Integrated Global–Local Finite Element Assessment of Stern Boss Structural Integrity Under Realistic Trim and Stability Conditions

This study presents a traceable global–local finite element (FE) framework for assessing stern-boss structural integrity under operationally derived loading. Two production-scale MSC Nastran models—a shell-dominant model and an otherwise equivalent global model with a locally solid stern-boss region—were compared under five vessel-specific states generated from trim-and-stability weight, buoyancy, hydrostatic, ballast, and machinery-load distributions. Baseline-to-fine mesh changes were limited to 0.68% for the shell model and 1.07% for the solid model. Both models reproduced the same global deformation mode, while the solid model predicted 5.9–6.3% greater maximum vertical deflection. Within a common stern-boss assessment region, the shell and solid peak von Mises stresses were 42.1–70.9 MPa and 41.5–72.4 MPa, respectively, with differences confined to −1.4% to +2.3%. By contrast, stresses extracted at the stern-tube interface were 17.2–26.8 MPa in the shell model and 29.7–46.6 MPa in the solid model, demonstrating the importance of three-dimensional constraint, transverse shear, and through-thickness response at the local interface. The governing design-draught/APT-full condition produced a solid-model deflection of 46.8 mm and a regional stress of 72.4 MPa. Its nominal SS400 yield-utilization ratio was 0.308, whereas the LR rule-based inverse safety-factor index ranged from 1.3 to 2.1 and identified surrounding panel buckling as the more restrictive limit state. The shell model reduced wall-clock time by 38.6% and is therefore appropriate for global screening, while the solid representation is required for interface-level assessment. The framework constitutes a numerically verified, digital-twin-compatible baseline; independent validation against measured structural or shaft-line data remains necessary.