Where in the past the operational cost of a ship was dominated by crew costs, this is now taken over by the bunker costs due to the ever increasing oil prices. To reduce operational costs, ships should be optimised in fuel efficiency considering environmental conditions and operational parameters. As a first step the speedpower relation should be established by means of speed trials upon delivery by the yard. A transparent and accurate industry standard for conducting and analysing speed trials is discussed. New methods to determinethe added resistance of ships in waves are presented and a consistent practice for speed trials is proposed. The second step in optimizing the vessel performance is to monitor the speed-power performance of the vessel in service. The vessel however is navigating under continuously changing loading and environmental conditions. By a proper analysis of the results and a display to the crew, the in service fuel consumption can be reduced by e.g. optimum trim of the vessel, engine settings or timely cleaning of the propeller.