Dynamic behaviour of a flexible yacht sail plan

• Dynamic Fluid–Structure Interaction of a sail plan is modelled in harmonic pitching.
• Aerodynamic forces oscillations show hysteresis phenomena.
• Neglecting the structural deformation underestimates the forces oscillations.
• Both aerodynamic and structure inertia affect loads in the rig.

A numerical investigation of the dynamic Fluid–Structure Interaction (FSI) of a yacht sail plan submitted to harmonic pitching is presented to address both issues of aerodynamic unsteadiness and structural deformation. The FSI model – Vortex Lattice Method fluid model and Finite Element structure model – have been validated with full-scale measurements. 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, suggesting that some energy is exchanged by the system. The area included in the hysteresis loop increases with the motion reduced frequency and amplitude. Comparison of rigid versus soft structures shows that FSI increases the energy exchanged by the system and that the oscillations of aerodynamic forces are underestimated when the structure deformation is not considered. Dynamic loads in the fore and aft rigging wires are dominated by structural and inertial effects. This FSI model and the obtained results may be useful firstly for yacht design, and also in the field of auxiliary wind assisted ship propulsion, or to investigate other marine soft structures.