modern sailing ships is mainly performed by fin keels or centerboards and rudders. The mechanics of the occurrence of lift on a wing with a symmetrical profile, which are keels, skewers and rudders, was considered in chapter II (see p. 67). It should be noted that the value 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 lifting force with minimum drag, namely at low angles of attack.

Tests of aerodynamic symmetrical airfoils have shown that thicker airfoils (with a larger section thickness ratio t to his chord b) give more lift than thin ones. However, at low speeds, such profiles have a higher drag. Optimum results on sailing yachts can be obtained with a keel thickness t/b= 0.09÷0.12, since the lifting force on such profiles depends little on the speed of the ship.

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 with a maximum thickness located at a distance of 50% of the chord from the nose also has good qualities (Fig. 195).

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

Ordinates of the recommended section profiles for yacht keels and daggerboards

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

Keel extension 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 amount of lift and drag of the keel. With a keel lengthening of about λ = 0.6, a leading edge inclination of up to 50° can be allowed; at λ = 1 - about 20°; with λ > 1.5, the keel with a vertical leading edge is optimal.

The total area of ​​\u200b\u200bthe keel and rudder for effective counteraction to drift is usually taken equal to from 1/25 to 1/17 of the area of ​​\u200b\u200bthe main sails.

“Fair wind!” - wish all sailors, and completely in vain: when the wind blows from the stern, the yacht is not able to develop maximum speed. I helped make this diagram. Vadim Zhdan, professional skipper, racer, organizer and host of yacht regattas. Read the tooltips on the diagram to find out.

2. The thrust of the sail is due to two factors. Firstly, the wind simply presses on the sails. Secondly, slanting sails, installed on most modern yachts, when flowing around with air, work like an airplane wing, and only it is not directed upwards, but forward. Due to aerodynamics, the air moves faster on the convex side of the sail than on the concave side, and the pressure on the outside of the sail is less than on the inside.

3. The full force generated by the sail is directed perpendicular to the canvas. According to the vector addition rule, it is possible to distinguish the drift force (red arrow) and the thrust force (green arrow) in it.

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

9. gulfwind－ the wind is blowing perpendicular to the direction of travel.

11. jibe- the same one favourable wind blowing from the stern. Contrary to expectations, not the fastest course: here the lift 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 in the same way

No less important than the resistance of the hull is the traction force developed by the sails. In order to more clearly imagine the work of sails, let's get acquainted with the basic concepts of sail theory.

We have already talked about the main forces acting on the sails of a yacht sailing with a tailwind (gybe) and with a headwind (haul). It was found that the force acting on the sails can be decomposed into the force that causes the yacht to roll and drift downwind, the drift force and the thrust force (see Fig. 2 and 3).

Now let's see how the total force of the wind pressure on the sails is determined and what the forces of traction and drift depend on.

To imagine the work of a sail on sharp courses, it is convenient to first consider a flat sail (Fig. 94), which experiences wind pressure at a certain angle of attack. In this case, vortices form behind the sail, pressure forces arise on the windward side of it, and rarefaction forces appear on the leeward side. Their resulting R is directed approximately perpendicular to the plane of the sail. For a correct understanding of the operation of a sail, it is convenient to present it as the resultant of two components of forces: X-directed parallel to the air flow (wind) and Y-perpendicular to it.

The force X, directed parallel to the air flow, is called the drag force; it is created, in addition to the sail, also by the hull, rigging, spars and crew of the yacht.

The force Y, directed perpendicular to the air flow, is called lift in aerodynamics. It is 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 force increases, for example, to a value Y1, then, as shown in the figure, the resultant lift and drag will change by R and, accordingly, the thrust force T will increase to T1.

Such a construction makes it easy to verify that with an increase in drag X (for the same lifting force), thrust T decreases.

Thus, there are two ways to increase the traction force, and hence the speed on sharp courses: an increase in the lifting force of the sail and a decrease in the drag of the sail and the yacht.

In modern sailing, the lifting force of the sail is increased by giving it a concave shape with some “pot-belliedness” (Fig. 96): the size from the mast to the most deep place The "belly" is usually 0.3-0.4 of the width of the sail, and the depth of the "belly" is about 6-10% of the width. The lifting force of such a sail is 20-25% greater than that of a completely flat sail with almost the same drag. True, a yacht with flat sails goes a little steeper towards the wind. However, with "pot-bellied" sails, the speed of advance into the tack is greater due to the greater thrust.

