Froude number (Fn)

The Froude number $Fn = V / \sqrt{g L}$ is the dimensionless speed governing free-surface (wave-making) similarity, with $V$ in m/s, $g$ gravity, and $L$ waterline length. Equal $Fn$ between model and full scale reproduces the wave pattern, the basis of Froude’s law of comparison in towing-tank testing. Displacement ships cruise near $Fn = 0.15$ to $0.30$; the hull speed where wavelength matches $L$ sits around $Fn = 0.40$.