ABSTRACTWater wheels can be efficient hydropower converters at low heads and low discharges. The paper presents the results of a study of a breastshot water wheel in a laboratory experimental channel. The obtained data show that breastshot water wheels can reach a maximum efficiency of about 75%, at a rotational velocity which is about 60% of the runaway velocity. The maximum efficiency is constant over a wide range of flow rates. A dimensional analysis is then carried out and a simple relationship for estimating the maximum mechanical power output of geometrically similar wheels is determined, relating the Newton number (dimensionless maximum power output) to the Reynolds number (function of the hydraulic conditions and channel geometry).