An induction generator has been designed for use in an open centred tidal turbine. Mechanical constraints in the turbine design enforces a large magnetic gap in the generator, which inherently limits the power factor. The large diameter of the machine means it is likely to be constructed out of a number of segments, which can each be modelled independently and approximated to either linear segments or individual rotary machines with the same pole to air gap ratio. An analytical tool to design an induction machine for this unique set of requirements is presented and compared to results from commercial software. Detailed design options are investigated using finite element analysis before a final design is presented.