NACRA 17 is a small foiling catamaran used in Olympic sailing. Sailing in the NACRA 17 is truly unsteady, especially in the foiling conditions. Dynamic velocity prediction has gained a lot of attention in recent years due to the unsteady nature of foiling sailboats. Different approaches have been applied to predict the motions of foiling or skimming boats in unsteady conditions. Typically, the aerodynamic forces are computed by the quasi-steady theory assumption. Static coefficients are used and the applied velocities are corrected with boat motions. The hydrodynamic forces of a fully foiling boat can be computed in a similar fashion, but this method fails to include the effect of changes in the free surface as well as added mass. This paper proposes an approach where dynamic coefficients of the NACRA 17 hydrofoils are determined based on unsteady Computational Fluid Dynamics (CFD) simulations with a free surface. In the simulations the foil is moved in prescribed motions in one degree of freedom (DOF) at a time and dynamic coefficients are extracted for different frequencies of motion. All coefficients are combined in a coupled system of equations, and the equations can be solved in 6 DOFs in the time domain. A number of test scenarios with time varying conditions are simulated and compared with full dynamic CFD simulations. The initial investigation of the approach shows promising results, but more investigations are needed to validate the approach and to identify possible limitations.

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