When a boat or ship moves through the water, a wave is formed at the bow. This wave is known as bow wave. Hull speed is the speed at which the bow wave's wavelength equals the length of the boat/ship.
Image by Serenthia, Public Domain
Hull speed (knots) equals 1.34 times the square root of the length of waterline (LWL, originally Load Waterline Length) in feet. Expressing mathematically, hull speed =