And the Gerr's Formula gives a different outcome as well. That's because there are many formulae. And none are totaly correct, nor any totaly wrong. Even Bowditch.
In fact, the standard Bowditch Formulae can have the multiplication factor (normaly seen as 1.34) range from 1.18 through to 1.42. This factor is determind by hull design. Not all displacement hulls push through the water
as easily as each other. A big influence is Bow design, but other hull influences can affect the actual figure. The main point of hull speed is the wave length in relation to the hull length. Remember that the hull is "floating" and thus the way it sits in the fluid flow also changes as speed changes. This alters fluid flow and can either aid or increase drag. The water
flow itself also alters. This also has an affect. Then there is a "skin drag" affect that comes into play with longer hulled vessels.
That's all I really know. Anymore is beyond my limited knowledge in this field. A Naval Archetect would be better ask here. Jeff??? you still around??
By the way, the initial formula I posted is what is commonly used by many propellor manufacturers today. As I understand it, it is a more Accurate representation of Hull speed.