Since 1 January 2023, every commercial ship on international voyages has been living with two constraints written into MARPOL Annex VI: a technical design index, the EEXI, and an annual operational rating, the CII. The first was settled once, at a survey. The second is replayed every year and depends on what the ship actually burns at sea — in other words, on its technical condition.
Many owners fight the wrong battle. They look for the answer in weather routeing or in the next alternative fuel, while a fouled propeller, a blocked charge air cooler and three compressed air leaks quietly eat several points of efficiency. CII rating maintenance is the fastest lever, the least capital-intensive, and the one most fully under the crew's control. This article sets out the framework, links technical condition to consumption, and shows how to turn that into a maintenance plan and into reporting data a verifier will accept.
The framework in plain terms: Annex VI, EEXI, CII, SEEMP Part III
Chapter 4 of MARPOL Annex VI, on energy efficiency, was strengthened by amendments applicable since 1 January 2023. Three instruments now coexist: a design index, an operational indicator, and a management plan.
EEXI: a technical snapshot, taken once
The Energy Efficiency Existing Ship Index applies the EEDI logic to existing ships of 400 GT and above in the types listed in Annex VI. They calculate an attained EEXI in grams of CO₂ per tonne-mile — from installed power, propulsion type and capacity — and compare it with the required EEXI for their category.
Verification took place once, at the first IAPP survey on or after 1 January 2023, and results in an International Energy Efficiency (IEE) Certificate; many ships complied through Engine Power Limitation (EPL) or Shaft Power Limitation (ShaPoLi). The EEXI remains a frozen theoretical calculation: it says nothing about what the ship will burn next year. Certificate follow-up belongs in a certificates module.
CII: a rating replayed every year
The Carbon Intensity Indicator applies to cargo ships, ro-pax and cruise vessels of 5,000 GT and above on international voyages: annual CO₂ emissions divided by transport work. The metric is AER for most cargo ships, cgDIST for passenger and cruise ships.
The attained CII is compared with the required CII for the year, derived from a 2019 reference line reduced by a factor Z. That factor was 5% in 2023 and rises by two points a year to 11% in 2026. Resolution MEPC.400(83), adopted in 2025, extends the curve: 13.625% in 2027, 16.25% in 2028, 18.875% in 2029 and 21.5% in 2030. A ship whose consumption stays flat will therefore see its rating slide.
The result is a rating from A to E: A major superior, B minor superior, C moderate, D minor inferior, E inferior. A ship rated D for three consecutive years, or E in a single year, must submit a corrective action plan showing how it will return to C or better, included in SEEMP Part III and approved by the flag Administration or its recognised organisation. The penalty is not financial in IMO terms — it is commercial, because charterers, financiers and insurers now ask for the letter.
SEEMP Part III: the document that commits the owner
The Ship Energy Efficiency Management Plan has three parts: energy efficiency management (I), the fuel oil data collection plan (II) and, since 2023 for ships subject to CII, the implementation plan showing how the required CII will be met over three years (III). It is audited and carries a Confirmation of Compliance under regulation 26; ships had to have a Part III covering 2026-2028 approved before 31 December 2025. This is where maintenance measures — drydocking interval, propeller cleaning policy, hull performance monitoring — belong officially. A Part III listing measures with no maintenance plan behind them is an empty document.
What Europe adds for EEA port calls
If your ship calls at a European Economic Area port, two EU instruments sit on top of the IMO regime, with one major difference: they carry a price.
EU ETS. Since 2024, ships of 5,000 GT and above surrender allowances for 100% of emissions on voyages between two EEA ports and at berth, and 50% on incoming or outgoing voyages. Phase-in ran in steps: 40% of verified 2024 emissions, 70% of 2025, then 100% from 2026 emissions onwards. From 2026 the scope also covers methane and nitrous oxide, which weighs on LNG-fuelled ships because of methane slip. Every tonne of fuel saved through better maintenance is an allowance cost avoided immediately.
