Modelling the wind, sail and rig interactions on a sailing yacht is a complex subject, because the quality of simulation depends on the accuracy of both structural and fluid simulations which strongly interact. Moreover, the sails are submitted to highly unsteady sollicitations due to waves, wind variations, course changes or trimming for example, but sometimes also due to the unsteadiness of the flow itself (vortex shedding,…). The problem for downwind sails is even more complex because the flow is often detached from the sails, and the sails are subject to large shape changes. A specific dynamic coupling has been developed between a RANSE code from Ecole Centrale de Nantes for the aerodynamics (ISIS-CFD) and a FEM code from K-Epsilon for the structure (ARA) specialized to simulate the aeroelastic problem of yacht sails and rig. In this paper, the particular issues of coupling, remeshing and transfer of forces from one code to the other are detailed. An experimental comparison is made on a well controlled test case with an original experiment developed by IRENav Ecole Navale, consisting in a square of spinnaker fabric mounted on two carbon battens which are moved in a forced oscillation. The good agreement of numerical results with experimentals results permits to be confident to go ahead and investigate a full example of application on a racing yacht spinnaker.