HF radar is a remote sensing technique for measuring surface currents and ocean wave directional spectra. This paper describes the extension of a land based system to enable surface current measurements from a slowly sailing ship. To compensate the ship's motion, the Global Positioning System (GPS) has been used. Due to additional error sources compared to the land based system, the accuracy of measured currents is slightly reduced from 2 to 5 cm/s to 3 to 10 cm/s.