Composite materials are good candidates for hydrofoils manufacturing, ensuring a good balance between strength and weight. In the high performances sailing yacht domain, hydrofoils are thin structures, highly loaded that experience sig- nificant displacements. This study investigates experimentally and numerically the influence of the laminate layup on the hydrodynamic performances of a surface piercing hydrofoil. Four hydrofoils with a constant chord, geometrically identical with different composite layups are mechanically characterized and tested in a hydrodynamic flume. The foils are designed to have a significant tip displacement of 5 to 10% of the span. Experimental results highlight a bending-twisting effect that leads to significant change in the hydrodynamic performances of the structures. Two different FSI numerical approach: from a potential code coupled with beam theory to the full coupling of a shell structural code and a VOF hydro model with free surface are presented and the first one is compared to the experiments with great results. The two approaches are two com- plementary bricks in the design process to compute the effect of passive deformation on hydrodynamic performances of the foils and therefore the yacht stability.

While sailing offwind, the trimmer typically adjusts the downwind sail "on the verge of luffing", letting occasionally the luff of the sail flapping. Due to the unsteadiness of the spinnaker itself, maintaining the luff on the verge of luffing needs continual adjustments. The propulsive force generated by the offwind sail depends on this trimming and is highly fluctuating. During a flapping sequence, the aerodynamic load can fluctuate by 50% of the average load. On a J/80 class yacht, we simultaneously measured timeresolved pressures on the spinnaker, aerodynamic loads, boat and wind data. Significant spatio-temporal patterns are detected in the pressure distribution. In this paper we present averages and main fluctuations of pressure distributions and of load coefficients for different apparent wind angles as well as a refined analysis of pressure fluctuations, using the Proper Orthogonal Decomposition (POD) method. POD shows that pressure fluctuations due to luffing of the spinnaker can be well represented by only one proper mode related to a unique spatial pressure pattern and a dynamic behavior evolving with the Apparent Wind Angles. The time evolution of this proper mode is highly correlated with load fluctuations. Moreover, POD can be employed to filter the measured pressures more efficiently than basic filters. The reconstruction using the first few modes allows to restrict to the most energetic part of the signal and remove insignificant variations and noises. This might be helpful for comparison with other measurements and numerical simulations.

A numerical investigation of the dynamic fluid structure interaction (FSI) of a yacht sail plan submitted to harmonic pitching is presented to analyse the system's dynamic behaviour simplifications and rigging adjustments on aerodynamic forces.