Beyond uniform roughness: Ship resistance with spatially non-uniform hull surface conditions

Traditional ship resistance models often assume uniform hull surface roughness, potentially misrepresenting the heterogeneous fouling patterns observed in real-world operations. To address this limitation, we investigate the hydrodynamic impact of spatially non-uniform roughness on a KRISO Container Ship (KCS) hull using Computational Fluid Dynamics (CFD) simulations. Seven hull surface conditions were investigated, including a smooth baseline and six types of roughness distributions: uniform, linear gradient, non-linear gradient, random, direct-shear, and inverse-shear. All cases were designed to have the same arithmetic mean hull surface roughness, allowing isolation of the effects of spatial roughness distribution. Among the tested configurations, the linear gradient distribution exhibited the most favourable resistance characteristics, whereas the shear-based and random distributions showed relatively minor differences from the uniform case. Spatial roughness patterns significantly influenced boundary layer growth and wake development. Uniform, random, and shear-based distributions induced thicker boundary layers and delayed wake recovery, whereas the linear gradient case resulted in weaker momentum loss and faster wake recovery. These findings indicate that even under identical arithmetic mean roughness conditions, the spatial distribution of hull surface roughness can significantly affect resistance characteristics. Explicit modelling of roughness patterns is therefore essential for accurate performance prediction and motivates further experimental validation and integration with propeller-hull interaction and free surface effects. • A CFD-based framework is proposed to evaluate resistance under spatially non-uniform hull roughness. • Linear gradient roughness yields up to 12 % reduction in total resistance compare to uniform roughness. • Shear stress-based roughness maps result in less than 1.5 % resistance deviation from uniform cases. • Boundary layer thickening correlates with wake recovery delay and resistance increase across roughness distributions. • Findings offer new insights for modelling fouling patterns and evaluating ship performance via CFD.