Propulsors improved simultaneously with the advent of new types of vessels and ships.

paddle

With the advent of the first small boats, man realized that he would need a means that would push his vessel. Initially, these were oars, which, by immersing them in water and moving them, produced the desired effect - the boat moved. The need for speed forced ancient shipbuilders to increase the number of oars and rowers. A striking example of this is a galley, up to 12 meters long, with up to six oarsmen from among slaves or coasters on each of the 96 oars.

kochet

Oars come in roller, pair and double-blade types. They are used on boats, dinghies and other watercraft as a last resort for movement. During rowing, the middle part of the oar is inserted into the hole - the kettle, where it is fixed and creates a stop.

ACTIVE MOTORS

sail

We know that for thousands of years, sailors were aware of another type of propulsion - the sail. It is also an ancient and popular type of propulsion that uses wind power. Basically, there are two types of sails: straight - trapezoidal in shape, located symmetrically relative to the mast, and oblique - triangular or trapezoidal in shape, which are attached to one side of the mast.

A straight rig is one that has straight main sails (barque, barquentine).

Vessels with oblique rigs are those whose main ones are oblique sails (schooner, iola, ketch, etc.).

Yachts are most often equipped with triangular sails, which are called “Bermuda” sails.

yacht with Bermuda sails

There are also mixed sailing equipment, in which sails of all the above types are used.

mixed-rigged ship

Another type of sail that has become widespread in our time can be considered a kite. In essence, this is also a sail, but of a slightly different shape. In the shipping company Beluga Projects“This type of propulsion is already saving them on fuel costs for commercial vessels.

cargo ship of the Beluga Projects company

Forced to constantly visit areas of the ocean with developed storm conditions in search of wind, they often found themselves in severe storms and storms. Over time, technical imperfections played a role, and a further increase in the size of merchant ships could no longer be supported by sailing ships - they reached their maximum. They were replaced by other technically more advanced ships that met the needs of that time, and became museum ships.

JET PROPULSIONS

paddle wheel

paddle steamer, Vancouver, Canada

On the first steamships, shipbuilders began to use a paddle wheel as the main propulsion device. But this is perhaps the most unsuccessful of all movers. Due to the numerous shortcomings of the paddle wheel, which included frequent breakdowns and low efficiency due to “jumping out” of the water during rolling, the paddle wheels did not perform their functions conscientiously and took last place among other types of propulsors.

appearance of the propeller

The idea of ​​creating a perfect and universal propulsion, as always, was not new, you just had to be in the right place and right time. Such a person turned out to be Isambard Brunel, to whom, in my opinion, shipbuilders are indebted to this day. Despite numerous opinions of skeptics, he, having studied in detail the work of the invention of the ancient Greek scientist Archimedes, created a propeller, the operation of which he demonstrated on a steamship " SS Great Britain».

Since then this mover received the widest distribution. Made from various materials By changing the number and angle of blades, the propeller was improved and took a leading position among other propellers.

So, a propulsion device is a device that converts power from an engine (energy source) into the work of forward movement of a ship or vessel.

CLASSIFICATION OF PROPULTORS FOR SHIPS AND VESSELS

Distinguish active propulsors: sails that provide the movement of the vessel due to the direct influence of the force created by the energy source - wind, and reactive, creating a driving force by throwing masses of water in the direction opposite to the movement of the ship.

The latter are divided into lobed (wheeled, screw, fin, winged) And water-flowing (water-jet and hydrojet).

BLADE MOTORS

Typical propeller screw consists of a hub with blades located on it. Its operation is based on the hydrodynamic force created by the pressure difference on the sides of the blades. Any concentric section of the blades represents an element of the aircraft's main wing. Therefore, when the propeller rotates, the same forces arise on each element as on the wing.

principle of operation of the propeller

The flow flowing around the convex side of the blade (suction side) is slightly compressed, and as a result its movement accelerates. The flow flowing around the concave side of the blade (discharge side), encountering an obstacle on its way, slows down the speed somewhat. In accordance with Bernoulli's law, on the suction side of the blade the flow pressure drops and a rarefaction zone appears. At the same time, on the discharge side of the blade, on the contrary, a zone of increased pressure appears. As a result of the pressure difference on the sides of the blade, a hydrodynamic force is generated. As a result of long-term research, it was found that the main part of the hydrodynamic force, about 70 percent, is created due to the vacuum on the suction side of the propeller blades and only 30 percent due to the pressure on the discharge side of the blades. The projection of the hydrodynamic force onto the propeller axis is the propeller thrust. This force is perceived by the blades, which transmit it to the ship or vessel through the hub and propeller shaft.

Since the blades have a helical surface, when the propeller rotates, water is not only thrown back, but also twisted in the direction of rotation of the blades. Meanwhile, the task of the propeller is only to throw away the water, without rotating it, creating a reactive impulse - the traction force. A significant portion of the power supplied to it from the engine is spent on twisting the flow and overcoming the resistance of rotation of the propeller in water. Therefore, the efficiency of the propeller, equal to the ratio of the power expended to create propeller thrust (net power), to the total power expended to rotate the propeller, will always be less than one.

