A system for detection of turbulent air flow across a sail of a sailboat, comprising at least one base module arranged to be attached to said sail; an elongate body arranged to move in said air flow, which is attached to said at least one base module at one end; a power source on said at least one base module; at least one sensor on said at least one base module arranged to detect movement of said elongate body; and a signalling device arranged to output signals to a sailor on said sailboat in response to signals received from said at least one sensor; wherein said base module comprises at least one radiator which is arranged to transmit electromagnetic or optical radiation in the direction of said elongate body moving in said air flow; and said at least one sensor on said base module is arranged to detect reflected radiation from said elongate body moving in said air flow.