Speed ​​sailing yachts. Motor or sailing yacht

Back in the early fifties, many yachtsmen were prejudiced about installing a gasoline or diesel engine on a sailing yacht. The engine on the yacht was considered completely useless and even dangerous (in terms of fire) cargo; It was said that the motor tends to fail at the most critical moments. Most yacht captains called the union of motor and sail unnatural.

Now the situation has changed. It is difficult to find a cruiser yachtsman who would deny the need for an auxiliary engine. The vast majority of cruising yachts are supplied with motors, if not during construction, then during subsequent conversion.

The motor is certainly necessary when the yacht has to enter the harbor along a narrow winding fairway, tacking against the headwind. They involuntarily remember him even when the sails hang helplessly, and you need to urgently return to the yacht club, going against a strong current. And what about anchoring and anchoring in narrow places, crossing canals and under bridges, sailing in calm? In all these cases, the motor not only facilitates maneuvering, but also saves time, which can then be used to go a hundred miles further or to explore the sights on the coast.

Tourists sailing on boats, on the contrary, often have a desire to set sails in order to use a fresh tailwind, save fuel, and just take a break from the constant engine noise and vibration.

The features of the combined motor sailing ships, combining the qualities of a sailing yacht and a boat. The two extreme poles of such combined vessels are a yacht with an auxiliary motor of small power and full sailing equipment and a boat with a powerful engine and auxiliary sail (mainly to give the vessel stability in rough seas). Intermediate type ships considered in this article will hereinafter be referred to as motor sailboats.

Boat and yacht. It is easy to compare the main performance motor boat and a sailing yacht in the form of a table. 1.

Table 1. Comparison of the main qualities of a sailing yacht and a boat

When designing a motor sailboat, the goal is to achieve high speed, both under power and under sail, and to maintain the shallow draft of the boat, which makes many shallow harbors and bays accessible. From a sailing yacht, high seaworthiness, economy and a long cruising range must be preserved, as well as good conditions habitability.

In coastal cruising, a keel yacht (7-10 m long on waterline) without a motor shows average speed at the transition from 3 to 5 knots. On a motor sailboat, you can get a stable average speed of 3-4 knots more, which allows you to cover an extra 50-80 miles per day; there is no need to maneuver in a weak headwind or wait out the calm hours at sea. On the other hand, if the crew of the boat is often forced to refuse to go to sea, especially with a strong headwind and a big wave, on a motor sailboat you can safely go into tight hauled sails with reefed sails.

How best to combine positive qualities in one vessel? Is it right to put a powerful engine on a sailing yacht or advanced sailing equipment on a boat?

It is known that a sailing vessel can develop an acceptable speed if its sail area S (m²) is in a certain ratio with the displacement D (m³) and the wetted surface Ω (m²). These ratios should not be less than:

S 1/2 /D 1/3 = 3.8÷4.2; S/Ω = 2÷2.5,

and the first of them characterizes the propulsion of the yacht in strong wind, and the second - in the weak.

The yacht will be able to carry such an optimal windage if it has good stability, which is ensured by a deeply sunken heavy false keel (its weight is 35-50% of the total displacement). Naturally, when sailing under a motor, such stability is not needed, and the “transportation” of a false keel will require unproductive expenditure of engine power; spars, sails and equipment become the same useless cargo in this case.

To create sufficient drift resistance, the hull of the yacht must have large area lateral resistance (14-18% of the sail area). Therefore, the wetted surface of the yacht's hull is larger than that of a boat of the same dimensions, and to achieve the same speed as the boat, more engine power will be required. The advanced rigging and spars of the yacht increase air resistance, which also requires additional power to overcome. The contours of the yacht, designed for sailing at a relatively low speed and with a roll, do not allow you to develop a higher speed under the motor, no matter how much its power increases.

On the other hand, if you put the sailing rig of a yacht of the same size on the boat, the result is unlikely to be satisfactory. Due to the absence of a false keel and the high location of heavy loads (engine, fuel reserves, developed superstructures), the stability of the boat will be clearly insufficient for sailing and a reduction in the sail area will be required. It will not be possible to go steeply to the wind on it, since the lateral resistance of its hull is small. The contours of its underwater part are not designed for swimming with a list and drift. A propeller with a large diameter and wide blades will greatly slow down sailing. And the hull of the boat itself, designed to move at one and quite high speed, will have more resistance than the hull of the yacht.

From what has been said, it is clear that a boat under sail cannot achieve the same tacking and sailing qualities as a yacht, just like a yacht with a powerful motor can achieve the speed of a boat of the same size and with an engine of the same power. When designing a motor sailboat, a compromise must be found between these types of vessels and preference should be given to one or another individual quality.

