The Other Fuel Bill: The Auxiliary Tonnes Nobody Reads

Ask where a ship's fuel goes and the industry answers with one engine. The main engine is the fuel bill; the generators are a rounding error; the chief runs two at sea because that is what a careful chief does; and nobody in the office has ever seen the auxiliary tonnes as a line of their own. The tonnes are there all the same. IMO's GreenVoyage2050 programme, in its page on improved auxiliary engine load (open), puts it plainly: the auxiliary engines 'can represent up to 15% of the total fuel consumption for a vessel with a diesel-mechanical configuration', with 'normally 2 or 3 auxiliary engines on a diesel-mechanical vessel, and 4 to 6 auxiliary engines on diesel-electric vessels'. The same page prices the cure: at the right load it 'should be possible to reduce the specific consumption by 5 – 15 g/kWh for over 50% of the auxiliaries' operational time, i.e. 2.5 – 7% reduction', inside a potential it puts at '0 – 20% on fuel consumption on auxiliary engine', for a total ship saving 'estimated to be between 0.2-4%'. And the capital: 'There are no direct costs related to improved engine load', only crew training, guidelines or software tools. This lesson sizes the bill before anyone spends anything on it.

One definition, used throughout the course. The auxiliary share is auxiliary-engine fuel divided by main-engine plus auxiliary-engine fuel. Boilers are left out of both the top and the bottom of that fraction: their fuel is real, but it belongs to our course Auxiliary Boilers and Steam (available shortly), and mixing it in would blur the one number this lesson wants to isolate.

The ten-minute estimate

A first number needs no meter, only a published table and a calculator. The Fourth IMO GHG Study 2020 (free PDF, full report and annexes) lists in Table 17, page 68, auxiliary-engine power in kilowatts by ship type, size and operating mode (at berth, anchored, manoeuvring, at sea), built from Starcrest's vessel boarding programme reports and ClassNK's fuel data. Annex B, Table 4, gives the baseline specific fuel consumption the study burns them at: 195 g/kWh on HFO for auxiliary engines built in 2001 or later (185 on MDO), against 175 for a slow-speed two-stroke of the same age.

Take a bulk carrier of 60,000 to 99,999 dwt. Table 17 gives 410 kW at sea, 240 kW at berth, 400 kW anchored and 1,100 kW manoeuvring. The hours are this lesson's assumptions, the only thing here to argue with: 6,000 at sea, 1,500 at berth, 1,000 at anchor, 260 manoeuvring, 8,760 in all. Multiply and add: 2,460 MWh at sea, 360 at berth, 400 at anchor, 286 manoeuvring, 3,506 MWh a year. At 195 g/kWh that is 683,670 kg, call it 684 tonnes. Now the main engine beside it. MAN Energy Solutions' Market Update Note of 18 December 2023 (free PDF) works a Kamsarmax with a 6S60ME-C10.5 at 9,000 kW SMCR and 'the typical three auxiliary engines onboard' of about 900 kWe each. Slow-steam that engine at 50% for the same 6,000 sea hours at 175 g/kWh, the Annex B baseline, which the study defines as the engine's best point: 4,500 kW for 6,000 hours is 27,000 MWh, 4,725 tonnes. The ship burns 5,409 tonnes in its engines, and the auxiliaries are 684 / 5,409 = 12.6% of it. That is 'up to 15%' in one ship's numbers, and it took ten minutes. Lesson 6 applies the study's own part-load correction to that engine, 181.8 g/kWh at 50%, 4,908 tonnes, and the share becomes 12.2%: half a point, which a ten-minute estimate is allowed, and the flat 175 stays here so the arithmetic stays in the head.

Now an oil or product tanker of 20,000 to 59,999 dwt, from the same table: 510 kW at sea, 720 at berth, 520 anchored, 600 manoeuvring. The 2,700 kW that Table 17 lists at berth in its boiler column is steam for cargo heating and pump turbines, not the generators', and it belongs to Auxiliary Boilers and Steam (available shortly). Same hours: 3,060 + 1,080 + 520 + 156 = 4,816 MWh, 939 tonnes at 195 g/kWh. Against a 7,000 kW main engine at 60% for 6,000 hours at 175 g/kWh, 4,410 tonnes, the share is 939 / 5,349 = 17.6% (178.1 g/kWh, 4,488 tonnes and 17.3% with lesson 6's correction). The tanker beats the bulker not because its generators are worse but because its port hours are cargo hours: 720 kW at the berth, three times the bulker's 240, with the main engine cold, which is 140 of the 255 extra tonnes. Another 117 come from 100 kW more at sea, anchor and manoeuvring nearly cancel each other (23 more, 25 fewer), and the smaller main engine adds about a point to the share.

