Being a crucial component of ship propulsion systems, aeroengines, and industrial fluid equipment, the propeller impeller has complex geometric shape, strict performance requirements, and extremely high machining difficulty. Combining my practical experience in numerical control machining, the paper systematically expounds on the entire process of propeller impellers, from material selection, modeling design, machining path generation to finishing and quality inspection. By using multi-axis CNC technology and process planning technology, impeller machining not only meets the requirements of high precision and high surface quality but also demonstrates the outstanding advantages of modern manufacturing technology in machining complex curved surface parts.
The History of the Propeller
The British ship “Archimedes” used a screw propeller in 1836, a long wooden screw-shaped rod. At the first trial, it traveled at a speed of 4 nautical miles per hour. Since an underwater obstacle broke the screw rod, there was only a short piece left. When the shipbuilding engineer Smith was despairing, the ship abruptly increased its speed to 13 nautical miles per hour. This incident inspired the shipbuilding engineers. They transformed the long screw rod into a short one and also transformed the short screw rod into a blade shape, and thus the propeller was created.
Materials and Structure of the Propeller
Ship propellers are made of anti-corrosive material because they are operating directly in seawater, which will cause rapid corrosion. Aluminum alloys and stainless steel are the materials used to make ship propellers.
The other materials in widespread use are nickel, aluminum, and bronze alloys, which are 10-15% stronger and lighter than other materials.
The manufacturing process of a propeller involves mounting a number of blades onto the hub by welding or forging as a complete unit. Forged blades have high strength and reliability but are more expensive than welded blades.
Marine propellers are composed of a number of sections of helical surfaces, which interact with one another to rotate in the water through the helical effect.
Classification According to the Number of Attached Blades
The efficiency and application range of a propeller are directly related to the number of blades. Conventional propellers can be classified into 3-blade propellers, 4-blade propellers, 5-blade propellers, and even 6-blade propellers according to the number of blades. Generally speaking, the fewer the number of blades, the higher the efficiency of the propeller, yet the structural strength and bearing capacity will also decrease. To meet the requirements of various working conditions and vessel types, the aspects of efficiency, strength, maneuverability, and cost should be taken into account to select an appropriate propeller structure.
2-Blade Propeller
Theoretically, the most efficient is the 2-blade propeller since it has the least resistance in the water and can be used for ships aiming for high speed and fuel economy. However, the 2-blade propeller is not strong under heavy load and has low load-carrying capacity, with a high tendency to deform or break. Therefore, it is not commonly used in the merchant ship business and can be seen mostly in light boats or small sports boats.
3-Blade Propeller
The 3-blade propeller is the most commonly used and is installed on a large number of small and medium-sized ships. The key features are:
Low production cost: Usually manufactured through casting aluminum alloy, it is convenient and low-cost.
Well-balanced performance: With fine high-speed performance, it is suitable for medium-speed navigating requirements.
Good accelerating performance: With sensitive response during the course of starting or accelerating.
General low-speed maneuverability: Maneuverability and stability are slightly worse when traveling at low speeds, especially in complex sea states.
4-Blade Propeller
The 4-blade propeller performs even better than the 3-blade propeller and is installed mainly on vessels with higher requirements, e.g., commercial yachts and working boats. Its features are:
Relatively high manufacturing cost: Most are made from stainless steel alloys, which increase strength and lifespan.
Excellent maneuverability: It runs more smoothly especially at low speed and is suitable for application conditions that require frequent starting and stopping or high mobility.
Applicable to complex sea conditions: It is designed with higher holding power and anti-interference ability when sailing in rough seas.
Good fuel economy: It has higher propulsion efficiency in the medium and low-speed range, thus saving fuel consumption.
5-Blade Propeller
With the demand for smoothness and quietness on the increase, the 5-blade propeller has emerged as the first choice for both warships and merchant ships that require high performance. Its features include:
Higher production cost: The technology for processing is complex, and the material requirements are stringent.
Minimum vibration: Since the number of blades is higher, the thrust is smoother, and it significantly reduces vibration and noise.
Good holding ability under high sea states: It still enjoys good propulsion and directional stability under rough seas.
Applicable to large and medium-sized vessels: Such as cruise vessels, heavy cargo vessels, etc., with both comfort and efficiency being taken into account.
6-Blade Propeller
The 6-blade propeller is a high-grade structural form extensively used in today’s large vessels, especially very large container vessels and special vessels. Its advantages are centered around the following points:
High manufacturing cost: Precise layout and high machining complication.
Minimum noise and vibration: The large number of blades distributes the thrust, reducing interference with the ship structure and personnel.
Very good adaptability: It can still provide stable and strong propulsion in extreme sea states.
Decreased induced pressure field: Due to the more denser configuration of blades, pressure on each blade separately is lesser, enhancing the life of the propeller and reducing the agitation of the water body at the same time.
Classification According to Blade Pitch
Pitch of a propeller can be defined as the displacement achieved when the propeller makes 360-degree rotation. Classification of propellers according to pitch is as below.
