Source: adapted from Notteboom, T. and P. Carriou (2009) "Fuel surcharge practices of container shipping lines: Is it about cost recovery or revenue making?". Proceedings of the 2009 International Association of Maritime Economists (IAME) Conference, June, Copenhagen, Denmark.

Fuel Consumption by Containership Size and Speed

Fuel consumption by a containership is mostly a function of ship size and cruising speed, which follows an exponential function above 14 knots. For instance, while a containership of around 8,000 TEU would consume about 225 tons of bunker fuel per day at 24 knots, at 21 knots this consumption drops to about 150 tons per day. While containership operators would prefer consuming the least amount of fuel by adopting lower speeds, this advantage must be mitigated with longer shipping times as well as assigning more ships on a pendulum service to maintain the same port call frequency. The main ship speed classes are:

• Normal (20-25 knots; 37.0 - 46.3 km/hr). Represents the optimal cruising speed a containership and its engine have been designed to travel at. It also reflects the hydrodynamic limits of the hull to perform within acceptable fuel consumption levels.
• Slow steaming (18-20 knots; 33.3 - 37.0 km/hr). Running ship engines below capacity to save fuel consumption, but at the expense a additional travel time, particularly over long distances. This is likely to become the dominant operational speed as more than 50% of the global container shipping capacity was operating under such conditions as of 2011.
• Super slow steaming (15-18 knots; 27.8 - 33.3 km/hr). Also known as economical speed. A substantial decline in speed for the purpose of achieving a minimal level of fuel consumption while still maintaining a commercial service.
• Minimal cost (12-15 knots; 22.2 - 27.8 km/hr). The lowest speed technically possible, since lower speeds do not lead to any significant additional fuel economy. The level of service is however commercially unacceptable, so it is unlikely that maritime shipping companies would adopt such speeds.

The practice of slow steaming emerged during the financial crisis of 2008-2009 as international trade and the demand for containerized shipping plummeted at the same time as new capacity ordered during boom years was coming online. As a response, maritime shipping companies adopted slow steaming and even extra slow steaming services on several of their pendulum routes. It enabled them to accommodate additional capacity with a similar frequency of port calls. It was expected that as growth resumed and traffic picked up maritime shipping companies would return to normal cruising speeds, but this appears not to be the case. In an environment of higher fossil fuel prices, maritime shipping companies are opting for slow steaming for cost cutting purposes, but using the environmental agenda to further justify them. Slow steaming practices have become the new normal to which users must adapt to.

Slow steaming also implies adapting engines that were designed for a specific optimal speed of around 22-25 knots, implying that for that speed they run at around 80% of full power capacity. Adopting slow steaming requires the "de-rating" of the main engine to the new speed and new power level (around 70%), which involves the timing of fuel injection, adjusting exhaust valves, and exchanging other mechanical components in the engine.

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