An apparatus is disclosed for a turbine for generating electrical power from water or air flow comprising at least one rotor disk having a plurality of hydrofoil blades, a guide vanes, a cylindrical housing, and a generator means. A rim generator comprising a magnet race rotor rim and fixed stator coils in the housing is used. The apparatus is fitted with a screen to stop the ingress of debris and marine life, and a skirt augmenter device to reduce the Betz effect. The apparatus is preferably for sub-sea deployment and driven by tidal currents, but may be powered by river current or wave driven air or by wind. The apparatus may be deployed on at least one telescoping pole, tethered to the sea-bed and kept buoyant by buoyant concrete in the housing, inserted in a dam, under a barge or in a tidal power array.