Froude scaling

Froude scaling is the model-test rule that holds the Froude number Fn = V / sqrt(g L) equal between model and ship, so gravity-driven wave-making is dynamically similar at both scales. A model of scale lambda is therefore towed at speed V_m = V_s / sqrt(lambda). Because the Reynolds numbers cannot also be matched (the model runs far below ship Re), only the wave-dependent residuary resistance scales directly by Froude; the viscous friction is corrected separately with the ITTC-1957 line and a form factor. This split is Froude’s hypothesis, the foundation of the ITTC-1978 extrapolation from a single model test to full-scale power.