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Improving the performance of Vertical Axis Wind Turbines (VAWTs) is key to make VAWTs commercially successful. Scientists have investigated several interesting concepts for that purpose: increasing the swept area, especially to limit the losses due to 3D effects, or using flap and pitch systems to control the flow around the blades [1, 2]. Nenuphar, with the Twinfloat concept(see Figure 1), propose to take advantage of all these concepts combined with an aerodynamic effect called counter-rotating effect [3]. The turbine is made of two 2.5MW VAWT on only one floater, to reach a rated power of 5MW. The proximity of the two rotors generates a contraction of the stream tubes that flow in the area between the rotors, thus increasing the air flow rate going through both the swept areas and thereby the performance of the VAWT. This concept has also other advantages:smaller rotors that make easier their fabrication,their installation and Operating and Maintenance (O&M) operations, floater motions reduction thanks to specific control laws and wake reduction leading to an increase in the capacity factor.Simulations presented in this paper are carried out by Computational Fluid Dynamics (CFD) or by a Vortex Panel Method to investigate the stream tubes contraction caused by counter-rotating wind turbines and the impact of the distance between each rotor.This paper presents the main results of this study and the positive impact of counter rotating turbines compared to single turbine performance for floating conditions.