The propeller construction, types, principle and operation mechanism.
The propeller is a device whose intended purpose is to create thrust pressure required for propulsion of the vessel.
The principle and operation mechanism of the propeller
Characteristics. The blade of the propeller. Diagram of forces and velocities on the propeller blade
Screw design. The number of blades. The diameter of the screw. Spoiler
Other important parameters and the performance of the propeller
Types of propellers. The fixed pitch propellers. The controllable pitch propellers.
The advantages and disadvantages of propellers
The propeller of the ship:
The propeller is a device whose intended purpose is to create thrust pressure required for propulsion of the vessel. This result is achieved due to the simple physical processes: a rotating shaft of the motor is converted into a force pushing the water transport on which it is placed, which ensures the motion of the ship.
If we consider this in detail, that during the rotation of the propeller each blade captures a lot of water and throws it back, telling her a given angular momentum, the reaction force of this drop of water transfers momentum to the rotor blades, the bladesin turn, – the propeller shaft through the hub, and the propeller shaft, furthermore, – the hull of the ship through the main thrust bearing.
The solution of this problem (ensuring the movement of the ship) gave the propeller another name – mover, but what he kind, made of what material and what construction is, depends on the speed and type of turn transport.
The principle and mechanism of operation of the propeller:
The basis of the mechanism of operation of the propeller – transform rotational shaft of the engine of the vessel in the force, forcing him to move, i.e. creating of the water column peculiar emphasis, which is like a normal boat, and the multi-ton cruiser can push and start (and later continue) the process.
The main component of the screw – blades, from the correct location which depends on the course of the machine. When construction begins rotation, on the surface of the blades are created by certain forces:
– on the side facing in the direction of motion (suction), there is a vacuum;
– on the side located against the course (forcing) – increased pressure of the water mass.
The difference in the resulting from different parties and pressure forms the required force (Y), called the lift. She, in turn, consists of forces directed in the direction of movement of the machine (P) and perpendicular to the vessel (T), so that:
– achieved the desired focusing for the operation of the screw;
– forms a torque whose overcoming is imposed on the engine.
Of great importance is the angle of attack of the blade profile (α), which is in the range of 4-8 degrees. The angle of attack is the angle formed between the velocity vector of the flow of water approaching the blade and the pressurizing surface of the blade. A higher setting will increase the torque and hence the engine performance will be spent for nothing. By reducing encounter the opposite situation: decrease the lifting force and emphasis, which will lead to under-utilization of engine power.
Characteristics. The blade of the propeller. Diagram of forces and velocities on the propeller blade:
On the specified drawing shows a diagram of forces and velocities on the blades of the propeller right rotation, where:
R – the force that creates the emphasis of the propeller,
T – power, the generator torque,
Y – lift force
W – water flow,
VA – velocity of the translational movement,
Vr is the peripheral speed of the screw. Vr = 2·π·r·n. Thus, the larger the value r of the propeller, the more the circumferential speed Vr, and therefore the total velocity W,
r is the radius of the propeller,
n – the number of revolutions of the propeller, R/h,
α – angle of attack
N – step screw. Pitch is the movement of any point of the blade along the axis for one full turn of the screw,
H·n – the theoretical speed of the screw along the axis. It represents the product of step screw on the rpm.
A mandatory part of the design of the propeller – the presence of the blades and the hub on which they are located. To obtain the desired angle of attack and the screw installation of the blades on the hub is:
– with equal distance between them,
– with the same angle of rotation relative to the plane of rotation.
Themselves blade may have a small or medium elongation, depending on the size and design of the machine, which will be established mover. In order for the screw was set in motion, it is placed on the propeller shaft, rotation of which provides the engine of the machine through the hub. When rotating blades grab the water and throw it, resulting in formation of the desired physical forces and impulses, the emphasis of water and, as a result, the water body of the vehicle starts the turn with thrust bearing.
The number of propeller blades:
The main difference in the design of the propeller is the number of blades, providing coefficient of performance (COP) of the device. So, the highest efficiency has a propeller with only two blades, but it is effective only at small disk relationship (about 0.5). At higher step disc of the relationship to 1-1. 5 (the ratio of the straightened blades to the area of the disk) to ensure the durability of the blades is very difficult, so use only water-vessels, where the load on the screw closer to the minimum (a racing yacht) or a screw is used as an auxiliary means of propulsion (sailing and motor vessels).
On small vessels the most widely propellers with 3 blades. Four and five blade propellers are usually used on large vessels, ocean liners, where their main tasks are not speed transport, and the provision of silence and vibration reduction.
The diameter of the propeller:
The diameter of the propeller is determined by the diameter of the circle, which describe the ends of the blades, located on the engine. Depending on the size of the vessel for which they are intended, the diameter size can vary from several tens of centimeters up to 5 meters.
“Giants” of the latter type are usually equipped with ocean liners, for the propulsion of which requires a considerable size of screws and cost of appropriate physical strength.
The name of this part of the structure translates to “invader” and completely justifies it. The spoiler is a curved edge located at the outgoing path of the blades on the propeller, and its main purpose is to increase the ability of the mover to capture the liquid. The presence of the spoiler is very important in applications where the motor is mounted very high and running trim has large angles.
Also install the “invader” allows you to:
– to further raise the nose of the vessel if it is installed on the lines of the angle of inclination of the blades;
– to increase blade pitch when installing it on the external and outgoing edges.
An important caveat: the installation of a spoiler reduces the number of revolutions of the screw in an average of 200-400 per minute, which requires a corresponding reduction step in an average of 1-2 inches.
