Abstract
The International Maritime Organization introduced two energy efficiency indexes to reduce greenhouse gas emissions from ships. One of the short-run solutions is to reduce ship speed. The present research calculates the cost-effectiveness of reducing CO
2
emissions and the improved Energy Efficiency Design Index (EEDI) corresponded to the reduced Energy Efficiency Operational Indicator (EEOI) due to slow steaming. As a case study, RO-RO cargo vessel has been investigated. Reducing ship speed by 10% and 40% will reduce CO
2
emission by 27.05% and 78.39% with cost-effectiveness of 121.2 $/ton CO
2
and 287.6 $/ton CO
2
, respectively. The attained EEDI values will be improved by 2.04% and 35.81% with the reduced EEOI values by 8.76% and 70.65%, respectively. Although ship slow steaming by 40% would comply with the required EEDI for the first and the second phases, the complied EEDI value needs to be further reduced by 7% in the year 2025.