Features of the movement of displacement ships at high speed. Every yachtsman, of course, knows that when a yacht moves, waves form around its hull. The height and length of these waves increase as the speed of the yacht increases (Fig. 1), and their number, which fits the length of the vessel, decreases. Sometimes you can see how racing yachts, for example, the class "P-5.5", go only on one wave (neighboring ridges are located in the bow and stern, and the sole is near the midship). This position means that the yacht has reached its maximum speed if its weight, contours and sail area do not allow it to go into planing mode. It seems that the ship is not able to climb the crest of the wave that it created itself. Nevertheless, light yachts - "Flying Dutchmen" and "stars" - in a fresh wind can overcome this barrier and planing, being on only one ridge, which is now located near the midship. Similar phenomena are observed on boats with a gradual increase in their speed.

It is easy to see that the pattern of wave formation depends not only on the speed of travel, but also on the length of the ship: the shorter the ship, the lower the speed of the wave barrier phenomenon. Therefore, in shipbuilding, the speed of ships is usually characterized by relative speed, or the Froude number,

where v - vessel speed, m/s; L - length along the waterline, m; g - acceleration of gravity, equal to 9.81 m / s², √ - square root.

This value characterizes, first of all, the intensity of wave formation near the hull at a given speed and the proportion of motor or sail power necessary to create these waves. For example, if a yacht is said to be moving at a speed of Fr = 0.29, the shipbuilder knows that no matter how long it is:

Approximately two transverse bow waves fit along the length of the yacht;

The power required to create waves is about 50-60% of the total required engine power (the rest is spent on overcoming the friction of the hull skin on the water and the hull vortex resistance).

Similarly, when the Froude number Fr = 0.4 ÷ 0.5, the moment comes when the vessel goes on two adjacent crests of the same wave, and the resistance to movement from wave formation reaches 90% of the total hull resistance. This speed represents the very barrier that only light planing yachts or boats with the appropriate contours and engine power can overcome. On fig. Figure 2 shows a plot of the yacht's resistance (in the form of the towing power required to overcome it) on the relative speed. It can be seen that in the range Fr = 0.3÷0.5, the resistance increases sharply with the slightest increase in speed. That is why the power developed by the sails is usually only enough to achieve a certain speed v = 2.2÷2.4√L knots. (which corresponds to the relative speed Fr = 0.38÷0.39). It is obvious that an increase in the speed of a yacht under a motor beyond this limit without any change in contours and a decrease in displacement will require an exorbitant increase in the power of the motor, and, consequently, an increase in its dimensions and weight, fuel reserves and displacement of the vessel as a whole.

Therefore, the speed of motor sailboats under the motor usually does not exceed the value v = 2.7√L. At this speed, you can get a satisfactory compromise between sailing qualities and propulsion under the engine.

In table. 2 shows the values ​​of the maximum and economic speed for motor-sailing yachts of various lengths according to DWL.

Table 2. Economic and maximum speeds of motor sailboats

When the ship moves at a speed above v = 2.7√L (Fr = 0.45), it forms, as already noted, a wave whose length exceeds the length of the ship, and the top is near the ship's midsection. Such a wave causes the vessel to trim to the stern, which, in turn, leads to an increase in the stern wave and, ultimately, to a sharp increase in water resistance to the movement of the vessel. In order to counteract trim, the stern of the vessel must have a wide transom and a flat bottom with gentle, almost horizontal buttocks. Due to this shape of the hull, a lifting force is created on the bottom, which levels the vessel, and with a further increase in power, it squeezes it out of the water, putting it into planing mode.

However, such contours of the stern are unacceptable for a motor sailboat, since when sailing with a roll (under sail), a large amount of stern causes a trim on the bow; as a result, the hull and keel of the yacht take the wrong position (angle of attack) when tacking and do not allow you to go steeply to the wind, and the stream formed behind the stern slows down the movement of the yacht.

Thus, having considered the features of the movement of heavy displacement vessels, which are usually tourist boats and yachts, we can draw the following conclusions:

The maximum achievable speed under sail for yachts is v = 2.2÷2.4 √L knots;

The engine power for a motor-sailing yacht with good tacking qualities should not exceed the value necessary to develop a speed v = 2.7 √L knots;

If a vessel is designed for high speed under power, it cannot be expected to have a satisfactory ability to tack.

Types of motor sailboats. Depending on the magnitude of the speed developed under the motor, and the role assigned to the sail or motor on a given vessel, all motor-sailing yachts can be divided into four main types.

I. Boats with an auxiliary engine. These are, in essence, ordinary cruising yachts, on which the motor plays a secondary role and is installed solely to facilitate entry and exit from the harbor, passage along the fairway, mooring, etc. The motor is chosen with minimal power, weight and dimensions. The speed under the motor in these cases does not exceed the value v = 1.8÷2.0 √L knot. (5-6 knots for most cruising yachts). The supply of fuel is also small, as a rule - for 20-30 hours. continuous operation of the engine, i.e. 100-200 miles.

