A ship's propulsion unit is a special device for converting the work of the main engine or other energy source into useful thrust, which ensures the forward motion of the ship. Ship propulsors include propellers, paddle wheels, water jets and wing propulsors. Propeller screw is a hydraulic mechanism whose blades capture seawater and impart additional speed to it in the direction opposite to the movement of the vessel. In this case, the hydrodynamic forces arising on the blades create an axial resultant force called the propulsion thrust. The propulsion thrust is transmitted to the ship's hull through a thrust bearing rigidly connected to it. For a propeller with guide devices (nozzles), the thrust is created both by the blades and by a coarse nozzle. Propellers are the most common type of propulsion. They can have 2-7 blades located at equal distances around the circumference of the hub, and represent a load-bearing wing formed by part of the helical surface. Depending on the design, the following types of propellers are distinguished: with integral blades (solid and welded), with removable blades and controllable pitch propellers (CPP). Solid propellers, which have the simplest design, are widely used in the river fleet. Propellers with removable blades have begun to be used in recent years in cases where, due to operating conditions, frequent blade failures are possible. The removable blade has a flange at the root section, with which it is attached to the hub. The characteristics of a propeller blade and a single wing element (see paragraph 46) are determined by the same forces with the difference that the propeller considers the total thrust and the resulting force of resistance to rotation of all blades. If the speed of the flow incident on the propeller is U p (Fig. 57), and the radial speed is ωr, then the angle of attack of a given element of the blade section α l is determined by the angle between the resulting speed U 1 and the line of zero lift (LNL). The lifting force and the force of any resistance are reduced to the resultant Υ in. One of its projections gives the force of the useful thrust of the screw P in, and the second gives the force of resistance to rotation R in p. The moment of force R in p relative to the axis of the propeller is overcome by the main engine of the vessel. Propellers have a relatively low mass, small size, are reliable in operation, inexpensive to manufacture and allow the use of most low-speed main engines without gearboxes; their efficiency reaches 70%. Paddle wheels (Fig. 58) have a horizontal axis of rotation perpendicular to the direction of movement, and, as a rule, are located along the sides of the vessel. With paddle wheels, the thrust force is created on the plate and on the wing. The intersection of the plate creates a force FH, the projection of which on the direction of movement creates a useful stop PH. The advantage of paddle wheels is their fairly high efficiency and the ability to create significant thrust at low drafts. Rowing wheels with rotary plates have the highest efficiency value of 50-60%. The disadvantages of paddle wheels are the complexity and bulkiness of the design, large mass, forced increase in the width of the vessel, and frequent damage, especially when moving in rough seas and ice conditions. In a water-jet propulsion system, the necessary useful traction force is created due to the increment in the amount of motion that the sea water receives in the water-jet complex. water is sucked in by the propeller through the intake hole in the bottom of the vessel and thrown out at increased speed through the pressure section of the jet pipe, creating a reactive net thrust force acting in the direction opposite to the direction of jet emission. At the end of the pressure part of the water jet pipe there is a special reversing device with a balancing rudder. The use of such a rudder allows you to steer the vessel, as well as move in reverse without reversing the main engine due to the change in direction of the ejected stream of water. The main advantages of water-jet propulsion are the creation of significant useful thrust with a shallow draft of the vessel, as well as the absence of moving parts outside the hull, which makes it possible to reliably protect the propulsion from damage when the vessel is sailing on small rivers. A winged propulsion unit is a disk rotating around a vertical axis, along the circumference of which there are wing-shaped rotary blades at equal distances (variable winged propellers have 4-8 such blades). Thanks to the rotation of the blade, an optimal angle of attack is ensured and the necessary useful thrust of the propulsion is created. To do this, the blades located on the front semicircle of the drive wheel have their incoming edges outward, and those on the rear - inward. With this movement of the blades, all normals to them intersect at one control point A-center. Each blade is in a complex movement: it rotates around its own axis, around the vertical axis of the rotor and moves translationally with the ship. When a flow of water flows around the blade at a speed of U 1 at an angle of attack α l, a lifting force Υ L is created, the projection of which on the direction of movement of the vessel gives the useful thrust force of the blade R L and the force of resistance to rotation R in p. By moving the control center, for example, from points A and A 1, it is possible to rotate the blades around their own axis, thereby changing the direction and value of the force of the total thrust of the propulsion, and therefore, ensure a change in the speed of the vessel and its rotation without the use of rudders, without changing the frequency rotation and direction of rotation of the main engine. Wing propellers have a large mass, a complex design and are inferior to propellers in terms of efficiency, so they are used only on ships whose maneuverability is particularly demanding (on seaport tugs, ferries, etc.).