Rice. 96. Sail profile

Note that for pot-bellied sails, not only traction 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 ones. Therefore, the “pot-bellied” sail of more than 6-7% in strong winds is unprofitable, since an increase in roll and drift leads to a significant increase in hull resistance and a decrease in the efficiency of the sails, which “eat up” the effect of increased thrust. In light winds, sails with a “belly” of 9-10% are pulled better, since due to the low total wind pressure on the sail, the roll is small.

Any sail at angles of attack greater than 15-20 °, that is, at yacht courses of 40-50 ° to the wind and more, allows you to reduce lift and increase drag, since significant turbulences form on the leeward side. And since the main part of the lifting force is created by a smooth, without turbulence, flow around the lee side of the sail, the destruction of these turbulences should have a great effect.

They destroy the turbulences that form behind the mainsail by setting the staysail (Fig. 97). The air flow entering the gap between the mainsail and the staysail increases its speed (the so-called nozzle effect) and, with the correct adjustment of the staysail, “licks” the whirlwinds from the mainsail.

Rice. 97. Staysail work

The profile of a soft sail is difficult to keep the same at different angles of attack. Previously, dinghies were equipped with through armor passing through the entire sail - they were made thinner within the “belly” and thicker towards the leech, where the sail is much flatter. Now through armor is installed mainly on iceboats 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 rinsing along the luff.

If only the sail is the source of lift, then drag is created by everything that happens to be in the air flow around the yacht. Therefore, the improvement of 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 amount of drag on a sail depends on its shape. According to the laws of aerodynamics, the drag of an aircraft wing is the smaller, the narrower and longer it is with 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 you to use the riding wind.

The drag of a sail depends to a very large extent on the condition of its leading edge. The luffs of all sails must be tightly wrapped to prevent the possibility of vibrations.

It is necessary to mention one more very important circumstance - the so-called centering of the sails.

It is known from mechanics that any force is determined by its magnitude, direction and point of application. So far, we have only talked about the magnitude and direction of the forces applied to the sail. As we will see later, knowing the application points is essential to understanding how sails work.

The wind pressure is unevenly distributed over the surface of the sail (its front part experiences more pressure), however, to simplify comparative calculations, it is considered that it is distributed evenly. For approximate calculations, the resultant force of wind pressure on the sails is assumed to be applied to one point; it is taken as the center of gravity of the surface of the sails when they are placed in the diametrical plane of the yacht. This point is called the center of windage (CP).

Let's dwell on the simplest graphical method for determining the position of the CPU (Fig. 98). Draw the sail of the yacht in the right scale. Then, at the intersection of medians - lines connecting the vertices of the triangle with the midpoints of opposite sides - find the center of each sail. 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 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 grotto lay the area of ​​the staysail, and from the center of the staysail - the area of ​​the grotto. 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 to find the center of windage

As we have already said, the drift force (we will consider it applied in the center of the windage) is counteracted by the force of the lateral resistance of the yacht's hull. The lateral resistance force is considered to be applied at the center of lateral resistance (CLC). The center of lateral resistance is the center of gravity of the projection of the underwater part of the yacht on the diametrical plane.

The center of lateral resistance can be found by cutting out the outline of the yacht's underwater part from thick paper and placing this model on a knife blade. When the model is balanced, lightly press it, then turn it 90 ° and balance it again. The intersection of these lines gives us the center of lateral resistance.

When the yacht is going without a roll, the CPU should lie on the same vertical line with the CBS (Fig. 99). If the CPU lies ahead of the CBS (Fig. 99, b), then the drift force, shifted forward relative to the lateral resistance force, turns the bow of the vessel into the wind - the yacht bears away. If the CPU is behind the CBS, the yacht will turn with its bow to the wind, or be driven (Fig. 99, c).

Rice. 99. Yacht alignment

Both excessive bringing to the wind, and in particular bearing away (improper centering) are harmful to the course of the yacht, as they force the helmsman to work the steering wheel all the time in order to maintain the straightness of the movement, and this increases the resistance of the hull and reduces the speed of the vessel. In addition, incorrect centering leads to a deterioration in controllability, and in some cases to its complete loss.

