If you want to move forward, you have to push from backward. Well, propeller does this work in ships. But, Will its construction vary from ship to ship? If yes, what are pitch propellers? what are its types? Pros and cons of FPP and CPP? Why CPP is preferred over FPP or vice versa? We will look after them in this article.
Pitch on a propeller: It is the distance that a propeller can move the ship in one complete revolution.
- Higher the pitch- higher will be the speed (less acceleration)
- Lower the pitch- higher will be the acceleration (less speed)
Pitch Propeller types: 1. Controllable Pitch propeller (CPP) 2.Fixed pitch propeller (FPP)
When the ship is running ahead, both the propellers work and look the same. The difference comes when we want to reduce the speed or to move the ship in astern (Reverse) direction.
Controllable Pitch Propeller(CPP): To reduce the speed of the ship, the propeller blades change their angle of attack at minimum angle. To move the ship in astern direction, the propeller completely alter their blades angle in opposite direction, as shown in the figure below.
Fixed Pitch Propeller(FPP): when we want to reduce the speed of the ship, we have to slow the rotation of the propeller. To move the ship in astern direction, we literally stop the engine and reverse the direction of rotation of the propeller.
PROS & CONS of pitch propellers:
- A non-reversible engine can be used for both forward and astern direction of the ship using CPP, by just changing the angle of blades.
- Speed of the propeller can be reduced simply without altering the rpm of the main engine. So, the speed of the ship can be handled from the bridge directly.
- CPP is complicated in installation and requires constant inspection.
- CPP requires very high initial cost of installation.
- The length and diameter of hub is larger in CPP.
Conclusion: All modern ships are installed with Reversible main Engine. A ship with Reversible main engine prefers FPP over CPP. This reduces the initial cost of installation and maintenance cost. Propellers can be classified on the basis of various factors. In this article, we have seen how a propeller can be classified on the basis of its pitch, why it is preferred, what is FPP & CPP, and their advantages over each other.
Facts:
- Emma Maersk is propelled using the largest propeller, weighing around 131 Tons.
- Oasis of ships has rotatable-multi propellers, which makes the manoeuvring simple.
- Titanic was propelled using 3 propellers, each having three blades. Each propeller was powered by separate engines.
To know how are propellers manufactured, watch the videos below:-
Authored By:- Cdt. Krishnasai Venkatesan, TMI
