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Marine Propeller Trivia

Views: 5     Author: Site Editor     Publish Time: 2020-04-20      Origin: Site

Introduction of propeller

By the hub and a number of radial fixed to the hub of the propeller, commonly known as the vehicle blade. The propeller, which is mounted below the stern line and rotated by the main engine, pushes the water to the back of the ship and uses the water's reaction to propel the boat forward. The propeller is simple in construction, light in weight, high in efficiency, and protected below the waterline.

A GTS has one or two propellers. A ship of high propulsive power may increase the number of propellers. Large fast passenger ships have two to four oars. Propeller generally have 3 ~ 4 blades, diameter according to the ship's horsepower and draft, the lower end does not touch the bottom, the upper end does not exceed the full load waterline. The speed of propeller should not be too high. The speed of ocean freighter is about 100 RPM. The speed of small speedboat is as high as 400 ~ 500 RPM, but the efficiency will be affected. Propeller materials are generally used manganese bronze or corrosion-resistant alloy, stainless steel, nickel-aluminum bronze or cast iron.

A propeller with a disk-shaped spiral surface that drives a ship forward. The utility model is composed of a paddle blade and a hub connected with the paddle blade. Three leaves, four leaves and five leaves are commonly used. It includes single propeller, ducted propeller, counter-rotating propeller, tandem propeller, controllable pitch propeller, supercavitating propeller and large skewed propeller. The propeller is usually mounted aft (underwater) . Marine propellers are mostly made of copper alloys, cast steel, cast iron, titanium alloys or non-metallic materials. The research on marine propeller is divided into theoretical and experimental aspects. The momentum theorem, blade element theory, lifting line theory, lifting surface theory, boundary element method and other theories and analysis methods are available, which can accurately predict the hydrodynamic performance of propeller and carry out theoretical design. The experimental research is mainly through the model test to study the performance of the propeller, drawing propeller design chart. The design method of marine propeller can be divided into two categories: theoretical design method and graphic design method.

Since the 1960s, the ship tends to be large-scale, after the use of high-power main engine, the stern vibration, structural damage, noise, erosion and other problems caused by propeller-induced vibration have attracted the attention of many countries. The basic reason of propeller exciting vibration is that the load on the propeller blade is heavier, and when the propeller is working in the non-uniform wake behind the ship, it is easy to produce local unstable cavitation, as a result, the pressure, amplitude and phase of the propeller acting on the hull are constantly changing.

Classification of propeller

On the basis of the common propeller, in order to improve the performance, better adapt to various navigation conditions and make full use of the main engine power, the following special propeller was developed.

Controllable pitch propeller

The controllable pitch propeller can adjust the pitch according to the need, give full play to the power of the main engine, improve the propulsion efficiency, and do not change the main engine rotation direction when the ship retrogrades. The pitch is adjusted by turning each blade by a mechanism in the hub, which is operated mechanically or hydraulically. The controllable pitch propeller has good adaptability to the change of blade load, and is widely used in tugboats and fishing boats. For the general transport ship, the ship-engine-propeller can be in a good matching state. But the hub diameter of the controllable pitch propeller is much larger than that of the ordinary propeller, and the section of the blade root is thick and narrow.

Ducted propeller

A circular duct with wing-shaped section is added to the outer edge of the common propeller. This catheter is also known as the Kohlrabi Catheter. The vessel is fixedly connected with the hull, and the vessel is connected to the rotating rudder bar and acts as a rudder vane. The ducting can improve the propulsive efficiency of the propeller because of the high velocity and low pressure inside the ducting, and the pressure difference between the ducting and the outside of the ducting forms additional thrust on the pipe wall, the flow around the tip of the propeller is limited, and the ducting can reduce the wake contraction behind the propeller and reduce the energy loss. However, the performance of the ducted propeller is poor. The fixed duct propeller increases the turning diameter of the ship, and the rotary duct can improve the turning performance of the ship. Ducted propellers are often used to propel ships.

Tandem propeller

Two or three ordinary propellers are mounted on the same shaft and rotated in the same direction at the same speed. When the diameter of the propeller is limited, it can increase the blade area and absorb more power, which is beneficial to reducing vibration or avoiding cavitation. The tandem propeller has more weight and longer shaft, which makes the arrangement and installation more difficult and less applied.

Counter-rotating propeller

The two ordinary propellers are respectively mounted on the concentric inner and outer axes and rotated in the opposite direction at the same speed. The efficiency is slightly higher than that of the single propeller because the loss of wake rotation can be reduced, but the shafting structure of the propeller is complex and has not been used in large ships. 5 STRAIGHT BLADE PROPELLER: It is composed of 4 ~ 8 vertical blades. The upper part of the straight-blade propeller is a disc-shaped one, and the blades are evenly arranged along the circumference of the disc. The bottom of the disc is connected with the hull plate Zippin. When the disc rotates, the blades swing around their own vertical axis, in addition to rotating around the main axis, thus generating thrust in different directions, therefore, the ship can be turned in situ, without the rudder, the ship does not have to reverse when the change in the main engine steering. However, due to complex structure, high price and easily damaged blades, it is only used in a few port ships or ships with special requirements for maneuverability.

Propeller installation

The propeller is mounted on the shaft in two ways. One is keyless mounting and the other is keyless mounting.

With the development of shipbuilding technology, the keyless installation of propellers is more and more used in ships because the keyless installation and connection structure of propellers and shafts avoids the stress concentration of shafts caused by the machining of keyways on propeller shafts, step-by-step replacement of keyless installation. Keyless hydraulic sleeve installation method of the propeller, as shown in figure. The propeller is sheathed on the stern shaft, the hydraulic nut is installed, and the high-pressure oil from the oil pump enters the fitting place between the stern shaft and the propeller by the oil groove of the inner hole of the propeller hub through the high-pressure oil pipe of the oil pump. At the same time, to the hydraulic nut to supply high-pressure oil, hydraulic nut to produce forward thrust, so that the propeller forward. When the propeller is pushed to the specified position, the high pressure oil in the hub cone hole is first put away, and then the high pressure oil in the hydraulic nut is released.

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