Air Pollution Minimization Through Reduction of Vessel Speed at Sea

Reducing emissions from ocean-going vessels as they sail near populated areas is a widely recognized goal, and vessel speed reduction regulations is one of several strategies that is being adopted by regulators and port authorities. A Californian research shows that slowing the speed of cargo ships near coastlines could dramatically cut ships’ air pollution.

The study conducted by the Bourns College of Engineering, University of California measured the emission benefits associated with greenhouse gas and criteria pollutants when merchant vessels were operating at reduced speed.

The emissions were measured from one Panamax and one post-Panamax class container vessels as their vessel speed was reduced from cruise to 15 knots or below. The results showed that:

  1. Vessels’ speed reduction to 12 knots yielded carbon dioxide (CO2) and nitrogen oxides (NOx) emissions reductions (in kg/nautical mile (kg/nmi)) of approximately 61% and 56%, respectively, as compared to vessel cruise speed.
  2. The mass emission rate (kg/nmi) of PM2.5 was reduced by 69% with vessels’ speed reduced to 12 knots alone and by 97% when coupled with the use of the marine gas oil (MGO) with 0.00065% sulfur content.

Furthermore, emissions data from vessels while operating at sea are scarce and measurements from this research demonstrated that tidal current is a significant parameter affecting emission factors at lower engine loads.

Emissions factors at ≤20% loads calculated by methodology adopted by regulatory agencies were found to underestimate particulate matter (PM2.5) found in the air and NOx by 72% and 51%, respectively, when compared to emission factors measured in this study.

The total pollutant emitted in the emission control area (ECA) calculated during the study and the emission benefits were estimated as the vessels’ speed reduction zone was increased from 24 to 200 nmi. Total pollutants emitted (TPE) CO2 and particulate matter (PM2.5) estimated for large container vessels showed benefits for CO2 (2–26%) and PM2.5 (4–57%) on reducing speeds from 15 to 12 knots, whereas TPECO2 and TPEPM2.5 for small and medium container vessels were similar at 15 and 12 knots.

Studies worldwide have linked particulate matter to deaths from respiratory disease and heart attacks. Particulates specifically from ocean-going vessels have been linked to an increased number of premature deaths.

In addition, the shipping industry is responsible for 3 percent of the world’s carbon dioxide emissions, according to the International Maritime Organization. Shipping emissions are expected to grow 2 to 3 percent every year over the next three decades [PDF] as shipping traffic grows, according to the IMO.

Sources: Environmental Science & Technology, Maritime Connector


  1. Always a bit cautious about accepting the data as presented.
    It might be more interesting to see how this relates to the per tonne carried.
    If a vessel steams at half speed, it can only carry half as much freight in a year which means that another vessel is carrying that freight. So the emissions reductions are not quite so good as at first appears.
    Then there is cost. It may be that fuel costs are reduced but so are the earnings per vessel. They are obviously not working at their expected potential.
    This maybe OK now when so many vessels are laid up (I think Maersk have the most?) but how will things change when older vessels are scrapped and the industry recovers?

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