This study uses an activity-based method to investigate the fuel consumption and corresponding CO2 emissions of Capesize, Panamax, Supramax, and Handysize dry bulk carriers. The emission and energy reductions are estimated for speed reductions of 10%, 20%, and 30%. The CATCH (cost of averting a tonne of CO2-eq heating) model is applied to evaluate the cost efficiency of speed reduction. Results show that speed reductions of 10%, 20%, and 30% reduce fuel consumption by 27.1%, 48.8%, and 60.3% and CO2 emissions by 19%, 36%, and 51%, respectively. Speed reduction leads to emission reductions, with greater reductions for larger ships. CATCH values are positive, indicating that reducing speed increases cost. Line C3 of Capesize is used to determine the optimal ship number and operational speed under energy conservation. The minimum number of vessels in service is 9, with an average operational speed of 14.53 knots and one port call per week. If speed is reduced by 10%, 20%, and 30%, one, two, and four additional ships are needed, respectively. (C) 2013 Elsevier Ltd. All rights reserved.