32. Technical operation of the fleet. Main goals The main tasks of technical operation of the fleet (TEF) are a set of scientifically based organizational technical and technological methods for its maintenance and use. Technical method of maintenance: The main tasks of technical equipment are: 1. Increasing the durability and reliability of operation of all types of ships and reducing downtime for technical reasons 2. Reducing operating costs 3. Systematic implementation of measures to modernize the fleet
1. A set of works performed by the crew during the operation of the vessel, in accordance with technical and operational indicators Maintaining and monitoring the parameters of technical equipment and the use of operating materials (fuel, oil), consumables are included in the operation of soda equipment
PET is a combination of the following components: 1) Technical use 2) Maintenance 3) Repair 4) Technical operation management 2. The work includes: external and internal inspection of connections???? , adjustment (of clearances and response sizes), disassembly, cleaning of components and parts, removal, adjustment of operating conditions, partial replacement of worn parts, wetting or lubrication, cleaning. 3. A set of works performed by the crew and employees of onshore enterprises, which ensure the restoration of the vessel’s operability at certain time intervals with or without the vessel being taken out of service. * Planned (capital, medium, current, warranty, maintenance) * Unscheduled (emergency, recovery, inter-flight)
The most important components of ship power plants include power transmission elements. This refers to all the elements involved in transmitting torque from the crankshaft or rotor in turbines to the propeller. A typical diesel power plant with two medium-speed diesel engines is shown in the figure. It includes couplings, a single-stage gearbox, shafting and a propeller. In power plants with low-speed diesel engines, there is no gearbox; in turbine and power plants with high-speed diesel engines, two- and three-stage gearboxes are installed. Diesel and turboelectric power plants use electric motors.
Diesel-geared power plant with medium-speed diesel engines
1 - coupling; 2 - gearbox; 3 - shafting; 4 - propeller
The coupling connects the nodes that perform rotational movements. The coupling is designed to transmit torque from the drive shaft to the driven shaft, as well as to smooth out minor longitudinal, radial, angular deviations and torsional vibrations. Depending on the design, purpose and principle of operation, rigid (blind), elastic, friction, hydrodynamic and electromagnetic couplings are distinguished. All types of couplings are found in ship installations depending on the type, power and design of the main engine. In installations that do not have transmission mechanisms (for example, in low-speed diesel engines), rigid couplings are most often used (Fig. a, b). In the heated state, the flanges of the rigid coupling are pressed onto the shaft or onto the cone and are additionally secured with a parallel key. In power plants with a gearbox, the connection between the gearbox and the engine, as well as with the propeller shaft, is carried out on the engine side most often through a coupling, and on the propeller side - through a disconnect coupling. In Fig. e shows an elastic coupling. It consists of two bases connected to each other by flexible gaskets made of special rubber. Such couplings are screwed to the shaft flanges. They can transmit moments regardless of the direction of rotation. Due to flexible liners, alignment is possible when the shafts are skewed relative to each other.
The operation of hydrodynamic couplings is based on the hydraulic principle, shown schematically in Fig. With. You can think of it like this: a pump driven by an engine sucks liquid from a reservoir and forces it into a turbine. Liquid under a certain pressure flows through the turbine blades, causing it to move, and then flows back into the reservoir. With the same sizes of pump and turbine rotors, the unit works as a hydraulic coupling; with different sizes, it turns into a torque converter transmission, which allows you to change the speed of rotation of the driven shaft. In practice, the rotors of pumps and turbines are located in a special housing (Fig. d). The action of a hydrodynamic coupling is based on the energy exchange between two coupling halves (Fig. d) using the working medium and fluid circulation. This circulation only occurs when the primary side and the turbine have equal rotation speeds. For hydraulic couplings used on ships, this slip ranges from 1.5 to 3%.
Ship couplings
a, b - rigid (blind) couplings: 1 - half coupling; 2 - flange; 3 - keyway with key. c - fluid coupling diagram: 1, 2 - pumps; 3 - tank. d - diagram of the fluid coupling (turbo coupling); e - flexible coupling. 4 - flange; 5 - coupling element. f - electromagnetic clutch.
Electromagnetic induction sliding clutches are also quite often used in marine main engines. The principle of operation of such a coupling is to use the torque resulting from the effect of a rotating magnetic field on induced currents. The inner part of the coupling is located on the drive shaft. The pole windings are fed with direct current through brushes and slip rings. The outer part of the coupling has a squirrel cage winding. When the external part, driven by the engine through the shaft, begins to rotate and is excited, it, together with the shaft connected to it and leading, for example, to the gearbox, enters the region of rotation of the magnetic field. Due to this, induction currents arise in the squirrel cage winding of this part of the coupling. These currents, interacting with the magnetic field lines, cause the occurrence of a torque, as a result of which the outer part of the coupling begins to rotate together with the inner one. Thus, rotation, power and torque are transmitted from the engine to the gearbox shaft. The squirrel cage part of the coupling must - similarly to the hydrodynamic and electromagnetic coupling - rotate slower than the rotating magnetic field, since if the rotation speed of both parts were the same, induced currents could not arise and transmission of torque would not be possible. Therefore, in this case, the so-called coupling slip occurs. The main engine gearbox must transmit torque and change its speed so that it has the optimal value necessary for normal operation of the propeller. On ships, mechanical gearboxes consisting of gears are most often used. With the introduction of the planetary gearbox, it became possible to significantly reduce the size and overall weight. Recently, new ships are increasingly using planetary gearboxes in power plants with medium-speed diesel engines, gas or steam turbines.
