Hydrodynamic and Trim Analysis of a Dihedral Bulbous Bow on a Longliner Hull

This study evaluates the hydrodynamic performance of a displacement-type FAO longliner fishing vessel fitted with a surface-piercing dihedral bulbous bow. Unlike conventional submerged bulbs, this configuration partially emerges at the free surface. Hydrodynamic behaviour was analysed under heavy- and light-load conditions using both computational and experimental fluid dynamics. Results show that the dihedral bulb significantly reduces total resistance beyond a critical speed of approximately 6 knots, whilst also affecting dynamic trim and vertical hydrodynamic forces. Full-scale effective power was estimated by extrapolating model results according to ITTC procedures. This study confirms that dihedral bulbous bows are well suited for retrofit applications on small fishing vessels under 20 m in length, achieving maximum resistance reductions of about 18% at higher speeds. These gains translate into notable fuel savings and reduced greenhouse gas emissions, making the retrofit both economically and environmentally advantageous.