Slow steaming: The other side of the coin

Although many shipowners are turning to slow steaming to cut fuel consumption and emissions, this strategy could have negative effects as well, design and engineering firm Houlder has warned.

Owners who are not informed by data analysis, risk a series of ‘unintended financial consequences’. The practice could even hinder progress towards International Maritime Organisation (IMO) greenhouse gas (GHG) reduction targets, according to the London-based consultancy.

As explained, slow steaming can have a positive, negative or neutral impact on vessel efficiency, depending on the ship’s original design parameters, hydrodynamic performance and unique operational profile.

Houlder’s research using Clarksons’ data (figure 1) indicates a noticeable trend in the reduction of average service speeds across different vessel types – oil tankers, bulkers, and container ships – from 2012 to 2024. Specifically, container ships experienced the most significant decrease, with a reduction of 1.50 knots (-11%) by 2024, followed by bulkers and oil tankers, which saw reductions of 1.01 knots (-9%) and 0.74 knots (-6%) respectively.

Speed and power are related by a power law, meaning that a reduction in speed results in a disproportionately larger reduction in power consumption. This relationship makes it a viable way to reduce greenhouse gas (GHG) emissions.

“Any speed reduction could result in a significant departure from a vessel’s original design parameters, so the benefit of corrective measures should be considered too. For example, a simple hull retrofit or propellor optimisation to align with the vessel’s new operating conditions could be the difference between slow steaming delivering minimal or real benefits,” Iebum Shin, Data Analytics Lead at Houlder, cautioned.

The analyzed data shows that while subject vessel types have reduced speed, the rate of reduction varies, reflecting differing impacts of slow steaming across vessel categories.

The associated speed changes from 2012 (figure 2) underscore the unevenness in speed reduction, emphasizing the need for a detailed and tailored approach when considering slow steaming as a strategy for reducing GHG emissions.

“It’s important to carry out a ship-specific analysis to establish what the real saving will be from a range of speed reductions and to consider these against the other, perhaps unintended, or unseen, consequences,” Shin added.

Slow steaming has largely been popular because it is relatively simple to implement. In theory, it doesn’t require extra capital expenditure (CAPEX) and, as long as the rest of the fleet is doing it to the same degree, then market economics should push the freight rate up to compensate the owner or charterer for the reduced annual cargo carrying capacity. Houlder highlighted that, in reality, slowing down is not necessarily a no-, or even low-cost option. There is often hidden CAPEX for the ship to address the consequences of slow steaming, such as modification to the turbocharger – and if engine de-rating is considered (as opposed to mechanical/software power limiters), CAPEX should be expected to be more significant.

There can also be unintended consequences of slow steaming on operating expenditure (OPEX) meaning that the expected fuel savings (and resulting OPEX reductions) may never be realised. Also, consistently operating outside the ship’s design parameters may mean costly engine maintenance is required more often – for example, cold corrosion, and fouling on the exhaust gas boiler, injector and piston rings.

“There is real potential for significant financial impact in additional days on hire or in reduced cargo revenue if the market doesn’t compensate with increased freight rates. If you can improve the ship’s efficiency without a high cost and without slowing down, this will offer a competitive advantage in terms of earning capacity,” Rupert Hare, CEO of Houlder, said.

“Ultimately, if the fleet slows down, it will need to expand to maintain the same cargo-carrying capacity. More ships can easily result in more GHG emissions for the same cargo miles, hindering progress towards IMO GHG reduction targets. While slow steaming can lower emissions in some circumstances, it can also deter investments in a more ambitious, long-term, sustainability strategy – including investments in energy efficiency technology, optimised operations and new fuels.”

Checking assumptions on slow steaming and understanding the ship’s sweet spots should be the minimum evaluation. The latest computational fluid dynamics analysis based on digital twin technology can help shipowners realize a more ambitious long-term sustainability strategy.

• Reducing Ship Speeds Has Multiple Benefits, New Study Shows
• UK: Slow Steaming Is Not Good Enough as Low Carbon Alternative