Combined CFD-EFD methods applied to determining the form factor of vessels with very low length to beam ratio

• The scale effect on the value of 1 + k is significantly more pronounced for the fishing vessel than for the KCS. The difference between the 1 + k value at model scale and full scale is approximately 40% for the fishing vessel, while it is approximately 8% for the KCS. • At full scale, aircraft-derived drag estimation methods better capture the form factor behavior of low L/B. • Using an EFD-CFD combined method, with a two CFD-DB form factor approach, yields effective power estimates around 15% higher than the ITTC method. This study investigates the influence of scale effects on form factor determination for a fishing vessel with a low length-to-beam (L/B) ratio, using a combined Experimental Fluid Dynamics (EFD) and Computational Fluid Dynamics (CFD) approach. The research compares the behavior of the form factor between a fishing vessel and the well-known KCS benchmark hull. Results show that scale effects have a more significant impact on the fishing vessel, particularly due to increased viscous pressure losses in the stern region. While the non-dimensional frictional resistance component remains similar between both hulls, the non-dimensional pressure component differ significantly, highlighting the influence of hull shape on pressure recovery. The comparison with empirical methods reveals that traditional marine formulations may not adequately capture full scale form factor of hulls with very low length to beam ratio. Instead, the form factor obtained using aircraft drag estimation approaches shows better agreement with CFD predictions at full scale. Based on these findings, the adoption of distinct form factor values at model and full scale is recommended for vessels with similar geometric characteristics, in contrast to conventional extrapolation practices that assume a constant form factor. This methodology may improve the accuracy of effective power predictions and support more reliable design evaluations.