Mechanical marine gearbox
a - summing; b - planetary. 1 - high pressure turbine shaft; 2 - low pressure turbine shaft; 3, 5, 8, 9 - central sun gears; 4 - carrier; 6 - free epicycle; 7 - shaft; 10 - brake epicycle; 11 - free carrier; 12 - hollow shaft; 13 - gears (3rd stage); 14 - propeller shaft drive gear; 15 - propeller shaft; 16 - propeller
The shaft line connects the drive motor to the propeller. The propeller shaft, which, depending on the location of the engine room on the ship, may consist of one or more parts connected through blind couplings, must transmit engine rotational torque to the propeller. The propeller shaft rests on radial bearings. The end part passes through the sealing gland, which protects the propeller shaft tunnel from sea water. A propeller is fixed to the cone-shaped end part of the propeller shaft (Fig. a). The axial pressure acting from the propeller and transmitted further through the shaft is absorbed by the thrust bearing. The operating principle of a thrust bearing is shown in Fig. d-e. This old type bearing consists of a pressure ridge interacting with the supporting surfaces; the supporting surfaces are filled with metal. On the forward stroke, one surface of the pressure ridge functions, on the rear - another.
Shafting
a - general view; b - coupling half; c - thrust bearing; d, e - operating principle of the thrust bearing. 1 - propeller shaft; 2 - oil seal; 3 - half-bearing; 6 - bulkhead seal; 7 - coupling; 4 - intermediate shaft; 5 - support thrust bearing; 8 - thrust shaft
The propeller is currently almost the only type of propulsion. It consists of several blades radially mounted on a hub. As the propeller rotates around its axis, a pressure force is generated on the blades, which ultimately causes the ship to move. The characteristic value of a propeller is its pitch. Its theoretical value, i.e. without taking into account slip, depends on the angle of attack of the propeller blade. To achieve good interaction between the main engine and the propeller, it is necessary that the parameters and especially the propeller pitch have certain values. Optimal interaction will only be achieved under a certain load condition of the vessel and under certain weather conditions (wind, waves, etc.). If these values deviate from the specified values, then the interaction between the engine and the propeller does not produce the result intended in the design. In practice, this means that the interaction between the engine and its associated propeller will be most efficient, for example, when the boat is fully loaded and in good weather. On vessels operating in changing conditions, such as tugs or fishing vessels (free-sailing, trawling), the propulsion system must be adapted to the relevant operating conditions. At the same time, it would be possible to simultaneously use the full power of the drive motor under different load conditions.
Ship propulsion
a - propeller with fixed blades; b - adjustable pitch screw; c - propeller in the nozzle; d - coaxial propellers
Fixed pitch propeller blades are cast together with the hub or firmly screwed to it (see Fig. a). You can change the pitch on adjustable-pitch propellers (Fig. b). The propeller blades are located on curved disks and are mounted on the propeller hub so that they can rotate. The use of CVS allows the use of non-reversible engines as marine engines. They can also operate at a constant speed, since in this case all maneuvers can be carried out by changing the angle of attack, i.e., from the largest pitch of the propeller at the forward speed, when the blades are in such a position that despite the rotation of the propeller, thrust does not appear (and therefore the ship does not move) until the position of the blades corresponds to astern. At first, CV propellers were used only on tugs, fishing and special vessels, and only later they began to be installed on merchant fleet vessels. By installing a rotary propeller, greater efficiency of power plants is achieved, the ability to use the full engine power at different loads, as well as the possibility of using non-reversible internal combustion engines or steam turbines without reverse turbines. The advantages also include the ability to reverse at full engine power.
Sometimes on ships (especially on river vessels) the propeller is installed in a nozzle (see Fig. c). This design improves the performance of the propeller and increases efficiency. The diameter of a ship's propulsion unit can reach 9 m, and its weight can reach 50 tons. Controlled pitch propellers have a smaller diameter. The predominant number of ships have only one propeller installed in the center plane of the ship. There are also twin-screw ships that are driven by either two low-speed or four medium-speed diesel engines, in the latter case one propeller is driven by two engines. In rare cases, three-screw ships are built, for example, torpedo boats, on which two onboard propulsors are driven by high-speed diesel engines through a gearbox, and the middle propeller is driven by a gas turbine. Some large passenger ships and warships, such as aircraft carriers, are equipped with four symmetrically arranged propellers. With the ever-increasing power of main engines, propellers of very large diameters are required, which leads to technological and production difficulties. To counteract this and improve efficiency, they are trying to “install thrusters rotating in opposite directions (see Fig. d). In this case, complex devices are required, such as hollow propeller shafts and special gearboxes. Along with propellers, wing propulsors have recently been used. They consist of several rotating hinged paddle-shaped blades of variable profile mounted on a flat impeller. The impeller is driven by the main motor through a hypoid gear reducer. The rotating paddle-shaped blades create a thrust force acting in a direction depending on the angle of the blades, as shown in Fig. A. During operation of the propulsion unit, the angle of attack of the blades can be smoothly changed.
