News / Benefits of slow-steaming on emissions exaggerated, studies find

© Norman Chan |

The emission benefits of slow-steaming may have been “grossly overestimated”, according to studies by Simpson Spence & Young, corroborated by research from Clarksons last week.

Although it has been assumed that ship speed has a cubic relationship with fuel consumption, with consumption increasing exponentially as speed increases, real-world evidence has shown this largely academic assumption does not necessarily pan out in reality.

This threatens to undermine many of the maritime industry’s long-held beliefs, and even brings into question the assumptions underlying the IMO CII regulation.

“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,” said Clarksons lead analyst Jon Leonhardsen on Friday.

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

Simpson Spence & Young (SSY) head of research Roar Adland referred to work on the subject by himself and colleagues in 2020. That study found the so-called ‘cubic law’ – a function of ship speed and hull resistance – is really only true “near the design speed of vessels” he said.

“The fuel consumption curve is flatter than the cubic rule at current sailing speeds and below, which makes further speed reduction much less impactful,” he explained. “Go below 9-10 knots and you may actually get the inverse effect, increasing emissions per tonne-mile.

“This was a pretty controversial claim when we first made it. It flies in the face of 100 years of naval architecture tradition for a start, but it seems to be increasingly accepted and mainstream now, apparently also in the big shipbroking houses.”

Slow-steaming is the preferred strategy for many carriers to comply with CII measures, which will be more stringent for older vessels. MSC, for example, is expecting CII-related slow-steaming to take up some 10% of cargo capacity throughout the global box fleet. But more ships needed in times of demand would increase emissions.

Xeneta market analyst Emily Stausbøll told The Loadstar last week a slow-steaming-driven capacity drop would not increase global emissions today, when demand is dropping precipitately.

“In today’s market, where we’re seeing an 8% decline in volumes, demand is so low… that there isn’t really that same need for extra capacity,” she said.

However, in the longer-term, Ms Stausbøll admitted, slow-steaming to comply with CII and other emissions regulations will increase the amount of ship capacity needed for the same amount of cargo, adding: “Ultimately yes, slow-steaming is a problem because it requires more capacity.”

This means that slow-steaming, though it might decrease emissions from individual ships in some cases, actually increases the number of ship engines burning fuel and emitting carbon, per given unit of cargo. This will cause an overall rise in CO2 emissions from the sector.

However, Peter Sand, analyst at Xeneta said: “I would definitely say the models are not being radically changed or undermined by this. By taking older vessels and slowing them down, you can cut fuel consumption by a third. But on newer ships, which are already built with better fuel economics, it is not like they can achieve that dramatic an increase.

“If we wanted to achieve the most [CO2] efficient way of transport, we would hold the ships until they are full and then start steaming. But that’s really not how supply chains work – so I think that ideal can be called off.

“The fact that carriers are now adding more ships into their services, they would only do so if they cut costs. So fuel costs remain the single most important cost item for operators.”

Mike Wackett, sea freight consultant for The Loadstar, said: “The re-emergence of the debate on the questionable benefits of super slow-steaming comes at a bad time for carriers endeavouring to convince their customers that the deployment of additional ships per loop to absorb their surplus capacity, and the subsequent increase in transit times for shippers, will cut emissions.”