This article is from the Boats FAQ, by John F. Hughes with numerous contributions by others.
A displacement-hull boat whose waterline has length L (in feet) will
have a "hull speed" that is K SQRT(L) knots, where K is a number
between about 1.2 and 1.4 for most conventional cruising hulls. Small
planing dinghies, large planing sleds, scows, and other designs
(including catamarans) will not fit well into this formula, so you
should ignore it. The formula assumes a lot of things, but all in all
it does pretty well for figuring whether your Bristol 40 will keep up
with a Catalina 30 in moderate winds (or vice versa).
The hull speed, by the way, can loosely be thought of as the speed at
which the boat, in order to go faster, has to start "climbing up" over its
bow wave, which takes a lot more power. (jfh)