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 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, a 33% decline. While shipping lines 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. Most containerships are designed to travel at speeds around 24 knots.
- Slow steaming (18-20 knots; 33.3 – 37.0 km/hr). Running ship engines below capacity to save fuel consumption but at the expense of an additional travel time, particularly over long distances (compounding effect). This is likely to become the dominant operational speed as more than 50% of the global container shipping capacity operated under such conditions as of 2011.
- Extra slow steaming (15-18 knots; 27.8 – 33.3 km/hr). Also known as super slow steaming or economical speed. A substantial decline in speed to achieve a minimal fuel consumption level while still maintaining a commercial service. It can be applied to specific short-distance routes.
- 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. However, the level of service is 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 ships 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. However, 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 justify them further. Slow steaming practices have become the new normal to which users must adapt to.
Slow steaming also involves adapting engines designed for a specific optimal speed of around 22-25 knots, implying that they run at around 80% of full power capacity for that speed. 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. The ongoing practice of slow steaming is likely to impact supply chain management, maritime routes, and the use of transshipment hubs. For instance, slow steaming has different impacts depending on the type of trade involved. Low-value goods in containers such as waste products (a dominant American export) are less impacted than the retail trade, which is more time-sensitive.