The propeller to reduce resistance when running under the motor must have a minimum allowable diameter, narrow blades; usually place the propeller in the sternframe and rudder window.

The power of the auxiliary engine to achieve the specified speed is usually 1.2÷2.0 hp. With. per 1 ton displacement of the yacht. In this case, the weight of the motor does not exceed 3% of the displacement D, and the weight of the fuel reserves is 2% D. Therefore, the installation of the engine does not affect either the stability of the yacht or its tacking qualities. The weight of the false keel is maintained within 35-45% D.

II. with a preference for sailing qualities. When designing ships of this type, the designer usually strives to combine good tacking and propulsion under sail with a relatively high speed under power. One of these sailboats is shown in Fig. 3.

From yachts with an auxiliary motor, motor sailboats of the type in question differ in a more powerful motor (4÷5.5 hp / t) and, therefore, more speed stroke under the motor (2.2÷2.4√L knots), as well as an increased cruising range under the motor (up to 800-1000 miles for a yacht about 15 m long). Here the engine plays the same basic role as the sails, so the driving performance under the motor is given more attention. Often this type of yacht is called "50/50" (i.e. 50% of the yacht and boat).

On fig. 4 shown theoretical drawing motor sailboat, the main elements of which are indicated in Table. 3 (for comparison, data for a yacht of type I and a seaworthy boat with the same length according to DWL are shown next).

Table 3. Comparison of the considered types of ships

The contours of this motor sailboat are characterized by a shallow draft, short overhangs, a straight keel line, a transom stern that is wider than usual for yachts. The collapse of the frames in the bow and the outlines of the deck line are typical for motor yachts. The waterlines in the bow have a sharper entry angle (pointing), and the buttocks in the stern rise at a smaller angle to the waterline than that of a sailing yacht.

In connection with the installation of a powerful diesel engine, the weight of the false keel has been reduced to 30% D. The propeller is located in a large stern window, behind the vertical star post and has a significant diameter. This arrangement of the screw contributes to its efficiency increase and more complete use of power. Naturally, the reduced stability, as well as the trimmed underwater part of the DP, do not allow carrying full windage. On larger yachts of this type, a centerboard is often installed to improve the tacking qualities. The daggerboard option is a good compromise between sail and motor: when sailing under the engine, the daggerboard can be removed and thereby reduce the wetted surface of the hull.

To reduce air resistance when sailing, the volume of superstructures is sought to be minimized.

Of the relationships characteristic of this type of ships, one can also note the parameter

S 1/2 /D 1/3 = 3.5÷3.9,

while for type I yachts this value is higher (3.8÷4.4).

III. Motor-sailing yachts with a preference for boat qualities. In this case, the speed under the motor plays a primary role and reaches v = 2.7÷2.9 √L knots. As already noted, at this speed the ship receives a strong trim to the stern, so a wide transom stern with gentle lines of buttocks is preferable. The required engine power increases to 6.5 ÷ 9 hp / t, which makes it necessary to reduce the weight of the false keel to 15-25% D.

The draft is taken such as to accommodate a propeller of the required diameter (usually T=11÷13% L).

Since the shape of the hull still turns out to be unsuitable for steep tacking, they abandon the centerboard device, and increase the volume of superstructures. The sail area is relatively small:

S 1/2 /D 1/3 = 2.8÷3.4.

Sails are intended mainly for sailing full courses in a fresh wind and stabilizing the movement of the yacht on rough seas.

An example of a vessel of the type under consideration is the "Search" (Fig. 5 and 6) - a seaworthy yacht designed for long-distance voyages. She has good sailing performance both under power and under sail. The main elements of the yacht are given in table. 4 (for comparison, the data of the yacht with the auxiliary motor "Khortitsa" are shown nearby).

The hull of the yacht, by the nature of the contours, approaches the shape of a seaworthy boat (straight keel line, short overhangs, high freeboard, stern with a wide transom partially submerged in water). The propeller with a diameter of 850 mm is located behind the sternpost in a large window.

"Search" carries half the windage than a yacht with an auxiliary motor. The sails are relatively wide, with a low center of sail, designed for sailing in full courses.

Table 4. Comparison of two representative vessels

IV. Boats with auxiliary sails. If the boat is designed to sail at sea or on big lake, it makes sense to install sails of a small area on it, first of all, to improve seaworthiness on a wave (primarily to increase course stability, mitigate pitching and make it possible to lie in a drift). In a fresh wind, the boat can go (without a motor) at a low speed to the backstay or even tack, moonlighting with the engine. The sail area is assumed to be about 5 m² / t for boats with a displacement of up to 5 tons; 4÷3 m²/t for boats with a displacement of 5-10 t and 2.5÷3 m²/t for large ships.