Wing propeller
a - operating principle; b - Voith-Schneider propulsion unit (side view); c - Voith Schneider propulsion unit (top view); d - tug with a Voith-Schneider propulsion unit in the bow of the vessel; e - tug with Voith-Schneider propulsion in the stern of the vessel
I - “Stop”; 2 - “Forward”; 3 - “Reverse”; 4 - “Turn to port”; 5 - “Turn to left” (in reverse); 6 - “Turn to starboard”; 7 - control mechanism; 8 - drive; 9 - blades; 10 - distribution levers and rods
The vane propulsion unit can serve both as a propulsion propulsion unit and as a rudder. A vessel equipped with two symmetrically located propulsors can move in any direction. The disadvantage is that the paddle-shaped blades protruding below the bottom of the vessel are often damaged. Paddle propulsion is mainly used on port tugs and pilot vessels, as well as on port service vessels. The power of such installations is small: the maximum is 2200 kW.
When starting to study existing ship propulsors, it is necessary to define this concept. Ship propulsion- this is a device for converting the work of a ship's power plant into thrust, ensuring its forward motion. The propulsion thrust is generated due to the reactive forces that arise when the working medium is thrown in the direction opposite to the direction of the forward motion of the vessel. Based on the nature of the working medium, propulsors are currently conventionally divided into hydraulic (working medium - water), air (air) and gas-jet (water-air mixture). In turn, hydraulic propulsors are divided into bladed (oar, propeller, paddle wheel plates, etc.) and non-bladed (gas-jet propulsors). An intermediate place in this classification is given to the water-jet propulsion system.
Hydraulic propulsors are widely used on all displacement vessels, air propulsors - on high-speed vessels such as hovercraft and ekranoplanes. Of the listed propulsors, the manual examines in more detail the propeller (as the main propulsion device used on ships) and the water-jet propulsion system. The propulsion thrust is generated due to the reactive forces that arise when the working medium is thrown in the direction opposite to the direction of the forward motion of the vessel. The nature
working environment, propulsors are currently conventionally divided into hydraulic (working medium - water), air (air) and gas-jet (water-air mixture). In turn, hydraulic propulsors are divided into bladed (oar, propeller, paddle wheel plates, etc.) and non-bladed (gas-jet propulsors). An intermediate place in this classification is given to the water-jet propulsion system. Hydraulic propulsors are widely used on all displacement vessels, air propulsors - on high-speed vessels such as hovercraft and ekranoplanes. Of the listed propulsion devices, the manual examines in more detail the propeller (as the main propulsion device used on ships) and the water-jet propulsion system.
In the previous paragraph, we said that there are five main types of power plant on a ship, and each of them is characterized by its own shafting scheme, i.e. a mechanical system for transmitting rotation of the engine crankshaft to the propeller (propeller). Let's consider in order (Fig. 107):
1. The vessel is equipped with a conventional stationary converted high-speed engine, which is located in the center of the cockpit, in the midship frame area. The crankshaft is connected through a gearbox (to reduce the speed) by a straight propeller shaft with a propeller (linear shafting diagram). The installation is easy to maintain, efficient, simple, and does not require additional design solutions.
2. The same engine is located at the stern of the ship. With this arrangement, a number of advantages are lost and new ones appear (space in the cockpit, reduced noise in the cabin). A major drawback is the constant trim to the stern and the need to use an angular gearbox (V-shaped, or angular shafting design).
3. Shafting scheme with a sterndrive (Z-shaped gear) combining the advantages of a stationary engine and a PLM (high engine power, good seaworthiness, tilting of the column when hitting an obstacle, ease of work with the propeller and maintenance of the column, exhaust of gases into the water etc.) has one major drawback - high cost.
4. The use of a water-jet propulsion system makes life easier for the navigator due to the absence of any parts protruding below the keel of the vessel, but complicates it quite a bit due to changes in the vessel’s performance and, above all, deterioration in controllability. The engine is installed slightly further from the stern than in the previous two cases, which reduces the trim to the stern, eliminating the need for a coupling and reversing clutch
5. The shaft line of outboard boat motors has an L-shape, in which the engine is connected to the propeller (propeller) through a gearbox using an intermediate, so-called. torsion bar, shaft (spring). PLM does not take up useful cockpit space, is easy to maintain and is quite cheap
In a number of shafting options under consideration, the gearboxes used allow simultaneous reversal of the propellers - changing the direction of rotation to the opposite. In general, reversal is carried out in three ways: by reversing the main engine, engaging reverse gear and reversing the propulsion itself. Main engine reverse- changing the direction of rotation of the engine crankshaft to the opposite direction, and, accordingly, changing the direction of propeller thrust. This reverse is ensured by the reversing device of the engine itself, the main part of which is a movable camshaft, which ensures a given sequence of fuel supply to the cylinders, as a result of which the engine crankshaft begins to rotate in the opposite direction. Reverse gear- this is a gear with the help of which the direction of rotation of the propeller shaft (the propeller shaft on which the propeller is mounted is called the propeller) changes to the opposite direction while the direction of rotation of the engine crankshaft remains unchanged
Reversal is achieved through reversible gear reducers, hydraulic transmission or couplings, which make it possible to disconnect a part of the gearbox with one direction of rotation and connect it with another. On boats, reverse gearboxes (reversible clutch) are used - a special mechanism that ensures a change in the direction of rotation of the ship's propeller shaft while maintaining the same direction of rotation of the crankshaft of the ship's engine with the inclusion of a gearbox in the design to reduce or multiplier to increase the number of shaft revolutions. The reverse gearbox is connected to the crankshaft by flange connections via an intermediate shaft or directly (see Fig. 108), the driven shaft is connected to the propeller shaft. The gearbox cavity is filled with oil, to check the presence and level of which there is a level indicator (measuring ruler). Propulsion reverse- changing the direction of the thrust created by the propeller is ensured by turning the blades of the controllable pitch propellers (CPP).
