CII Rating Explained: 6 Engine Room Changes (2026)

Updated 30 September 2026 · cii, marpol annex vi, engine room, energy efficiency

CII rating explained for ship engineers, in one line: the Carbon Intensity Indicator (CII) is the International Maritime Organization's (IMO) annual score for how much carbon dioxide a ship emits against the transport work it does, and from 2026 that score changes what your engine room measures, loads and maintains. Your fuel figures become compliance evidence that class and flag inspectors can check. Main engine and generator loading now affects the ship's yearly letter grade. Your maintenance records end up feeding that same grade. The rating divides the carbon dioxide the ship emitted in a calendar year by the transport work it delivered — broadly, cargo carried multiplied by distance sailed — in that same year. Nothing about the engines themselves changes. What changes is how precisely you have to log what they burn, and how directly that log now affects a number the company answers for.

Most explainers stop at "slow steam and update the SEEMP." SEEMP stands for Ship Energy Efficiency Management Plan — the onboard document that fixes how the ship measures its fuel use and what it plans to do about it. This article is written for the chief engineer and the watchkeepers, the officers and engineers who stand bridge and engine room watches, not for the office. It covers which readings you now log differently, which loads you can trim yourself, where the data goes wrong, and which decisions belong to the master, the superintendent (the shore-based manager who oversees the ship's technical performance for the owner) or the charterer (the company that leases the ship and sets its speed and schedule for a voyage).

CII rating explained in plain engine room terms

The Carbon Intensity Indicator is an operational score, meaning it is recalculated every year from what the ship actually burned and actually carried. It divides the carbon dioxide the ship emitted in a calendar year by the transport work it did in that year. The American Bureau of Shipping (ABS), in its summary of the IMO Data Collection System (DCS — the IMO's fuel-reporting scheme) and SEEMP updates read on 30 September 2026, says the rules generally apply to ships of 5,000 gross tonnes and above. Gross tonnage measures a ship's enclosed internal volume, not its weight. ABS notes some ship types are excluded. GreenCalculus and Sustainable Ships, in pages read on 30 September 2026, describe the grading scale: every ship gets a letter from A (best) to E (worst) each year, and the performance needed to hold each letter gets stricter over time.

The top half of the ratio comes from fuel. Every tonne burned by the main engine, the generators and the boiler is converted into carbon dioxide using a conversion factor: a fixed number, set by the IMO, that says how many tonnes of carbon dioxide come out of one tonne of a given fuel grade when it is burned. So almost the whole top half of the ratio is engine room data.

The bottom half is transport work, which for most ship types is built from cargo capacity carried and distance sailed. The engine room has very little say over it.

That split is the most useful thing in this whole article. A ship with a spotless engine room can still slide down the scale because it spent weeks at anchor burning fuel while covering no distance. A tired engine room on a ship that sails full and steady can look better than it deserves. Once you see the ratio this way, most of the arguments onboard about "whose fault the rating is" make more sense.

How the target tightens each year

The target moves down every year, so a ship that just holds its current performance will slowly lose its grade. The Signal Group, in its CII compliance guide read on 30 September 2026, puts the 2026 required CII at roughly 11% below the 2019 reference level — the baseline emissions figure that every later year's target is measured against. GreenCalculus repeats that 11% figure in a page read the same day. ABS, in the update read on 30 September 2026, says the reduction for 2027 to 2030 moves in annual steps of 2.625% under IMO Resolution MEPC.400(83). MEPC stands for the Marine Environment Protection Committee, the IMO body that adopted that resolution. The resolution sets the reduction trajectory; the sources reviewed for this article do not fully spell out every provision inside it, so treat the 2.625% figure as the headline, not the whole text.

The A to E boundaries themselves depend on ship type and size, so there is no single "C rating" fuel-burn number that applies to every vessel. A bulk carrier's C boundary and a container ship's C boundary are different figures. Ask the office for the exact boundaries that apply to your ship and keep them next to your fuel log.

Treat every figure in that table as something to re-check against your flag and class guidance before you rely on it.

What changes in the engine room under CII

The changes to a real watchkeeping routine fall into six areas: fuel measurement, main engine loading, auxiliary load, performance trending, maintenance records and the SEEMP paperwork. None of them needs new machinery. They change how you log, how you load and what you can prove afterwards.

Fuel figures become compliance evidence

Your daily fuel figures now feed a regulated annual score. That means they have to be as defensible as your oil record book. ABS, in the update read on 30 September 2026, says a revised SEEMP Part II and a Confirmation of Compliance had to be onboard by 1 January 2026. That deadline applied to ships under flags that implemented the amendments on 1 August 2025. SEEMP Part II is where the ship's fuel measurement method is written down, and that method is what you are now held to.

In practice, this changes the daily routine in four ways.