As an example, let's call the Passagemaker seaworthy boat (Fig. 7 and 8), designed for long-distance sea and ocean voyages. Engine power is small - only 40 liters. With. (1.6 hp/t); accordingly, the speed is low - 7.5 knots (2√L), but the fuel supply is 5.5 tons (22% D), which provides a huge cruising range - 2400 miles. Only 2.3 kg of fuel is consumed per mile.

The maximum length is 15.3, and according to the design waterline - 14.0 m; width 4.9 m, draft 1.53 m, displacement of the Passagemaker 25 tons, and the weight of the false keel is only 3.3 tons (13% D). The sail area is about 50 m².

The contours of its hull are typical for motor seaworthy yachts with low speed (sharp waterlines at the bow, a bottom with a large deadrise at the transom, a straight keel line). The high freeboard and developed superstructures are also typical. This theoretical drawing can be taken as a basis for designing a motor sailboat of a shorter length (9-10 m).

It should be noted that yachts of this type are often equipped with low bilge keels, which significantly reduce drift under sail, and in addition, serve as effective roll dampers.

D. A. Kurbatov, 1966

WIND DRIVING FORCE

The NASA website published very interesting materials about various factors influencing the formation of lift by an aircraft wing. There are also interactive graphical models that demonstrate that lift can also be generated by a symmetrical wing due to flow deflection.

The sail, being at an angle to the air flow, deflects it (Fig. 1d). Going through the "upper", lee side of the sail, the air flow travels a longer path and, in accordance with the principle of the continuity of the flow, moves faster than from the windward, "lower" side. The result is less pressure on the lee side of the sail than on the windward side.

When gybeing, with the sail set perpendicular to the direction of the wind, the pressure increase on the windward side is greater than the pressure decrease on the lee side, in other words, the wind pushes the yacht more than it pulls. As the boat turns sharper into the wind, this ratio will change. So, if the wind is blowing perpendicular to the boat's course, an increase in sail pressure to windward has less effect on speed than a decrease in pressure to leeward. In other words, the sail pulls the yacht more than it pushes.

The movement of the yacht occurs due to the fact that the wind interacts with the sail. The analysis of this interaction leads to unexpected, for many beginners, results. It turns out that the maximum speed is achieved, not at all when the wind blows exactly behind, but the wish for a “tailwind” carries a completely unexpected meaning.

Both the sail and the keel, when interacting with the flow, respectively, of air or water, create a lifting force, therefore, to optimize their work, wing theory can be applied.

WIND DRIVING FORCE

The air flow has kinetic energy and, interacting with the sails, is able to move the yacht. The work of both the sail and the wing of an aircraft is described by Bernoulli's law, according to which an increase in the flow velocity leads to a decrease in pressure. When moving in the air, the wing separates the flow. Part of it bypasses the wing from above, part from below. An aircraft wing is designed so that the airflow over the top of the wing moves faster than the airflow under the underside of the wing. The result is that the pressure above the wing is much lower than below. The pressure difference is the lift force of the wing (Fig. 1a). Due to the complex shape, the wing is able to generate lift even when it cuts through the flow, which moves parallel to the plane of the wing.

The sail can only move the yacht if it is at a certain angle to the flow and deflects it. The question remains as to which part of the lifting force is associated with the Bernoulli effect, and which is the result of flow deflection. According to the classical theory of the wing, the lift force arises solely as a result of the difference in flow speeds above and below the asymmetric wing. At the same time, it is well known that a symmetrical wing is also capable of creating lift if it is installed at a certain angle to the flow (Fig. 1b). In both cases, the angle between the line connecting the anterior and posterior points of the wing and the direction of the airflow is called the angle of attack.

The lift force increases with the angle of attack, however, this dependence only works when small values this corner. As soon as the angle of attack exceeds a certain critical level and the flow stall occurs, numerous vortices are formed on the upper surface of the wing, and the lift force sharply decreases (Fig. 1c).

Boaters know that gybe is not the fastest course. If the wind of the same strength is blowing at a 90 degree angle to the course, the boat is moving much faster. On a jibe, the force with which the wind pushes against the sail depends on the speed of the yacht. With maximum force, the wind presses on the sail of a yacht standing still (Fig. 2a). As the speed increases, the pressure on the sail drops and becomes minimal when the yacht reaches its maximum speed (Fig. 2b). The maximum speed on a jibe is always less than the wind speed. There are several reasons for this: firstly, friction, in any movement, some of the energy is spent on overcoming various forces that impede movement. But the main thing is that the force with which the wind presses on the sail is proportional to the square of the speed of the apparent wind, and the speed of the apparent wind on the gybe is equal to the difference between the speed of the true wind and the speed of the yacht.