Efficiency propellers fluctuates in the range of 0.5 - 0.7. The upper limit is considered very high and achievable on low-speed, large-diameter propellers. For high-speed propellers of small diameter, the efficiency rarely exceeds 0.5.

Propeller screw always coordinated with the engine, otherwise there will be a pointless loss of power. In addition, there are non-reversible motors that are not able to change the direction of rotation of the shaft. In such cases there is controllable pitch propeller. Its hub contains a mechanism that rotates the blades to a given angle and holds them in that position. Rotation of the blades allows you to change the traction force at a constant speed of rotation of the propeller shaft and vice versa, maintain a constant traction force at different frequencies of rotation of the shaft, and also generally change the direction of the thrust (reverse) with a constant direction of rotation of the propeller shaft.

To transmit high power, two- and three-shaft installations are often used, and some large ships, such as aircraft carriers, are equipped with four symmetrically arranged propellers. Sometimes guide nozzles are used, which, at low propeller speeds, provides an increase in thrust of up to six percent.

a) - a propeller with fixed blades; b) - adjustable pitch screw; c) - propeller in the nozzle; d) - coaxial counter-rotating propellers;

azipod

steering column

To increase the maneuverability of some vessels, universal propulsors, the so-called active rudders, called “ azipod" Steering column type " azipod"includes a small propeller with its own electric motor. Rotating around its axis, the screw creates a stop and thereby increases the torque acting on the steering wheel.

"Azipod" type propulsion device

Unfortunately, the high cost of the design limits the scope of application movers like " azipod”, but they are worth the money spent. They are used on icebreakers, modern cruise ships, oil drilling platforms and other types of ships.

fin propulsion

fin propulsion

To maintain the stability of a ship or vessel, shipbuilders equip their “creations” with small keel-shaped stabilizers protruding from both sides of the ship’s hull. In their image and likeness they are similar to the fins of huge whales, for which they received the appropriate classification. Each of them has a streamlined shape, thanks to which it cuts through the waves without slowing down the ship. The principle of operation is very simple - fin propulsors installed at an angle produce the same effect as the wings of an airplane - either immerse the ship's hull deeper or raise it higher. When waves try to tilt the ship one way or the other, the keel stabilizers tilt the hull in the opposite direction of the roll. This gives the vessel stability even in large waves.

wing propulsors

principle of operation of a vane propulsion

Wing propellers have found application, primarily in thrusters. They combine the functions of propulsion and rudder and represent a rotor installed at the same level with the bottom of the vessel and rotating around a vertical axis, along the circumference of which from 3 to 8 blades perpendicular to its surface, made in the form of wings, are located at equal angular distances. Rotating together with the rotor, the blades periodically rotate around their own axis. The blades are rotated in such a way that at each position a force is created on it, which has the greatest projection in the direction of movement of the vessel. This is achieved when the conditional perpendicular to the chords of the blades intersect at one point, which is the control center. Moving the control center along an axis perpendicular to the direction of movement of the ship changes the magnitude and sign of the stop. Thus, winged movers have the same properties as an adjustable pitch propeller. By arbitrarily moving the control center in a plane parallel to the waterline plane, you can change the direction of the stop vector in the range from 0 to 360 degrees. To rotate the blades and move the control center, a mechanical drive is used, located in the propulsion housing and controlled by a hydraulic system.

wing propulsion

In terms of efficiency, as well as complexity and weight and size characteristics winged mover inferior to propellers, and therefore used as an effective thruster.

They are used on vessels whose maneuverability is subject to increased requirements (tugs, fishing vessels, minesweepers, etc.).

WATER FLOW MOTORS

water jet propulsion

water jet propulsion

Water-jet mover(water jet) is a water pump impeller placed in a water flow channel through which water is thrown out at an increased speed along the axis of the propeller. The main advantages of such propulsors include: good protection from mechanical damage and the ability to avoid cavitation, protection from objects floating on the surface of the water area, less hydrodynamic noise compared to screw propulsors, which is very important for submarines. located inside or outside the ship's hull. The efficiency of a water jet propulsion system depends on the shape of the water conduits, the location and design of the water intakes.

They are usually used on ships operating in shallow waters, or serve as a thruster to improve the maneuverability of ships.

pump type propulsors

pump-jet type propulsion

In general, submarines began to use a new type of propulsion - pump-jet, which means pump-type propulsion. There are two types of them:

-mover pump type with pre-twisting - the stator (base of the nozzle) is located in front of the rotor;

-mover pump type with subsequent spin-up when the rotor is located in front of the stator.

types of pump jet propulsion

1) - rotor; 2) - nozzle; 3 - stator; 4) - base of the nozzle; 5) - stator-base of the nozzle;

The qualities of both types of propulsion are the same, but mover pump type with pre-twisting has better cavitation characteristics, although it is structurally more complex.

hydrojet propulsion

In a hydrojet propulsion system, the energy of compressed air or combustion products supplied to the water conduit through a nozzle is used to accelerate the flow of water. Feature such devices - the absence of a shaft line and a mechanical working element. There are:

thermal- direct-flow (steam-water mixture is formed in a chamber into which steam or hot gas is supplied, creating a driving force);

pulsating(piston type with a pulsating gas-water combustion chamber, with a reactive gas-water pipe of an explosive type, etc.);

ejection and others that use the energy of cold compressed gas, accelerating the flow of the water-air mixture. Used in civil shipbuilding.

