How does a Ship move forward? - Propeller and Thrust explained

Introduction

            Modern ships are fitted with Propeller(s) at the aft (back portion) of the ship. The propeller looks like a fan and is fitted to the underwater portion of the ship. The ship moves forward when the propeller is rotated.

Working Principle

The working of a propeller is similar to how we swim. While swimming, we use our hands to push the water backwards to move forward. The force which pushes us forward is called as Thrust. A ceiling fan sucks the air above the fan and pushes it down. Similarly, a propeller sucks the water present in front of it and throws it backwards. By pushing the water back, the propeller gets a thrust. Thrust is the force which pushes the ship forward. Propeller acts like an axial water pump developing axial thrust.

Force and Thrust

            Thrust is the force which acts on the propeller because of the resistance of water. Here is an example to help you understand the difference between force and thrust. If you try to push the wall sitting on a rolling chair; you will be pushed back. The push you give on the wall is called force and the force which pushes you back is called thrust. Thrust is also a force which acts on a direction; opposite to the direction of application of force.

Thrust Transmission

            Similarly, a propeller exerts a force on the water and it gets back thrust because of the resistance of the water. This thrust pushes the propeller forward. This thrust has to be transmitted to the structure of the ship or else the propeller alone will move forward and pierce the hull of the ship. The thrust is transmitted to the hull by Thrust block.

How does a SHIP float?

             Ship is a very big steel structure but still it floats in water. If you drop a steel plate in a tank, it will sink but if you make a small steel cubical tank, it will float in water.

             If you throw a stone it goes down into the water; but if you stick a piece of thermocole to the stone, it will not sink. By sticking the thermocole, you are making the whole body ( stone + thermocole) lighter than the same volume of sea water. The same happens with the ship. The ship is made of steel, but it has lots of empty space filled with air. Here, air acts as the thermocole making the whole structure lighter and the ship now floats with water.

                Any body which is lighter than the sea water will sink into the sea. Steel is heavier than sea water. But still we can make it float by increasing its volume without increase in weight. When you make a small steel container, the weight of the structure includes the air which is present inside the container. When you compare the weight of the container with the weight of same volume of water; the volume of water will be heavier than the empty steel container. This makes the steel container to float.

            A ship is not a mould of steel. Only the structure is steel and it has lots of empty space filled with air. Air is very light weight ( about 1.27 kg/m³  and density of water is 1000 kg/m³ ) when compared with sea water. So, the weight of sea water for the space occupied by the ship (volume) is very much higher than the weight of the ship. This makes the ship float in water.

            This also explains why a ship sinks. If the ships bottom part gets ruptured, water starts coming in filling the space which was previously occupied by air. This makes the ship heavier and it goes down the waterline.

             Friends, I have tried to make the concept as simple as possible.Please give me your suggestions to improve my writing. Thank you 

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