Propeller – Types, Construction & Efficiency

Definition

The propeller is an essential part of the conventional ship propulsion system. It is the rotating fan form structure that propels the ship by utilizing the power from the main engine.

Generally, a vessel may have one, two and in rare cases three propellers. This depends on the speed and manoeuvring requirements of the subject vessel.

What does a Propeller do?
By converting the transmitted power to rotational motion, the propeller generates a thrust that induces momentum to the water. Hence, resulting in a force that acts on the ship and pushes it forward.

How it works?

Ships are propelling following Bernoulli’s principle and Newton’s third law. Thus, we have a pressure difference between the forward and aft side of the blade which accelerates the water.

The engine rotates the propeller through the shaft arrangement. Hence, the radiating blades, which have a particular pitch form a helical spiral, transform the rotational power into linear thrust. 
This linear thrust is a result of the pressure between the surface in front and back of the blades. Therefore, a mass of fluid accelerates in one direction, creating a reactive force that helps the body attached to the propeller move. 

For the ship to move in the reverse direction, the propeller has to produce thrust accordingly. So, depending on the design pitch, the shaft may rotate in the same or opposite direction to move the vessel astern.

We will see later how.

Propeller Types

The categorization of propellers is typically in terms of pitch or number of blades.
At first, let’s define them by the number of blades.

A) Number of Blades

The most commonly used propellers are the 4 bladed and 5 blades designs.

The propeller with the minimum number of blades will have the highest efficiency i.e. 2-blade propeller.
However, 2-bladed propellers are not capable for merchant ships due to the strength factor and considering the heavy loads from the ship, sea and weather.

B) Pitch of Blade:

The Pitch of a propeller is 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 (FPP)

The blades in the fixed pitch propeller are permanently attached to the hub. In other words, the operator cannot adjust the position of the blades and thus the position of the pitch.
They are usually copper alloy.

FPPs are robust and reliable since the system doesn’t incorporate any mechanical and hydraulic connection as in Controlled Pitch Propeller (CPP). Hence, the manufacturing, installation and operational costs are lower than the CPP type.
However, the FPP lacks in manoeuvrability aspects.

Therefore, this propeller type is typically fitted in a ship with minimal manoeuvrability requirements.

– Controllable Pitch Propeller (CPP)

With CPP type propellers, we may alter the pitch by rotating the blade about its vertical axis. This is achieved by means of mechanical and hydraulic arrangement. This allows driving the propulsion machinery at constant load without a reversing mechanism since the pitch matches the required operating condition. Thus, providing better manoeuvrability and engine efficiency.

On the other hand, it is a complex and expensive system in both installation and operational manner. Moreover, the pitch can stuck in one position, making it difficult to manoeuvre the engine. Nevertheless, it is possible that the hydraulic oil in the boss for pitch control may leak out and cause pollution.

However, the CPP efficiency is slightly lower than the same size FPP. This is due to the larger hub required to accommodate the blade pitch mechanism and pipings.

Propeller Size 

As a general rule, the larger the propeller diameter, the more efficient it is.

But the real dimension of the propeller will depend on the type of ship and the following factors:

1. Aft body construction and vessel design
2. Clearance requirement between tip and hull
3. General ballast condition (ie. tankers and bulkers have smaller propeller size than containers)
4. The design draught

Materials and Construction

Due to the operational conditions of propellers, they need corrosion-resistant materials to withstand seawater in long term. The materials used for making marine propellers are an alloy of aluminium and stainless steel.

Alternatively, alloys of nickel, aluminium and bronze which are 10-15 % lighter and have a higher strength factor.

A propeller is constructed in sections of helicoidal surfaces acting together to rotate through the water with a screw effect.

The blades are attached to the hub or boss by welding or forging in one piece. Note that, forged blades are highly reliable and stronger. On the contrary, they are expensive, compared to welded.

Propeller Shafts

The ship engine connects to the propeller via different shafts connected together, as follows:

1. Thrust Shaft
2. Intermediate Shaft
3. Tail Shaft

Thrust Shaft

The crankshaft of the main engine is first connected with the thrust shaft. Passing through the thrust bearing which is transferring the produced thrust to the ship’s structure. The bearing lubricates by the main engine lub-oil system.

The material of the thrust shaft is usually solid forged ingot steel. 

Intermediate Shaft

The thrust shaft then connects to a long intermediate shaft. This comes in parts and assembles together using solid forged couplings.

The length and number of intermediate shafts joined together depends on the position of the main engine. A larger ship will have more distance between the main engine and the propeller.

Likewise thrust shaft, intermediate is usually from the same steel. 

Tail Shaft 

As the name suggests, it is the end part of the shafting arrangement and carries the propeller. The tail shaft is on a lubricated, sealed stern tube bearing as it protrudes out of the ship’s engine room into the open sea. 

The lubrication system can be either oil-based or water type. The tail shaft transmits the engine power and motion drive to the propeller. The material of the tail shaft is usually high strength duplex stainless steel alloy. 

Heavy Running

Heavy running occurs when engines reach the maximum continuous rating before they reach full rev/min. FPPs can suffer heavy running due to the following reasons.

• Propeller/hull fouling, damage
• Engine ageing
• High pitch on the propeller
• Change in normal operating conditions
• Modifications to the ship.

The propellers are designed according to the trial curve. However, ships in service operate according to the service curve which is significantly above the trial curve.

When the effect becomes too serious, engine overload is inevitable. Thus, resulting in higher fuel consumption and wear to the pistons, liners and valves, hence higher maintenance costs. The solution by means of modification is within reach and has proven to be very effective.

A tailor-made solution, to increase the rotation rate of the shaft by decreasing the mean pitch. This is realized by lifting the trailing edge after a section from the trailing edge is removed.
Hence, a trailing edge cutting will prevent the overloading of the engine by adapting the mean hydrodynamic pitch.

Advancements

Modifications around the propellers can offer a major improvement in the energy-saving potential of the vessel. Therefore, shipowners are considering such in order to comply with EEXI & CII in the upcoming years.

Energy Saving Devices provide a direct increase in vessel propulsion efficiency by reducing hull resistance and improving propellers’ thrust. In particular, they comprise devices applicable before or after the propeller while also high-performance hull coatings to reduce resistance.

References

1. Martelli M, Figari M, Altosole M, Vignolo S. Controllable pitch propeller actuating mechanism, modelling and simulation. Proceedings of the Institution of Mechanical Engineers, Part M: Journal of Engineering for the Maritime Environment. 2014;228(1):29-43.
2. Kerwin, Justin E.. “MARINE PROPELLERS.” (2008).
3. Goutam Kumar Saha, Md. Hayatul Islam Maruf, and Md. Rakibul Hasan , “Marine propeller modeling and performance analysis using CFD tools”, AIP Conference Proceedings 2121, 040012 (2019) https://doi.org/10.1063/1.5115883
4. Molland, A., Turnock, S., & Hudson, D. (2011). Propeller Characteristics. In Ship Resistance and Propulsion: Practical Estimation of Propulsive Power (pp. 261-295). Cambridge: Cambridge University Press. doi:10.1017/CBO9780511974113.015
5. https://www.wartsila.com/marine/build/propulsors-and-gears/propellers
6. Photo: Wartsila