There is disclosed a sail for producing propulsive power or lift in a fluid, where the sail comprises a first layer and a second layer where the first layer and the second layer are connected along at least one corresponding side, and the first layer and the second layer define a cross-sectional shape of the sail substantially perpendicular to a longitudinal direction of the sail. The shape of the first layer is adjustable and/or the shape of the second layer is adjustable such that the cross-sectional shape of the sail is adjustable. There is also disclosed a vessel or a sailboat comprising such a sail.