Tuesday, 17 April 2018

Types of Propellers

Introduction

A propeller is a fan like rotating structure generally at the aft of a ship imparting thrust to propel the ship. Most ships use the conventional screw propeller. But in some ships depending upon the need and requirement, different types of propellers are used.

The different types of propellers with varying characteristics are listed below.

Paddle Wheel

Image Courtesy- Google Images.


 It is a very simple type of propulsion system. It consists of wheels with paddles attached at its periphery. It has its axis of rotation about the transverse direction of the ship above the waterline. The paddles get immersed in water only when they are at the bottom of the wheel. As the paddles rotate in water, they accelerate it, experiencing a reactive thrust from the water which is transmitted to the ship.
 It is of two types


  • FIXED: These are simple and easy to construct. In this type, the paddles are firmly attached to the wheel. This system requires wheels of large diameter so that they can enter the water at large angles. Large diameter means low rpm and more weight. This is the main drawback of this system.


  • FEATHERING: In this system, the paddles are attached at the ends of the wheel in such a manner that the angle of entrance is much larger than that of fixed type.By this principle, the diameter of the wheel can be reduced.

Tandem propeller

Image Courtesy- Google Images.


In this arrangement, two propellers are mounted on a single shaft turning in the same direction. Tandem propellers are fixed so as to reduce loading on a single propeller as it can lead to cavitation. Here the thrust is divided between the two propellers.
In normal loading, they are not of much use but in heavy loading, they produce better loading than a single propeller.
Generally, size and number of blades are kept the same

Overlapping propeller


Image Courtesy- Google Images.


It has the same advantage as a tandem propeller as the load is divided between two propellers. There are two propellers with their shafts placed at a horizontal distance less than the diameter of either propeller.
They have higher hull efficiency because they work in a region of a higher wake. The advantage over twin screw is that no extra appendage is required to support and hence the resistance is reduced.
Sometimes the mutual interaction between the propellers may, however, result in more cavitation.


Controllable Pitch Propeller

Image Courtesy- Google Images.


In this type of propeller, the blades are not directly fixed to the boss but attached to separate spindles. The spindles can be turned about the axis and so the pitch of the propeller can be altered. These are mainly used in ships requiring full power at varying speeds and resistances.These are mainly used in tugs, ferries, icebreakers etc.

Some advantages over the conventional fixed propeller are
  • They provide better acceleration, stopping and manoeuvring properties.
  • Non reversing propulsion machinery may be used thereby reducing its cost, weight and space occupied.
  • At all loading conditions, the full power of machinery can be used.
  • The speed of the ship can be varied without altering the speed of the main engine.
  • Speed can be directly controlled from navigation bridge.
  • It is easy to replace damaged blades.
Some disadvantages are:
  • The control mechanism is very complex.
  • It has high initial cost.
  • Maintainance costs are also high



Ducted Propeller

Image Courtesy- Google Images.


In this type a non -rotating duct surrounds a screw propeller.
The gap between the propeller and duct is very minute.
These are mainly of two types:




  • ACCELERATING TYPE: They increase the velocity of flow of water to the propeller.


  • DECELERATING TYPE: They decrease the velocity of flow of water to the propeller.

Some advantages of ducted propeller over normal propeller are:
  • Better course stability
  • Less effect of load and speed variation on efficiency
  • Fewer chances of damage to the propeller
  • Improved efficiency at high loading
Some disadvantages of the ducted propeller are
  • More chances of cavitation
  • Poor astern performance


Supercavitating Propeller


Image Courtesy- Google Images.


Supercavitating propellers are used when the design criteria of the propeller are such that cavitation cannot be mitigated. It can produce very high thrust at same efficiency without cavitation, corrosion and noise. Ships with high engine power, speed and rpm deploy such propellers.The back of the propeller blade is covered by vapour filled cavity.There is a separation of flow on the back at the leading and trailing edge.The main objective is that the back of the blade should not be in contact.
 These type of propellers have however less blade strength owing to a thin leading edge of the blade.
They also don’t work properly at low speeds.

Surface Piercing Propeller

Image Courtesy- Google Images.


These type of propellers are partly submerged in water.
These are fitted just at the end of the ship rather than under it. The propeller shaft is just above the water surface. Since no extra appendages are required the drag resistance is considerably reduced.

Some advantages of the surface piercing propeller are:

  • It requires less power to achieve the same speed as compared to the fully submerged propeller.
  • Cavitation is considerably reduced.
  • Since it is not vulnerable to cavitation, they can have low blade area


Some disadvantages of the surface piercing propeller are:
  • Since the blades enter and leave the surface at each revolution, it is subjected to periodic loading and can lead to fatigue.
  • They have very poor astern performance.
  • Difficult to operate at low speeds


Contra-rotating propeller


Image Courtesy- Google Images.


It uses two propellers placed on two coaxial shafts.The propellers rotate in opposite directions.It helps to reduce the rotational energy losses caused in the slipstream.
Some advantages of contra-rotating propellers are
  • Loading is shared between two propellers and hence cavitation is reduced.
  • Efficiency is higher than a single propeller.
  • Less pressure fluctuations and noise

Some disadvantages are:
  • Greater weight of machinery at the aft
  • Complexity of gearing and coaxial shafts
  • Sealing of the shafting system is difficult


Azimuth propeller


Image Courtesy- Google Images.


An azimuth propeller is a configuration of marine propellers placed in pods which can rotate at any horizontal angle.
Some advantages of azimuth propellers are:
  • Rudder use is not required
  • Excellent manoeuvrability
  • Good astern performance
  • Good speed control
  • Vibrations are less

This is a video made by Team LearnShipDesign on some basic type of propellers. Hope you all like it.


Article By- Anil Kumar Singh


1 comment:

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