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Marine Propeller Tips Collection.
Jul 14, 2018

Introduction to the propeller


A propeller consisting of a hub and a plurality of blades fixed radially to the hub, commonly known as a vane. The propeller is installed below the stern waterline, and the power is rotated by the main engine to push the water toward the ship, and the ship's reaction force is used to push the ship forward. The propeller is simple in construction, light in weight and high in efficiency, and is protected below the waterline.


Ordinary transport vessels have 1 or 2 propellers. Pushing a ship with a high power can increase the number of propellers. Large fast passenger ships range from sculls to four oars. The propeller generally has 3 to 4 blades, the diameter depends on the horsepower and draught of the ship, the lower end does not touch the bottom of the water, and the upper end does not exceed the full water line. The propeller speed should not be too high, the ocean cargo ship is about 100 rpm, and the speed of the small speedboat is as high as 400 to 500 rpm, but the efficiency will be affected. The propeller material is generally made of manganese bronze or a corrosion-resistant alloy, or stainless steel, nickel aluminum bronze or cast iron.


A disc-shaped spiral-faced propulsion device that drives the ship forward. It consists of a paddle and a hub connected thereto. Commonly used are three leaves, four leaves and five leaves. Including single propeller, long blade propeller, counter-rotating propeller, tandem propeller, adjustable pitch propeller, supercavitating propeller, large side inclined propeller, etc. The propeller is usually installed at the stern (underwater). Marine propellers are mostly made of copper alloy, and are also made of cast steel, cast iron, titanium alloy or non-metallic materials. The research on marine propellers is divided into two aspects: theory and experiment. Theoretical theory now has momentum theory theorem, leaf element body theory, lift line theory, lift surface theory, boundary element method and other theoretical and analytical methods, which can accurately predict the hydrodynamic performance of the propeller and theoretical design. The research in the test is mainly to study the propeller performance through model test and draw the propeller design map. The design method of marine propellers is divided into two categories, namely, theoretical design methods and map design methods.


Since the 1960s, ships have become larger, and after the use of high-powered main engines, the stern vibration, structural damage, noise, and erosion caused by propeller excitation have attracted the attention of various countries. The root cause of propeller excitation is that the propeller blade load is aggravated, and local unstable cavitation is easily generated when working in a non-uniform wake after the ship, resulting in constant changes in the pressure, amplitude and phase of the propeller acting on the hull. Propeller classification


On the basis of ordinary propellers, in order to improve performance, better adapt to various navigation conditions and make full use of host power, the following special propellers have been developed.


Adjustable pitch propeller


Referring to the pitching paddle, the pitch can be adjusted as needed to fully utilize the power of the host; the propulsion efficiency is improved, and the direction of rotation of the main engine is not changed when the ship is reversing. The pitch is adjusted by mechanically or hydraulically manipulating the mechanisms in the hub to rotate the blades. The pitch control paddle has better adaptability to blade load changes and is used more on tugs and fishing boats. For general transport vessels, the ship-machine-paddle can be placed in a good match. However, the hub diameter of the pitch paddle is much larger than that of the ordinary propeller. The section of the blade root is thick and narrow. Under normal operating conditions, the efficiency is lower than that of the ordinary propeller, and the price is expensive and the maintenance is complicated.


Catheter propeller


A circular duct with a wing-shaped cross section is added to the outer edge of the common propeller. This catheter is also known as a Koch's catheter. The fixed conduit of the conduit and the hull is fixed, and the conduit is connected to a rotatable guide which acts on the rotating rudder stock and acts as a rudder blade. The duct can increase the propulsion efficiency of the propeller because the internal flow velocity of the duct is high and the pressure is low. The pressure difference between the inside and outside of the duct forms an additional thrust on the duct wall; the gap between the duct and the propeller blade is small, which limits the tip of the blade. Flow loss; the conduit reduces the wake shrinkage after the propeller and reduces energy losses. However, the reversing performance of the catheter propeller is poor. The fixed duct propeller increases the ship's swing diameter, and the convertible duct can improve the ship's swing performance. Catheters Propellers are often used to push ships.


Tandem propeller


Mount two or three common propellers on the same shaft and rotate in the same direction at the same speed. When the diameter of the propeller is limited, it increases the area of the blade and absorbs a large amount of power, which is beneficial for damping or avoiding cavitation. The tandem propeller has a large weight and the paddle shaft extends for a long time, which increases the difficulty in arrangement and installation, and has less application.


Counter-rotating propeller


The two ordinary propellers are mounted one after the other on the inner and outer shafts of the concentric, and rotate in opposite directions at the same speed. Because it can reduce the wake loss of the wake, the efficiency is slightly higher than that of the single paddle, but its shafting structure is complicated and has not been applied on the big ship. 5 straight blade propeller: consists of 4 to 8 vertical blades. The upper part of the straight blade propeller is disc-shaped, the blade is evenly installed along the circumference of the disc, and the bottom of the disc is flush with the hull plate. When the disc rotates, the blade rotates around the main axis and also rotates around its own vertical axis. Swing, which produces thrust in different directions, so that the ship can be turned in place without having to turn with the rudder, and the ship does not have to change the steering of the main engine when it is reversed. However, due to the complexity of the mechanism and the high price, the blades are easily damaged and are used only for a few port ships or ships with special requirements for handling performance. Propeller installation


There are two ways to install the propeller on the shaft, one is a keyed installation and the other is a keyless installation.


With the continuous development of shipbuilding technology, due to the structure of the propeller and the shaft without key connection, the stress concentration of the shaft is avoided by machining the keyway on the propeller shaft. Therefore, the ship is more and more used to install the screw without key, and gradually replaces the key installation. The method of installing the propeller without a key hydraulic sleeve is shown in the figure. The propeller is sleeved on the boring shaft, and the hydraulic nut is installed. The high-pressure oil pumped by the oil pump passes through the high-pressure oil pipe from the oil groove of the inner hole of the hub into the joint surface of the boring shaft and the propeller, so that the hub is elastically deformed and swollen. open. At the same time, high pressure oil is supplied to the hydraulic nut, and the hydraulic nut generates forward thrust to move the propeller forward. When the propeller is pushed to the specified position, the high-pressure oil of the hub taper hole is first discharged, and then the high-pressure oil in the hydraulic nut is discharged. Due to the recovery of the elastic deformation of the hub, the propeller is tightly fitted on the boring shaft.


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