UK shipbroker Clarksons has warned that the effectiveness of slow-steaming on Carbon Intensity Indicator (CII) compliance is being “grossly over-estimated” by shipowners.

The London-listed company said its green transition team has produced “breakthrough” analysis into the relationship between speed and fuel efficiency.

A large share of the global fleet is expected to reduce sailing speed in order to meet the IMO measure’s targets.

Clarksons believes many of the traditional methodologies being relied upon for decision-making are inaccurate, however.

It is advising clients to re-examine any CII-related decisions and how calculations are being made to ensure that they are maximising the upside potential of vessel utilisation and mitigating downside risk as far as possible.

The green transition team, led by Kenneth Tveter, argues cutting speed may not necessarily improve CII ratings.

And lead analyst Jon Leonhardsen said: “The reason for the misconception is that calculations are based on textbook speed-consumption curves which have an exponential growth across an entire speed range.”

“However, if you factor in all of the consumers of a ship and the variables of real-world sailing conditions, the curve becomes less exponential (or flatter) at lower speeds,” he added.

Tveter believes this means the marginal fuel saving for distance sailed becomes lower, and this is ultimately what matters for CII performance.

“So, whilst some ships are needing to slow down significantly to comply, for others speed reduction isn’t viable at all and other costly measures and improvements need to be done,” he added.

No plans in place?

Clarksons claims its new CII assessment modelling tool has in some cases proven to be 30% more accurate than traditional modelling methodologies in predicting real-world fuel consumption, especially at lower speeds.

“Whilst the introduction of CII in January wasn’t a surprise to the shipping industry, it has been met with mixed reaction and many ship operators do not have plans in place for effective management,” the broker said.

The lack of clarity and the complexity around how CII calculations are made can easily lead to inaccurate decision-making or procrastination, the company added.