Abstract: A system wherein a vessel (12) is moored by catenary mooring lines (14, 15, 16) that include polymer cable lengths (30) that can stretch under tension, which indicates the amount of polymer stretching. In the absence of polymer line stretching, a measured actual drift or excursion of the vessel from its initial position (as determined by a GPS system) would result in a predetermined nonstretch inclination (graph 50) of the top (42) of a first mooring line (14) from the horizontal. If the inclination is greater than the predicted nonstretch inclination, this indicates that the polymer line has stretched, with the amount of stretch indicated by the increase in inclination over the predicted nonstretch inclination.