Numerical investigation of the wake-control mechanisms and pre-duct parameter sensitivity in a ship propulsor with propeller boss cap fins

The wake behind a single-screw ship contains both non-uniform axial momentum and near-hub swirl-related kinetic-energy loss. This study investigates a combined pre-duct and propeller boss cap fins (PBCF) configuration using unsteady Reynolds-averaged Navier–Stokes simulations coupled with the volume-of-fluid method and a sliding-mesh approach. A local wake-energy diagnostic framework is established by coupling axial momentum utilization, circumferential kinetic-energy proportion, non-axial kinetic-energy density, and positive-contribution statistics. Based on this framework, the complementary interaction between the duct-induced axial momentum enhancement and PBCF-induced near-hub swirl attenuation is clarified, and the sensitivity of the duct outlet diameter, chord length, axial position, and duct angle of attack is quantitatively ranked. The Duct-PBCF configuration increases the near-hub axial momentum utilization by approximately 9.1% and 5.0% at sections S2 and S3, respectively, compared with the bare propeller. It also reduces the circumferential kinetic-energy proportion by about 14.96% at S2 compared with the duct-only configuration. Among the duct parameters, the overall sensitivity follows A > D > X > L, with normalized weights of 0.36, 0.28, 0.21, and 0.14, respectively. Within the tested range, Dd/Dd,0 = 0.9 yields the largest efficiency improvement of approximately 3.2%, whereas a moderately positive duct angle of attack improves the thrust–torque balance. These results indicate that the Duct-PBCF design should be formulated as a multi-objective matching problem rather than as the optimization of a single performance index, involving propulsive performance, axial momentum enhancement, and vortex-energy reduction.