This paper addresses the dimensional analysis of experimental data for the impulse turbine and the use of that data to create a model to predict the performance characteristics of any size of turbine under a range operating conditions. The model assumes that the performance of the turbine is a function of flow coefficient only. The model is used to compare the performance of different turbines at the scaled-up level and under varying conditions of axial velocity and angular velocity. Also, the model is used to identify the optimum turbine rotational speed, for maximum output power, at practical sizes over a range of input power levels. This paper discusses issues relating to the sizing and optimum operating point of the impulse turbine over variable sea conditions which oblige the turbine to operate over a design range rather than at a single design point and shows how this optimum operating point may be obtained. (C) 2003 Published by Elsevier Ltd.