Advancements in ship propulsion driving efficiency 

To achieve the International Maritime Organization’s target of net-zero greenhouse gas emissions by 2050, the industry will need to utilise greener fuels and a range of different technologies. Innovative propulsion systems are being developed to improve fuel efficiency, reduce emissions and cater to varying vessel requirements. Let’s explore the different types of ship propulsion.

A ship propulsion system refers to the technology that propels a ship through the water, converting energy into thrust to overcome resistance. This process is based on Newton’s third law of motion, which states that every action has an equal and opposite reaction. A propulsion mechanism exerts force on the water, which in turn generates thrust that moves the vessel.

The core component of any propulsion system is the engine, which can vary widely in type depending on the ship’s size, purpose and energy requirements. The propulsion process involves generating mechanical energy – often through combustion, electricity or gas – that is transferred to propellers or jets that push the ship forward.

The maritime industry offers a wide range of propulsion systems, each with its unique advantages. These can be broadly categorised as traditional, hybrid or alternative energy systems.

Diesel engines remain the most common ship propulsion type, particularly for cargo ships, tankers and bulk carriers. They are reliable and efficient and can be scaled to power ships of various sizes. There are two main types: two-stroke and four-stroke engines, with two-stroke engines typically used in large cargo vessels and four-stroke engines in smaller vessels.

Also known as marine electric propulsion, electric ship propulsion utilises an electric motor powered by generators. The generators can be driven by various fuel types, including diesel or renewable energy sources.

Diesel-electric ship propulsion is a popular variant, commonly used on submarines and large passenger ships. One of the significant benefits of electric ship propulsion is its efficiency in managing energy consumption, particularly in vessels with variable speed requirements, such as ferries and cruise ships. These systems also contribute to lower noise levels and fewer emissions.

Widely used in military vessels and high-speed ferries, gas turbines operate similarly to aircraft turbines, using compressed air mixed with fuel to generate power.

Gas turbine ship propulsion provides significant advantages in terms of power-to-weight ratio and quick acceleration, making it ideal for ships that require speed and agility. However, gas turbines are less fuel-efficient than diesel engines, especially at lower speeds, which limits their application in commercial shipping.

Liquefied natural gas (LNG) has gained momentum due to its price competitiveness, abundant supply and lower emissions compared to traditional fossil fuels. Studies also show that LNG is the most efficient of fuels. Numerous vessels providing services in the LNG industry have been retrofitted with dual-fuel engines, which have proved to be extremely efficient. 

LNG ship propulsion systems produce significantly fewer sulphur oxides and nitrogen oxides, making them more environmentally friendly. Additionally, LNG has a well-established infrastructure, ensuring a consistent and reliable fuel supply.

Marine hybrid propulsion combines conventional fuel engines with electric motors or batteries, offering a versatile and fuel-efficient propulsion option. Hybrid systems allow ships to switch between power sources depending on operational needs, such as using electric power in environmentally sensitive areas or when operating at low speeds. This approach is particularly advantageous for vessels such as tug boats, ferries and offshore support vessels.

Emerging technologies, such as hydrogen fuel cells and wind-assisted propulsion, are also being explored. Hydrogen fuel cells convert hydrogen into electricity, offering a zero-emission alternative. Wind-assisted propulsion uses wind to supplement traditional power systems and reduce fuel consumption. While these systems are not yet widespread, they are very promising for long-distance shipping in particular.

Innovations in electric ship propulsion, marine hybrid propulsion, LNG propulsion and alternative propulsion systems represent a significant step forward in reducing the environmental impact of the shipping and cruise industries. With ongoing advancements in fuel efficiency and alternative energy sources, the future of ship propulsion looks set to be cleaner and quieter.