Hydrodynamic interference mechanism of small waterplane area twin hull retrieving disc-shaped submersibles via a nonlinear ship seakeeping method

The small waterplane area twin hull (SWATH) vessel, due to its excellent seakeeping performance, is increasingly regarded as a promising recovery platform. However, the hydrodynamic interference mechanism between the SWATH vessel and the disc-shaped submersible in the near field is not yet well understood. Additionally, existing computational methods struggle to balance efficiency and accuracy. To address this, the present research develops a nonlinear ship seakeeping method (NSSM) that integrates viscous corrections with potential-flow theory. This method is used to systematically analyse the effects of different wave directions, horizontal spacings, and wave frequencies on the six-degrees of freedom loads on the submersible. The results show that the SWATH interference exhibits a dual effect of ‘shielding’ and ‘amplification,’ with gap resonance under head seas conditions amplifying sway, while the geometric symmetry of the hull suppresses roll. Interference is comparatively low for most modes at horizontal spacings of 2.5–3.0 m. The time-domain analysis showed that the maximum absolute heave displacement and pitch angle under Sea State 3 were 0.529 m and 12.131°, respectively. Sea State 2 produced the smallest six-degree-of-freedom time-history response ranges among the three conditions analysed, providing relatively favourable motion conditions for deployment operations. These findings clarify the near-field hydrodynamic interference characteristics between the SWATH vessel and the disc-shaped submersible and provide guidance for deployment and recovery operations.