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Two-stroke low-speed marine engines are direct-coupled crosshead diesels that still power nearly all deep-sea cargo ships, with the largest unit rated 75,570 kW. Three designers, Everllence, WinGD and J-ENG, now sell LNG, methanol and ammonia variants, and the first ammonia-fuelled two-stroke ships entered service in 2026.
Four-stroke marine engines drive cruise ships, ferries, offshore vessels, tugs and the generator sets of every large ship, with the Wartsila 31 holding a Guinness record at 169.4 g/kWh. Between 2024 and 2026 Wartsila, Everllence, Rolls-Royce mtu, Caterpillar MaK and HD Hyundai HiMSEN all added methanol or ammonia versions of their base platforms.
Marine gas turbine propulsion uses aeroderivative turbines such as the 25,100 kW GE LM2500 and the Rolls-Royce MT30 for warships and fast craft where power density matters more than fuel cost. Naval frigate and destroyer orders in 2025-2026 drive the market, while a 16 MW ammonia turbine targets a full test by end-2027.
Marine steam turbine propulsion was the standard LNG carrier prime mover because boilers burn boil-off gas simply, but at about 30% thermal efficiency it trails DFDE plants above 42% and two-stroke gas engines by more. Record scrapping in 2025 and widening lay-ups in 2026 leave nuclear as the only growth route for marine steam.
Marine waste heat recovery turns exhaust and cooling-water heat into electricity or shaft power, with full steam-cycle systems recovering about 5% of fuel energy and Siemens reporting fuel savings of up to 12% on container ships. Since 2024 the growth has shifted to modular organic Rankine cycle units and hybrid turbochargers that work at low load and suit retrofits.