Automatic release system on a supply- or tug-vessel, comprising a rotational towing hook (10) supported on a vertical axel (16), for receipt of a towing hawser (12), and a releasing unit for releasing the towing hawser (12) from the towing hook (10). A registration unit (14) is installed above the towing hook (10) working area, said registration unit (14) being a tiltable or liftable hoop plate, where the registration unit (14), under influence of vertical angular movement in the towing hawser's (12) horizontal direction in relation to the towing hook (10), is arranged to trigger release of the releasing unit.