A cross-flow HKT can be deployed in rivers and canals having a shallow depth of water to harness the kinetic energy from free stream water. Few studies have been carried out on the hybrid HKT having a straight-bladed Savonius rotor in order to enhance the performance of the rotor. However, there is a scope to investigate the effect of radius ratio, attachment angle and water flow velocity on the average power coefficient of a hybrid HKT having a helical-bladed Savonius rotor. Under the present study, the effect of radius ratio, attachment angle and water flow velocity has been analysed numerically on a configuration of a hybrid HKT comprised of three Darrieus rotor blades and two Savonius helical blades. The simulations on different configurations of the hybrid HKT are performed using ANSYS v15. It is observed that the radius ratio and attachment angle significantly affect the energy harnessing capability (average power coefficient) of the hybrid HKT. The optimum radius ratio and attachment angle values for the best-configured hybrid HKT are 0.4 and 90°, respectively. The maximum average power coefficient of the hybrid HKT model with a radius ratio of 0.4 is enhanced by 46.2%, 27.6% and 5.2% compared to models with a radius ratio of 0.2, 0.8 and 0.6, respectively. The hybrid HKT with an attachment angle of 90° is found 4.3% and 4.8%, more efficient than the hybrid configurations with attachment angles of 30° and 150°, respectively. It has been observed that the hybrid HKT is found to have better efficient for low values of water flow velocity. Further, a significant effect of the Savonius blade profile is also found on the performance of the hybrid hydrokinetic turbine rotor.