A CFD-based propellers systematic series for full-scale performance extrapolation

This study investigates scale effects on marine propeller performance through a large number of CFD calculations aimed at assessing the limitations of conventional extrapolation methods. A systematic series of 672 propeller geometries, derived from a parent design by varying key parameters such as blade number, expanded area ratio, pitch distribution, and tip loading, is analysed using RANSE simulations under fully turbulent conditions. Each configuration is evaluated over multiple advance coefficients and scales, covering a wide range of Reynolds numbers. Including the effects of surface roughness, this results in a database exceeding 26,000 tested geometries and operating conditions. Results show that the CFD predicted frictional component of scale corrections, when fully turbulent boundary layer is assumed at any propeller size, is generally consistent with the trends by the ITTC, 1978 procedure. However, significant discrepancies arise when total performance is considered. CFD calculations reveal that pressure-related contributions exhibit a strong and systematic dependence on scale, which is not accounted for in conventional formulations. Overall, they play a crucial role in the scaling process since the combination of frictional and pressure contributions as evaluated through CFD calculations lead to non-monotonic trends and, in some cases, to an increase in full-scale torque, contradicting the commonly assumed reduction based solely on frictional considerations. The magnitude and direction of scale effects are found to depend strongly on geometrical parameters, particularly expanded area ratio and pitch, with the inclusion of surface roughness that modifies the magnitude of scale corrections but does not alter the underlying trends. The findings highlight the need for improved full-scale prediction methods explicitly accounting for pressure contributions, with data-driven approaches suggested as a promising direction for future development.