Reading EEOI without being misled

Why the largest emitter on a list is rarely the least efficient ship on it.

Absolute emissions answer a simple question: how much carbon did this ship put into the atmosphere last year? It is a real number and it matters. It is also almost useless for judging a vessel, because it mostly measures how much work the ship did.

A container ship on continuous liner service will out-emit a handysize bulker that spent four months idle. That tells you about trade patterns, not about engineering or seamanship.

The efficiency ratio

EEOI, the Energy Efficiency Operational Indicator, divides emissions by transport work: grams of CO₂ per tonne of cargo carried one nautical mile. A lower figure means more freight moved per unit of carbon. Because it is a ratio, it lets a 300-metre vessel be compared with a 100-metre one, provided both are the same type of ship doing the same kind of work.

Across types the comparison collapses. A cruise ship's transport work is measured in passengers, a car carrier's in vehicle units, a tanker's in tonnes of cargo. Those units do not convert. Every ranking on this site is therefore calculated within a ship type and within a single reporting year, never across them.

The outlier problem

EEOI is a division, and the source data contains returns where reported transport work is close to zero: a ship that spent the year in lay-up, or one that carried almost nothing while still burning fuel to keep systems running. Dividing by a near-zero denominator produces figures in the millions.

Measured across the published dataset, the median EEOI is around 12 and the 99.9th percentile is roughly 4,100. Values above 100,000 are therefore withheld on this site rather than displayed. They are not evidence of an extraordinarily dirty ship. They are evidence that the ratio has stopped meaning anything for that return.

Design index versus operation

Many vessels also report a technical efficiency figure, an EEDI or EIV. That is fixed at construction and describes the hull and machinery as designed. It says nothing about how the ship is actually run. A vessel can hold an excellent design index and a poor EEOI by sailing fast, waiting at anchor, or running half empty. Read the two together.