How sails generate forces

We propose a new paradigm of the aerodynamics of sails that reconciles our understanding of the force generation mechanisms of both upwind and downwind sails, and that is applicable to both attached and separated flow conditions. Sail experience significant flow separation. Even when the flow appears to be attached, this is sometimes only in the time averaged sense. In these conditions, the underlining as- sumptions of thin airfoil theory and lifting line theory are violated. There is therefore a need to develop an intuitive understanding of the force generation mechanisms that does not rely on these assumptions. This paper aims to address this issue by proposing a new paradigm based on the impulse theory. The force generation mechanism can be intuitively associated with the vorticity field, which can be gathered with computational fluid dynamics or particle image velocimetry. This paradigm intuitively reconciles key results of traditional wing aerodynamics, and provides sail designers a measurable objective to modify a sail shape also in separated flow conditions. It will hopefully underpin both a deeper understanding of sail aerodynamics and the development of low order models for new design tools