The sail deformation is due to the fluid motion: the aerodynamic pressure field deforms the sail surfaces and this, in its turn, modifies the flow field around the sails. From a mathematical viewpoint, this yields a coupled system that comprises the incompressible Navier-Stokes equations with constant density ρ = ρair (3-4) and a second order elastodynamic equation that models the sail deformation as that of a membrane. More specifically, the evolution of the considered elastic structure is governed by the classical conservation laws for continuum mechanics