The aim of this research is to quantify the membrane deformations and their impact on performance for a ribbed wing sail. A 1m x 0.8m rectangular planform NACA0012 foil was designed to replicate a single section of a wing-sail. Two foils were manufactured based on this geometry, one out of solid foam and one using a rib and membrane structure. These were tested in the R.J. Mitchell closed return 3.6 m x 2.5 m wind tunnel at the University of Southampton. Their aerodynamic performance was assessed over a range of angles of attack using a six-component force balance showing the overall performance of the membrane wing was reduced by between 5-11% depending on the analysis conducted. A stereo camera system was used to perform Digital Image Correlation (DIC) in order to quantify the full field deformation of the membrane wing structure whilst under aerodynamic load. This showed membrane deformations of up to 15% of the section thickness. The experimental membrane displacements were then used to create a deformed wing sail geometry, removing the effect of foil bend and twist, allowing a CFD investigation of the impact of membrane deformations alone. This indicated that the static membrane deformations resulted in a decrease in performance of up to 1.3% compared to the rigid aerofoil.