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Propellers – Types, Construction and Working

What is Propeller ?        

Propeller

A propeller of ship is a fan like rotating structure which is used to propel the ship by using the power generated and transmitted by the main engine of the ship. Thrust created by the rotting propeller is used to move the ship from one point to another. The thrust generated by the rotation of propeller is totally depends on the number of blades and the speed of rotation of propeller. Ships are fitted with one, two or rarely three propellers depending upon the speed  and manoeuvring requirement of the vessel.

Types of Propeller

Propellers used on Merchants Ships are classified on the basis of several factors stated as follows:

Classification by number of blades  attached:

1. Three blade propeller:

  • Gives a good high-speed performance.
  • The acceleration is better than other types.

2. Four blade propeller:

  • Has a better holding power in rough seas.
  • 4 blade propeller provides a better fuel economy than all the other types.

3.  Five blade propeller:

  •  Vibration is minimal from all the other types.
  • 5 blade propellers have better holding power in rough seas.

4. Six blade propeller:

  • Manufacturing cost is high
  • Vibration is minimal from all the other types.

Classification by pitch of blade:

1.Fixed Pitch Propeller : It is the most common type propeller used on merchant ships. The twist in the blade results in the forward motion of ship with each rotation of propeller. Fixed pitch propellers are robust and reliable as the system doesn’t incorporate any mechanical and hydraulic connection as in Controlled Pitch Propeller (CPP). The manufacturing, installation and operational costs are lower than controlled pitch propeller (CPP) type.

Propeller

2.Controlled Pitch Propeller (CPP) : It is type of propeller fitted with mechanism which enables blades to move simultaneously while rotating to vary the pitch angle and thus the pitch. The maneuverability of Controlled Pitch Propeller is efficient than Fixed Pitch Propeller

Construction of Ship’s Propeller

Marine propeller are made from corrosion resistant material. The material used for construction of marine propeller are an alloy of aluminium and stainless still. Other materials are also used like alloys of nickel, aluminium and bronze which are 10~15% lighter than other materials and have higher strength.

The propeller’s construction process involves connecting several blades to the hub or boss by forging or welding in one piece. Forged blades are highly stable and have higher strength but are costly compared to welded ones. A marine propeller is assembled by sections of helicoidal surfaces acting together to twist through the water with a screw effect.

The ship Propeller is connected to main engine via different shafts,which are given as follows:

  1. Thrust Shaft
  2. Intermediate Shaft
  3. Tail Shaft

There are many variables for designing the propeller such as pitch angle, flow angle, chord distribution at the blade span and twist distribution. Blade elements theory will be used for designing the propeller.

Working of Propeller

A ship propels on the basis of Bernoulli’s principle. A ship moves from one place to another with the help of movement of propeller. Propeller is connected to the main engine via shaft which helps to rotate the propeller with the help of power generated by main engine.

As the propeller starts to rotate the movement of blades which are set at particular pitch produce the thrust. The thrust from the propeller is transmitted to move the ship through water and push the ship forward and astern depending upon direction of rotation of propeller.

Worlds Largest Ship Propeller

The firm Mecklenburger Metallguss GmbH (MMG) of Germany is considered a world leader in the design and production of ship propellers for large container vessels.

In 2006, MMG produced the world’s largest ship propeller for the Emma Maersk. Measuring a stunning 1,302 feet (397m) long, the Emma Maersk is one of the longest container ships ever constructed. The metal used to cast the Emma Maersk propeller is an alloy of copper, aluminum, nickel, iron, and manganese. The six-blade, single-piece propeller took one and a half years of development and planning alone.

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