The paper opens by considering five reasons for monitoring the performance of ships and then discusses the problem of measuring ship speed accurately. A generalized system for speed performance monitoring is described in detail from collecting the data to presenting the analysis to management. A less satisfactory system based on fuel consumption is outlined for those ships without torsionmeters. The estimation of voluntary and involuntary speed loss of large tankers in heavy weather is discussed as an example of results from a monitoring system: a simple formula is given for involuntary speed loss in heavy weather. Fouling and roughness are well known causes of speed loss and these are considered in the light of recent research. The paper closes with a description of the power diagram and the results from the calculation of optimum docking intervals.