When hull speed is exceeded, a vessel in displacement mode will appear to be climbing up the back of its bow wave.
The concept of hull speed is not used in modern naval architecture, where considerations of speed/length ratio or Froude number are considered more helpful.
When the Froude number grows to ~0.40 (speed/length ratio ~1.35), the wave-making resistance increases further from the divergent wave train.
This very sharp rise in resistance at speed/length ratio around 1.3 to 1.5 probably seemed insurmountable in early sailing ships and so became an apparent barrier.
It dramatically simplifies the units on the constant before the radical in the empirical equation, while giving a deeper understanding of the principles at play.
Such hull forms are commonly used by canoes, competitive rowing boats, catamarans, and fast ferries.
Ultra light displacement boats are designed to plane and thereby circumvent the limitations of hull speed.