What Is the Type of Propeller and Construction of Propellers?

A propeller is a rotating fan-shaped structure designed to propel a ship using the power generated and also transmitted by the ship’s main engine. The transmitted force is converted from rotational motion to thrust, which imparts momentum to the water and produces a force that acts on the ship and pushes it forward. The ship is propelled by Bernoulli’s theorem and also Newton’s third law. The thrust of the Propeller is transmitted through a transmission system consisting of the main engine crankshaft, the intermediate shaft and its bearings, the stern tube shaft and its bearings, and finally, the rotational motion produced by the Propeller itself, which propels the vessel. A ship may be equipped with one, two, or rarely three types of propeller, depending on the speed and maneuvering requirements of the ship.

Propeller Materials and Construction 

Ship propellers are made of corrosion-resistant materials because they operate directly in seawater, where corrosion is accelerated. The ship’s Propeller is made of aluminum and stainless steel alloy. Other common materials include nickel, aluminum, and bronze alloys, which are 10-15% lighter and stronger than other materials.

The manufacturing process for the Propeller involves attaching multiple blades integrally to the hub or hubs by welding or forging. Forged blades are reliable and strong, but they are more expensive than welded blades. A ship’s Propeller consists of sections of helical surfaces that work together to rotate underwater in a screw effect.

Propeller Types 

propellers are classified based on several factors. The classification of an different types of propellers is shown below.

Classification according to the number of blades installed

Propeller blades vary from 3-blade propellers to 4-blade propellers to 5-blade propellers. However, three-blade and four-blade propellers are most commonly used. However, four-blade and five-blade propellers are most commonly used.

Propeller efficiency is highest for propellers with a minimum number of blades. H. Two-blade propeller. However, two-blade propellers are not used in commercial ships in order to achieve a strength factor and take into account the high loads exposed to the ship, sea and weather.

3-Blade Propeller 

The 3-Blade Propeller has the following features:  ・Manufacturing cost is lower than other types.

  1. Usually made of aluminum alloy.
  2. Provides excellent high-speed performance.
  3. Better acceleration than other types.
  4. Low-speed handling is not very efficient.

4-Blade Propeller  

The 4-blade Propeller has the following characteristics: 

  1. Manufacturing costs are higher than 3-blade propellers.
  2. Four-blade propellers are usually made of stainless steel alloys.
  3. Increases strength and durability.
  4. Provides excellent handling and also performance at low speeds.
  5. Improves holding power in rough seas.
  6. Four-blade propellers have lower fuel consumption than all other types.

5-Blade Propeller 

The 5-Blade Propeller has the following characteristics: 

  1. Higher overall manufacturing costs.
  2. All other types have minimal vibration.
  3. A 5-blade Propeller provides excellent holding power in rough seas.

 6-Blade Propeller 

  1. High manufacturing costs 
  2. All other types have minimal vibration.
  3. The 6-blade Propeller provides excellent holding power in rough seas.
  4. Using a six-blade propeller reduces the induced pressure field across the Propeller.

 Large container ships are mainly equipped with five or 6-blade propellers.

Classification by Blade Pitch

Propeller pitch can be defined as the displacement that occurs each time the Propeller makes a complete rotation of 360°. The classification of propellers by pitch is as follows.

Fixed Propeller 

The blades of a fixed propeller are rigidly connected to a hub. Fixed propellers are cast, and the blade position and pitch position are permanently fixed and cannot be changed during operation. Usually made of copper alloy.

Fixed-pitch propellers are robust and reliable because the system does not include mechanical and hydraulic connections like controlled pitch propellers (CPP). Manufacturing, installation, and operating costs are lower than controlled pitch propeller (CPP) types. The manoeuvrability of fixed propellers is also not as good as that of CPP. This type of Propeller is installed on ships that do not have good manoeuvrability requirements.

Adjustable Pitch Propeller 

Adjustable pitch propellers allow the pitch to be changed by rotating the blades around a vertical axis using mechanical and also hydraulic mechanisms. This allows the pitch to be changed to match the required operating conditions,also allowing the prime mover machine to be driven at a constant load without the need for a reversing mechanism. This improves manoeuvrability and also improves engine efficiency. 

