To expand offshore wind to regions of deep waters, floating foundation technology is currently being developed by many academic and industrial players worldwide. An important tool in the engineering design process is computational fluid dynamics (CFD), which can assess hydrodynamic loads on a proposed floater design and predict its motion response in the often harsh wave environment at sea. When it comes to coupling of incompressible fluid and a rigid body, CFD is still a computationally costly tool. The CFD engineer is often faced with a painful choice between excessive simulation time and reduced accuracy at best or numerical instability at worst. An important problem is the so-called added mass instability, which is a companion of partitioned coupling strategies. This instability kicks in when the instantaneous added mass surpasses the body mass. We have developed a coupling method, dubbed FloatStepper, which solves this problem by preceding each real time step with a probe time step of prescribed acceleration and an instantaneous added mass calculation. This allows decomposition of forces and torques into an added mass component and everything else. The result is a coupling method that eliminates the need for iteration within the motion solver and is stable all the way down to zero body mass. Similar strategies were previously employed, e.g. in Shigunov et al. [1] for aircraft ditching with 3-DoFs and forces derived from empirical formulae, and in Devolder et al. [2] for CFD coupled with 1-DoF heaving of a floating object. FloatStepper works for full 6-DoF coupling and is released as an extension module to the widely used open source CFD toolbox OpenFOAM. In the talk we will present the FloatStepper algorithm, demonstrate its capabilities with a number of validation cases, and discuss further improvements to the current implementation. We appreciate funding from Innovation Fund Denmark (grant 8055-00075B). REFERENCES [1] Shigunov, V., Söding, H. and Zhou, Y., 2001. Numerical Simulation of Emergency Landing of Aircraft on a Plane Water Surface. 2nd International Conference on High-Performance Marine Vehicles HIPER’01. [2] Devolder, B., Troch, P. and Rauwoens, P., 2019. Accelerated numerical simulations of a heaving floating body by coupling a motion solver with a two-phase fluid solver. Computers & Mathematics With Applications, 77(6), pp.1605-1625.