In recent years, increasing attention is being given to the study of hydrokinetic turbines for power generation due to the use of clean energy by using renewable sources. This paper aims to present a general methodology for the efficient design of horizontal axis hydrokinetic turbines with variable rotation. The approach uses the Blade Element Method (BEM) for determining the power coefficient of the turbine. The modeling of the hydrokinetic rotor is coupled with the model of the drive line of the system, including the multiplier and the electric generator. Therefore, the modeling of the whole system comprises the hydrodynamic information of the rotor and the characteristics of the inertia of whole system, frictional losses and electromagnetic torque of the generator. The results of numerical simulation are obtained for the rotational speed of the rotor as well as the results of the torque, mechanical and electrical power.