In this study, the hydrodynamic performance of a twin-blade hydrofoil has been numerically and experimentally investigated in three dimensions for tip speed ratios ranging between 1.5 and 5.5. The optimum geometric and flow parameters leading to the maximum value of the C-L/C-D ratio, which is the major design parameter of the wind and hydrokinetic turbines, have been determined. At a design flow velocity of 2 m/s (Re=3 x 10(5)) the maximum power coefficient of 0.457 was obtained at the tip velocity ratio of 3.5 at optimum geometric parameters of h/c(1) =0.667, c(1)/c(2)= 0.671 and the angle of attack of 3 degrees. The maximum torque of 224 Nm was obtained at the tip speed ratio of 2.5 for a prototype that has been built and tested during the experimental studies. The experimental studies were conducted in a towing tank, by which a power coefficient of 0.424 at the tip speed ratio of 3.48 was obtained. This speed ratio of 3.48 is about 7% lower than the numerical results. The optimum tip speed ratio of 3.5 is quite low when compared with the optimum tip speed ratio of 5.0 for the three bladed standard wind turbines. The advantage of employing twin-blade hydrofoils or airfoil is the potential of achieving better engineering designs and applications of wind and hydrokinetic turbines, which can be used for power generation purposes starting-up at lower wind and hydraulic current velocities. (C) 2015 Elsevier Ltd. All rights reserved.