Propeller screw- a device that converts the rotation of the engine shaft into a stop - the force that pushes the ship forward. It consists of a hub and several (two or more) blades. The blade of a ship's propeller is a hydrodynamic profile that operates at a certain angle of inclination to the water flow, discarding it and thus creating a thrust. The blade has an incoming and outgoing edge
Rice. 108. Two types of angular gear for a stationary engine:
a - with a short intermediate driveshaft;
b - fixed to the engine.
and working (discharge) surface. The physical essence of the operation of the propeller is quite simple - when rotating, a vacuum is formed on the surface of its blades facing towards the movement of the vessel, and increased water pressure is formed on the surfaces facing backwards. The pressure difference creates a force, one of the components of which moves the ship forward. The thrust largely depends on the angle of attack of the blade profile. The optimal value of this angle for high-speed boats is 4 - 8°.
Basic concepts when considering the topic and characteristics of a propeller:
Screw pitch- geometric movement (distance) of any point of the blade along the axis for one full revolution of the propeller, provided that it makes it in a relatively solid environment.
Screw diameter- diameter of the circle into which the straightened propeller blades are inscribed (Fig. 109)
Step ratio- ratio of screw pitch to diameter
Disk ratio- the ratio of the area of the straightened blades (without a hub) to the area of the disk, the diameter of which is equal to the diameter of the propeller (Fig. 111). The pitch and disk ratios are the main parameters of the hydrodynamic characteristics of the propeller, which determine the degree of use of engine power and the achievement of the maximum possible speed by the ship. Each propeller of a specific size and fixed pitch has its own propeller characteristic. In principle, each vessel hull and engine must have its own optimal propeller. The process of calculating a propeller is complex and is based on the use of existing graphs and diagrams for determining the diameter and pitch of the propeller depending on the power on the shaft. For light loads and high speeds, a two-blade propeller is usually selected, for normal loads (on boats) - a three-blade propeller, for heavy loads and low speeds - a four-blade propeller. The use of a five-bladed propeller significantly reduces vibration.
Screw slip- a phenomenon that occurs when a propeller operates in a water environment under load, and is the difference between the calculated pitch of the propeller and the actual distance traveled per revolution. Slip is almost never less than 15% of the propeller pitch, in most cases it is 30%, sometimes about 45-50% of the propeller pitch.
Propeller efficiency (efficiency)– the ratio of usefully used power to expended engine power depends mainly on the diameter and speed of rotation of the propeller. Efficiency is an assessment of the efficiency of the propeller; its maximum value can reach 70-80%, on small ships 45-50%. Knowing the efficiency of the propeller is necessary to calculate the designed speed of the vessel. The efficiency of propellers is also calculated using numerous graphs and diagrams, the basis of which is the power factor (load factor) - the ratio of the product of the engine power given to the propeller by its rotation frequency to the forward speed of the propeller in the associated flow
Most propellers operate with load factors ranging from 1 to 10. The load factor structure shows that low engine power, low rpm, and high speed lead to high propeller efficiency. Direction of rotation of the propeller (Fig. 110) in navigation (right - clockwise, left - counterclockwise) install looking from stern to bow when the propeller is operating in forward motion and is determined only for forward motion.
Cavitation- the phenomenon of “boiling” of water and the formation of steam bubbles on the suction side of the propeller blade. When the bubbles collapse, enormous local pressures are created, which causes the blade to chip. During long-term operation, these destructions reach large values, which negatively affect the operation of the screw. The second stage of cavitation is the appearance of a continuous cavity on the blade, which can sometimes close even beyond its boundaries.
The thrust developed by the propeller falls due to a sharp increase in drag and distortion of the shape of the blades. When the pitch and diameter of the propeller change more or less than the optimal values, moments arise when the engine is either unable to rotate the propeller at a higher speed (does not develop rated power), or, conversely, not only develops, but easily exceeds the rated crankshaft speed , and since the propeller stop is small, the ship still does not develop high speed. In this case, the concepts come into force light heavy) screw, which are also among the screw characteristics, o. which were mentioned above.
