Floating Offshore Wind Turbines (FOWTs) can have a very unsteady aerodynamic behaviour at sea. However state-of-the-art aerodynamic models used for FOWTs usually assume a steady and inviscid flow around the rotor. The induction factor is then computed using Froude-Rankine Actuator Disk theory, also called momentum theory. For a Horizontal Axis Wind Turbine (HAWT), the Blade Element Momentum theory (BEM) may miss important unsteady phenomena when the rotor strongly interacts with its wake [1]. For a Vertical Axis Wind Turbine (VAWT), an equivalent momentum method is the Double Multiple Streamtube (DMS) theory [2]. Despite its higher CPU cost, the Free Vortex Wake (FVW) theory is an alternative to momentum theories as it takes into account the vorticity shed in the wake by the blades to compute the induction. The aerodynamics is thus entirely unsteady. This study presents a comparison between the DMS and the FVW theory for a two-straight-bladed Floating VAWT mounted on the OC3Hywind SPAR platform [3] for several of the OC3 project load cases [4]. The study shows that the DMS solver is not able to predict important unsteady aerodynamics phenomena when the rotor/wake interaction is strong. The motions of the turbine's platform are significantly impacted and the dynamic loads acting on the rotor blades are very different. It could lead to design issues, especially for blade design.