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Application Of Marine Propeller
Aug 23, 2018

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 400-500 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.

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.

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.

1 Adjustable pitch propeller: referred to as the pitch control paddle, the pitch can be adjusted as needed, and the power of the main engine can be fully utilized; the propulsion efficiency can be improved, and the rotation direction of the main machine can be not changed when the ship is reversed. 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.

2 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 the rotating rudder stock and the rudder blade acts as a rotator. 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. Pipeline propellers are often used to push ships. 

3 tandem propellers: Two or three common propellers are mounted on the same shaft and rotate in the same direction at the same speed. When the diameter of the propeller is limited, it can increase the area of the blade and absorb more 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.

4 Counter-rotating propellers: 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 complex 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, the high price, the blade is easily damaged, and it is only used for a few port ships or ships with special requirements for handling performance.