Free Vibrations of Taut and Slack Marine Cables

The natural frequency and damping of several typical marine cables were measured in air and in water and are reported in this paper. The changes in natural frequency and mode shape that occur in both media as the cable sags from a taut configuration are in agreement with a recently developed linear theory. A modal crossover predicted by the theory was also observed as the cable tension was decreased. The onset of the catenary behavior has been identified and it coincides with at least an order of magnitude increase in the structural damping of the cables. The fluid dynamic contributions to the virtual mass and the damping are found to be independent of the sag in the cable and are comparable to measurements for taut cables reported in the literature.