Hull Speed is not theoretical, as many people believe, it's a definition.
As a displacement boat moves through the water, the bow pushed the water out of the way, both to the side and ahead. Since water is practically incompressable, this causes the water to pile up at the bow. As a boat moves through the water, it also leaves a hole where the stern use to be, this causes a building of a pressure difference between that area and the surrounding area, but especially between bow and stern.
As the water pressure seeks to equalize, it rushes around the hull, like moving through a restriction in a pipe. Thus, it forms ripples as the hull continues to move. Each wake crest is a high pressure and each trough is a low. The water at the high is moving almost with the hull, then rushes astern to get back to fill the lows created behind. The water moving through the trough is move in reverse of the hull's movement.
There is a frequency, in this wave making, that is consistent, based on weight and viscosity of the fluid. Therefore, as the hull moves faster through the water, the wavelength gets longer to maintain that frequency. Once the hull reaches the point where the wave length and the hull length are equal, the boat is balanced in its wake. After that, the faster the boat goes, the wavelength continues to extend because of the law of conservation represented by Bernoulli's equation. Volume going in equals volume coming out. Because you can, again, think of the hull as a length of pipe of various diameters.
When the wavelength is longer than the hull, to go faster requires part of the energy moving the boat to drive her up hill onto the bow wake (submarines go faster under water because they aren't fighting gravity). You can cheat it a little by moving the center of gravity aft, but since this tends to lift the bow out of the water, it could also effectively shorten the waterline. That's why you see power boats riding bow high at such slow speeds until the weght over balances across the crest and they level out in a plane.
One of the goals of a designer is to flatten out that wave by either making the hull shoal drafted or skinny or both. This decreases the volumetric movement of water resulting in a lower amplitude and an easier bow wave to climb. Fat deep heavy boats won't plane, light flat bottomed skinny boats will plane easier.
Of course the hull shape changes as a sailboat heels and this changes the waterline length but the dynamics that defines hull speed is the same.
- Will (Dragonfly)
