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I am sure Robert (as a Naval Architect) can explain this much better than I can (being a Marine Engineer), but I suspect he calculated the hull speed using an equation like:
Hull Speed (kts) = 1.3 to 1.4 X Square Roof of Waterline Length (in feet)
Hull speed in a displacement (non-planing) hull is when the resistance due to wavemaking basically limits further speed increases. Below that speed, the frictional resistance dominates. The very rough rule of 2 Hp per 1000 lbs generally provides sufficient power to get to hull speed plus a little margin.