Hmmm... i think the baby is being thrown out with the bath water here!!
Hull Speed....as DC rightly points out is a "mythical" term that most amateurs and none naval architects use. They use it to describe "what they think is happening" - hydrodynamically - and then incorrectly apply it to all boats.
It is based upon their perception of what they see....and understand... with their limited knowledge of hydrodynamics, especially high speed hydrodynamics, as well as "historical" observations thrown into the mix.
Before the days of light weight structures and high speed engines, going fast was not really possible. And I mean fast...i.e planning speeds.
Thus hulls of that "era" were predominately classical hull shape, that is to say, carrying large amount of weight for the length = displacement hulls, as it is carrying a lot of weight for its length!
These hulls all - in general - fall into the same type of shape - no surprises there. Since that is what the SOR seeks....
This shape being bluff or similar bows, and a stern with changing shape from the keel to the transom to facilitate a smooth flow of water - as much as possible - into the prop.
Something resembling a hull shape like this:
And endless variations of this.
The problem comes, as Bajansailir notes, the classical law of diminishing returns. So what does this mean?
The faster this type of hull form (displacement hull) goes the lower the pressure at the stern. The shape of the hull created for the water flow into the prop, accelerates the water = lower pressure.
Im over generalising here to get the basic message across.
Thus as the pressure gets lower, the water level, relative to the static waterline reduces, i.e gets lower. So the resulting wave profile, the increase in amplitude of said wave profile, "pulls down" the hull, or is what is termed squat.
A concomitant effect of less wave profile supporting the hull means the static equilibrium must move.
As the speed increases this effects becomes more and more pronounced. This is observed by extreme squatting or generally noted as a large trim.
The law of diminishing returns is that no matter how much additional power is provided for the hull to go faster, it can't. It is all about the flow of the water around and under the hull.
The curve of resistance climbs very steeply.
In tank testing of landing craft you see this effect very quickly and the resistance curve approximates the 7th power....ie near vertical above a certain/critical speed.
So this 'critical speed' is a cause and effect of the wave provide along the length of the hull.
The speed of a wave is simply = sqrt(g.L/2.Pi)
L = wave length = LWL.
Thus the wave speed = 1.25 sqrt(L)
In metric units, so this "hull speed" is really the "wave speed".
This has been incorrectly attributed with all hulls and thus their magic speed they cannot exceed, simply owing to ignorance of the hydrodynamics of what is occurring.
So..the question then becomes, well how does a hull go faster than this "wave speed" of 1.25xL, where L = LWL.
Simple... change the shape of the stern.
This encourages the flow of water aft and to separate from the hull at the transom, rather than "stick" to the hull.
We have designed many high speed catamarans (as have many others) which "technically" are displacement hulls.
However changing the shape of the stern allows the hull to go faster than this "mythical" notion of a brick wall of a speed limit.
It is a simple as that.... in a nut shell.
This has nothing to do with beam, which is a function of residuary resistance.
But as noted from the outset, if you reduce the beam, to increase the L/B ratio...and if the displacement remains the same, the draft must increase.
This equates to an increase in WSA (generally).
Thus what you have gained in reduction of residuary resistance (lower B) you loss in the roundabouts of additional frictional resistance (deeper T).
Ergo, not much overall effect - except as small speed range sweet spots - because the overall resistance is governed by the length-displacement ratio (L/D).
