Large-scale towing response of a medium-sized USV towing a light-mass UTCD in moderate seas: numerical study

The growing demand for flexible and cost-effective platforms for ocean observation has highlighted the potential of medium-sized unmanned surface vehicles (USVs) for towing light-mass underwater communication devices (UTCDs). However, the large mass mismatch between the USV and the UTCD may induce significant towing-response variations and transient cable loading. In this study, a low-order coupled numerical model based on the lumped mass method (LMM) and linear potential flow theory is adopted to investigate the large-scale towing response of a medium-sized USV towing a light-mass UTCD under towing speeds and moderate sea states. The numerical framework is validated against published experimental data under steady towing conditions. The effects of towing speed, UTCD mass and sea state on the towing response are systematically examined. Results show that UTCD mass and towing speed jointly influence the steady-state towing configuration and transient towing-response characteristics of the system. Higher towing speeds and lighter UTCDs exhibit larger displacement offsets together with more pronounced transient towing-response variations. Increasing the UTCD mass from 36 kg to 50 kg reduces the horizontal and vertical displacement offsets by up to 15% and 35%, respectively, at 6 knots, and by 84% and 98%, respectively, at 1 knot. Under low towing speeds, transient cable tension is strongly influenced by wave-induced USV motion, where cyclic slack-taut behavior leads to significant transient tension increase. At a towing speed of 1 knot, the maximum transient tension increases from below 6000 N in sea state 4 to approximately 9000 N in sea state 5. In contrast, mean steady-state cable tension grows monotonically with towing speed. Wave excitation exerts a stronger influence on the USV motion than towing speed or cable reaction within the investigated operating conditions, while the transient lateral towing load acting on the USV shows nonmonotonic dependence on towing speed. The present results provide preliminary insights into towing parameter selection and large-scale towing-load estimation for medium-sized USV-based underwater towing systems with light-mass UTCDs under moderate sea states.