Propulsion of Cruise Liners and Other Ships – How These Floating Sea Mammoths Move


Welcome aboard, maritime enthusiasts! Ever gazed at a massive cruise liner cutting a majestic path through the ocean and wondered, “Just how does that behemoth keep moving?” You’re not alone. In this blog post, we dive into the fascinating world of ship propulsion – the force that keeps these sea leviathans afloat and moving. The importance of propulsion systems in the movement of cruise liners and other ships can’t be overstated. In this journey, we’ll navigate through the history of ship propulsion, swing by the various types of propulsion systems, and explore how they work. We’ll also take a look at the innovating strides in modern ship propulsion, and discuss the environmental considerations that are steering the future of this technology. Strap in, and let’s set sail on this enlightening voyage!

Propulsion, in the simplest terms, refers to the action or process of pushing or driving an object forward. In the context of ships and cruise liners, propulsion is the mechanism that enables these massive structures to move across bodies of water. It involves harnessing various forms of energy to create a force that propels the ship forward, overcoming factors such as water resistance and ship weight. Thus, propulsion is a vital element in maritime travel, acting as the ‘engine’ that powers the movement of ships.

The importance of propulsion in cruise liners and other ships cannot be overstated. Primarily, it enables these enormous structures to navigate across vast oceans, carrying passengers or cargo from one point to another. Without it, ships would be static objects, incapable of serving their primary functions. But it’s not just about movement, propulsion systems also play a crucial role in ship maneuverability. They allow ships to adjust their course, avoid obstacles, dock, and navigate through narrow passageways. Moreover, the type of propulsion system can also significantly impact a ship’s fuel efficiency, operational costs, and environmental footprint, making it a critical factor in the overall performance and sustainability of maritime operations.

Ships have been in use as a means of transport since early times. Ships are believed to have originated from rafts. Earlier ships used oars and were moved using sheer manpower. Later they were powered by wind with the help of sails. Then steam, diesel, and now even nuclear power are being used. Ships have undergone tremendous change over the years. The basic structure and parts of a modern cruise liner are shown in the figure.

Diagram of Modern Ship

Various parts of a modern ship.
1. Funnel
2. Stern
3. Propeller and rudder
4. Portside (right side is starboard side)
5. Anchor
6. Bulbous bow
7. Bow
8. Deck
9. Superstructure

Motions of ships require heavy power generation and an effective propulsion system. Without suitable means of propulsion, ships will be just floating iron mountains and nothing more. What causes a ship to move through the water? Propellers have this responsibility.


propellers of USS America
Propeller and rudder of SS Great Britain – Wikimedia Commons | User:TheLongTone

The propulsion of ships normally occurs with the assistance of a propeller. It has three or four (or sometimes even more in the case of propellers aimed at working under low noise conditions) blades. These blades are twisted and poking out at angles from a central hub (a rounded region on which the blades are attached). The hub is connected to a shaft which is connected to the ship’s engine. Usually, there will be two propellers and sometimes even more, and they are provided at the rear end of the ship. Propellers of ships and cruise liners are comparatively very large in size and require heavy torque for rotation.

How does a Propeller Work?

Marine propulsion system
Source: Blair Snow, Wikipedia,

Sir Isaac Newton stated in one of his works “Actio est reactio”. This sounds very familiar. This is Newton’s third law. Propellers work based on Newton’s third law which states that for every action there is an equal and opposite reaction. When the propeller is made to rotate, it draws water from the front of the ship and accelerates it towards the rear or backside of the ship. Because of the momentum imparted by the propeller to water, it experiences an equal backward momentum. In other words, water exerts a force on the propeller in the opposite direction. This force is called thrust. The thrust developed on the propellers is transferred to the ship’s structure through the main shaft which ends in a thrust bearing. In this way, propellers generate sufficient thrust to move the ship forward. Explanation of propulsive force generated by a propeller is very complex and is beyond the scope of our present topic. Earlier ships used rudders to help them steer. A rudder is basically a large flap provided at the back of the propeller. For steering, the rudder is inclined at an angle which changes the direction of expelled water. This results in a change in the direction of reactive force (sideways). This in turn helps the ship to turn.

Engine: The Heart of the Ship

View from the main engine room inside a ship.
Source:vHervé Cozanet, Wikipedia under the Creative Commons Attribution-Share Alike 3.0 Unported license.

Many engines were developed over the course of time to make ships faster and more efficient. Out of the many proposed theories steam engines and later diesel engines were put into practice. Turbines are also used sometimes in combination to generate sufficient power (also electricity for the ship). The workings of steam and diesel engines are explained below.

1. Triple-expansion steam engines

Steam engine inside a ship. The piston and crank are shown.

Earlier steam engines were of single expansion type (steam is made to expand only in one stage) but they were later modified to triple expansion types. Triple-expansion steam engines were the most common and advanced engines used in the early days. The chemical energy of coal is converted to thermal energy by burning coal to generate steam. This is done in very large boilers. Steam at high pressure from the boiler is supplied to the engine.

