Hydrodynamic Resistance and Stability Analysis of a Secondary Towed Body in a Two-Stage Deep-Towing System

This study numerically investigates the hydrodynamic resistance and stability of a two-stage towing system. A coupled fluid–structure interaction model comprising a primary body, catenary cable, and secondary body is evaluated under fully constrained, linearly constrained, and planarly constrained conditions. By simulating idealized infinite-depth operations, the inter-body wake interference, drag forces, and cable tension were analyzed at varying towing distances. Results indicate that as the towing distance increases, the secondary body gradually exits the highly disturbed wake region of the primary body. Consequently, the wake field becomes symmetrical, and both the hydrodynamic drag and mean cable tension progressively increase. Despite experiencing higher drag, a larger separation distance effectively diminishes reciprocating motions and significantly enhances the overall longitudinal translational stability of the system. This comparative assessment intentionally focuses on resistance-dominated wake shielding and translational stability. Rotational dynamics including explicitly extracted lift forces and trimming moments are reserved for future unconstrained six-degree-of-freedom studies.