HOW PROPELLERS ARE MADE

The most large propellers reach the height of a three-story building, and their manufacture requires unique skills. At the time when the screw steamship was created " SS Great Britain“It took up to 10 days to make the propeller molds. Today, thanks to the availability computer technology an automated manipulator does this in a couple of hours. The shape of the propeller is entered into the computer, and a diamond drill at the end of the manipulator cuts out a perfect copy of the blade from huge foam blocks with an accuracy of 1 mm. A mixture of sand and cement is then placed into the finished model to create an accurate impression. After the concrete has cooled, the mold, consisting of two halves, is joined together and metal molten to 3000 degrees is poured. The propeller cannot be made of anything. The propeller must be strong enough to withstand thousands of tons of pressure without corroding in salty seawater. The most common propeller materials are steel, brass and bronze. IN last years Plastics began to be used for the same purpose.

An alloy of non-ferrous metals for propellers, called " kunial" It has the strength of steel, but resists corrosion much better. Kunial can remain in water for decades without rusting. To give the alloy extreme precision, 5% nickel and 5% aluminum must be added to 80% copper, as well as 10% other metals. Melting is carried out at a temperature of 3200 degrees.

After passing quality control, a “cocktail” of molten metals is poured into a mold. To avoid air getting into the structure, the metal is poured in an even stream. After two days the mold cools down. The blades are then released from the mold.

The efficiency of a propeller depends on the smooth and streamlined shape of the blades. The surface of the part cast from the mold is imperfect and covered with casting crust. A laser meter is used to determine the layer thickness. After which the excess layer is removed using a tungsten carbide cutter. The propeller is then polished to a perfectly smooth surface until it is 1.6 micromm. As a result, the surface acquires the smoothness of glass.

Propeller screw- the product is purely individual and for each modern vessel or ship must have an optimal shape in order to slide and capture the required amount of energy, taking into account the operating conditions. The main problem of all propellers is cavitation. The thing is that under water, when they rotate on the blades, an area of ​​​​low pressure appears, in which water literally begins to boil, even at low temperatures. Therefore, the propellers are tested on special stands, where the optimal propeller operating parameters are selected and the correct blade angle is checked.

No matter how sad, but incredibly beautiful propellers doomed to hard work, hidden from human eyes under the waves of the sea. Thus, of all types of existing movers plays a leading role propeller screw, and there is no reason to believe that a more effective replacement will be found for it in the coming years.

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§ 13. Ship propulsors

On ships, the following propellers are used: propellers, winged propellers and water-jet propulsors. Sails, paddle wheels and other propulsion devices are also used.

According to the principle of operation, propulsors are divided into active ones, which include sails that directly convert wind energy into forward motion of the vessel, and reactive ones - all the rest, since the persistent pressure they create is obtained as a result of the reaction of masses of water thrown in the direction opposite to the movement of the vessel.

The most common due to their simplicity of design and operation, compactness, reliability in operation and the highest efficiency are propellers. Depending on the design, they are divided into two types: solid screws(the hub with the blades is manufactured together) and propellers with removable blades, used on ships sailing in ice. Such propellers are called fixed-pitch propellers, while propellers that have mechanisms that turn the blades in the hub and change the pitch of the propeller are called controlled-pitch propellers.

Step by step The length of a screw is the path in the direction of the axis that passes through any point on the surface of the screw in one revolution.

Fixed pitch propellers- VFSh (Fig. 27) are manufactured in one piece (one piece), cast, welded or stamped, and they consist of the following main elements: hubs, which is a bushing that fits onto the cone of the propeller shaft neck, and blades(from 3 to 6), radially located on the hub. The lower part of the blade connecting it to the hub is called the blade root; the upper part is the top or end; the surface of the blade facing the ship's hull is called the suction surface, the reverse surface is called the discharge surface, which in most cases is a regular helical surface. The intersection of these two surfaces forms the edges of the blades.

Rice. 27. Fixed pitch propeller (FPP) and scheme for creating thrust pressure by the elementary platform of the propeller blade.

Propellers of right and left rotation are used, they are distinguished by general rules: If the screw is screwed in a clockwise direction, then it is called a right-hand rotation screw, and if it is screwed in a counter-clockwise direction, it is called a left-hand rotation screw.

When the propeller rotates, its blades throw masses of water to one side. The reaction of this water is perceived by the pressure surface of the blade, which creates a thrust for the propeller, which is transmitted through the hub and propeller shaft to the thrust bearing, converted into a force that moves the vessel.

To understand how persistent motion occurs when the propeller rotates (Fig. 27), let us consider the forces that act on the elementary area of ​​its blade, moving in a circle with a speed v 0 And simultaneously moving with the ship at a speed v 1 . The angle a formed between the resultant of these forces v and the chord of the elementary area of ​​the blade under consideration will be the angle of attack creating on it lift R. If we decompose this force into components, then one component, the force P, acting in the direction of movement of the vessel, will be the thrust force, and the second, the force T, acting in a circle in the direction opposite to the rotation of the propeller, creates a moment relative to its axis, which is overcome by the ship's engine.