Two more cases exceed the generic figure, and the course leans on the first. A chemical tanker whose deepwell cargo pumps run on hydraulic power packs puts the whole discharge load on the switchboard, so its port hours are its heaviest generator hours (Table 17 gives its 20,000 to 39,999 dwt class 790 kW at berth against 660 at sea): that ship becomes the course's worked default from lesson 2. And a container ship carrying reefers, Table 17's 8,000 to 11,999 TEU class at 1,800 kW at sea and 1,150 kW at berth, is the other ship on which 'up to 15%' is a floor rather than a ceiling.

Why the office never sees it

The number was always on board; it was never on a page of its own. That is changing. The IMO DCS consumer split of resolution MEPC.385(81) separates auxiliary-engine fuel from main-engine fuel, and underway from not underway, for every ship from calendar 2026: the first mandatory auxiliary cell covers 2026 and reaches the office by 31 March 2027, and only a ship delivered on or after 1 August 2025, or one whose flag applied the amendments early, has one for 2025 (MEPC.1/Circ.913, via DNV). EU MRV separates the fuel burned at berth, not who burned it. Once the cell arrives it is the first number an analyst reads, per ship, per year. Our course Fleet Data & Performance Monitoring owns that split in its lesson "What the Regulators Did to Your Data", together with a finding this course inherits: main engines are usually well metered, auxiliary engines 'less consistently', so the cell you read may come from a flow meter or from a sounding and a guess. (The regimes themselves are the territory of our free course Maritime Regulations, Explained, lesson "IMO DCS: The Global Ledger".)

Two things make the true bill larger than the estimate. The first is physics: a genset is a medium-speed four-stroke, and our course Marine Engineering & Main Engines explains in "Why the Two-Stroke Rules Deep-Sea" why its grams are always worse than the main engine's, by three to six points of thermal efficiency, which is the 195 against 175 above. The second is load. The 195 is a fleet baseline at the engine's best point; the maker's own table for a MAN L23/30H (project guide, free PDF) reads 192 g/kWh at 75% load and 217 at 25%, a 13% cliff, and a chief running two sets 'for safety' usually stands near the bottom of it. Lesson 2 opens that curve; until then, the share above is a floor, not the bill.

The prize in tonnes

GreenVoyage2050's 0.2 to 4% is a range wide enough to hide in, so convert it before anyone quotes it, and check it against the page's own limits. On the worked bulker's 5,409 tonnes it is 11 to 216 tonnes a year, but 216 tonnes would be nearly a third of the bulker's 684 tonnes of auxiliary fuel, past the page's own ceiling of 20% of it. The page's mechanism, 2.5 to 7% of the auxiliary fuel, gives 17 to 48 tonnes a year, and that is the range to carry for this ship; on the tanker's 939 tonnes it is 23 to 66. Lesson 3 works the default ship, the chemical tanker above, and arrives at 57.5 tonnes and about 31,600 USD a year at 550 USD a tonne, from running one set where two run today, with no purchase order attached. The three questions our free course Energy-Saving Technologies teaches in "The Measures Landscape: Five Families and a Sceptic's Checklist" apply here as everywhere. Percentage of what: of the engines' fuel, boilers excluded, which is why the definition above matters. Measured how: kilowatt-hours off the switchboard and fuel off the auxiliary meter, not the noon report's estimate. Net of what: net of nothing, which is the point: no capital to amortise, only a standing order to change.

The investigation map, with price tags

The course is the investigation, in the order an analyst runs it; each lesson ends on one decision with a price beside it.

• The bill (this lesson, nil): decide whether the ship's auxiliary share earns a file at all.
• The curve (lesson 2, nil): decide which SFOC curve, and which expensive band of it, go into the plant's fuel arithmetic.
• The count (lesson 3, a PMS setting): decide the power management system's start-and-stop table and the standing order for calm-weather passages.
• The sharing (lesson 4, a PMS function the equal-split default leaves unused): decide whether to let paralleled sets carry unequal loads, and at what set point.
• The load balance and the meters (lesson 5, a document and modest metering capital): decide which switchboard meters to fit and which generator-load trigger to run.
• The shaft generator (lesson 6, 520,000 to 3,500,000 USD to install, GreenVoyage2050's range): decide when to run the PTO, when to stop it, and whether a retrofit pays.
• The port stay (lesson 7, one set, a harbour set, or a plug at 50,000 to 2,000,000 USD vessel-side, again GreenVoyage2050): decide a per-port plan, with shore power as the limiting case.
• The audit and the wrapper (lesson 8, the company's routines): decide the energy performance indicators that keep the other seven alive, which lesson 8 argues is the dearest lever of all, because it costs attention every month rather than money once.

Monday morning

Put the auxiliary tonnes on the same page as the main engine's, per ship, for last year: from the DCS consumer split where the ship has one, from Table 17 and the hours in the abstract log where nothing better exists. Any ship whose auxiliaries exceed a tenth of the engines' total, or 500 tonnes a year, opens a file. The course workbook, for enrolled learners, carries the arithmetic from lesson 2 onward. Lesson 2 starts with the file's first page: that ship's generator, and its curve.