I have a skinny pocket cruiser with only 8.5' beam ... but it's within upper limit of the 3x or 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
S = 1.34 x SQRT(LWL) = 6.7 knot
S = LWL/(3 x BWL) x SQRT(LWL) = 5.2 knot
FWIW ... my hull clearly follows the first formula as it's efficient "coasting" or hull speed, as opposed to efficient "planning" speed, is 6.5 to 7.0 knots. Interesting enough, at the optimum throttle settings while coasting (1400 RPM) or planing (3400 RPM) I get the same 2.5 MPG on my gasoline powered VP 8.1L 375 HP engine (not talking about GPH here).
Best ... Richard