Rice. 28. Controllable pitch propeller (CPP) with a rotary crank mechanism for changing the pitch. 1 - propeller blades; 2- hub; 3- propeller shaft; 4 - slider with a rod; 5 - connecting rod pin; 6 - vane bearing; 7 - propeller fairing.

The use of adjustable pitch propellers allows the use of non-reversible main machines on ships with a simplified maintenance system, which reduces the wear of their cylinders by approximately 30-40% (arising in reversible machines from frequent changes in the operating mode and direction of rotation), allows for fuller use of the machines’ power and maintains high propeller efficiency value.

Rice. 29. Wing propeller: a - design diagram; b - placement of the propulsion device on the ship. 1 - carrier disk; 2 - rotating blades; 3 - driven gear that rotates the disk; 4 - hydraulic device for controlling the pendulum lever; 5 - pendulum lever, changing the position of the blades around its axis; 6 - propeller shaft with drive bevel gear.

Wing propeller(Fig. 29) is a structural device consisting of a horizontally rotating cylinder with 6-8 sword-shaped, streamlined blades vertically located on it, rotating around their axes with a pendulum lever controlled from the wheelhouse.

When the disk rotates on the blades, like on a wing, a lifting force arises, the component of which creates persistent pressure. When the blades are turned, the value of the thrust and its direction change, which makes it possible to vary the direction of movement of the vessel without the help of a rudder (a rudder is not installed on a vessel with this propulsion), as well as the amount of thrust of the propulsion from “Full forward” to “Full back” or to stop the vessel, without changing the speed and direction of rotation (without reverse) of the main power plant.

The efficiency of a winged propeller is almost equal to the efficiency of a propeller, but a winged propeller is much more complex in design. Protruding blades often break. However, in Lately This propulsion device is increasingly used, providing ships with good maneuverability, allowing them to work freely in narrow spaces.

Water jet propulsion belongs to a series of water-flowing propulsors. Modern water jet propulsors are made of three types: with the release of a water jet into water, into the atmosphere and with semi-underwater release.

The propeller works like a pump, drawing water into a channel through a pipe running into the bottom of the hull in front of the propeller. To protect against foreign objects getting on the screw, a protective grille is strengthened at the beginning of the channel.

To reduce losses from twisting the water flow by the propeller and increase the efficiency of the propulsion unit, a counterpropeller is installed behind the propeller. The direction of the vessel's progress is changed by shifting the reverse rudder.

The efficiency of such a propulsion device is only 35-45%, and the absence of any protruding parts in the underwater part of the vessel provides it with greater maneuverability in shallow water, in narrow waters and on clogged fairways. For a vessel with such a propulsion, even floating objects through which it moves freely are not an obstacle.

The listed advantages of water-jet propulsion made its use especially convenient on river vessels, primarily on timber rafting.

In recent years, water-jet propulsion has begun to be used on high-speed vessels, such as hydrofoils, which reach speeds of up to 95 km/h.

The use of modern steam and gas turbines allows the successful use of water-jet propulsion on large sea vessels, where, according to calculations, the propulsive efficiency can reach about 83%, which is 11% higher than the propulsive efficiency of a propeller designed for the same vessel.

The disadvantages of vessels with this propulsion include the loss of the vessel's carrying capacity by the weight of the pumped water and loss of volume interior spaces occupied by the channel.

How does a propeller work? The propeller converts the rotation of the engine shaft into thrust - a force that pushes the ship forward. When the propeller rotates, a vacuum is created on the surfaces of its blades facing forward - in the direction of the movement of the vessel (suction), and increased water pressure on those facing backwards (pumping). As a result of the pressure difference on the blades, a force Y arises (it is called lifting). By decomposing the force into components - one directed towards the movement of the vessel, and the second perpendicular to it, we obtain the force P, which creates the thrust of the propeller, and the force T, which generates the torque, which is overcome by the engine.

The thrust largely depends on the angle of attack a of the blade profile. The optimal value for high-speed boat propellers is 4-8°. If a is greater than the optimal value, then the engine power is unproductively spent on overcoming a large torque, but if the angle of attack is small, the lift force and, consequently, the thrust P will be small, and the engine power will be underutilized.

In a diagram illustrating the nature of the interaction between the blade and water, a can be represented as the angle between the direction of the velocity vector of the flow W flowing onto the blade and the discharge surface. The flow velocity vector W is formed by the geometric addition of the vectors of the translational movement velocity Va of the propeller together with the ship and the rotational speed Vr, i.e., the speed of movement of the blade in a plane perpendicular to the propeller axis.

Helical surface of the blade. The figure shows the forces and velocities acting in one specific cross section blade located at a certain radius r of the propeller. The circumferential speed of rotation V depends on the radius at which the section is located (Vr = 2× p × r× n, where n is the speed of rotation of the propeller, rev/s), while the translational speed of the propeller Va remains constant for any section of the blade. Thus, the larger r, i.e., the closer the section under consideration is located to the end of the blade, the greater the peripheral speed Vr, and therefore the total speed W.

