Current robotic sailing relies on sensing wind direction and moving the sails to a position that is appropriate for that relative wind angle. The research to date shows that a correctly tuned sail in the classic wing shape is essential for maximum speed over water. This paper relates research on sensors used to determine when sail trim is incorrect. With improper sail trim, the sail luffs. This luffing produces turbulence which reduces the efficiency of the sail. By instrumenting the sail with sensors to detect when sails begin to luff, the robot can determine when the sail is improperly trimmed and, potentially, take corrective action.