A series of bi-directional horizontal axis tidal turbine (HATT) rotors was prototyped. The geometry and motion parameters of the turbine series cover a wide range of tidal turbine operating conditions targeted to the tidal flow speed probability distribution of the Bay of Fundy, NS Canada. A generic yet novel wind/tidal turbine rotor design and optimization procedure was developed and used for the optimization process. In the process, optimum annual energy production, as a benchmark, was obtained and used for comparison by taking into account both rotor hydrodynamic power production performance characteristics and annual tidal inflow speed probability distribution. The diameter of the full-scale bi-directional HATT rotor series developed is 20-m. Optimum values of various variables were obtained for maximum annual energy production. These optimum values include: uniform pitch-diameter-ratio (p/D) distributions. nominal pitch values with different curve forms of non-uniform p/D distributions, shaft rotational speeds under fixed rotor diameter, diameter at fixed tip-speed-ratio (TSR), a combination of optimum shaft speeds at corresponding inflow speeds and optimum uniform p/D distribution at a fixed rotor diameter, number of rotor blades at fixed chord length, rotor solidity at fixed number of blades, and the combination of number of blades and solidity for both optimum constant pitch distribution and optimum linear pitch distribution. For the 20-m bi-directional HATT, a substantial improvement in power coefficient C-pow was obtained, from 0.28, as a result of the primary optimization process (for both optimum uniform p/D distribution and shaft speed n), to the final maximum of 0.43. This corresponds to a 56% increase in annual energy production of 1230 MW h, from 2188 to 3418 MW h, an improved energy production equivalent to the energy generated from 600 tonnes of coal (at 2 kWh per 1 kg coal from a standard thermal power plant). The annual energy production from the optimized 20-m bi-directional HATT before the deduction of mechanical and electrical conversion loses, is 3418 MW h, equivalent to the electric energy generated from 1550 tonnes of coal. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.