An experimental investigation of drift forces and coupled-motion effects for a 2D ship cross-section in regular waves

• Experimental study on drift sway force of a ship section in beam-sea waves. • Three cases: (A) fixed, (B) free in 3 DOFs, (C) free with one DOF restricted. • Mean sway force from CFM and DPI methods, with insights into motion effects. • Sway restriction increases drift via wave reflection and roll resonance. • New benchmark dataset with uncertainty analysis for model validation. Two-dimensional experimental studies were conducted on a ship-shaped section with vertical sides to investigate hydrodynamic forces and motions in regular beam-sea waves. The tests examined three scenarios: (A) a fixed section, (B) a section with three degrees of freedom (DOFs), and (C) a section with two DOFs. The study primarily focuses on the mean wave-drift horizontal force, complemented by an analysis of wave-frequency and higher-order wave loads, along with first-order motions. The experiments aim to replicate and extend the setup of Nojiri and Murayama [1975], using a Froude-scaled model tested in a 2D wave flume. A single load-cell system was used for the fixed condition, while a custom linear guide-based rig was implemented for the floating cases. A rigorous calibration process, including wave-generation optimization, decay tests, and uncertainty quantification, ensured high experimental accuracy. Automated Python routines were developed to identify steady-state conditions, perform frequency-domain analyses, and estimate uncertainties in the tests. The results showed good agreement with linear theory and numerical predictions for first-order loads and motions. The mean-drift sway force was both measured directly and estimated through conservation of fluid momentum and compared against reference results. Restricting individual degrees of freedom revealed a strong sway-roll coupling effect, particularly near the roll resonance. Restraining sway was found to increase the drift force both by enhancing wave reflection and by amplifying roll resonance motions. The possible role of finite-water effects on the loads and motions was also examined. The dataset generated in this study is openly available and with the intention to serve as a valuable benchmark for validating numerical models and hydrodynamic theories related to wave-body interactions in beam-sea conditions.