1. Use one measurement method per consumer. If SEEMP says flowmeters for the main engine and tank soundings for the boiler, stick to that all year. Switching methods mid-year creates jumps a verifier will query.
2. Record fuel type against each consumer. Every fuel grade has its own carbon conversion factor. After a changeover for an emission control area, the log has to show which consumer burned which grade, and from when.
3. Reconcile the remaining fuel on board at fixed points. Compare flowmeter totals with soundings at every bunkering, and at whatever interval your company sets. Write down the reason for any gap before it grows.
4. Keep flowmeter calibration and density corrections on file. A figure that looks right but has no calibration record behind it is weak evidence.

If your daily consumption method is shaky, start with how to calculate main engine SFOC and daily fuel consumption onboard. The same flowmeter and sounding logic feeds the CII figure. Bunker quantities and grades come off the delivery note, so the MARPOL Annex VI fuel sampling and bunker delivery note checklist is part of the same chain of evidence.

Main engine loading becomes a rating decision

Speed and load have more effect on the rating than anything else the engine room touches, because the main engine burns most of the fuel. Guidance from Navtor, and an energy efficiency paper published by Maritime Cyprus, both read on 30 September 2026, list slow steaming and lower engine loads as common CII measures. Neither gives a fuel saving figure that applies to a specific engine, so be wary of anyone quoting a universal number.

Speed is usually set by the master, the company or the charterer, not by the engine room. Your part is what long periods at low load do to the engine. Extended low-load running tends to bring turbocharger and exhaust gas boiler fouling, scavenge space deposits, and auxiliary blower operation into play. It may also need changes to cylinder oil feed settings. Follow your engine maker's low-load guidance and service letters. Log the hours spent at low load, and plan turbocharger washing and scavenge inspections around those hours. That record lets you tell the office what slow steaming is costing in maintenance.

Auxiliary load is the part you control most

The auxiliary plant is the CII lever that sits most fully in the engineer's hands. Navtor, in guidance read on 30 September 2026, names lower auxiliary electrical demand as a way to improve a vessel's CII. Unlike speed, most of these calls are yours.

Start with generators. Run the fewest sets that keep the safety margin required by your power management settings and electrical load analysis. Lightly loaded generators burn more fuel for each kilowatt they produce. Next, look at the loads that run on habit: extra seawater pumps in cold water, engine room fans running at full speed, steam tracing, and fuel tanks heated above what the purifiers and viscosity actually need. Air compressor run hours are a cheap indicator of leaks. The boiler matters too, above all in port, where every tonne it burns adds carbon without adding distance.

There is one firm limit on all of this. Never trim below what the safety management system requires for manoeuvring, restricted waters or blackout protection. A blackout in a channel costs far more than any rating.

Performance trending becomes early warning

Engine performance monitoring is now your first sign that the rating is drifting. Guidance from Navtor and the Maritime Cyprus paper, both read on 30 September 2026, names main engine efficiency, fuel consumption monitoring, hull and propeller condition, weather routing and speed optimisation as the relevant areas.

The engine room usually sees hull fouling before anyone else does. If the engine needs more power for the same speed and draft in similar weather, and specific fuel oil consumption (SFOC) is steady, the problem is outside the engine — likely the hull or propeller. If SFOC itself is rising, look at injectors, turbocharger condition, air cooler pressure drop and indicator readings. A simple trending sheet, or one of the best apps for marine engineers, makes those patterns easier to spot than a paper log.

Maintenance records carry more weight

Maintenance never appears on the CII statement itself, but it shows up in the fuel figure. The Maritime Cyprus paper, read on 30 September 2026, identifies fouled hulls, damaged or inefficient propellers and poorly performing machinery as causes of higher fuel use. It gives no universal percentage for any of them, so do not quote one to the office.

For the engine room, that makes fuel valve overhauls, turbocharger washing, air cooler cleaning, boiler soot blowing and steam line insulation repairs part of the rating story. Separator performance belongs on that list as well. The fuel oil purifier troubleshooting checklist is useful here because poor separation eventually shows up as injector wear and higher consumption. Record the date and a reading before and after each job. If the ship ever needs a corrective plan, those pairs of readings are the evidence the engine room did its share.

SEEMP paperwork and the corrective plan

A poor rating creates formal follow-up work, and the engine room supplies most of the data behind it. A LinkedIn post by Praveen Kumar, read on 30 September 2026, says a ship rated E, or rated D for three consecutive years, must submit a corrective action plan. That is the only source found for this specific trigger. It is a LinkedIn post, not an IMO resolution text — the same MEPC.400(83) resolution cited above for the reduction trajectory does not, in the sources reviewed here, get quoted directly on the corrective-action-plan trigger. Treat the E/D-for-three-years figures as unverified until your flag or class confirms them against the primary IMO text.

Measures in corrective plans often land on the engine department anyway, whether as hull and propeller work, auxiliary load reduction or tighter monitoring. Keeping your logs tidy all year means you are not rebuilding them under pressure. For the reporting side, a round-up of digital tools every chief engineer should know covers the kind of software that makes year-end data less painful.

