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)