Apparently, there are two formulas to determine the theoretical hull displacement speed. Older one, for the "fat" boats 3x to 4x as long as they are wide, and newer one for "skinny" hulls and catamaran type hulls. Mind you, the concept of hull speed is not used in modern naval architecture, where considerations of speed-length ratio or Froude Number are considered more helpful. Nevertheless …

Old formula: S = 1.34 x SQRT(LWL)
New formula: S = LWL/(3 x BWL) x SQRT(LWL)

Where:
S = speed in knots
LWL = Load Waterline Length (waterline length)
BWL = Beam Waterline Length (wet beam width)

I have a skinny pocket cruiser with only 8.5' beam and it is within upper limit of the 3x to 4x as long as wide criterion. Just for kicks ... I thought I will calculate the hull speed based on both formulas:

LWL = 25'
BWL = 8'
S = speed in knots

Old formula: S = 1.34 x SQRT(LWL) = 6.7 knot
New formula: S = LWL/(3 x BWL) x SQRT(LWL) = 5.2 knot

My hull clearly follows the first formula as its efficient "coasting" or displacement speed, as opposed to efficient "planning" speed, is 7.2 knots. Interesting enough, at the optimum throttle settings while coasting at 1400 RPM or planing at 3400 RPM, I get the same 2.5 MPG on my gasoline powered VP 8.1L 375 HP engine … not talking about GPH here.

1000 RPM     05.3 MPH     03.4 MPG     01.6 GPH
1400 RPM     07.2 MPH     02.5 MPG     02.9 GPH
3400 RPM     30.2 MPH     02.5 MPG     12.1 GPH

Here is the kicker … the boat goes 5.3 MPH and I get 3.4 MPG at 1000 RPM.
So what is my hull displacement speed? Which formula applies?