Feuerhahn, JonasJonasFeuerhahnKrüger, StefanStefanKrüger2026-10-282026-10-282026-0645th International Conference on Ocean, Offshore and Arctic Engineering, OMAE 2026978-0-7918-8953-4https://hdl.handle.net/11420/65576Underwater detection is an important capability for many vessels, particularly naval and research ships, where accurate and reliable data is essential. To enable this capability, vessels are often equipped with variable depth sonar (VDS) systems that are typically towed behind the ship. The deployment and retrieval of such towed bodies can exhibit complex and potentially uncontrolled dynamic behaviour—not only while entering or exiting the water but also in the underwater near-field of the vessel. For instance, if the towed body enters the propeller wash, it may experience uncontrollable oscillations, making a safe retrieval impossible. Therefore, calculating and predicting the motion behaviour and the influence of external forces acting on the system during deployment and retrieval is of great importance. This paper presents a comprehensive and modular approach for modelling the dynamic motion behaviour of a VDS system deployed from a shipborne crane. The model accounts for ship-induced excitations, external forces and the complex oscillatory motion of the towed body. Ship motions are simulated using the E4-ROLLS method, while the VDS dynamics are formulated within a multi-coordinate framework based on the system’s differential equations of motion. The model combines ship-induced excitations at the deployment point as well as hydrodynamic damping effects due to water resistance. Furthermore, external forces acting on the VDS during underwater operation are considered, particularly those arising from interactions with the vessel’s propeller wash and surrounding environment. The proposed approach provides a valuable tool for enhancing the deployment strategies and operational performance of variable depth sonar systems under realistic sea conditions.enseakeepingunderwaterlifting-line theoryship designTechnology::600: TechnologyConference Paper10.1115/omae2026-178575