FuelEU Maritime. Regulation (EU) 2023/1805, in force since 1 January 2025 for ships above 5,000 GT calling in the EEA, works not in tonnes but in greenhouse gas intensity of the energy used on board, in gCO₂eq/MJ, well-to-wake. The baseline is 91.16 gCO₂eq/MJ, with reductions of 2% from 2025, 6% in 2030, 14.5% in 2035, 31% in 2040, 62% in 2045 and 80% in 2050. A compliance deficit costs around EUR 2,400 per tonne of VLSFO equivalent, with pooling, banking and borrowing (the latter at a 10% surcharge). Note the common misreading: burning less fuel does not lower GHG intensity, but it shrinks the energy base the deficit applies to.
And the IMO Net-Zero Framework? The framework, which would introduce a global GHG fuel intensity standard with a pricing mechanism, has still not been adopted. The extraordinary MEPC session of October 2025 was adjourned for a year, and MEPC 84 (27 April to 1 May 2026) failed to reach a compromise. Work continues in intersessional groups, with MEPC 85 expected at the end of 2026 and entry into force not before 1 March 2028 at the earliest. Check the status of this text as you read: it is the only item here still open.
Key takeaway: EEXI is a frozen calculation, CII an annual rating whose target tightens by roughly two points a year, SEEMP Part III the document that commits you to means. In Europe, ETS prices the tonne of CO₂ and FuelEU prices fuel intensity. In all four cases, technical condition is a direct input.
Where the fuel actually goes
Hull and propeller: the dominant item
Frictional resistance accounts for a dominant share of propulsion power at service speed. A light slime film typically costs a few per cent of power at constant speed; established macrofouling — weed, barnacles, calcareous growth — can push demand up by tens of per cent. The gap between those two states is mostly the time you let pass.
The propeller deserves separate treatment: a few tenths of a micron of extra roughness on the leading edge and blade back degrade propulsive efficiency disproportionately. Moderate fouling commonly costs a few per cent of power — and it is the cheapest intervention on the list.
Machinery, steam, compressed air, shafting
In the engine room, degradation is insidious: slow, and it triggers no alarm.
- Fuel injectors. Eroded nozzles atomise poorly: specific fuel consumption rises, exhaust gas temperatures drift, liner wear accelerates.
- Turbocharger. Deposits on the compressor wheel and turbine cut air flow and collapse scavenge air pressure. Periodic washing is one of the highest-return routines on board.
- Charge air cooler. Fouled, it lets scavenge air temperature climb: less dense air, poorer combustion. Air-side pressure drop and temperature difference across the cooler are the best indicators.
- Heat exchangers and coolers. When fouled, pumps work harder and the engine drifts out of its optimum range. A heat transfer coefficient from routine readings exposes the trend.
- Boilers. Excess air sends heat up the funnel; monitoring flue gas O₂ and outlet temperature allows fine tuning.
- Steam insulation and traps. Missing lagging or a trap stuck open dissipates energy around the clock that the boiler must replace. The trap round is the best effort-to-gain task on board.
- Compressed air leaks. A few millimetres of leakage at 7 bar keeps compressors cycling and wastes real power continuously.
- Lighting and electrical auxiliaries. LED conversion, restored variable speed drives, shutting down idling equipment: small gains, but permanent. On passenger ships, hotel load alone justifies dedicated monitoring.
- Shafting. Misalignment, a running-hot bearing or an over-tightened seal create direct mechanical losses, visible in bearing temperatures and vibration.
Drydocking and propeller cleaning: two direct CII levers
Two logics overlap. Class and safety first: bottom surveys run in cycles, with two in any five-year period and a maximum interval of thirty-six months between them. That interval is a regulatory floor, not an energy optimum.
Performance second. Since 2023 the IMO Biofouling Guidelines (resolution MEPC.378(80)) recommend a ship-specific biofouling management plan and record book. The reasoning is economic: the cost of in-water cleaning is weighed against the cumulative fuel penalty over the period fouling would have developed. In warm water, with frequent idle periods or low service speeds, the calculation almost always favours intervention. Three decisions shape a hull's carbon performance:
- The antifouling system and its fit with the real trading pattern. A self-polishing coating designed for continuous service at 14 knots will not deliver on a ship idle alongside a third of the time.