Propellers are made of bronze, brass, stainless and carbon steel, and cast iron. Plastic is used for propellers of small ships. Metal screws are made cast and then finished (processed).
The problem of taking into account the changing resistance of the ship's hull when its load changes and using the engine more efficiently under these conditions is quite successfully solved by using a variable-pitch propeller (multipitch propeller, not to be confused with a variable-pitch propeller - VRP). The propeller hub is metal, the interchangeable blades are made of polyamide resins (the propeller hub has recently been made from them). The blades have rigidly fixed fingers (Fig. 112), which pass into the holes at the end of the nose of the hub 6 and fit into the grooves of the driver 4, which has a measuring scale.
When any blade rotates around its axis, all blades simultaneously rotate in the direction of increasing (decreasing) the propeller pitch. The blades are secured in the selected position with nut 3. Bushing 5 has an internal diameter equal to the diameter of the motor propeller shaft. The screw is secured against axial movement in the bushing by nut 3 and locking screw 8. The operation of changing the pitch takes 3-5 minutes with skill and does not require approaching the shore and removing the screw. For the Vikhr PLM, such propellers were produced by the Black Sea Shipyard.
Rowing adjustable pitch screws They are distinguished by the complexity of the device, a massive hub and high cost, since the rotation of the blades to change the pitch of the propeller is carried out remotely, during operation (rotation). Such propellers were discussed when we talked about changing the mode of movement of the vessel from “full forward” to “stop” and “full back” only with the help of a propulsion device. Advantages of a rotary propeller: the ability to use the full engine power in various modes of vessel motion and obtain the entire range of speeds without changing the direction and speed of rotation of the propeller shaft; saving fuel and increasing engine life. Disadvantages of rotary propellers: design complexity, reduced engine efficiency due to the increased hub size and distortion of the blade profile when they rotate at intermediate operating modes, low efficiency in reverse. To increase the efficiency of the propeller on heavy displacement ships, it is often used ring profiled nozzle(Fig. 113), which is a closed ring with a flat-convex profile. The area of the inlet section of the nozzle is larger than the output area, the screw is installed in the narrowest place and with a minimum (0.01 D of the screw) gap between the edge of the blade and the inner surface of the nozzle . When the screw is operating, the suction flow increases the speed due to a decrease in the flow area of the nozzle, as a result of which the sliding of the screw decreases. Additional emphasis is also created on the nozzle itself (due to water flowing around it, similar to a wing). The action of a water-jet propulsion system is based on Newton's well-known law: the mass of water thrown by the propulsion device into the stern creates, in the form of a reaction, persistent pressure that moves the vessel forward.
Water jet propulsion (water cannon) can be imagined as a powerful pump that takes water from under the bottom and throws it out behind the transom from a nozzle above the water. A water jet differs from a propeller only in that the propeller (pump wheel) is installed in a pipe inside the vessel. Steering the ship and moving in reverse in this case is carried out in various ways. Our most applicable control method is to rotate the jet in the outlet nozzle using a double-leaf reversible steering device, consisting of two flat plates (rudders) connected to each other and hinged on a reversible box. In this case, when moving forward, the rudders are shifted parallel to each other, changing the direction of the ejected jet in one direction or another; when moving astern, the ship is not controlled. It is possible to use a rotating nozzle and a reversing damper, as well as a rotating water jet (Fig. 114), which significantly increases the maneuverability of the vessel. Water jets are used primarily on light high-speed boats, where high power is combined with the light weight of the boat.
Propellers are very rarely used on small vessels due to low efficiency, large sizes and a large number of other shortcomings and problems that designers encounter when designing a vessel with such a propulsion device. Propellers are indispensable in the manufacture of amphibious vessels (Fig. 115, 116), hovercraft, i.e. such vessels for which the underlying surface may be a swamp, snow, ice, smooth sand, etc. Two-blade propellers are most often used. There are corresponding formulas for calculating propeller thrust, blade width, pitch, diameter and other characteristics of the propeller. Propellers for boats are most often made of wood, glued from slats.
Concluding the topic of propulsors and summing up briefly, we can say that the maximum speed, the greatest efficiency and reliability, as well as the greatest thrust of the existing propulsors is created by the propeller. The smallest draft and material losses for the boatmaster when touching the ground are achieved when using water-jet propulsors, and simplified installation and ease of maintenance are possible when using outboard motors and sterndrives.
When moving at a certain speed V, the ship experiences a force
environmental resistance R (water and air), directed towards
the side opposite to its movement.
Hydrodynamic forces applied to the surface elements of the hull of a moving ship can be decomposed into two components: tangential and normal.
The tangential component is called the friction force, and the normal component is called the pressure force. In the figure, the friction force τ and the pressure force P act on the selected element of the wetted surface of the vessel. Projecting all elementary friction forces onto the direction of velocity
movements of the vessel and summing them over the entire wetted surface, we obtain the resultant friction forces - friction resistance RTR, due to the action of viscous forces.
The resulting projection of pressure forces on the direction of movement speed
vessel V, taken over the entire wetted surface, determines the pressure resistance RD, which is determined by the density and viscosity of water.