The engine basically consists of three cylinders. High-pressure steam

Working of triple expansion engine.
Source:Wikipedia, User: Emoscopes

is supplied to the first cylinder. Steam expands pushing the piston down. The steam from the first high-pressure cylinder then goes to an intermediate cylinder where it again expands. The resulting low-pressure steam then goes to the last cylinder where it undergoes the final expansion. Steam is made to expand in three successive stages hence the name triple expansion engine. Triple expansion engines provided greater speed as well as better heat conversion (to mechanical energy). The pistons of the three cylinders are connected to the main propeller shaft and they work together to rotate the shaft as shown in the animation shown alongside.


2. Diesel Engines

Diesel engines are nowadays widely used to power cruise liners and other ships. Diesel engines became increasingly popular because of their high output power, better efficiency and reduced fuel consumption. But the diesel engines used in ships are different from the ones used in other automobiles. As seen earlier the propellers are very large and hence rotating them requires very high power (or torque). So ships diesel engines are very large in size weighing up to 2000 tons! The working speed of these engines varies from 60 to 200 rpm and they use low-grade heavy fuel (Bunker C fuel). A typical diesel engine consists of a piston fitted in a cylinder arrangement. The number of cylinders varies depending on the power output required.

Massive diesel engine ” Bertholf” being placed inside a ship. Compare the size of men and the engine.

Working of Diesel Engines

Diesel engines used for ship propulsion are essentially 4 stroke engines. Their working involves the following sequence of processes, which are illustrated in the animation given alongside.

1. Air is sucked inside the cylinder during the downward motion of the piston because of the partial vacuum created within the cylinder.

2. This air is then compressed to very high pressure during the next upward stroke of the piston (here “stroke” means the motion of the piston within the cylinder).

Working of a 4-stroke diesel engine.
Tosaka, Wikipedia under the Creative Commons Attribution 3.0 Unported license.

3. At this instant diesel is sprayed in the form of fine droplets. Due to the high pressure inside the cylinder, diesel gets ignited. The air-fuel mixture undergoes combustion, expanding the gases which in turn push the piston down.

4. During the next upward stroke, the burnt-out gases are expelled out.

The combined action of various pistons on a propeller shaft.
Attribution: Tmmore 
under Creative Commons Attribution-Share Alike 3.0 Unported license.

This cycle is repeated. Since there is a power stroke for every four strokes of the piston, the engine is called a four-stroke engine. The reciprocating motion of the piston is converted to rotational motion by means of a connecting rod attached to a crank (the rotating metal disc shown in the animation). The crank rotates the shaft which in turn drives the propeller. A number of pistons are connected to the shaft and they all work together to turn the shaft. The manner in which the pistons are connected is made clear with the animation shown alongside.

RMS Titanic: The Unsinkable Ship

Titanic before her maiden voyage, docked at Southampton

RMS Titanic, the world’s largest, fastest, and most luxurious cruise liner at that time still comes to everyone’s mind when you first hear the term cruise liners. It was an engineering marvel at that time. In those days steam engines were used to run the ship. Diesel engines were not yet in use. She had two reciprocating engines and a turbine. The reciprocating engines were triple expansion type engines which have been explained above.

Steam generated in massive boilers and is fed to the engine. Exhaust steam from the engine is fed to a turbine. Steam is expanded within the turbine which causes the turbine to rotate. This rotating turbine drives a 4-blade propeller (17 feet diameter) situated at the center line of the ship.  Each engine drove a three-bladed propeller (23.5 feet diameter) on the starboard and port side of Titanic. The reciprocating engines can be seen clearly in James Cameroon’s movie.

As we all know, the Titanic hit an iceberg and sank into the icy waters of the Atlantic during her maiden voyage. Immediately after sighting the iceberg the engines were put at full astern (made to work in reverse to create a breaking effect), but she couldn’t maneuver away for the ship was too close.

Advanced Propulsion Systems in Modern Cruise Liners and Ships:

Oasis Of the Seas.
Baldwin 040. Image Credit 
Azimuth Thrusters
Image Credit

Although propellers are still in use, they have undergone many technical changes. Nowadays, modern cruise liners are equipped with azimuth thrusters. Azimuth thrusters are simply the propellers attached to pods, but they are capable of rotating through 360 degrees. Hence they help ships to steer more quickly and effectively. The entire propeller is made to rotate at the desired angle in order to steer the ship. Azimuth thrusters removed the use of rudders in modern ships. This type of propeller system is used in Oasis of the Seas –  the largest, most luxurious, and fastest cruise liner in the present world. The ship is capable of turning 360 degrees without any change in its position!!  Had RMS Titanic been equipped with one of these, her story would have been quite different.


The evolution of ship propulsion systems over the years has been nothing short of remarkable. From the rudimentary reciprocating engines of the Titanic era to the innovative azimuth thrusters in modern cruise liners like Oasis of the Seas, these advancements have significantly enhanced navigational precision and efficiency. It’s intriguing to speculate how different the fate of Titanic might have been with today’s technology. As we continue to push the boundaries of marine engineering, who knows what the future of sea transportation might hold? We can only anticipate even more incredible breakthroughs and improvements in this field.

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