- The effective interval between drydockings, driven by hull performance data rather than the class calendar alone: an earlier docking can pay for itself in fuel.
- The propeller cleaning policy. Polishing is planned in port, costs a fraction of hull cleaning and restores a meaningful share of propulsive efficiency. Every six to twelve months is reasonable; on exposed ships, measurement should trigger the job.
One caution: in-water cleaning is increasingly regulated by coastal States and port authorities over invasive species and biocide release, so location and method must be checked port by port.
Measure before you act
None of this can be proven without measurement, and shipboard fuel data is often too coarse, too late, and not tied to an operating state. The minimum foundation has four elements.
- Reliable tank soundings, time-stamped and consistent: the reference for any fuel balance. A tanks module that stores level history removes unexplained month-end discrepancies.
- Mass flow meters on main engine, generator and boiler supply lines: this is what separates propulsion from auxiliaries.
- Running hour and production counters: generators, compressors, pumps, fresh water and steam. A counters module removes double entry between the engine log and the PMS.
- Shaft power measurement where fitted: without it you cannot separate hull and propeller degradation from sea and loading effects.
On indicators, ISO 19030 provides a recognised framework: expected power for a given speed, draught and trim is compared with actual power, with adverse weather filtered out. The result is a performance drift expressed as excess power in per cent — and that, not raw consumption, should trigger a cleaning.
Maintenance lever, expected effect, recommended frequency
The orders of magnitude below are indicative, as commonly observed in service. They vary with ship type, trading area, speed profile and fuel quality: use them to rank priorities, not as a promise of result.
| Maintenance lever | Expected effect on consumption | Recommended frequency |
|---|---|---|
| Propeller polishing | A few % of propulsion power recovered, almost immediate | Every 6 to 12 months, or on performance drift |
| In-water hull cleaning | A few %; considerably more if macrofouling is established | Quarterly inspection, action on threshold |
| Drydocking and antifouling renewal | Hull returned to its reference condition | Class cycle (2 bottom surveys / 5 years), adjusted on performance |
| Fuel injector check and renewal | A few % of specific consumption, exhaust temperatures stabilised | Maker's running hours plus exhaust temperature trend |
| Turbocharger washing | Scavenge air pressure restored, 1 to a few % | Compressor wash weekly to fortnightly |
| Charge air cooler cleaning | Lower scavenge air temperature, around 1% on specific consumption | Monthly checks, cleaning at pressure drop threshold |
| Heat exchanger and cooler cleaning | Less pumping power, machinery temperatures in range | Six-monthly to annual, or on heat transfer drift |
| Boiler combustion tuning | Around 1% to a few % of boiler fuel | Monthly O₂ and flue gas checks, six-monthly tuning |
| Steam insulation and trap rounds | Continuous losses removed, 24/7 effect | Quarterly trap round, annual lagging inspection |
| Compressed air leak survey | Compressor running hours directly reduced | Six-monthly survey, line isolated |
| LED lighting and drive overhaul | Small but permanent gain on electrical load | One-off programme, then annual check |
| Shaft alignment and bearings | Reduced mechanical losses between engine and propeller | Continuous monitoring, verification at drydock |
Building an energy-aware maintenance plan
A conventional planned maintenance system protects availability and safety. An energy-aware plan adds one dimension: it treats consumption as a monitored parameter, like a bearing temperature. Five steps, extending the method in our four-step preventive maintenance plan guide.
- Identify the energy-critical equipment: hull, propeller, shafting, main engine, boilers, compressors, steam system, electrical generation.
- Attach one or two performance parameters to each item, not just an interval: charge air cooler pressure drop, temperature difference across an exchanger, flue gas O₂.
- Set trigger thresholds — the move from calendar-based to condition-based work, covered in our article on moving from corrective to condition-based maintenance.