The pressures on the surface of the vessel are distributed unevenly: in the bow
They are larger in parts, smaller in the aft part. This pressure difference creates
pressure resistance, which in turn is divided into two parts.
The first part is the shape resistance RF, caused by the influence of the viscosity of the liquid, the second part is the wave resistance RB, which depends on the intensity of the wave movements of the liquid caused by a moving vessel.
Each vessel has certain protruding parts (rudders, brackets and
propeller shaft fillets, bilge keels, etc.). Water resistance,
caused by them is called the resistance of the protruding parts RВ.Ч. In addition, the ship experiences air resistance RВОЗ, distributed over the surface of the moving ship.
Thus, the total resistance to the movement of the vessel is summed up from the following components:
R = RTR + RB + RF + RV.Ch. + RWHO (1)
To determine each component of the impedance
Various methods are used. Frictional resistance is determined
by calculation based on the boundary layer theory. Resistance
shapes and wave resistance, combined under the general name of residual resistance Ro, are determined by experimental methods by testing ship models in experimental pools.
In practical calculations, the total resistance to the movement of the ship
calculated by the formula:
R = C × (ρπV2 / 2) × (S + SHF), (2)
where C is the total resistance coefficient;
S - wetted surface of the bare body;
SHF - wetted surface of protruding parts;
ρ - density of water;
V is the speed of the ship.
By analogy with formula (1), the impedance coefficient can be presented as a sum of coefficients:
C = CTP + CB + SF + S.H. + CWHO or C = CTP + CO + MW.H. + WHO
where Co is the residual resistance coefficient.
Therefore, the total resistance of the ship is:
R = (CTR + CO + SHF + SVOZ) × (ρπV2 / 2) × (S + SHF) (3).
The driving force Pe is created and maintained by ship propulsors, which convert the mechanical energy of the forward motion of the vessel.
During operation, the propulsion unit influences the flow surrounding the vessel, and the ship's hull changes the flow in the area where the propulsion unit is located.
Useful power developed by the propulsion: N=Pvp
Power consumed by the propulsion Np=Mw
(M-torque transmitted to the propulsion from the engine, w-angular speed of rotation of the propeller)
Ship propulsors are hydraulic or hydrojet based on their operating principle. Recently, air propulsion has been used (hovercraft are equipped with propellers).
The action is based on throwing away masses of the surrounding medium: water or air, in the direction opposite to the direction of movement of the vessel. This is accomplished by communicating the quantity of motion to the masses processed by the working elements of the propulsion unit. The reaction of the thrown masses is perceived by the parts of the propulsion device. Its component in the direction of movement of the vessel is called the propulsion stop. The forces arising on the elements of the propulsors can be created both due to the resistance forces when the propulsion moves in the environment, and due to the lifting force, which is similar in nature to the lifting force of the main wing.
Propellers are used on ships of various types and purposes; they are distinguished by their simplicity of design, satisfactory operational reliability and relatively high efficiency. Varieties of propellers include adjustable pitch propellers, coaxial counter-rotating propellers, and tandem propellers.
The wing propulsion device has specific properties.
Water jet propulsors
There are other types of propulsors, among which are gas-jet and rotary.
Ship propulsors are special devices that convert the energy of the main engines into the driving force (useful thrust) necessary to overcome the resistance of the environment to the movement of the vessel and ensure a given speed of its movement.
According to the principle of operation, ship propulsors are hydrojet, because they create a driving force due to the reaction of masses of water thrown by the working parts of the propulsion unit - the blades - in the direction opposite to the movement of the vessel. Currently, the following main types of ship propulsors are used in water transport: propeller, paddle wheel, winged and water-jet propulsors. The propeller serves as the main type of propulsion
for sea vessels. It consists of several blades located on the hub at equal angular distances from each other. The number of propeller blades ranges from 2 to 6. In order to prevent vibrations of the stern end of single-rotor ships, the number of propeller blades is taken to be at least four. The diameter of the propellers of large modern ships reaches 6 - 8 m.
There are three main structural types of propellers: solid propellers (solid), propellers with removable blades (prefabricated) and propellers with rotating blades - controllable pitch propellers (PVP). The propeller characterizes its pitch. The pitch of the screw is the distance by which the point of the screw will move during one full revolution of the screw when rotating it in an absolutely rigid body. Propellers, depending on which direction they rotate, are of left and right pitch. Unlike VFSh blades, adjustable-pitch propeller blades can rotate around their longitudinal axis and change the pitch, which makes it possible to use the full engine power at the optimal rotation speed in any mode of movement of the vessel. The calculation of a propeller consists of determining its geometric characteristics (diameter, pitch, disk ratio and number of blades), which provide the highest propulsion qualities to the vessel in its main operating mode. Thus, for a transport vessel, the specified characteristics should provide the highest speed, and for a towing vessel, the highest thrust on the hook with full use of the power of the main engines.