On a gulfwind course (at 90º to the wind), sailing yachts are able to move faster than the wind. Within the framework of this article, we will not discuss the features of the pennant wind, we will only note that on the Gulfwind course, the force with which the wind presses on the sails depends to a lesser extent on the speed of the yacht (Fig. 2c).

The main factor that prevents the increase in speed is friction. Therefore, sailboats with little drag can reach speeds much faster than the wind, but not on a gybe. For example, a buer, due to the fact that skates have negligible slip resistance, can accelerate to a speed of 150 km / h with a wind speed of 50 km / h or even less.

The Physics of Sailing Explained: An Introduction

ISBN 1574091700, 9781574091700

We have successfully completed the transition through Atlantic Ocean December 23rd. We covered about 2750 miles in 21 days and 21 hours. It turns out that the average speed of the yacht was 5.2 knots, such a modest result was due to the fact that the average wind was 9 knots, so we have nothing to be ashamed of. Compared to the average Pacific crossing speed of 6.9 knots, Slick is a bit confused. I do not know anyone who would show better results.

We were an hour late because Matthew's filling fell out while we were eating a farewell horribly undercooked paella. That's another story. The transition started normally, the wind did not want to help us, so we had to go a little under the motor. In the morning we decided to stop at La Restinga (Hierro Island). In this beautiful little port we refueled full tank fuel and move on. It's a pity we left so quickly, because it was a delightful secluded town on a wild volcanic island.

As soon as we got out, the wind picked up, so we covered a long distance on the first night. It was cold, but everything was fine with the wind. Several days passed, the wind became more and more changeable, we had to go more with the engine running. When we downloaded the first GRIB forecast, it showed that we had problems - a baric trough appeared next to us. Luckily my friend Norm from CYC kept an eye on the weather and kept us updated even without GRIB. To get out of the hollow, which took on the scale of a cyclone, it was necessary to quickly go south. Maybe this is the very last seasonal hurricane that everyone is so afraid of, who crosses the Atlantic in December? The storm had every chance of becoming one, and I prudently headed south.

By the time the storm broke, we hadn't had time to get south of it. We were touched southern edge storm, and for the next 24 hours the wind reached a force of 44 knots, were big waves It was raining constantly, and lightning flashed in the distance. Slick bravely weathered the storm. The cyclone faded away and left behind a motionless area of ​​low pressure that blocked all winds for us. We were calm for six days. If I had acted differently, things could have been much worse. We went under the motor, sometimes set sails, drifted. It turned out that in such conditions the yacht went fastest when the sails were set up strangely: Heidi's big gennaker was on the bowsprit, the 142% genoa was set to leeward, and two reefs were taken on the mainsail so as not to obscure this whole shaky structure from the wind. So we did 5 knots gybe with a wind force of 8-10 knots. If the wind picked up a little, then we went faster. With this sail setting, the overall speed to the destination was the highest.

The wind was picking up strength, then dying, he teased us. We kept the engine running for a while until we ran out of fuel, leaving five gallons in reserve for emergencies. We also got into the opposite direction. According to all maps and sailing directions, we had to be in a fair current with a strength of from half a knot to two. We had half a node towards. We drifted, the nights were long, and we walked no more than 86 miles in a day. I complain too much, I've met people in pacific ocean who crossed it at an average speed of 4 knots.

The current suddenly changed direction in the direction we needed, and the wind increased for a long time. First we did 125 miles in a day, then 150, and in the last two days before Barbados, we did almost 180 miles a day. On approach to land, we slowed down in order to enter the bay in daylight. I think that if we had pushed, Slick would have covered all 200 miles in a day.

This did not happen. We slowed down, not because we were approaching the shore, but because about a week ago I discovered that the ceiling of the saloon was diverging at the central bulkhead in front of the chart table. At night, I woke up in fear at every creak and thought that something important had broken. After sailing 30,000 miles across the ocean and facing storms, Slick showed his traitorously flexible Beneteau nature. I did not want to urge an old friend, fearing that he would break something for himself. The mainsail broke and slowly spread. On the mainsail, the armor jumped out and almost flew away, the fasteners of the slider frayed and broke, the clew of the first reef was torn. There were no other breakdowns. But the ill-fated crack in the wardroom did not give me peace. At the start of the trip, the bookshelf guard rail was too small and fell out. Now she is holding on again. When the yacht is going through a wave, especially under the engine, the hull breathes. It is strange that under sail it creaks less, which cannot but rejoice me.

