Overall, speed reductions are a quick, easy and effective way to achieve emissions reductions from ocean-going vessels.
Slow steaming is when ships travel at 20 knots or below rather than 24 or 25 knots. Given the recent increases in oil prices, speed reduction makes sense not only environmentally but also economically.
Emissions, especially those of carbon dioxide, are directly proportional to fuel consumption. Greater speeds require increased fuel consumption. Consequently, slowing down, even by a small amount, can result in significant fuel savings and emissions reductions.
The IMO calculated that a speed reduction of just 10 percent across the global fleet by 2010 would result in a 23.3 percent reduction in fuel consuption and emissions per unit of time. Hapag-Lloyd found that slowing some of their ships by just five knots, or 20 percent, resulted in savings of around 50 percent on fuel costs. Slow steaming alone could reduce carbon dioxide emissions in some instances by as much as 70 percent when speed is halved. In addition, by 2013, reducing speeds to utilize laid-up vessels could total an approximate 30 percent reduction in emissions. Restrictions on vessel speed would reduce emissions of carbon dioxide, black carbon, nitrogen oxides, and nitrous oxide.
Compared to other forms of transport, ships traveling at slow speeds have been found to be far more efficient and less polluting -- roughly ten times more efficient than trucks and at least a hundred times more efficient than air transport. As ship speeds increase, much of this efficiency is lost. Ships traveling at very high speeds have been found to have similar energy demands to those of airplanes.