Yacht sail systems are subjected to low speed and transitional flow. Because of the supporting structure (mast and boom) of the sail system and the curvature of sail membrane, sail systems also have a partially separated flow. In this work, it is introduced the sectional method as a sail system flow analysis tool. The sectional method uses the surface discretization and it is based in the simultaneous approach for viscous-inviscid interaction but, it works independently to the initial panel mesh and inviscid panel method calculation, permitting the adjustment of sectional points to have a better local convergence. The sectional method is applied to the detection of separated flow regions by means of integral boundary layer parameters investigation. The investigation is used in cases of weak separation and strong separation, when analysing mast and sail configurations. The weak separation detection is applied to a three-dimensional sail shape in a sail design problem: the study of parameters such as twist and section curvature in order to control separation on the sail.