A sensor apparatus for a sail includes a unique sail identifier and an in-use sensor, the in-use sensor determining whether the sail is in an in-use condition or a dormant condition. The in-use sensor includes a light sensor for measuring light falling on the sail and one or more accelerometers for measuring orientation and vibration of the apparatus, wherein an in-use condition is determined from reviewing whether measurements from the light sensor and one or more accelerometers exceed threshold values.