An experimental technique to accurately quantify the deformation and the bend-twist coupling of high performance composite foils under fluid loading is presented. The experimental results are reproduced in a Computational Fluid Dynamic (CFD) environment to assess the impact of board deflection and changes in pitch angle on vertical force generated in the C-foils while sailing under increased hydrodynamic pressure. A three dimensional Digital Image Correlation (DIC) methodology suitable for use within a wind tunnel is developed. The technique allows for the measurement of full-field deflection during fluid-structure interaction (FSI) experiments. Combined with DIC technique, the C-foil tip vortex is investigated using Particle Image Velocimetry (PIV) to correlate the variation of the vortex position and strength to the deflection of the board. These techniques, combined with CFD investigations allow potential changes in structural behaviour to be assessed with regard to improving the performances of the foils in sailing conditions. Experimental results are presented for a high performance curved foil from a NACRA F20 catamaran tested within the University of Southampton RJ Mitchell wind tunnel. The fluid regime is chosen to have a Reynolds number equivalent to light upwind sailing conditions (Rn=6.66x105: boat speed of 6 knots) with a fifth of the fluid loading experienced in the water. Curved foils provide both a hydrodynamic side-force to counteract the aerodynamic forces of the sails and a vertical lift force to reduce the wetted surface area and hence the resistance. It is therefore necessary to investigate from a sailor point of view the influences of the side force and vertical coefficients that the change in effective angle of attack and of pitch will give to the stability and the performances of the catamaran.

A numerical investigation of the dynamic Fluid Structure Interaction (FSI) of a yacht sail plan submitted to har-monic pitching is presented to analyse the effects of motion simplifications and rigging adjustments on aerodynamic forces. It is shown that the dynamic behaviour of a sail plan subject to yacht motion clearly deviates from the quasi-steady theory. The aerodynamic forces presented as a function of the instantaneous apparent wind angle show hysteresis loops. These hysteresis phenomena do not result from a simple phase shift between forces and motion. Plotting the hysteresis loops in the appropriate coordinate system enables the associated energy to be determined. This amount of exchanged energy is shown to increase almost linearly with the pitching reduced frequency and to increase almost quadratically with the pitching amplitude in the investigated ranges. The effect of reducing the real pitching motion to a simpler surge motion is investigated. Results show significant discrepancies on the aerodynamic forces amplitude and the hysteresis phenomenon between pitching and surge motion. However, the superposition assumption consisting in a decomposition of the surge into two translations normal and collinear to the apparent wind is verified. Then, simulations with different dock tunes and backstay loads highlight the importance of rig adjustments on the aerodynamic forces and the dynamic behaviour of a sail plan.