Turbines and methods for their use, for generating power utilizing the flow of water or other fluids. In certain embodiments, turbines with helical blades oriented to efficiently intercept off axis or angular fluid flow, such as presented by whirlpool water flow patterns. In certain preferred embodiments, turbines having helical blades with major blade surfaces oriented to take advantage of whirlpool or circular-angle flow patterns, depending on geographic location of installation in the northern or southern hemisphere. In still other embodiments, turbines in which the pitch of rotor and/or helical blade surfaces is variable.