Numerical and experimental hydrodynamic assessment of a dihedral bulbous bow retrofit for a semi-planing fishing vessel

• Dihedral bulbous bow achieves 28-32% resistance reduction at semi-planing speeds (Fr > 0.38). • Combined pressure reduction and trim modification mechanisms validated through EFD-CFD approach. • Critical speed thresholds identified: 7 knots for resistance, 9 knots for trim divergence. • Performance benefits most pronounced under heavy-load condition with deeper bulb immersion. • Validated methodology applicable to high-speed fishing vessels operating up to Fr = 0.59. This study investigates the improvement in hydrodynamic performance achieved through the integration of a dihedral bulbous bow into a semi-planing fishing vessel, with particular emphasis on calm-water resistance and dynamic trim behavior. The results indicate a total resistance reduction of up to 28% under light-load condition and up to 32% under heavy-load condition. A dihedral bulbous bow is a developable surface bulbous bow appendage that partially pierces the free surface rather than being fully submerged. Both experimental towing tank tests and numerical simulations using computational fluid dynamics (CFD) are conducted to evaluate the retrofit effectiveness. The vessel examined is a traditional artisanal longliner fishing boat, as classified by the Food and Agriculture Organization (FAO), operating at high speeds with Froude numbers approaching 0.5, significantly higher than conventional displacement-type fishing vessels. The results emphasise the critical influence of dynamic lift forces on hull performance at elevated Froude numbers, where semi-planing behavior becomes dominant. Implementation of the dihedral bulbous bow demonstrates measurable improvements in hydrodynamic efficiency through favorable modifications to the wave pattern and pressure distribution around the hull. The combined effects of bow-generated lift, dynamic trim adjustment, and altered flow field contribute to reduced total resistance. Comparative analysis reveals distinct differences in the fluid dynamic behavior between the original hull and the retrofitted configuration, providing valuable insights for the design optimisation of high-speed fishing vessels.