Computational Fluid Dynamics Analysis of the Effect of Trim Angle on Planing Stepped Hulls

Abstract

This study investigates the hydrodynamic influence of trim angle variation on a stepped planing hull using computational fluid dynamics. The analyses were performed in ANSYS Fluent under steady-state RANS conditions with the SST k–ω turbulence model and a Volume of Fluid multiphase setup. Two scenarios were examined: a hull with no trim and a hull trimmed by 1.6°. The computational domain consisted of 3,572,223 elements with high mesh quality metrics, ensuring accurate free-surface and near-wall resolution. Quantitative results show that in the non-trimmed configuration, the maximum flow velocity reached 20.8 m·s⁻¹, with an average of 17.0 m·s⁻¹ near the hull, while the 1.6° trimmed case exhibited a slightly lower mean surface velocity of 15–16 m·s⁻¹ but a more elongated wake. The maximum turbulence kinetic energy decreased from 18 m²·s⁻² to 9.6 m²·s⁻², indicating reduced turbulence intensity and a more stable wake structure. The total drag force decreased dramatically from 22,592.89 N to 3,009.294 N, representing an 86.68% reduction in drag under identical conditions. This improvement is attributed to flow acceleration beneath the bow and the redistribution of pressure and viscous forces. However, this unusually large reduction reflects the idealized fixed-hull simulation conditions and far exceeds typical empirical improvements, indicating that caution is needed in interpreting these results. The results suggest that minor positive trim angles can substantially lower hydrodynamic resistance in this stepped planing hull configuration. Compared to prior studies, the magnitude of drag reduction observed here far exceeds reported values, largely because a simplified hull model is used at a high Froude number with the hull fixed in trim and heave.