As a typical sailboat with a displacement hull moves through the water, it creates a series of waves at the bow. The distance from the crest of one wave to the crest of the next is called the "wavelength".
Hull speed is the speed at which the wavelength of the boat's bow wave is equal to the boat length. As boat speed increases from rest, the wavelength of the bow wave increases, and usually its crest-to-trough height increases as well. When hull speed is reached, a boat in pure displacement mode will appear trapped in a trough behind its very large bow wave.
- Boats with extrmely narrow hulls, such as competitve rowing boats and catamarans, are not subject to the same hull speed restrictions as boats with a wider beam.
- Boats that are capable of planing, through a combination of speed, weight, and hull design, are not subject to hull speed restrictions once they enter a planing mode and begin "skimming" over the water.