Again, not everyone had this. light wind. My buddy Kress on the yacht Conversations left the day before us and rode the tail of a cyclone that hit Tenerife. He was incredibly lucky with the winds, and even after fleeing south from the baric trough, his lead was so great that he was only calm for two or three days. Another yacht, Scope, left the day after us, and unlike us, they no longer got into a tailwind from a cyclone, but first sailed a week against the wind, and then for a whole week they had a calm, accompanied by constant thunderstorms with lightning. Another yacht capsized and dismasted just because they left at the wrong time. A difference of one day can affect the transition very much if you leave a day earlier or stay a day late.

To be continued …

KEEL

It has long been observed that yachts with more lateral drag sail better than those with the same frontal and lateral drag. The designers were tasked with increasing the lateral resistance without changing the frontal one. Keel turned out to be a very successful decision.

Over the years, shipbuilders have experimented with its shape and size in an effort to achieve maximum efficiency. It turned out that a long and narrow keel works best, and this is due to the fact that its main function is to create lift when moving in a stream of water. The keel is symmetrical, so it is only able to generate lift if the direction of motion does not exactly coincide with the longitudinal axis of the yacht, i.e. the ship moves with some lateral drift. It is due to lateral drift that the keel crosses the flow at an angle called the angle of attack. The consequence of this is an increase in the flow path from the "upper", windward side. Due to this, in accordance with the theory of the wing, on the windward side there is an increase in the flow velocity and a decrease in pressure. On the leeward side of the keel, there is a decrease in the flow velocity and, accordingly, an increase in pressure.

A long and narrow wing works much more efficiently than a wide and short wing. This statement is true for both the sail and the keel, which, in fact, are wings, only located vertically. The explanation for this phenomenon is the vortices that form at the end of the wing and create additional resistance to movement. With the same area, a longer and narrower wing has more lift, and the cost of vortex formation is less.

Due to the higher density of water compared to air, the role of the keel shape is especially important. With the same hydrodynamic properties, a narrow and long keel can have a much smaller wetted surface area, and hence less resistance. The most striking example of the application of this principle are yachts contenders for the America's Cup, but for a regular pleasure or cruising yacht, such a keel can become a serious problem due to the depth limit in their navigation zones (Fig. 3).

FORCES OF RESISTANCE

There is a rather complex set of forces that impede the movement of the yacht. Water resistance to hull movement. Since water molecules are attracted to each other and to the surface of the body (van der Waals forces), any movement is accompanied by the expenditure of energy to overcome these forces. The layer of water at the very surface of the hull is called the boundary layer, the speed of its displacement is maximum. As you move away from the surface of the hull, the rate of displacement of water layers decreases, i.e. there is a speed gradient. The cost of energy to overcome the resistance of water is proportional to the area of ​​the wetted surface and the speed of movement.

The forces of friction of a liquid are fundamentally different from the forces of friction between solids. In order to reduce friction between the surfaces of solids, they can be polished and lubricated. This will reduce the protrusions on the surface and replace contact between solid parts with contact with lubricating fluid molecules. Lubrication of the case does not make sense in principle, since it moves in a liquid medium. Polishing the body also does not eliminate the need to separate water molecules. Conclusion: the most effective way to reduce friction is to reduce the area of ​​the wetted surface.

Formation of turbulence is a well-known flow phenomenon. When moving at a low speed, there are no disturbances, turbulences in the flow, it is smooth, i.e. laminar. As the flow velocity increases, displacements of molecules relative to each other appear in it, uniformity disappears, and vortices appear. When the critical level is reached, the number of eddies sharply increases, and the flow stalls. As a result, the pressure difference on different sides of the wing decreases, which leads to the disappearance of lift. At the end of the 19th century, the English engineer Osborne Reynolds proposed a formula, the result of which is a dimensionless quantity characterizing the moment of transition from laminar to turbulent flow. It turned out that at a typical yacht speed of about 5 knots (2.4 m / s), turbulence begins for any yacht longer than half a meter.

Usually turbulence increases the total drag by a factor of four to five! An uneven, rough surface leads to the fact that turbulence occurs earlier and is more pronounced. Therefore, for fast yachts, it is very important that the surface of the hull is smooth. It is considered sufficient that the roughness of the case does not exceed 0.05 mm. Usually such a surface can be achieved if the sanded surface is covered with two layers of good paint.

For a wind speed of 5 m/s, which can be called typical, turbulence occurs with a sail width of more than 3 meters. Sailing stall is also very dangerous. If turbulence is formed during the movement of the air flow along the surface of the sail, the pressure difference on different sides of the sail disappears, along with it, the lifting force (thrust) of the sail also disappears.

