Hull speed - Wikipedia, the free encyclopedia
As a ship moves in the water, it creates
standing waves that
oppose its movement. This effect increases dramatically in full-formed hulls at a
Froude number of about 0.35, which corresponds to a speed-length ratio
(see below for definition) of slightly less than 1.20 (this is due to a rapid increase of resistance due to the transverse wave train). When the Froude Number grows to ~0.40 (speed-length ratio about 1.35), the wave-making resistance increases further due the divergent wave train. This trend of increase in wave-making resistance continues up to a Froude Number of about 0.45 (speed-length ratio about 1.50) and does not reach its maximum until a Froude number of about 0.50 (speed-length ratio about 1.70).
This very sharp rise in resistance at around a speed-length ratio of 1.3 to 1.5 probably seemed insurmountable in early sailing ships and so became an apparent barrier. This leads to the concept of 'hull speed'.
The 1.34 in the formula is hardly cast in stone. Over the years I have seen it used for different hull shapes from 1.1 all the way up to 1.6/1.7.
And best of all...it's a theoretical number until one is actually determined for a finished hull...
So using 1.34 to "guess" at a hull speed is just that...a guess.