Nonlinear weather optimal control of ships exposed to environmental disturbances is discussed. Emphasis is placed on weather vaning, that is the ship should be orientated such that the moment in yaw due to mean current, wind and wave forces is zero and at the same time maintain a fixed position and heading. The main problem in doing this is that the environmental forces acting on the ship are impossible to measure with sufficient accuracy. A new concept for weather optimal position control (WOPC) based on nonlinear control theory is invented and presented for the first time in this paper. Experiments with a model ship are used to document the performance of the controller. The proposed controller and the concept of WOPC were patented by the authors in cooperation with ABB Industri AS, Oslo in December 1998 under patent no. NO308.334.