Disadvantages include the possibility of oil contamination as the hydraulic oil in the hub used to control pitch can leak. This is a complex and expensive system both in terms of installation and operation. Additionally, the pitch may lock him in one position and make engine operation difficult.

However, the CP propeller has a larger hub to account for the blade adjustment mechanism and also piping, resulting in slightly lower propeller efficiency than the FP propeller of the same size.

Propeller Dimensions

In general, larger-diameter propellers are more efficient. However, the actual dimensions of the Propeller will vary depending on the type of vessel used and the following factors: 

  1. Aft hull construction and also ship design 
  2. Clearance requirements between the ship’s nose and also the hull 
  3. General Ship’s ballast condition. For tankers and bulk carriers, the propeller size is smaller compared to containers.
  4. Design draft of the ship 

Approximate Propeller Dimensions 

  1. For container ships, d/D = 0.74 
  2. For bulk carriers and tankers, d/D = 0.65 

where d- propeller diameter, D- design draft 

How do Ship Propellers Work?

For land vehicles, the propulsion system that drives the vehicle is different. In these systems, a motor drives a shaft attached to the vehicle’s tires to move it in front of the vehicle. However, ships traveling underwater do not have such tires or surfaces.

A ship moves underwater and a propeller is used to propel the ship forward or backward depending on the direction of rotation or pitch of the Propeller. The ship’s engine is connected to the ship’s Propeller through a shaft arrangement. When the engine rotates the Propeller, the radial blades set at a constant pitch draw a spiral like a screw. Converts rotational force into linear thrust.

This linear thrust acts on the water in such a way that as the propeller blades rotate, pressure is created between the front and rear surfaces of the water. This accelerates the fluid mass in one direction, creating a reaction force that helps propel the object (ship) attached to the Propeller forward.

To move the ship in the opposite direction, rotate the engine and Propeller counterclockwise. This reverses the thrust and causes the ship to move backward. However, the FP’s propeller motor is designed to always rotate clockwise when moving forward, so driving in reverse for long periods of time becomes inefficient.

For ships with CP propellers, the direction of rotation of the engine is not affected, so the reverse efficiency of the ship is better than with fixed pitch propellers.

Types of Propeller Shafts 

A ship’s engine is connected to the Propeller through various interconnected shafts. These can be named as follows: 

1. Thrust Shaft 

2. Intermediate Shaft 

3. Stern Shaft 

Thrust Shaft

The crankshaft of the engine is an first connected to the thrust shaft, through which the thrust bearing passes its main function. It is to provide thrust to the transferred ship structure. The thrust bearing housing has a similar construction to the main engine base housing, and the bearings are lubricated with oil from the main engine lubrication system. The material of the thrust shaft was usually solid forged billet steel.

Intermediate Shaft

The thrust shaft is connected to a long intermediate shaft. The intermediate shaft is supplied in parts and assembled using heavy-duty forged couplings. The length and number of intermediate shafts assembled depend on the location of the main engine. Larger ships have a greater distance between the main engine and the Propeller. The material of the intermediate shaft is an usually solid forged billet steel.

Tail Shaft

The Tail Shaft, was the name suggests, is the end portion of the shaft assembly that supports the Propeller. The stern shaft itself is supported by a lubricated stern tube bearing with seals and, together with the Propeller, projects from the ship’s engine room into the open sea.

Lubrication systems can be oil-based or water-based. The tail shaft transmits engine power and kinetic drive to the Propeller. The material of the tail shaft is typically a high-strength duplex stainless steel alloy.

Why the Propeller Rotates So Powerfully 

The types of propeller is powered by the engine to turn the ship and propel it in the desired direction. If the power supplied to the Propeller does not produce the same speed, the Propeller is considered to be in a difficult operating condition. This may be due to the following reasons: 

  1. Damage to propeller blades 
  2. Drag leading to changes in the wake due to contamination of larger hulls 
  3. Rough seas/rough seas 
  4. Ships sailing against the current 
  5. Ships sailing under light ballast 
  6. Ships sailing in shallow waters 
  7. Ships with a flat stern
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