End swirls, are another drag-increasing factor. They occur at the end of the wing, and on the yacht at the top of the sail or the bottom of the keel. Both air and water, moving along the sail or keel, will tend to equalize the pressure on opposite sides of the sail or keel, respectively, moving from an area of ​​high pressure to an area of ​​low pressure. Figure 4 shows a diagram of such a movement for the keel. On the one hand, the flow angle goes a little up, on the other a little down. As a result of the fact that on the trailing edge of the keel or sail the flows from both sides meet at a certain angle, the formation of vortices occurs, which intensify as they approach the top, and here they form an end vortex. The tip vortex leads to a redistribution of lift along the span of the wing, reduces its effective area and elongation, and reduces its dynamic quality.

On fig. Figure 5 clearly shows how vortices form at the tops of the masts during the race, which took place in dense fog, and Figure 6 shows the same vortices on the wings of the aircraft.

The wider the keel, the more energy is spent on vortex resistance. By making the keel narrow and long, designers increase the lift-to-drag ratio. The same happens with narrow and high sails, especially when moving in sharp courses. Long and narrow wings for gliders are made for the same reason. In order to reduce the braking associated with the formation of end vortices on the keel, additional horizontal wings are made. In aviation, such a device is called a winglet (Fig. 7), it helps to achieve the optimal distribution of lift over the wing area. Wing theory recommends the use of an elliptical or cone-shaped trailing tip, such as a bulb at the end of a keel, to minimize induced drag.

The keel of modern, non-racing yachts is a compromise between a convenient short and wide keel and a very efficient, high hydrodynamic, narrow and long keel, but difficult to use outside the racing range. A different kind of resistance results from water flow deviations during the course of the ship. First of all, it depends on the geometry of the case. It is clear that a narrow body has less resistance than a wide one. Any boat is a compromise between a minimum of resistance and providing the necessary space for passengers and cargo. For centuries, shipbuilders have searched for the ideal shape for a given volume in an effort to provide minimal hull drag. Even Isaac Newton dealt with this issue. The conclusion he came to is that the best form for the body is an ellipsoid of revolution with a truncated cone attached at the front.

Spatial computer modeling and hydrodynamic tests have shown that the optimum is a hull that smoothly expands from the bow and remains fairly wide at the stern. To ensure a smooth flow at the stern, many designers narrow and raise the rear of the hull. If the flow at the stern is not smooth, laminar, turbulence will create a significant resistance to movement.

BODY SPEED.

When moving, the hull creates a wave, the length and speed of which depends on the speed of the yacht. As soon as the movement begins, several short waves are formed on the water, which move along the hull. As the speed increases, the length of these waves also increases, and the number, along the length of the hull, becomes smaller (Fig. 8a). At some stage, the yacht reaches a speed at which the wavelength becomes equal to the length of the yacht's hull, i.e. a ridge near the bow, a depression in the middle of the hull, and a second ridge at the level of the stern (Fig. 8b).

With a further increase in the speed of the yacht, the wavelength also increases, therefore, the second crest will move further and further back, astern. As the second crest shifts back, the stern sinks into the depression between the crests. If you look at the hull from the side, it turns out that the bow is turned up, the stern is lowered down, and the yacht must constantly climb the wave, while the resistance to movement increases dramatically (Fig. 8c).

This type of resistance is called wave resistance. Of course for motor boat with a powerful engine and a flat bottom, the speed at which the stern reaches the middle (trough) of the wave is not a limit. Adding engine speed motor yacht, you can increase the speed and switch from displacement to planing mode. However, most sailing yachts do not have this capability, and in most cases the hull geometry does not provide for a planing mode. Therefore, for most yachts of the traditional form, the resistance of the wave turns out to be an insurmountable obstacle. This applies not only to sailing yachts, but to barges, tankers, large passenger ships, in a word, to everyone who is not able to plan.

The speed at which the wavelength becomes equal to the length of the hull along the waterline is called the speed of the hull. A further increase in speed is fundamentally possible, but without switching to a gliding mode, this is associated with very high energy costs. In practice, it is very rarely possible to accelerate the yacht to a speed one and a half times the speed of the hull.

Hull speed is determined by the formula - v=1.34√L,

where v is the speed in knots, L is the length in feet. So for a yacht with a waterline length of 20 feet (6 m), the maximum speed will be 6 knots. For a large cruising yacht with a 40ft (12m) waterline, the speed would be around 8.5 knots. For a 300 foot warship, the hull speed is 23 knots.

Comparing all the factors preventing the yacht from moving, we find that friction accounts for more than a third of the total resistance, another third is due to wave formation, about 20 percent is due to the formation of turbulences at the surface of the hull, 10 percent is the resistance associated with the formation of vortices at the trailing and lower edges keel. The rest falls on the resistance of the surface part (resistance of the spars, air turbulences formed by the sail, etc.). Of course, the ratio of these components can vary significantly depending on the shape of the hull, the conditions in which the yacht is moving, the course relative to the wind, etc.