Fixed Pitch Propeller
In a fixed pitch propeller, the blades are securely fixed to the hub. Fixed pitch propellers are cast, and both the blade position and the pitch position are permanently fixed and cannot be changed in flight. They are normally made of copper alloys.
Fixed pitch propellers are reliable and robust because the system does not have any mechanical and hydraulic connections like a controllable pitch propeller (CPP). The manufacturing cost, installation cost, and operating cost are lower than those of a controllable pitch propeller (CPP). Maneuverability of fixed pitch propellers is also lower than CPPs.
These propellers are installed on ships with low requirements for maneuverability.
Controllable Pitch Propeller
In a controllable pitch propeller, the pitch can be changed by moving the blades around the vertical axis through mechanical and hydraulic mechanisms. This is for operating the propulsion machinery at constant load without the need for a reversing gear since the pitch can be changed according to the operation conditions needed. So, maneuverability is improved, and the engine efficiency is also improved.
The hydraulic oil of the boss used in pitch control may leak. It is a complex and expensive system from both an installation and operational standpoint. In addition, the pitch may lock in one position, and it becomes problematic to operate the engine.
However, due to the need for a bigger hub to accommodate the blade pitching mechanism and pipes, a CP propeller’s propeller efficiency is slightly lower than that of an identical-sized FP propeller.
Propeller Size: As a rule, the larger the propeller diameter, the higher the efficiency. The size of the propeller fitted is, however, decided by the type of ship it is being fitted on and by the following considerations:
The structure and form of the aft section of the ship
The clearance requirements between the bow and the hull
The total ballast condition of the ship. For tankers and bulk carriers, the size of the propeller is relatively smaller compared to container ships
The designed draft of the ship
Approximate Values of Propeller Sizes
For container ships, d/D = 0.74
For bulk carriers and tankers, d/D = 0.65
Where d – propeller diameter, D – designed draft
How Does a Marine Propeller Work?
A ship sails in water, and the propeller is used to push the ship forward or backward, as per the direction of rotation or pitch of the propeller. The engine of the ship is connected to the propeller of the ship through a shaft mechanism.
As the engine makes the propeller rotate, the radiating blades, which are at a specific pitch, form a spiral shape, similar to a screw. In doing so, it converts rotational energy into linear thrust.
This linear impulse is transferred to the water, and when the blades of the propeller rotate, the front and back surfaces have a pressure difference. As a result, the acceleration of a large amount of fluid in one direction is due to the reaction force that propels the object (that is, the ship) attached to the propeller forward.
In order to reverse the direction of the ship, the engine and propeller will rotate counterclockwise. Thus, the thrust will be reversed, and the ship will move astern. However, the engine of an FP propeller always rotates clockwise when moving in the forward direction. Therefore, long-term reverse operation is impractical.
In the case of the ships with controllable pitch propellers, there is no effect on the direction of the engine, so the reverse efficiency of the ship is better than the fixed pitch propeller..
Types of Propeller Shafts
The propeller is attached with the ship engine through different shafts that are assembled together and can be termed as:
Thrust Shaft
Intermediate Shaft
Stern Tube Shaft
Thrust Shaft:
The engine crankshaft powers the thrust shaft, which is supported through the thrust bearing. The thrust bearing transfers the propeller thrust to the hull structure. It has the same housing as the main engine seat and is lubricated by the engine oil system. The thrust shaft is typically solid forged steel.
Intermediate Shaft:
Intermediate shaft stands next and consists of various sections welded together using forged couplings. Its length differs with respect to the distance between the propeller and the engine. Like the thrust shaft, it too is normally manufactured in solid forged steel.
Stern Tube Shaft:
At the end of the shaft line is the stern tube shaft, on which the propeller rides. It goes through a sealed, lubricated bearing and extends out to the ocean. It transmits power from the engine to the propeller and is typically made of high-strength duplex stainless steel. The bearing system may be oil- or water-lubricated.
KESU Propeller Impeller Machining Service
KESU high-precision propeller impeller machining service covers the fields of ships, aviation, and industrial fluid equipment. On the basis of advanced multi-axis CNC technology, from material selection, 3D modeling, machining path generation to fine grinding and inspection, we completely ensure the strength, efficiency, and lifespan of the impeller. Regardless of three-blade, four-blade, or high-level five-blade and six-blade structure, KESU can provide customized solutions to meet the requirements of complex working conditions, and it is a trustworthy manufacturing partner of propulsion systems.
Conclusion
The propeller impeller is a spinning fan-like structure that uses the power generated and transmitted by the ship’s main engine to propel the ship.
The transmitted power is converted from rotary motion to thrust, and the thrust gives momentum to the water and thus generates a force acting on the ship and propels it forward.
The sailing of a ship is based on Bernoulli’s principle and Newton’s third law of motion. There is a pressure gradient established between the rear and front of the blades, and water accelerates behind the blades.
