An aerofoil sail (30) for providing motive power to a waterborne vessel, the sail comprising a leading aerofoil portion (35a) and a trailing aerofoil portion (35b), and the sail comprising a spar (32), at least one of the aerofoil portions rotatably positionable, and the sail comprising a controller to control individually the angular position of at least one of the aerofoil portions relative to the spar, and the spar rotationally positionable about its longitudinal axis.