Estimation of the roll restoring moment variation from captive model experiment and Grim’s effective wave method in irregular oblique waves

This study investigates the ship’s RRMV (roll restoring moment variation) in irregular oblique waves through captive model experiments. Two working assumptions are proposed: (1) the variation in roll restoring moment is zero when the heel angle is zero, and (2) the wave excitation moment does not change with respect to the heel angle under identical irregular wave conditions. Rather than proving these assumptions hold exactly, this research uses captive model experiments and Grim’s effective wave method to reconstruct the RRMV, evaluating how closely the two cross-check against each other. In the experiment, roll moments were measured under captive conditions, and the data were processed using a denoising technique based on FFT (Fast Fourier Transform) to remove high-frequency noise. We analyzed the PDF (Probability Density Function) of Δ GZ at different heel angles. The observed consistency in the PDF indicates these assumptions are practically usable over the tested conditions. Furthermore, the improved Grim’s effective wave method demonstrates a powerful universal reconstructive capability: taking an arbitrary time series of any irregular wave profile as an input, it successfully reconstructs the complete time history of the RRMV, illustrating the flexibility and practical accuracy of this analytical framework against the experimental benchmarks.