Summing up, we can formulate the following rules - the faster the yacht moves, which has a longer and narrower hull, more sail area and less area wetted surface. Of course, such simple rules can lead designers to make long boats with cabins that do not provide even minimal comfort. But any design decision is a compromise between mutually exclusive wishes. For jibe it is desirable to have wide square sails that will easily capture the wind and a keel of minimal size. In contrast, high, narrow sails work best for upwind because they provide the best balance of lift and vortex loss. The keel on sharp courses should be long and narrow to create maximum lateral drag with minimum wetted surface. But such a keel is very uncomfortable off the race track or just in shallow water. Short keel with bulb or horizontal winglets, an excellent compromise that satisfies most boaters.

The Physics of Sailing Explained: An Introduction

The speed of the yacht on the surface of the reservoir depends on a huge number of factors. They are the propulsion of the vessel, which has a huge impact on the speed of the yacht under sail. Propulsion depends on the resistance to movement from the hull of the vessel and the driving force that is created by the sails. At low speeds (up to 2 knots), up to 80% of the driving force power is spent to overcome the friction force of the hull on the water, which significantly affects the speed of a sailing yacht. Among the factors affecting the average speed of a sailing yacht are the friction force, the size of the wetted surface, wave resistance, shape resistance.

When it comes to offshore sailing trips, it is usually fixed the distance that the yacht travels per day, and not the average traffic intensity. sailing ship. But the desire to measure and compare everything, or the simple habit of people to translate the speed of movement into the concept of speed, forces the “sea wolves” to adapt to such terms.

What slows down ships at different speeds?

The influence of various braking factors on the intensity of the movement of a vessel under sail changes with a change in speed. At low speeds, the friction of water on the surface of the vessel slows down the movement as much as possible, while accelerating, the effect of wettability increases. This concept is defined by the surface area of ​​the ship's contact with water. Experienced yachtsmen often heel the vessel while moving to reduce the wetting surface, which gives a gain in speed.

At 7 knots and above, the influence of wave resistance increases, which is determined by the magnitude of the water surface being raised by the bow of the vessel. This characteristic affects the speed of the sailing yacht together with the amount of form drag. The decrease in shape resistance largely depends on the care of the vessel, the smoothness of its surface, the ratio of the length, width and streamlining of the sides and underwater parts - the keel and rudder.

Exactly the same factors affect the movement of other types of sailing vessels for travelers, among which catamarans can be distinguished. The intensity of the movement of sailing catamarans is quite comparable with the movement of the ship. A catamaran under sail can lose in other characteristics, this concerns the stability of the vessel (there is a risk of trim on the bow at high speed - fall on the cheekbone) and crew comfort (but this is an amateur question). But if this problem is solved, then the speed of sailing catamarans is much higher than the ships under sail due to the smaller wetted surface and wave resistance.

The intensity of the movement of a sailing vessel: measurement features

The speed of a sailing yacht is measured in knots. Not in knots per hour, not in miles per hour, but simply in knots. This is the law. And he has an explanation. The very principle of measuring the speed of a yacht is to use a sector log, which is a triangular plank thrown overboard from the stern. The plank carried along a thin cable - a line, on which knots were tied at a distance of 50 feet from each other. The measurement lasted a certain time - from 15 s to a minute. By counting the number of knots that left behind the log in the allotted time, it was possible to find out the intensity of the vessel's movement, which, of course, was determined in knots.

The wind speed for a sailing yacht is also not measured in an unambiguous way, but is a conditional concept and is tied to different points. The wind blows much stronger as it rises above the ground. This is due to the deceleration of air flows when interacting with the water surface. Therefore, the intensity of the movement of the yacht under sail directly depends on the height of the sail. In addition, the direction of the wind does not coincide with the direction of movement of the ship, which experiences many other effects of air currents, in addition to just wind in the concept of land people. When intensified, the wind always blows in gusts, it does not have a calm and constant pressure, as at calm speeds. Therefore, for yachts there is the concept of pennant wind, the direction of which is determined by the pennant attached to the mast of the vessel under sail.

Speed ​​records

And yet people are trying to set a speed record on the water. Such competitions have been held since 1972 and are gaining popularity every season. Max speed sailing yachts are still being shown to not quite ordinary participants - surfers. The explanation for this is simple - the board and the helmsman have the best performance in all characteristics that determine speed on the water: they have less friction, a minimal wetting surface with better polishing, there is no wave resistance, and sail control is as fast and sensitive as possible. Today, speed records are being conquered by windsurfers and kitesurfers and are approaching 70 knots or 130 km/h! Sailing yachts and catamarans have something to strive for!