The implementation of International Maritime Organization regulations for CO2 emission reduction has oriented ships to sail at lower speeds. Speed reduction reduces fuel consumption of ships significantly. In order to perform efficient ship operation, energy demand throughout the whole engine room has to be decreased. The goal of the study is to minimize electrical power consumption during slow steaming operation, hence equivalent fuel consumption and emissions could be decreased. A computational study was carried out in order to clarify potential savings with respect to main engine load. The study quantifies the energy savings in main engine cooling system which has significant potential to reduce electrical power consumption in slow steaming operations. Operational requirements are determined with the help of realistic engine room simulator of a container ship which has waste heat recovery system. The values, obtained from the simulator were verified using the technical manual of a diesel engine, which have similar output power range. The results of calculations show that there is great potential to improve energy efficiency when variable pumps are used. Based on the results, from the main engine cooling system, 60% of electrical power demand reduction can be achieved. The power reduction saved by pumps, decreases considerable amount of marine diesel oil consumption as 296.2 tons where 367.4 tons of fuel needed by diesel generators or oil fired boiler. The reduction in fuel consumption is 80% that the saving in the central cooling system corresponds 948 tons of CO2 emission reduction and $207,300 cost saving. (C) 2019 Elsevier Ltd. All rights reserved.