This paper presents an analysis of the hydrodynamic considerations in the design of water dynamometers, with particular emphasis on high-speed dynamometers with high rotor tip velocities. Hydrodynamic tradeoff considerations for high rotor tip speeds are discussed.Quantitative design techniques are presented to estimate the hydrodynamic characteristics in water dynamometers. These techniques are strictly empirical and limited in view of design requirements. Recent demands for higher-speed, higher-power density, long-service-life dynamometers have introduced added hydrodynamic tradeoff problems which require further investigation.This paper clearly portrays the need for a research water dynamometer program to provide data necessary to advance water dynamometer design technology. A research and development program currently under way is discussed.