Enter the waterline length and the calculator gives the hull speed in knots and km/h. Add the speed reached on the sea trial for the speed-length ratio.
The formula
Hull speed in knots = 1.34 × √LWL, with the waterline length in feet. Good Old Boat explains it as a wave limit: the faster a boat goes, the longer its bow wave, and at hull speed the wave is as long as the waterline. To go faster the boat would have to climb the face of its own wave. The square root is why a boat with a 40 ft waterline doesn't sail twice as fast as one with 20 ft.
Wikipedia gives the constant as 2.43 with the waterline in metres, and notes that sources use anything from 1.34 to 1.51 in feet. The calculator shows the range.
Where it stops working
The rule describes displacement hulls. Planing hulls climb onto the surface and leave it behind, and Good Old Boat notes that long, slender multihull hulls make so little wave drag that the limit doesn't apply. Wikipedia adds that modern naval architecture uses the speed-length ratio or Froude number in its place.
On the sea trial
Hull speed tells you what to expect from a displacement boat at full power. A boat that falls well short of it at the engine's rated rpm is worth a closer look at the bottom and propeller, and at whether the engine reaches its rated rpm at all.
Citing it in a report
"Waterline length 30 ft 6 in, hull speed about 7.4 knots. Maximum speed on trial 6.1 knots at 2,900 rpm against a rated 3,600 rpm; the engine did not reach rated rpm. Recommend the propeller and bottom are inspected and the engine checked under load." Give the speed with the rpm reached, and the rated rpm beside it.