Since side Va in the triangle of speeds under consideration remains constant, then as the blade section moves away from the center, it is necessary to rotate the blades at a large angle to the propeller axis so that a maintains its optimal value, i.e., remains the same for all sections. Thus, a helical surface with a constant pitch N is obtained. Let us recall that the pitch of the propeller is the movement of any point of the blade along the axis in one full revolution of the propeller.

The drawing helps to visualize the complex helical surface of the blade. During operation of the propeller, the blade seems to slide along guide squares, which have a different base length at each radius, but the same height - pitch H, and rises in one revolution by the amount H. The product of the pitch and the rotation frequency (Hn) is the theoretical speed of movement of the propeller along the axis.

Vessel speed, propeller speed and slip. When moving, the ship's hull carries water along with it, creating a passing flow, so the actual speed of the propeller meeting the water Va is always slightly less than the actual speed of the ship V. For high-speed planing motorboats the difference is small - only 2 - 5%, since their hull slides along water and almost does not “pull” it along with it. For boats traveling at an average speed, this difference is 5-8%, and for low-speed, deep-draft displacement boats it reaches 15-20%. Let us now compare the theoretical speed of the screw Hn with the speed of its actual movement Va relative to the water flow.

The difference Hn - Va, called slip, determines the work on the mouth of the propeller at an angle of attack a to the water flow having a speed W. The ratio of slip to the theoretical speed of the propeller as a percentage is called relative slip:
s = (Hn-Va)/Hn.

The slip reaches its maximum value (100%) when the propeller is operating on a ship moored to the shore. The propellers of light racing motorboats have the least slip (8-15%) at full speed; for the propellers of planing pleasure motorboats and speedboats, the glide reaches 15-25%, for heavy displacement boats 20-40%, and for sailing yachts with an auxiliary engine, 50-70%.

Light or heavy propeller. The diameter and pitch of the propeller are the most important parameters on which the degree of use of engine power depends, and therefore the possibility of achieving highest speed progress of the ship.

Each engine has its own so-called external characteristic - the dependence of the power removed from the shaft on the crankshaft speed when the carburetor throttle is fully open. Such a characteristic for the Whirlwind outboard motor, for example, is shown in the figure (curve 1). Maximum power of 21.5 l, s. the engine develops at 5000 rpm.

The power that is absorbed by the propeller on a given boat, depending on the engine speed, is shown in the same figure not by one, but by three curves - screw characteristics 2, 3 and 4, each of which corresponds to a specific propeller, i.e. a propeller of a certain pitch and diameter.

When both pitch and propeller diameter are increased beyond optimal values, the blades become caught and thrown back too much. a large number of water: the thrust increases, but at the same time the required torque on the propeller shaft also increases. Propeller characteristic 2 of such a propeller intersects with the external characteristic of engine 1 at point A. This means that the engine has already reached the limit - the maximum value of torque and is not able to turn the propeller at a high speed, i.e. it does not develop the rated speed and its corresponding rated power. In this case, the position of point A shows that the engine produces only 12 hp. With. power instead of 22 hp. With. This propeller is called hydrodynamically heavy.

On the contrary, if the pitch or diameter of the screw is small (curve 4), both the thrust and the required torque will be less, so the engine will not only easily develop, but also exceed the rated crankshaft speed. Its operating mode will be characterized by point C. And in this case, the engine power will not be fully used, and operation at too high speeds is associated with dangerously high wear of parts. It should be emphasized that since the propeller stop is small, the ship will not reach the maximum possible speed. This screw is called hydrodynamically light.

A propeller that allows a particular combination of ship and engine to fully utilize the power of the latter is called agreed upon. For the example under consideration, this agreed the propeller has characteristic 3, which intersects with the external characteristic of the engine at point B, corresponding to its maximum power.

The figure illustrates the importance of choosing the right propeller using the example of the Crimea motorboat with the Whirlwind outboard motor. When using a standard motor propeller with a pitch of 300 mm, a motorboat with 2 people. on board it reaches a speed of 37 km/h. With a full load of 4 people, the speed of the boat is reduced to 22 km/h. When replacing the propeller with another one with a pitch of 264 mm, the speed with full load increases to 32 km/h. The best results are achieved with a propeller having a pitch ratio H/D = 1.0 (pitch and diameter are 240 mm): the maximum speed increases to 40-42 km/h, the speed with full load is up to 38 km/h. It is easy to conclude about the significant fuel savings that can be obtained with a reduced pitch propeller. If with a standard propeller with a load of 400 kg, 400 g of fuel is consumed for each kilometer traveled, then when installing a propeller with a pitch of 240 mm, the fuel consumption will be 237 g/km.

It should be noted that agreed upon There are an endless variety of propellers for a given boat and engine combination. In fact, a propeller with a slightly larger diameter but a slightly smaller pitch will load the engine just as much as a propeller with a smaller diameter and a larger pitch. There is a rule: when replacing a propeller matched with the hull and engine with another, with similar values ​​of D and H (the discrepancy is permissible no more than 10%), it is required that the sum of these values ​​for the old and new propellers be equal.

However, from this set agreed upon screws, only one screw, with specific values ​​of D and H, will have the greatest efficiency. This screw is called optimal. The purpose of calculating a propeller is precisely to find optimal diameter and pitch values.

