# Ship propulsion: Engineering methods

Diesel, LNG, spinning rotors or a reactor: four ways

Source page: https://www.materialinsights.org/energy/e_marine/methods/

What this tab covers The engineering methods that can do this conversion, compared on efficiency, scale, economics, limitations and reliability.

Four ways of propelling ships: fuel-oil diesel engines, LNG dual-fuel engines, wind assistance, and nuclear steam or electric drive.

#### Comparison

| Method | Efficiency | Scale | Economics | Limitations | Reliability |
|---|---|---|---|---|---|
| **Slow-speed two-stroke diesel (fuel oil)**^[14](https://wingd.com/media/2d4lb2aq/x92_20260402_1539.pdf)^[2](https://yadda.icm.edu.pl/baztech/element/bwmeta1.element.baztech-756de647-bc5e-4a75-80b1-3c95ad960df5/c/Korlak_Comparative_2_2023.pdf)^[11](https://transportgeography.org/contents/chapter5/maritime-transportation/containerships-operating-costs-panamax-post-panamax/) | about 51-54% (calculated from WinGD X92 fuel data) | up to tens of MW on the largest container ships, tankers and bulk carriers | lowest-cost propulsion; fuel 45-50% of operating cost | CO2, sulphur and NOx emissions; slow to change speed | very high; engines last the ship's life |
| **Four-stroke medium-speed diesel and diesel-electric**^[3](https://www.wartsila.com/media/news/02-06-2015-new-wartsila-31-engine-achieves-guinness-world-records-title) | best about 51% (calculated from 165 g/kWh at 42.7 MJ/kg, Wartsila 31 record claim) | 4.2-9.8 MW per engine (Wartsila 31) | more flexible but slightly less efficient than two-strokes | gearbox or electrical losses | redundancy from several engines |
| **LNG dual-fuel engines**^[2](https://yadda.icm.edu.pl/baztech/element/bwmeta1.element.baztech-756de647-bc5e-4a75-80b1-3c95ad960df5/c/Korlak_Comparative_2_2023.pdf)^[5](https://theicct.org/pr-real-world-methane-emissions-from-lng-fueled-ships-are-higher-than-current-regulations-assume-new-study-finds-jan24/) | above 50% for two-strokes; Diesel-cycle gas engines up to 2.9 points better than Otto-cycle at 75% load | large container ships, car carriers, LNG carriers, cruise ships | depends on LNG versus oil price | methane slip averages 6.4% in low-pressure four-stroke engines, against 3.1-3.5% assumed in rules | mature |
| **Wind assistance (rotor sails, suction wings, rigid sails)**^[6](https://www.norsepower.com/?p=4663)^[7](https://www.ship-technology.com/news/anemoi-rotor-sails-vloc/) | peak thrust equal to 7 MW of shaft power from two rotors (test); savings of up to 6% (ore carrier) to a projected 25% (North Sea ship) depending on route | retrofits on bulk carriers and ro-ro ships, e.g. five 35 m rotors on the Vale ore carrier Sohar Max (2024) | about 3,000 t CO2 a year saved on that ship (company figure) | depends on wind and route; deck space and air draught | limited long-term data published |
| **Nuclear propulsion (naval)**^[8](https://www.energy.gov/sites/default/files/2025-06/Gray%20Book_2025.pdf) | not published; the advantage is endurance, not efficiency | 97 US reactors in 11 carriers and 66 submarines (48 attack, 14 ballistic missile, 4 guided missile), end of 2024 | very high capital cost; no refuelling for decades | military only in practice; security, waste and port access | over 7,600 reactor-years and 177 million miles steamed |

Diesel engines win on cost and efficiency and will remain the backbone, increasingly as dual-fuel units that can later burn methanol or ammonia. LNG cuts CO2 at the stack but methane slip erodes the gain unless high-pressure engines are used. Wind assistance gives the largest single fuel saving available today on suitable routes, at modest cost. Nuclear propulsion is unmatched for endurance but confined to navies. IMO data show that shipping cut carbon intensity by about 21-29% between 2008 and 2018, with progress slowing to about 1-2% a year after 2015.

#### How each method works
**Slow-speed two-stroke diesel (fuel oil)**

Huge cylinders drive the propeller shaft directly at low rpm; turbocharged, burning cheap heavy fuel oil with scrubbers or low-sulphur fuel.
**Four-stroke medium-speed diesel and diesel-electric**

Faster, smaller engines drive the propeller through a gearbox or drive generators for electric motors; used on ferries, cruise ships and offshore vessels.
**LNG dual-fuel engines**

Burn natural gas either injected at high pressure (Diesel cycle) or premixed with air (Otto cycle), with a little diesel to ignite it.
**Wind assistance (rotor sails, suction wings, rigid sails)**

Spinning rotors or wing-like sails create a force from the wind that adds to propeller thrust, so the engine can throttle back.
**Nuclear propulsion (naval)**

A pressurised-water reactor heats water that raises steam for turbines; the Columbia-class submarine will use electric drive and a core that lasts the life of the ship.

#### Sources

1. [Comparative analysis of the heat balance results of the selected Tier III-compliant gas-fuelled two-stroke main engines (Korlak), Combustion Engines 193(2)](https://yadda.icm.edu.pl/baztech/element/bwmeta1.element.baztech-756de647-bc5e-4a75-80b1-3c95ad960df5/c/Korlak_Comparative_2_2023.pdf) Polish Scientific Society of Combustion Engines, 2023
2. [New Wartsila 31 engine achieves Guinness World Records title](https://www.wartsila.com/media/news/02-06-2015-new-wartsila-31-engine-achieves-guinness-world-records-title) Wartsila, 2015
3. [Real-world methane emissions from LNG-fueled ships are higher than current regulations assume (FUMES project)](https://theicct.org/pr-real-world-methane-emissions-from-lng-fueled-ships-are-higher-than-current-regulations-assume-new-study-finds-jan24/) ICCT, 2024
4. [Norsepower highlights Rotor Sail results at The Motorship Propulsion and Future Fuels webinar](https://www.norsepower.com/?p=4663) Norsepower, 2021
5. [Anemoi installs rotor sails on Vale's very large ore carrier](https://www.ship-technology.com/news/anemoi-rotor-sails-vloc/) Ship Technology, 2024
6. [The United States Naval Nuclear Propulsion Program (2025 edition)](https://www.energy.gov/sites/default/files/2025-06/Gray%20Book_2025.pdf) US Department of Energy / Naval Reactors, 2025
7. [Operating costs of Panamax and post-Panamax containerships (Drewry data)](https://transportgeography.org/contents/chapter5/maritime-transportation/containerships-operating-costs-panamax-post-panamax/) The Geography of Transport Systems (Rodrigue), 2022
8. [X92 IMO Tier II/Tier III engine data](https://wingd.com/media/2d4lb2aq/x92_20260402_1539.pdf) WinGD, 2026

## Other sections

- [Summary](https://www.materialinsights.org/energy/e_marine/index.md)
- [Overview](https://www.materialinsights.org/energy/e_marine/overview/index.md)
- [Correlation](https://www.materialinsights.org/energy/e_marine/correlation/index.md)
- [Science and limits](https://www.materialinsights.org/energy/e_marine/science/index.md)
- [Value chain](https://www.materialinsights.org/energy/e_marine/valuechain/index.md)
- [Unit economics](https://www.materialinsights.org/energy/e_marine/economics/index.md)
- [Equipment and suppliers](https://www.materialinsights.org/energy/e_marine/equipment/index.md)
- [Relationships](https://www.materialinsights.org/energy/e_marine/links/index.md)