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Seaplify March 22, 2026

CII rating seafarers need to understand, because it directly affects how your ship is operated, what operational decisions you make on watch, and increasingly, your career prospects. The Carbon Intensity Indicator (CII) is an IMO regulation that rates every large ship annually on its operational carbon efficiency. Ships rated D or E face serious consequences. Officers who understand CII and know how to improve their vessel’s rating are becoming increasingly valuable to operators.

this guide explains CII in plain language for seafarers, what it measures, how ships are rated, what it means for your daily work, and how to help your vessel achieve a better score.

What Is the CII Rating System?

The Carbon Intensity Indicator (CII) is a measure of how efficiently a ship converts fuel into transport work, tonnes of cargo moved per nautical mile, expressed as CO₂ emissions. It applies to ships of 5,000 GT or above on international voyages. CII is part of the IMO’s short-term GHG reduction measures under MARPOL Annex VI and became mandatory from January 2023.

each ship receives an annual CII rating of A, B, C, D, or E:

• A, Major superior performance
• B, Minor superior performance
• C, Moderate performance (compliant)
• D, Minor inferior performance (one year = corrective action plan required)
• E, Inferior performance (immediate corrective action plan required)

Ships rated D for three consecutive years, or E in any single year, must submit a Ship Energy Efficiency Management Plan (SEEMP) corrective action plan to their flag state. The stakes are increasing: from 2026, D and E ratings are expected to trigger stronger enforcement and potential commercial consequences with charterers.

How CII Is Calculated

However, cII is calculated annually using data from the IMO Data Collection System (DCS), which all ships over 5,000 GT must report to their flag state. The formula measures Annual Efficiency Ratio (AER), CO₂ emitted per deadweight tonne-nautical mile sailed during the reporting year.

The key factors that influence a vessel’s CII rating are:

• Fuel consumption, total fuel burned determines total CO₂ output
• Distance sailed, more miles sailed with the same fuel = better CII
• Cargo carried, higher utilisation improves the ratio
• Speed, higher speed burns disproportionately more fuel (cubic relationship)
• Port idle time, burning fuel at anchor or waiting counts against the ship
• Auxiliary engine use, hotel loads, reefer power, deck equipment all consume fuel

What CII Means for Officers and Crew on Watch

CII has made fuel efficiency a direct professional responsibility for deck and engine officers, not just a commercial concern for management ashore. Every decision on watch affects the ship’s annual rating.

“Officers who understand that slow steaming, trim optimisation, and auxiliary load management aren’t just company policy, they’re regulatory compliance, are the officers operators want to keep,” notes a fleet technical superintendent with 22 years of experience at sea and ashore.

• Deck officers, speed optimisation, weather routing, trim adjustments, reducing auxiliary use at anchor
• Engineer officers, main engine tuning, auxiliary efficiency, waste heat recovery, fuel switching
• Masters, voyage planning decisions, port approach optimisation, just-in-time arrival coordination with ports
• Chief Engineers, SEEMP implementation, fuel monitoring systems, technical modifications

CII and Your Career: What Operators Now Expect

Shipping companies with fleets of D- and E-rated ships face commercial pressure from charterers who increasingly specify minimum CII ratings in charter parties. This means operators are actively seeking officers who understand CII and can contribute to improving vessel ratings.

Practically, this means:

• Masters and Chief Engineers are increasingly evaluated on their vessel’s CII performance
• Knowledge of SEEMP Part III (CII improvement plans) is becoming expected at senior officer level
• Officers with green shipping training or decarbonisation credentials stand out in interviews
• Companies running LNG, methanol, or ammonia-fuelled vessels need officers who understand alternative fuel efficiency profiles

For a broader view of how decarbonisation is reshaping maritime careers, see our article on MARPOL Regulations: A Seafarer’s Quick Reference.

Practical Tips for Improving Your Ship’s CII Rating

• Just-in-time arrival, coordinate with port agents to avoid anchoring and burning fuel waiting for berths
• Speed optimisation, even a 1-knot reduction can significantly improve fuel consumption; discuss target speeds with the Master
• Trim optimisation, maintaining optimal trim for current loading conditions reduces hull resistance
• Auxiliary load management, switch off non-essential equipment at sea; reduce hotel loads where safe to do so
• Hull and propeller cleanliness, report fouling promptly; biofouling significantly increases fuel consumption
• SEEMP compliance, know your vessel’s SEEMP Part III targets and track them throughout the year

Frequently Asked Questions

For official maritime standards and further information, visit the International Maritime Organization (IMO).