Efficiency. The efficiency of a propeller is assessed by the value of its efficiency, i.e. the ratio of usefully used power to expended engine power.

Without going into details, we note that the efficiency of a non-cavitating propeller mainly depends on the relative slip of the propeller, which in turn is determined by the ratio of power, speed, diameter and rotational speed.

The maximum efficiency of a propeller can reach 70 ~ 80%, but in practice it is quite difficult to choose the optimal values ​​of the main parameters on which the efficiency depends: diameter and rotation speed. Therefore, on small ships, the efficiency of real propellers may be much lower, amounting to only 45%.

The propeller reaches maximum efficiency at a relative slip of 10 - 30%. As slip increases, efficiency quickly drops: when the propeller operates in mooring mode, it becomes equal to zero. Similarly, the efficiency decreases to zero when, due to high speeds at a small pitch, the screw stop is zero.

However, the mutual influence of the housing and the screw should also be taken into account. During operation, the propeller captures and throws significant masses of water into the stern, as a result of which the speed of the flow flowing around the aft part of the hull increases and the pressure drops. This is accompanied by the phenomenon of suction, i.e. the appearance of an additional force of water resistance to the movement of the vessel compared to that which it experiences when towing. Consequently, the screw must develop a thrust that exceeds the body resistance by a certain amount Pe = R/(1-t) kg. Here t is the suction coefficient, the value of which depends on the speed of the vessel and the contours of the hull in the area where the propeller is located. On planing boats and motorboats, on which the propeller is located under a relatively flat bottom and does not have a sternpost in front of it, at speeds above 30 km/h t = 0.02-0.03. On low-speed (10-25 km/h) boats and motorboats, on which the propeller is installed behind the sternpost, t = 0.06-0.15.

In turn, the ship’s hull, forming a passing flow, reduces the speed of water flowing onto the propeller. This takes into account the associated flow coefficient w: Va = V (1-w) m/s. The values ​​of w are easy to determine from the data given above.

The overall propulsive efficiency of the ship-engine-propeller complex is calculated by the formula:
h = h p h ((1-t)/(1-w)) h h m = h p h h k h h m Here h p is the efficiency of the screw; h k - body influence coefficient; h m - efficiency of the shafting and reverse gear transmission.

The housing influence coefficient is often greater than unity (1.1 - 1.15), and losses in the shafting are estimated at 0.9-0.95.

Screw diameter and pitch. The elements of a propeller for a particular vessel can be calculated only by having a curve of water resistance to the movement of a given vessel, an external characteristic of the engine and design diagrams obtained from the results of model tests of propellers having certain parameters and blade shapes. To preliminarily determine the diameter and pitch of the screw, there are simplified formulas, which make no sense to present here, because suggested to use more accurate methods for calculating the optimal propeller. These methods are based on the approximation (approximate representation) of graphic diagrams by analytical dependencies, which makes it possible to perform fairly accurate calculations on a computer and even on microcalculators.

The diameter of the propellers, obtained either by an approximate formula or by accurate calculations, is usually increased by about 5% in order to obtain a deliberately heavy propeller and ensure its consistency with the engine during subsequent tests of the vessel. To “lighten” the screw, it is gradually cut in diameter until the nominal engine speed is obtained at the design speed.

However, for propellers of small vessels this need not be done. The reason is simple: the loading of pleasure craft varies widely, and a propeller that is a little "heavy" or "light" at one displacement will become consistent at another load.

Cavitation and features of the geometry of propellers of small ships. The high speeds of motorboats and motorboats and the speed of rotation of the propellers cause cavitation - the boiling of water and the formation of vapor bubbles in the vacuum region on the suction side of the blade. In the initial stage of cavitation, these bubbles are small and have virtually no effect on the operation of the propeller. However, when these bubbles burst, enormous local pressures are created, causing the surface of the blade to chip. During prolonged operation of a cavitating propeller, such erosion damage can be so significant that the efficiency of the propeller will decrease.

With a further increase in speed, the second stage of cavitation begins. A solid cavity—a cavern—encompasses the entire blade and can even close outside of it. The thrust developed by the propeller falls due to a sharp increase in drag and distortion of the shape of the blades.

Propeller cavitation can be detected by the fact that the speed of the boat stops increasing, despite a further increase in rotation speed. The propeller makes a specific noise, vibration is transmitted to the hull, and the boat moves irregularly.

The moment of onset of cavitation depends not only on the rotation speed but also on a number of other parameters. So, the smaller the area of ​​the blades, the greater the thickness of their profile and the closer the propeller is located to the waterline, the lower the rotation speed, i.e., the earlier cavitation occurs. The appearance of cavitation is also facilitated by a large angle of inclination of the propeller shaft, defects in the blades - bending, poor-quality surface.

The thrust developed by the propeller is practically independent of the area of ​​the blades. On the contrary, as this area increases, friction with water increases and engine power is additionally consumed to overcome this friction. On the other hand, it must be taken into account that with the same emphasis on wide blades, the vacuum on the suction side is less than on narrow ones. Therefore, a wide-bladed propeller is needed where cavitation is possible (i.e. on high-speed boats and at high propeller shaft speeds).

