NOVELTY - The rotor has channel inlets forming a peripheral rotor inlet extending circumferentially around a peripheral envelope and extending axially between a rotor core at a base plane and a peripheral band at an intermediate plane over an inlet length. Rotor channels comprise a respective open channel outlet lying on an outlet plane circumferentially between circumferentially successive outlet edges. Channel outlets form an axial rotor outlet bounded radially within the peripheral band on the outlet plane, where the axial rotor outlet is divided sectorially into the channel outlet by rotor vanes.    USE - Turbine rotor for a fluid-driven turbine apparatus (claimed) i.e. instream tidal power generating apparatus towed below or behind a boat i.e. sail boat, deployed/immersed in a stream of flowing fluid e.g. water, air/wind and steam, to extract power from the flowing fluid into a rotating shaft.    ADVANTAGE - The apparatus smoothly accelerates, redirects and decelerates the fluid flowing through the apparatus, thus reducing or minimizing cavitation and turbulence in the flowing fluid, and hence increasing or maximizing the transfer of power from the flowing fluid to the rotor. The rotor is designed with rotor channel walls configured along preferably idealized fluid flow streamlines through the rotor channels. The intake channel, rotor channels, and exhaust channel are arranged in a proper manner so as to increase kinetic energy of the encaptured fluid, extract power, and regain static pressure of the fluid that is exhausted or discharged back into the flowing stream at the ambient pressure of the freely flowing stream. The rotor has relatively small-diameter and rotates at a relatively high rotational speed, thus maximizing the power transfer from the fluid to the rotor, and from the rotor shaft to an electrical generator, alternator, pump and motor. The rotor avoids the use of active movable control gates, guide vanes, variable pitch blades, and movable elements, so that the rotor on a shaft is the only moving component of the apparatus, thus generating power from a stream flow with a variable flow velocity in an easy manner. The rotor comprises a spiral channel formed by a spiral volute casing that is a fixed passive element that naturally directs the water radially into the rotor inlet when the water progresses around revolution of a spiral, so that active control gates and stator vanes are avoided, thus reducing cost and complexity of the apparatus, the chance of malfunction or breakdown, and the need for maintenance, and hence avoiding the need for active water flow regulation, superfluous drag, turbulence and cavitation. The rotor is fixed without variable pitch blades, and moves as a solid unit throughout an entire water flow channel, thus allowing the rotor to be simple and robust with efficient and effective design.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a method for designing a turbine apparatus.    DESCRIPTION OF DRAWING(S) - The drawing shows a perspective view of an instream hydro power generator apparatus.    Hydro power generator apparatus (1)    Intake duct (2)    Rotor housing (4)    Exhaust duct (14)    Exhaust duct elbow (15)