Ship propulsion: Engineering methods

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

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)¹⁴²¹¹	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-electric³	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 engines²⁵	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)⁶⁷	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	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

Comparative analysis of the heat balance results of the selected Tier III-compliant gas-fuelled two-stroke main engines (Korlak), Combustion Engines 193(2) Polish Scientific Society of Combustion Engines, 2023
New Wartsila 31 engine achieves Guinness World Records title Wartsila, 2015
Real-world methane emissions from LNG-fueled ships are higher than current regulations assume (FUMES project) ICCT, 2024
Norsepower highlights Rotor Sail results at The Motorship Propulsion and Future Fuels webinar Norsepower, 2021
Anemoi installs rotor sails on Vale's very large ore carrier Ship Technology, 2024
The United States Naval Nuclear Propulsion Program (2025 edition) US Department of Energy / Naval Reactors, 2025
Operating costs of Panamax and post-Panamax containerships (Drewry data) The Geography of Transport Systems (Rodrigue), 2022
X92 IMO Tier II/Tier III engine data WinGD, 2026