Abstract: A water brake which uses mechanical power to kinetically heat water to vapor or steam, and use thereof as a steam generator or cooling water conserving dynamometer or motion retarder. In the simplest embodiment, radial impeller vanes (5b) throw water against stator vanes (6e), whence the water rebounds to the impeller (5). The peripheral rebounding movement continues back and forth. Power dissipates as heat in the water causing the water to increase in temperature and to vaporize. The vapor, being lower in density and viscosity than is the water, flows to and out a central outlet (9) while the denser water is centrifugally separated from the vapor and retained in the peripheral rebounding motion. Water leaving as vapor is continually replaced through a cooling water inlet (8), allowing continuous operation over wide ranges of speed, torque, power, and steam generation rates, both at steady state and at controlled rates of change.