A hydrokinetic energy conversion system (HKECS) is provided, comprising design configurations suitable for efficient production of tidal or river in-stream kinetic energy into useful mechanical energy for tasks such as generating electricity or water pumping. The apparatus includes a set of blades with an airfoil cross sectional contour moving on a horizontal or vertical closed loop track, whereby the blades are propelled through the water by a net tangential lift force, rather than drag, to effectively convert the kinetic energy of flowing water to useful power.