Drag Forces and Flow-Induced Vibrations of a Long Vertical Tow Cable—Part II: Unsteady Towing Conditions

Abstract

Full-scale experimental data on the dynamics and flow-induced vibrations of a long vertical tow cable are analyzed. The data were measured while the surface ship was going through a series of starting, stopping, and backing maneuvers. The results of the study show that the amplitude of the flow-induced vibrations of the cable is strongly modulated during maneuvering operations. Maneuvering creates situations where different sections of the cable are translating at different speeds. This causes an “artificial” shear current which at times is severe, depending on the difference in speed between the top and bottom of the cable. The artificial shear is responsible for the intensification of the amplitude modulation above the level that is observed during steady-state towing conditions. The overall effect of the amplitude modulation is a reduction in the hydrodynamic drag forces. It is shown that the drag coefficient measured during maneuvering operations is lower than the steady-state value.