A propeller is a rotating fan like structure which is used to propel the ship by using the power generated and transmitted by the main engine of the ship. The transmitted power is converted from rotational motion to generate a thrust which imparts momentum to the water, resulting in a force that acts on the ship and pushes it forward.
A ship propels on the basis of Bernoulli’s principle and Newton’s third law. A pressure difference is created on the forward and aft side of the blade and water is accelerated behind the blades.
The thrust from the propeller is transmitted to move the ship through a transmission system which consists of a rotational motion generated by the main engine crank shaft, intermediate shaft and its bearings, stern tube shaft and its bearing and finally by the propeller itself.
Material and Construction of Propeller
Marine propellers are made from corrosion resistant materials as they are made operational directly in sea water which is a corrosion accelerator. The materials used for making marine propeller are alloy of aluminium and stainless steel. Other popular materials used are alloys of nickel, aluminium and bronze which are 10~15 % lighter than other materials and have higher strength.
The construction process of the propeller includes attaching a number of blades to the hub or boss by welding or forging in one piece. Forged blades are highly reliable and have greater strength but are expensive as compare to welded ones. A marine propeller is constructed by sections of helicoidal surfaces acting together to rotate through water with a screw effect.
Types of Propeller
Propellers are be classified on the basis of several factors. The classification of different types of propellers is shown below:
A) Classification by Number of Blades Attached:
Propeller blades may vary from 3 blade propeller to 4 blade propeller and sometimes even 5 blade propeller. However, the most commonly used are 3 blades and 4 blade propellers.
3 blade propeller:
A 3 blade propeller has following characteristics:
- The manufacturing cost is lower than other types.
- Are normally made up of aluminium alloy.
- Gives a good high speed performance.
- The acceleration is better than other types.
- Low speed handling is not much efficient.
4 blade propeller:
A 4 blade propeller has following characteristics:
- The manufacturing cost is higher than 3 blade propellers.
- 4 blade propellers are normally made up of stainless steel alloys.
- Have better strength and durability.
- Gives a good low speed handling and performance.
- Has a better holding power in rough seas.
- 4 blade propeller provides a better fuel economy than all the other types.
5 blade propeller:
A 5 blade propeller has following characteristics:
- Manufacturing cost is higher of all.
- Vibration is minimal from all the other types.
- 5 blade propellers have better holding power in rough seas.
B) Classification By pitch of the blade:
Pitch of a propeller can be defined as the displacement that a propeller makes for every full revolution of 360 ̊. The classification of the propellers on the basis of pitch is as follows.
Fixed Pitch Propeller:
The blades in fixed pitch propeller are permanently attached to the hub. The fixed pitch type propellers are casted and the position of the blades and hence the position of the pitch is permanently fixed and cannot be changed during the operation.
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. The manoeuvrability of fixed pitch propeller is also not as good as CPP.
Controllable Pitch Propeller:
In Controlled Pitch type propeller, it is possible to alter the pitch by rotating the blade about its vertical axis by means of mechanical and hydraulic arrangement. This helps in driving the propulsion machinery at constant load with no reversing mechanism required as the pitch can be altered to match the required operating condition. Thus the manoeuvrability improves and the engine efficiency also increases.
This drawback includes the possibility of oil pollution as the hydraulic oil in the boss which is used for controlling the pitch may leak out. It is a complex and expensive system from both installation and operational point. Moreover, the pitch can get stuck in one position, making it difficult to manoeuvre the engine.
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