Advantages and disadvantages of the V R Sh compared to a conventional propeller: the ability to change the position of the blades of the V R Sh allows you to change the thrust force of the propeller without changing the frequency and direction of shaft rotation from full forward to zero, and then to full reverse. This allows the use of a non-reversible engine on a vessel, which is easier to maintain and whose service life is significantly longer than a reversible engine. Due to the fact that there is no need to reverse to change the thrust force of the propeller, but rather just turn the propeller blades, which is done remotely from the bridge, the time it takes for the vessel to transition from one mode of motion to
another is significantly reduced. This improves the maneuverability of the vessel and simplifies engine operation. But the V R Sh is much more complex in design, which reduces its reliability and increases its cost. At the same efficiency, V R Sh have greater weight and size than conventional screws, which complicates their fastening.
The main geometric elements and characteristics that determine the action of a boat propeller include:
1. Diamert
propeller D=2R, which is the diameter of the circle described by the point of the blade farthest from the axis of the propeller. The diameter of a boat propeller is determined by the area hydraulic, or worker, propulsion sections.
2. Radius
boat propeller R=0.5D - the distance from the axis of the propeller to the most distant point, called edge blades.
3. Geometric
, or constructive
, screw pitch - H, characterizing the possible movement of the screw per revolution when moving without sliding. The constructive pitch of the propeller is determined by the pitch of the helical lines forming the discharge (aft) surface of the blade of the boat propeller, and is found by the formula: H=2πrtgѵ,
where r is the radius of the blade section under consideration; tgѵ - tangent of the step angle at radius r.
There are different types of boat propellers constant step H = const and variable H = ϝ(r), for which the pitch along the radius of the blade varies according to some law.
4. Constructive attitude
H/D is the ratio of the design pitch of the propeller to its diameter.
5. Disk ratio
Θ, representing the ratio of the total area of all z blades to the area of the disk swept by the propeller, Θ=A/Ad=(2zbcp(D-dc))/(πDD),
6. Given
, or relative
, the radius of the blade, which is the ratio of the radius of the pressure of the blade R0 to the largest radius of the propeller R. It is usually assumed that the center of pressure, which characterizes the point of application of the resultant of all forces to the blade, coincides with the center of gravity of the straightened surface of the blade.
7. Blade section profile
, by which we mean the cross-sectional trace of a boat propeller blade rectified onto a plane by a circular cylinder coaxial with it at a given radius.
To characterize the operating mode of the propeller as a whole, the concept of step is used. The linear advance hp of a screw is the path traversed by the screw in the axial direction during one revolution = vp/hp
The ratio of the linear advance to the diameter of the screw is called the relative advance or simply the screw advance. λp=hp /D=Vp/nD
Screw pitch is the distance the screw travels in one revolution in your rigid body.
The dimensionless hydrodynamic characteristics of the propeller, presented in the form of curves as a function of the relative advance, are called action curves. With their help, you can determine the thrust, torque, and efficiency of the screw under various operating modes.
The graph also shows λ1=H1/D - zero stop pitch ratio or hydrodynamic step ratio.
λ2=H2/D - step ratio of zero torque.
When λp ›λ2, k2‹ 0, the propeller operates in turbine mode, creating torque due to the flow energy.
In the range of changes in relative speed λ1 ‹ λp‹ λ2, the propeller cannot be used either as a propulsion device or as a turbine. The working area of the propeller as a ship propulsion unit is the range of relative movements 0 ‹ λp ‹ λ1, where P› 0
In each specific case, at design conditions, the propeller must operate in a range of relative advances corresponding to high efficiency values, which is ensured by the appropriate choice of geometric characteristics of the propellers.
One of the most important goals of model hydrodynamic tests of propellers is to obtain systematic experimental materials necessary for the design of propellers. These materials are obtained by testing specific series of screws. When developing a series of models, they strive to obtain a systematic change in their most important structural elements, which significantly affect the hydrodynamic characteristics of the propeller.
Such elements are: pitch ratio H/D, disk ratio A/Ad, number of blades Z, relative thickness of the blade Ω, shape of the blade sections, its contour.
The test materials are presented in diagrams containing the action curves of the screws of the series, differing only in the pitch ratio. The diagram shows the curves of the thrust coefficient K1 and efficiency as a function of the step.
Each series of screws, differing in pitch ratio, is represented by two diagrams: a diagram plotted in the k1-λp axes and a diagram plotted in the k2-λp axes.
The first diagram is called the hull diagram; it is used when the starting point for calculating the propeller is the towing resistance of the ship’s hull, and the power of the power plant required to ensure the speed of the ship specified in the technical specifications for design. A diagram constructed in the k2-λp axes is called a machine diagram. This diagram is used when the power of the power plant of the designed vessel is given, and the achievable speed is the desired value.
The simplest form of a screw design task, which allows one to unambiguously determine the geometric elements of the screw within a given series, is the case when the rotation speed n, the screw diameter D, the translational speed of the screw Vp, as well as the required thrust or available power on the screw Np are indicated. Using these values, it is possible to calculate the relative advance λp and the thrust coefficient k2, which define a single point on the diagram field that uniquely determines the pitch ratio and efficiency of the screw.