Slow Steaming

Slow steaming is the deliberate operation of a ship at a reduced speed to lower its fuel consumption. It rests on a basic physical fact: the power needed to push a ship through the water, and therefore the fuel it burns per hour, rises very steeply with speed — roughly with the cube of the speed for the propulsion power. As a result, cutting speed by a relatively small proportion cuts the fuel burned per hour by a much larger proportion, and even allowing for the longer time the voyage takes, the total fuel used for the passage falls substantially. This makes speed one of the most powerful levers a ship operator has over fuel costs and emissions.

The practice became widespread when high fuel prices and surplus shipping capacity coincided, particularly in the container trades, where operators found that running ships slower and adding a vessel to a service to maintain schedules could still save money overall. Very large reductions in speed are sometimes called super slow steaming. Beyond fuel cost, slow steaming reduces carbon dioxide and other emissions in direct proportion to the fuel saved, which is why it has become an important tool for meeting environmental targets and improving a ship's operational carbon intensity under the CII regime.

Slow steaming is not without complications. Running an engine for long periods at low load can cause problems such as fouling and cold corrosion if the machinery was designed for higher output, so engines may need de-rating, modification or careful management to operate efficiently and reliably at reduced power. Slower voyages also lengthen transit times, which affects supply chains, inventory and the number of ships needed on a service. Nonetheless, because it delivers immediate savings in both cost and emissions using existing ships, speed reduction remains one of the most effective and widely used measures for greener, cheaper operation, and proposals for regulated speed limits have been discussed as a climate measure.