For the top high performances foiling yacht design, the design process is a complex combination of free surface hydrodynamic simulation coupled with highly loaded composite structural analysis. Bend-twist coupling is a phenomenon that appears in composite materials, depending on its layup composition. It impacts the flow distribution and therefore fluid structure interactions applying on the hydrofoil. That coupling is a target effect for highly loaded structures to reduce the loads by reducing the flow 's angle of attack at the tip.

This study investigates experimentally and numerically the appearance of bend-twist coupling and its influence 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 length.

Experimental results highlight a bending-twist effect that leads to significant changes in the hydrodynamic performances of the hydrofoils. A numerical FSI approach from a potential code coupled with beam theory by finite elements, with modifications of stiffness matrices to consider bend-twist coupling effect is compared to the experiments and give great results (less than 10% of relative discrepancies on the displacements prediction and loads distribution).

 This approach is a strong brick to be used during the pre-design process, to compute the effect of passive deformation on hydrodynamic performances of the foils and therefore the yacht stability.