Machinery

Marine propulsion and engines

Published 28 August 2026 · updated 17 October 2026

Almost every ocean-going cargo ship above a few thousand deadweight has the same engine layout. One slow speed two stroke diesel engine turns one fixed pitch propeller (a propeller whose blades do not change angle). The shaft between them has no gearbox. This one layout carries most of the world's cargo. Once you know why it became the standard, the exceptions tell you what a ship is used for.

Why the slow two stroke won

A propeller works best when it is large and turns slowly. A two stroke engine burning heavy fuel also works very efficiently at low speed. So the propeller and the engine both work best at low speed. A designer who connects them directly can leave out the gearbox, along with its energy losses, its weight and its maintenance. A large two stroke engine turns at roughly 60 to 100 revolutions per minute and is bolted straight to the shaft.

The engine can run in reverse. So the ship goes backwards by running the engine backwards. It does not need a gearbox or a propeller with adjustable blades. The engine is built to burn residual fuel. This is the heavy, cheap part of crude oil that has little use on land. And the engine is very large. That is why the engine room of a Capesize is several decks tall, and why the engine casing sits where it does.

Arrangement	Where you find it	Why
Slow speed two stroke, direct drive	Bulk carriers, tankers, container ships, gas carriers	Uses the least fuel for each tonne of cargo moved each mile. No gearbox. Burns the cheapest fuel grade available
Medium speed four stroke with reduction gear	Ferries, ro-ro, smaller ships and ships for shallow water, multipurpose ships	Shorter and lighter, so it fits where a two stroke will not. Often several engines drive one shaft
Diesel electric	Cruise ships, offshore vessels, icebreakers, many gas carriers	The engines run at their most efficient load whatever the ship is doing. The ship needs the electrical power anyway
Steam turbine	Older LNG carriers, now rare	Burned the gas that boils off from the cargo. Replaced by dual fuel diesels that do this better

Consumption rises far faster than speed

This is the most useful fact for the commercial side. The power needed to push a hull through the water rises steeply with speed, roughly as the cube of the speed across the normal range. So a small cut in speed saves a large amount of fuel. Slow steaming (sailing slower to save fuel) was not a trend. It was simple arithmetic. It became the obvious choice as soon as fuel became expensive compared with freight rates.

This also explains how a time charter is written. Speed and fuel consumption are given as a warranty (a contractual promise). The warranty applies in defined weather, on a named fuel, at a named draught. It is a promise about performance in those conditions, not a measurement of the ship. So a performance claim is almost always an argument about whether those conditions applied. The weather routing report is the evidence.

The auxiliaries, and why port consumption is a separate line

The main engine is not the only thing that burns fuel. Diesel generators supply the ship's electricity. A boiler makes steam for heating and for cargo work. On a tanker, the cargo pumps also use a lot of power. That is why a ship's description has several consumption figures, not one: at sea loaded, at sea empty (in ballast), in port and idle, and in port and working. A voyage estimate that uses one average figure will be too high on some voyages and too low on others, by a different amount each time.

What the fuel rules did to the machinery

Three sets of rules under MARPOL Annex VI (the international rules on air pollution from ships) now decide what is in the engine room.

Sulphur. Since 2020 the global limit on sulphur in fuel is 0.50 per cent. Inside an emission control area it is 0.10 per cent. A ship can meet this in two ways. It can burn fuel that meets the limit. Or it can fit an exhaust gas cleaning system (a scrubber) and keep burning the heavy fuel. This is a large investment decision. It appears in the ship's description and affects its fuel costs for the rest of its working life.

Nitrogen oxides. Regulation 13 sets three tiers based on the date the ship's keel was laid (when building started). Tier III applies to newer ships inside designated NOx control areas. It is strict enough that the ship needs extra exhaust treatment equipment or exhaust gas recirculation. Engine tuning alone is not enough. So the ship's build date and its equipment decide whether it can trade in certain waters. The operator cannot choose otherwise.

Carbon. Every existing ship of 400 gross tonnage and above has had to calculate an Energy Efficiency Existing Ship Index (EEXI). Every ship also reports a yearly carbon intensity indicator (CII), which gives it a rating. The usual fix for a ship that fails the EEXI is an engine power limit. The maximum power is capped. The cap can only be removed through an override in an emergency.

That last rule looks like an engineering matter, but it affects chartering directly. A ship with a power limit cannot honestly promise the speed it used to reach, and its description has to say so. If the ship is fixed on its old speed warranty, a performance claim is likely later.

Dual fuel and the alternative fuels

Two stroke engines now come in versions that burn LNG, LPG, methanol or ammonia. They still use a small amount of liquid pilot fuel to start combustion. Dual fuel ships are a large and growing share of the ships on order. For a chartering desk, the important question is rarely how the fuel burns. It is whether the fuel is available, how the prices of the two fuels compare on the day, and who carries the risk if the ship cannot get the fuel it was fixed on. That is a clause, and it belongs in the recap (the written summary of the deal).

The trap

The common mistake is to read a machinery description as a list of facts about a ship. It is really a list of promises that depend on conditions: this speed, on this fuel, in this weather, at this draught, with the hull in this condition. Change any one of them and the number changes. That is why the date of the last drydocking and the fuel grade matter as much as the engine model when someone tests a performance figure.

References

IMO, International Convention for the Prevention of Pollution from Ships (MARPOL). imo.org
IMO, IMO 2020: cutting sulphur oxide emissions. imo.org
IMO, EEXI and CII: ship carbon intensity and rating system. imo.org
IMO, Improving the energy efficiency of ships. imo.org
Everllence (formerly MAN Energy Solutions), Two-stroke marine engines. everllence.com

Bunkers and fuel gradesTwo fuel grades, sometimes three, each with its own price. The largest number on a voyage estimate is also the one nobody guarantees. Emission control areasTen areas where the fuel a ship may burn has a stricter limit than the global one. Which lines are exact, which are traced, and which do not apply yet. Hull form and resistanceHow much fuel a ship burns depends on the shape of its hull, and on how clean that hull has been kept since the last drydocking. Time charter equivalentFreight per tonne cannot be compared with hire per day until you divide it by the voyage. This is the arithmetic that does it.

Neptune Atlas

A voyage estimate in Neptune Atlas is built from the speed and consumption you enter for each state of the voyage, laden, ballast and in port, so the machinery assumptions are visible and not hidden in one average. Change the speed and the daily figure updates at once. Every paid plan starts with 7 free days. A card is needed to start them, and cancelling before they end costs nothing.

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