An experiment was performed in the Yacht Research Unit’s Twisted Flow Wind Tunnel (University of Auckland) to test the effect of dynamic trimming on three IMOCA 60 inspired mainsail models in an upwind ( AW = 60°) unheeled configuration. This study presents dynamic fluid structure interaction results in well controlled conditions (wind, sheet length) with a dynamic trimming system. Trimming oscillations are done around an optimum value of CFobj previously found with a static trim. Different oscillation amplitudes and frequencies of trimming are investigated. Measurements are done with a 6 component force balance and a load sensor giving access to the unsteady mainsail sheet load. The driving CFx and optimization target CFobj coefficient first decrease at low reduced frequency fr for quasi-steady state then increase, becoming higher than the static state situation. The driving force CFx and the optimization target coefficient CFobj show an optimum for the three different design sail shapes located at fr = 0.255. This optimum is linked to the power transmitted to the rig and sail system by the trimming device. The effect of the camber of the design shape is also investigated. The flat mainsail design benefits more than the other mainsail designs from the dynamic trimming compared to their respective static situtation. This study presents dynamic results that cannot be accurately predicted with a quasi-static approach. These results are therefore valuable for future FSI numerical tools validations in unsteady conditions.