If we center the yacht as shown in fig. 99, a, that is, the CPU and the CBS will be on the same vertical, then the ship will be driven very strongly and it will become very difficult to control it. What's the matter? There are two main reasons here. Firstly, the true location of the CPU and CLS does not coincide with the theoretical one (both centers are shifted forward, but not equally).

Secondly, and this is the main thing, when heeling, the traction force of the sails and the force of the longitudinal resistance of the hull turn out to lie in different vertical planes (Fig. 100), it turns out, as it were, a lever that forces the yacht to be driven. The greater the list, the greater the propensity of the vessel to be driven.

To eliminate such a cast, the CPU is placed in front of the CBS. The moment of thrust force and longitudinal resistance arising with a roll, which causes the yacht to be driven, is compensated by the trapping moment of drift forces and lateral resistance with the forward location of the CPU. For good centering, the CPU has to be placed ahead of the CLS at a distance equal to 10-18% of the length of the yacht along the waterline. The less stable the yacht is and the higher the CPU is raised above the CBS, the more it needs to be moved forward.

In order for the yacht to have a good move, it must be centered, that is, put the CPU and CLS in such a position in which the ship on the hauled course in a light wind was completely balanced by the sails, in other words, it was stable on the course with the rudder thrown or fixed in the DP (it is allowed a slight tendency to bear away with a very weak wind), and with a stronger wind it had a tendency to roll. Every helmsman must be able to properly center the yacht. On most yachts, the tendency to luff increases if the hindsails are pulled over and the frontsails are lowered. If the forward sails are overdrawn and the hind sails are overdrawn, the ship will bear away. With an increase in the "pot-bellied" 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

4.4. The action of the wind on the sail

The boat under sail is affected by two media: the air flow acting on the sail and the surface of the boat, and the water acting on the underwater part of the boat.

Thanks to the shape of the sail, even with the most unfavorable wind (badewind), the boat can move forward. The sail resembles a wing, the largest deflection of which is 1/3-1/4 of the sail width away from the luff and has a value of 8-10% of the sail width (Fig. 44).

If the wind, which has direction B (Fig. 45, a), meets a sail on the way, it goes around it from two sides. On the windward side of the sail, the pressure is higher (+) than on the lee side (-). The resultant of the pressure forces forms a force P directed perpendicular to the plane of the sail or the chord passing through the front and rear luffs and applied to the center of the windage of the CPU (Fig. 45, b).

Rice. 44. Sail profile:
B - the width of the sail along the chord

Rice. 45. Forces acting on the sail and the hull of the boat:
a - the effect of the wind on the sail; b - the effect of wind on the sail and water on the hull of the boat

Rice. 46. ​​The correct position of the sail in different wind directions: a - close-hauled; b - gulfwind; in - jibe

The force P is decomposed into a thrust force T, directed parallel to the center plane (DP) of the boat, forcing the boat to move forward, and a drift force D, directed perpendicular to the DP, causing drift and roll of the boat.

The force P depends on the speed and direction of the wind relative to the sail. The more
If a
The effect of water on the boat largely depends on the contours of its underwater part.

Despite the fact that with a close-hauled wind, the drift force D exceeds the thrust force T, the boat moves forward. Here the lateral resistance R 1 of the underwater part of the hull affects, which is many times greater than the frontal resistance R.

Rice. 47. Pennant wind:
V I - true wind; В Ш - wind from the movement of the boat; B B - pennant wind

Force D, despite the opposition of the hull, nevertheless blows the boat off the course line. Compiled by DP and the direction of the true movement of the IP boat
Thus, the greatest thrust and the least drift of the boat can be obtained by choosing the most favorable position of the center plane of the boat and the plane of the sail relative to the wind. It is established that the angle between the DP of the boat and the plane of the sail should be equal to half
When choosing the position of the sail relative to the DP and the wind, the foreman of the boat is guided not by the true, but by the pennant (apparent) wind, the direction of which is determined by the resultant of the speed of the boat and the speed of the true wind (Fig. 47).

The jib, located in front of the forefoot, plays the role of a slat. The air flow passing between the jib and the foresail reduces the pressure on the lee side of the foresail and therefore increases its propulsive force. This happens only under the condition that the angle between the jib and the DP of the boat is slightly larger than the angle between the fore and DP (Fig. 48, a).