Impact Analysis of Hull Line Design on Fishing Vessels’ Vulnerability to Pure Loss of Stability

Pure Loss of Stability is one of the five typical stability failure modes identified in the Second-Generation Intact Stability Criteria by the IMO. This study investigates the influence of hull line variations on the vulnerability of a saury fishing vessel to pure loss of stability. Hull forms were parametrically modified using the Free-Form Deformation method, and an in-house code was developed to evaluate stability performance. The numerical framework was validated against the commercial ICS-HydroSTAB software (Version 1.0), demonstrating high computational accuracy and engineering applicability. Parametric sensitivity analysis was then conducted to examine the effects of geometric characteristics under both calm-water and wave-induced conditions. The results indicate that vulnerability in calm water is primarily governed by the maximum sectional area curve and the bow portion of the DWL half-breadth curve, while in waves it is influenced by both the maximum sectional area curve and the fore and aft portions of the DWL half-breadth curve. The half angle of entrance (E = 0.08) exhibits a comparatively minor effect, but its increase reduces the initial metacentric height and significantly elevates the risk of capsizing in waves. These findings provide useful references for hull form optimization and stability design.