The working, or straightened, area of ​​the blades is taken as a characteristic of the propeller. When calculating it, the width of the blade is taken, measured on the discharge surface along the length of the circular arc at a given radius drawn from the center of the propeller. The characteristics of a propeller usually indicate not the straightened area of ​​the blades A itself, but its ratio to the area Ad of a solid disk of the same diameter as the propeller, i.e. A/Ad. On factory made screws, the disc ratio value is stamped on the hub.

For propellers operating in pre-cavitation mode, the disk ratio is taken within the range of 0.3 - 0.6. For heavily loaded propellers on high-speed boats with powerful high-speed engines, A/Ad increases to 0.6 - 1.1. A large disk ratio is also necessary when making screws from materials with low strength, for example, silumin or fiberglass. In this case, it is preferable to make the blades wider than to increase their thickness.

The axis of the propeller on a planing boat is located relatively close to the surface of the water, so there are frequent cases of air being sucked into the propeller blades (surface aeration) or the entire propeller being exposed when sailing on a wave. In these cases, the propeller thrust drops sharply, and the engine speed may exceed the maximum permissible. To reduce the influence of aeration, the pitch of the propeller is made variable along the radius - starting from the cross section of the blade at r = (0.63-0.7) R towards the hub, the pitch is reduced by 15~20%.

Boat propellers usually have a high rotation frequency, therefore, due to high centrifugal speeds, water flows along the blades in the radial direction, which negatively affects the efficiency of the propeller. To reduce this effect, the blades are given a significant tilt towards the stern - from 10 to 15°.

In most cases, the propeller blades are given a slight saber shape - the line of the middle sections of the blade is curvilinear with a convexity directed along the direction of rotation of the propeller. Due to the smoother entry of the blades into the water, such propellers are characterized by less vibration of the blades, are less susceptible to cavitation and have increased strength of the entering edges.

The most widespread among the propellers of small ships is the segmented plano-convex profile. The blades of the propellers of high-speed motorboats and speedboats designed for speeds over 40 km/h have to be made as thin as possible in order to prevent cavitation. To increase efficiency in these cases, a convex-concave profile (“hole”) is advisable. The profile concavity arrow is assumed to be equal to about 2% of the section chord, and the relative thickness of the segment profile (the ratio of the thickness t to the chord b at the design radius of the screw equal to 0.6R) is usually taken within the range t/b = 0.04-0.10.

A two-blade propeller has a higher efficiency than a three-blade propeller, but with a large disk ratio it is very difficult to ensure the necessary strength of the blade of such a propeller. Therefore, three-blade propellers are most widespread on small ships. Propellers with two blades are used on racing ships, where the propeller is lightly loaded, and on sailing and motor yachts, where the engine plays an auxiliary role. In the latter case, it is important to be able to install the propeller in a vertical position in the hydrodynamic wake of the sternpost to reduce its resistance when sailing.

We have already seen the largest ships in the world, and even paid attention to the bow figures of the ships. But it seems that we missed perhaps the most important thing - the screws.

Interesting fact: When Edward Lyon Berthon invented the propeller in 1834, it was rejected and regarded by the Admiralty as “a cute little toy that could never, and never could, propel a ship.”

The largest ship propellers in the world

One of the largest ship propellers in the world was manufactured by Hyundai Heavy Industries for a vessel with a carrying capacity of 7,200 twenty-foot containers, owned by Hapag Lloyd. The height of a three-story building, 9.1 meters in diameter, the six-blade propeller weighs 101.5 tons. The following photo shows a 72 ton propeller installed on the Loannis Coloctronis tanker:

The largest ship propeller to date, weighing 131 tons, manufactured in the city of Waren on the Müritz River, is installed on the Emma Maersk - the largest container ship in the world, with a carrying capacity of up to 14,770 twenty-foot containers, a length of 397 m, a width of more than 56 m and a height of 68 m. with a powerful engine, the propeller allows the ocean giant to reach a speed of 27 knots (50 km/h).

These are the massive propellers and rudders of the Antarctic icebreaker Palmer, a research vessel operating in some of the harshest conditions on Earth:

Propellers installed on Eurodam - cruise ship:

These huge propellers belonged to the Titanic, one of the most famous ships in history. The liner had three propellers, each driven by a separate engine. The two outer propellers weighed 38 tons, and the central one weighed 17 tons:

The Titanic was one of the best ships of its time, but the Oasis of the Seas" Royal company The Caribbean is five times the size of the famous liner and is currently the largest passenger ship ever built. Naturally, a luxury ship must have propellers large enough to take it from the coast of Finland to its new home, Oasis of the Seas, in Fort Lauderdale, Florida:

Carnival Cruise Lines' Elation was also built in Finland and is currently based in San Diego, California. Next to the ship's propellers, the people responsible for their design and installation seem like pitiful midgets:

And this propeller is assembled in dry dock in San Francisco:

The next propeller belongs to another cruise ship, the Norwegian Epic:

Another example of the gigantic propeller needed to propel huge cruise ships like the Celebrity Solstice:

