Abstract: A device known as a Flow Stream Momentum Conversion Device (FSMCD) Power Rotor has been invented. The device makes use of the principle of Conservation of Momentum to convert fluid stream input momentum to device rotational power. A unique and non-obvious aspect of the fluid momentum conversion is circumferential discharge. This aspect maximizes power conversion in a given cross section of open stream flow. Power is extracted from the device as mechanical loading is applied to the rotating shaft resulting in a decrease in rotational speed and an increase is shaft torque. The device has practical usage in both wind and water flows. Under the condition of zero rotor shaft loading the device may be configured to make linear stream measurement of flow speed. This invention has applications in utilizing both unidirectional and bidirectional “fluid stream” flow. It is especially effective in tidal and wave action applications.