Neptune Atlas

Machinery

Marine propulsion and engines

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Almost every deep sea merchant ship above a few thousand deadweight is driven by one slow speed two stroke diesel, turning one fixed pitch propeller, on a shaft with no gearbox in it. That single arrangement carries most of the world’s cargo. Once you know why it won, the exceptions tell you what a ship is for.

Why the slow two stroke won

Propeller efficiency wants a large diameter turning slowly. Engine efficiency, in a two stroke burning heavy fuel, is also excellent at low speed. So the propeller and the engine want the same thing, and a designer who can match them directly gets to delete the gearbox, its losses, its weight and its maintenance. A large two stroke turns at roughly 60 to 100 revolutions per minute and is bolted to the shaft.

The engine is reversible, so the ship goes astern by running the engine backwards rather than through a gearbox or a controllable pitch propeller. It is built to burn residual fuel, the heavy end of the barrel that nothing on land wants. And it is enormous, which is why the engine room of a Capesize is several decks tall and why the machinery casing sits where it does.

ArrangementWhere you find herWhy
Slow speed two stroke, direct driveBulk carriers, tankers, container ships, gas carriersBest fuel economy per tonne mile, no gearbox, burns the cheapest grade available
Medium speed four stroke with reduction gearFerries, ro-ro, smaller and shallow draught ships, multipurpose tonnageShorter and lighter, so it fits where a two stroke will not. Often several engines on one shaft
Diesel electricCruise ships, offshore vessels, icebreakers, many gas carriersEngines run at their best load whatever the ship is doing, and the electrical power is needed anyway
Steam turbineOlder LNG carriers, now rareBurned the cargo boil off. Displaced by dual fuel diesels that do it better

Consumption rises far faster than speed

This is the single most useful fact on the commercial side. The power needed to push a hull rises steeply with speed, roughly as its cube over the normal service range, so a small reduction in speed buys a large reduction in fuel. Slow steaming was not a fashion. It was arithmetic that became compelling the moment fuel got expensive relative to freight.

It also explains the shape of a time charter. Speed and consumption are given as a warranty, in defined weather, on a stated fuel, at a stated draught. They are a promise about performance under conditions, not a measurement of the ship. A performance claim is therefore almost always an argument about whether the conditions held, and 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 burning fuel. Diesel generators supply the ship’s electrical load, a boiler raises steam for heating and for cargo work, and on a tanker the cargo pumps are a substantial load of their own. That is why a ship is described with several consumption figures rather than one: at sea laden, at sea in ballast, in port idle, and in port working. A voyage estimate that uses a single average will be wrong in both directions and by different amounts on every voyage.

What the fuel rules did to the machinery

Three regimes under MARPOL Annex VI now shape what is in the engine room.

Sulphur. Since 2020 the global cap on fuel sulphur content is 0.50 per cent, and inside an emission control area it is 0.10 per cent. A ship complies by burning compliant fuel or by fitting an exhaust gas cleaning system and continuing on the heavy grade. That choice is a capital decision that shows up in the ship’s description and in her bunker economics for the rest of her life.

Nitrogen oxides. Regulation 13 sets three tiers by keel laying date. Tier III, which applies in designated NOx control areas to newer ships, is strict enough to require after treatment or exhaust gas recirculation rather than tuning. So a ship’s ability to trade in certain waters is decided by her build date and by hardware, not by her operator’s intentions.

Carbon. Every existing ship of 400 gross tonnage and above has had to calculate an Energy Efficiency Existing Ship Index, and every ship reports an annual carbon intensity indicator that produces a rating. The common answer to a failing EEXI is an engine power limitation: the maximum power is capped, and can only be released through an override in an emergency.

That last one is a chartering fact dressed as an engineering one. A power limited ship cannot honestly warrant the speed she used to make, and the description has to say so. Fixing her on an old speed warranty is fixing a claim.

Dual fuel and the alternative fuels

Two stroke engines are now offered in versions that burn LNG, LPG, methanol or ammonia while keeping a liquid pilot fuel, and dual fuel tonnage is a large and growing share of the order book. For a chartering desk the interesting question is rarely the combustion. It is bunker availability, the price relationship between the two fuels on the day, and who carries the risk of the ship being unable to obtain the fuel she was fixed on. That is a clause, and it belongs in the recap.

The trap

The trap is reading a machinery description as a set of facts about a ship. It is a set of conditional promises: this speed, on this fuel, in this weather, at this draught, with a hull in this condition. Change any one of them and the number changes. Which is why the last drydocking date and the fuel grade matter as much as the engine model when a performance figure is being tested.

References

Bunkers and fuel gradesTwo grades and sometimes three, priced apart. The largest number on a voyage estimate is also the one nobody warrants. Emission control areasTen areas where the fuel a ship may burn is capped tighter than the global limit. Which lines are exact, which are traced, and which do not bite yet. Hull form and resistanceWhat a ship burns is set by the shape she drags through the water, and by how clean it has been kept since the last drydocking. Time charter equivalentFreight per tonne means nothing next to 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 instead of being folded into one average. Change the speed and the daily figure moves in front of you. 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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