- Schedule heavy jobs against the trading window: propeller during a long port stay, lagging during drydock, injectors before a busy season.
- Close the loop: compare the indicator before and after each job and record the result — the evidence that will feed the SEEMP Part III corrective action plan.
A quick test of your current plan: is propeller cleaning a planned task with an owner and a budget? Is there a daily fuel record separating propulsion, generation and boilers? Are charge air cooler thresholds written down? Can you retrieve three years of performance-related work history for a given ship?
Making the data fit for DCS and MRV reporting
CII, EU ETS and FuelEU all rest on the same raw material: verifiable fuel consumption data. The IMO Data Collection System has required ships of 5,000 GT and above to report annual consumption by fuel type, with distance travelled and hours under way, since 2019; EU MRV requires a comparable but more detailed set, voyage by voyage, verified by an accredited body.
The weak point is almost never the calculation — it is the traceability of the input. Hand-copied readings, gaps between soundings and bunker delivery notes, approximate running hours, missing periods around a crew change: this is what turns into a verifier's finding. A digital engine log answers exactly that: time stamp, identified author, value not editable after the fact.
A PMS built for shipboard use links the job to the measurement: "propeller polishing" is recorded with its date, parts and cost, and reads alongside the consumption curve. It keeps data continuous across crew changes, with a mobile app that works offline. And it produces the auditable history that SEEMP Part III and the MRV verifier require.
At Smart Sailors, the Tanks, Counters and Forecast modules were designed for that chain: bunker movements tracked, running hours centralised, maintenance due dates projected from hours actually accumulated rather than a theoretical calendar. An injector renewed at the right hour is fuel saved and a part not wasted. Over 400 vessels use the platform today, from yachts and ferries to port services and offshore.
FAQ
My ship is below 5,000 GT — does CII apply?
No. CII and SEEMP Part III apply to cargo ships, ro-pax and cruise vessels of 5,000 GT and above on international voyages; EEXI applies from 400 GT for the types listed in Annex VI. The absence of an obligation does not remove the economics: these levers produce the same savings on a harbour craft or fishing vessel as on a bulk carrier.
Can better maintenance really move the CII rating by a letter?
It contributes, but rarely alone. Moving between letters depends on the width of the rating bands for your ship type and size, and on where you sit within the band. A ship just below the C/D boundary can cross it on accumulated technical measures; a ship deep in D will need operational optimisation and, where justified, investment.
Which lever pays back fastest?
Propeller cleaning or polishing: low cost, feasible during a port call, measurable within days at sea. Turbocharger washing and a compressed air leak survey come next, both within the crew's reach. Drydocking delivers more, but it is an owner's decision, not a shipboard action.
Should we wait for the IMO Net-Zero Framework before acting?
No. That framework is not yet adopted and its timetable remains uncertain, while CII, EU ETS and FuelEU Maritime are already biting. Every tonne saved in 2026 cuts your EU allowance bill immediately and improves this year's rating.
How do we prove to a verifier that our fuel data is reliable?
Through the traceability of the chain: time-stamped readings attributed to an identified author, methodology documented in SEEMP Part II and the MRV monitoring plan, demonstrable consistency between bunker delivery notes, soundings and flow meters, and no gaps in the series. A system where readings are captured at source and cannot be edited retroactively meets that expectation; a shared spreadsheet never will.
Conclusion
Once a ship's carbon performance is rated annually, circulated and invoiced in Europe, technical condition stops being an engine room matter. A dirty propeller, a blocked charge air cooler and a leaking steam system have become lines on an allowance bill and lost points on a CII target that tightens every year to 2030 at least.
These levers are within the crew's reach, cheap in capital terms and immediately measurable — provided you have the data. Build the plan around performance parameters, set thresholds, record the work, and DCS and MRV reporting becomes a by-product of daily operation.
Want to connect your maintenance plan, counter readings and bunker levels in one tool designed by seafarers? Book a demonstration or start your 30-day free trial and test the approach on one ship before rolling it out across the fleet.
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