Standard airfoils historically used for wind and hydrokinetic turbines have lift coefficients of about 1.3 maximum at the stall angle of attack, which is about 12°. At these conditions, the minimum flow velocities to generate electric power are about 7 m/s and 3 m/s for wind turbine and hydrokinetic turbine, respectively. In this study, the double blade hydrofoil was investigated for potential hydrokinetic and wind turbine applications. Hydrodynamic performance of double-blade hydrofoil have been numerically and experimentally investigated. The 2D and 3D numerical analyses were carried out and the optimum geometric and flow conditions leading to the maximum value of the CL/CD ratio, which is the design parameter of the wind and hydrokinetic turbines have been determined. The maximum power coefficient of 0.457 was obtained at the tip velocity ratio of 3.5 at the optimum geometric parameters of h/c1 = 0.667 and c1/c2 = 0.671 at an angle of attack of 3°. It has been understood that the double-blade hydrofoil has very low optimum tip speed ratio of 3.5 compared to the optimum tip velocity ratio of 5.0 for standard, three-blade wind turbine applications. The experimental work conducted in a towing tank yielded the power coefficient of 0.424, which is about 7% lower. Hence, there is reasonable agreement between numerical and experimental values. This arrangement leads to the possibility of achieving engineering design of wind and hydrokinetic turbines that can be used for power generation at lower wind and current velocities.