Which CII decisions belong to the engine room, and which do not

Some CII levers belong to the engine room and many do not. None of the sources behind this article gives a formal method for splitting responsibility, so the table below shows common practice. Your safety management system (SMS) and SEEMP decide the actual split on your ship.

This table is worth having when the office asks why the rating dropped. If the answer is waiting time at anchor, the fuel log will show it.

Where CII data goes wrong on a real watch

CII data tends to go wrong in predictable operating conditions, and a note in the log fixes most of it. The sources behind this article do not document firsthand failure cases, so what follows describes how each condition affects the ratio. It does not report any measured result.

Manoeuvring brings fast load changes. Short-period flowmeter readings can look odd, so record the start and end of the manoeuvring period.

Port stays add fuel with no distance. The ratio gets worse even when the engine room is running as lean as it safely can.

Shaft generator operation moves electrical load from the auxiliary engines onto the main engine meter. The total fuel is unchanged, but the split between consumers shifts, so note when the shaft generator is in and out.

Weather deviations add distance and fuel together. Log them so the voyage can be explained later.

Engine power limitation or derating changes the speeds the ship can reach. Record any change in the limit setting.

Long low-load running may push some flowmeters toward the low end of their rated range. Check the meter manual for its accuracy band.

CII versus EEXI in one paragraph

CII and EEXI measure different things. Britannia P&I and ABS, in pages read on 30 September 2026, describe CII as an annual operational measure and EEXI as a measure of the existing ship's design or technical efficiency. For the engine room, EEXI usually means a one-off technical fix, such as a power limit set once and left alone. CII is the running score you feed every day with fuel data.

Where CII helps engineers, and where it falls short

CII gives engine room data a weight it rarely had before. That has real benefits, and it also has real problems.

On the plus side, maintenance requests become easier to justify because fuel savings now affect a regulated score the office has to answer for. Auxiliary savings finally count in a way the office can see on paper. Hull fouling evidence from the engine room gets a proper audience instead of being brushed off.

The problems are real too.

1. Waiting time you cannot control lowers the score, because the ratio punishes fuel burned with no distance covered.
2. Boundaries vary by ship type and size, so comparing your rating with a sister company's ship is often meaningless.
3. The score is annual, so a bad quarter shows up long after anyone can fix it.
4. The data burden grows with no extra crew, and the extra logging usually falls on the chief and the second engineer.
5. Slow steaming can move cost into maintenance, through low-load fouling that the rating itself does not count.

Who this guide is not for

This guide is for engineers on the watchkeeping and reporting side. If you are a superintendent building a fleet decarbonisation plan, you will need the full IMO resolution texts and your class society's guidance, not this summary. If your ship is below the 5,000 gross tonne threshold ABS quoted in its update read on 30 September 2026, CII will not apply directly. If you want a calculator that works out your ship's attained CII, this article only explains what goes into that number — it is not a calculator itself.

FAQ

Does the CII rating apply to my ship?

It probably does if your ship is a large cargo vessel. ABS, in an update read on 30 September 2026, says CII generally applies to ships of 5,000 gross tonnes and above, with exclusions. Check with your company or class, because ship type decides which exclusions apply.

What rating triggers a corrective action plan?

A LinkedIn post by Praveen Kumar, read on 30 September 2026, says an E rating, or a D rating for three consecutive years, triggers a corrective action plan. That claim has not been checked against an IMO text. Confirm the rule and the deadline with your flag administration.

Can the engine room improve a CII rating on its own?

The engine room can improve it but cannot control it. You control fuel data quality, auxiliary load, performance trending and maintenance. Speed, routing, cargo and waiting time sit with the bridge, the office and the charterer.

Does slow steaming always improve CII?

Slower speed generally reduces fuel consumption, which is why guidance from Navtor and Maritime Cyprus, read on 30 September 2026, lists it as a common measure. Neither source gives a saving for a specific engine. Long low-load running also brings fouling and maintenance costs, so log those hours.

Is CII the same as EEXI?

They are separate regimes. CII is an annual operational rating based on actual fuel burned and transport work. EEXI concerns the technical efficiency of the existing ship's design, which is usually addressed once through measures such as a power limit.

Verdict

CII turns the engine room into the main source of the ship's carbon evidence. You do not control the whole rating, but you control how trustworthy the fuel figures are, how lean the auxiliary plant runs, and how early fouling gets spotted. Log consistently, trim auxiliary load safely, record maintenance with readings before and after, and keep a clear note of every period the engine room did not control.

Related reading

1. Offline calculator apps for fuel, unit and power conversions
2. Online courses for marine engineers that are worth the time
3. Marine engineer salary by rank in 2026

About this article

Every regulatory figure in this piece is attributed to the publisher that stated it, along with the date it was read. Where a source was secondary, thin or unverified against a primary IMO text — as with the corrective-action-plan trigger — the article says so directly rather than presenting it as settled. That is the standard this site applies to CII rating explained for ship engineers and what changes in the engine room.