Other important parameters and the performance of the propeller:
The speed of rotation of the propeller depends on the intensity of the speed of the vessel on which it is installed, but this parameter has an optimal performance. The average is up to 300 rpm, for large ships the optimal performance is not above 200. This is because high speeds increase the wear of engine parts that feel the most stress, and this leads to breakdowns, unplanned repairs or final coming in disrepair and expensive mechanism.
To set the axis of rotation of the propeller is recommended in a horizontal plane, it improves the performance parameters. In the presence of the inclination of the propeller shaft there is a “scythe” the water flow around the blades, causing the performance of the propeller is reduced, and the higher the angle, the greater the loss of efficiency. First loss power is tangible when you see the difference in 10 degrees.
Special attention requires equipment of large and heavy vessels, used in industry or defense. So, for tankers, nuclear – powered icebreakers, aircraft carriers and other vessels of large displacement actual presence and the ability to transmit high power. To do this, they equip two or three shaft installations as well as installing a few screws. Most often it is 4 propeller, arranged symmetrically. One of the important parameters of screws for Arctic icebreakers considers the strength, because they should be able to crush the thick ice when moving not only forward but also backward.
The types of propellers:
Types of propellers a lot. They can be manufactured from different materials (steel, bronze, brass, cast iron, plastic), have a different design (- molded, removable or rotating blades) and other fundamental differences that affect their work and, directly, the motion of the ship on which they are installed.
Another parameter differences of the propeller – the ability to control the angle of attack of the blades of the propeller. On this principle they are divided into fixed pitch propellers and controllable pitch propellers.
Fixed pitch propellers:
Fixed pitch propellers (VFS) is the propulsion, which are the only and constant angle of the blades, due to their method of production. Such propellers are cast in one piece, so they have small dimensions and weight. Place them mostly on cars small displacement:
– small size;
– shipsdesigned for trade;
– ships requiring increased strength of the screw and other.
The movement of such vessels is a long-term move in one direction, and therefore the maneuverability of fixed pitch propellers as the main feature goes by the wayside.
A variation of this mechanism – screws with detachable blades. Step remains fixed, but the design implies not cast fabrication and mounting of the blades to the disk of the propeller in one position. This gives the possibility of replacing in case of breakage of individual parts (blades), not the whole device, and allows to produce solid propulsion with large diameter, one-piece casting which is quite difficult.
The controllable pitch propellers:
The controllable pitch propellers (CPP) suggest the possibility of change of rotation of the blades in the hub. The components of the fastening screw is made in such a way that due to the special drive of the blade can rotate around its axis and, if necessary, change the angle of attack. This is achieved by the possibility of actuator, known as a mechanism for changing the step (MISH).
The mechanism of change step can be:
Part of the mechanism of change step (MISH), with the exception of the manual include: the mechanism of rotation of the blades, placed usually in the hub of the screw; servomotor creating the effort to rotate the blades and placed on the area between the propeller shaft and main engine; feedback or a device that shows the amount of the new pitch.
In turn, the mechanism of rotation of the blades forming part of the mechanism of change step can be:
– gear – used on the screws of small diameters and courts, do not involve the development of high capacity;
– crank is a high degree of reliability and durability, is applied on the intense designs, high speed screws, etc.
Is the mechanism of rotation of the blades within the hub of the propeller, which is reflected both on its size and on the size of the screw.
The most frequently used drive is considered the hydraulic actuator with controllable pitch propellers. In it the rotation of the blades is produced by the influence of fluids with small viscosity, and the device mechanism is relatively easier. Another advantage of hydraulics is the ability to create great working capacity even on small and light propulsion.
By controlling the screw remotely, directly from the bridge, relieved and coordination of the movement of the ship. The use of a small, but powerful and strong propulsion even on the overall the courts had improved their handling characteristics and maneuverability that made it possible to coordinate the pitch with any speed machine. As a result of such action, the performance of the propeller is increased by several times, and this reduces the overall operating costs of the vessel.
The advantages and disadvantages of propellers:
Despite technological advances, the propeller is not the ideal mechanism. So, his job as propulsion is possible only under the condition that the rotation speed is constant or increasing, otherwise the blade face of the layer of water, will act as a brake, and quite active.
Although theoretical calculations of the efficiency of the propeller reach the indicators of 75 %, he is not able to achieve these parameters, and they are usually in the range of 30-50 %. To create a perfect screw with an efficiency of 100% is impossible, because his work depends on the environmental conditions that constantly change.
Interesting fact: although the propeller is greatly facilitated human control vessels and allowed to move on the machines of considerable dimensions, its efficiency is still inferior to ordinary oars, the parameters of which reach 60-65%. If we compare the engine of a paddle wheel, the advantage is still for the mechanical device (paddle wheel): its productivity is higher and the dimensions and weight – less. However, in the event of damage to repair the paddle wheels to hold not only possible, but easier. Repair is solid propellers is impossible, and teams requires appropriate equipment, skills and held in a dock.
The advantages of mechanical propulsion (paddle wheels) is attributed to its lower vulnerability, which provide the dimensions and material from which it is made, i.e., they break much much less. While it is more secure for the inhabitants of the water world and the overboard people. With regard to defence and military industry, there is undoubted leadership for propellers. So, area of the propeller under the water allowed to use for military purposes the entire surface of existing decks, as well as virtually eliminate the possibility of hitting the driver of enemy projectiles.
The history of invention and modernization of the propeller is rooted in antiquity, but only with the development of technical progress mankind has been able to mechanisms, prototypes of which are still used today. However, this industry continues to evolve: scientists and inventors looking for alloys and materials for enhanced performance propulsion and develop designs that could eliminate or reduce their weaknesses.
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