Here are the propellers of the Queen Elizabeth 2, known as QE2. Owned by Cunard Line (a British company that operates transatlantic and ocean liner cruise routes), the ship was launched in 1969 and removed from service in 2008:

Queen Mary 2 replaced QE2 as Cunard's flagship in 2004. Here are some of the spare QM2 propellers located on the boat's foredeck:

This is the propeller of another famous ship in history. The German battleship Bismark was launched in February 1939, shortly before the outbreak of World War II, and sunk by the British in May 1941 (image at left). The photo on the right shows a factory landscape and a propeller from an oil tanker during its construction in 1947:

Not as big, but no less interesting
The propeller of Japanese mini-submarines that attacked American aircraft carriers during the attack on Pearl Harbor in December 1941:

USS Fiske starboard propeller, 1946:

Technology is improving, of course, but big ships Large screws are still needed. This one is from the SS Great Britain, designed by Isambard Kingdom Brunel for the largest ship in the world (at the time of its launch in 1843). The ship crossed the Atlantic Ocean in 1845 in just 14 days, which was an absolute record at the time.

Workers shipyard studying one of the four brass propellers of the aircraft carrier USS George Washington. Each of the propellers weighs about 66,000 pounds and is 22 feet in diameter.

Devices designed to create persistent pressure perceived by the ship and which is the basis of its movement are called propulsors. There are movers various types: paddle wheels, winged propellers, propellers, etc.

The wing propulsion device is a disk equipped with three to four vertical rotating blades and located horizontally under the stern of the vessel on a vertical shaft. The disk is driven into rotation by an electric motor through a bevel gear. The use of wing propulsors ensures high maneuverability of the vessel in the absence of a steering device and allows reversing without reversing the engine. However, the structural complexity of such propulsors and their dimensions, which increase with increasing power of the ship’s power plant, do not allow their use for large
ships. Recently, self-propelled cargo cranes, some small vessels and thrusters of larger vessels have been equipped with Voitschneider-type wing propulsors.

The most widely used propeller for ships is the propeller. The main parts of the propeller (Fig. 81) are: 1 propeller hub with a conical hole inside and 2 blades, the number of which can be from two to six. Propellers are made with solid cast, removable and rotary blades.

Rice. 81. Propeller with solid blades.

Propellers with solid-cast blades (Fig. 81) are used mainly on merchant marine vessels. Such screws are characterized by their low weight and hub size, as well as higher strength in normal conditions operation.

Propellers with removable blades are installed on ships of the Arctic fleet, where, due to operating conditions, replacing a damaged blade as a whole is more convenient than replacing the entire propeller. In addition, such screws are used in cases where the diameter of the screw is large and its casting is difficult.

Propellers with rotating blades, otherwise called controllable pitch propellers (CPP), differ from conventional ones in that their blades are movably fixed in the propeller hub and can be rotated around their axis at a given angle using a special drive. This drive, or variable pitch mechanism (PVM), is usually located inside the propeller hub, so the hub is significantly larger than that of conventional propellers. The pitch change mechanism can be manual, mechanical, electromechanical, hydraulic and electrohydraulic. The MIS, with the exception of the manual one, includes: a mechanism for rotating the blades, usually located in the propeller hub; a servomotor that creates forces to rotate the blades and is located in the area between the propeller shaft and the main engine; feedback or device indicating the amount of new propeller pitch.

The blade rotation mechanism (Fig. 82) comes in two types: gear and crank, the latter being more reliable and used in all stressed propeller designs (high power and diameter, high-speed rotary propeller propellers of small diameter, etc.).

Rice. 82. Blade rotation mechanism: a - gear; b - crank.

The most common at present is the hydraulic MIS (Fig. 83), usually located in the shafting line. To turn the propeller blades, the energy of a liquid (most often low-viscosity oil) under pressure is used. The hydraulic drive of the MISH is distinguished by the relative simplicity of the device and the ability to create significant working forces with relatively small dimensions and weight of the installation.

Rice. 83. Design of a hydraulic drive.

In the hub 4 of the propeller there is a driver 1 of the rod 5, placed inside the hollow propeller shaft 6. The driver 1, in the groove of which the finger 2 is located on the butt of the blade, rotates the latter around its axis. To facilitate turning, the butt of the blade is seated in the hub socket on double-row tapered roller bearings 3. At the other end of the rod 5 there is a servomotor piston 7, connected feedback 8 with a movable coupling 12 and a piston of the distribution spool 11. Oil is supplied to the distribution spool 11 and the servomotor 7 through tubes 10 from the oil pump. The change in pitch of the propeller blades is controlled by lever 9, the lower end of which slides in the groove of the movable coupling. The hydraulic MIS allows you to control the propeller pitch from the navigation bridge using a remote pneumatic system.

The use of adjustable pitch propellers made it possible to significantly simplify the control of the vessel, reduce the size and weight of the main engines by eliminating steps and a reverse gear, and allow the vessel to reverse without changing the direction of rotation of the propeller shaft. In addition, the use of propeller propellers on vessels such as tugs, tankers and timber carriers allows the propeller pitch to be adjusted to any speed. This increases the efficiency of the power plant and makes it possible to more fully use the power of the main engines in various operating modes.