Generally, sailboats have displacement hulls, and their speed capabilities are traditionally thought to be limited by their waterline length, but that's not exactly true. Sailboats can, and often do, exceed their theoretical hull speed, especially when sailing off the wind. Therefore, you can get a very limited idea of a boat's speed capability by determining its hull speed, which can be calculated by using the following formula: Hull Speed = 1.34 x the square root of the LWL (length of the waterline)....
That's a good point to make out diferent types of cruising boats, in what concerns speed:
There are some sailingboats that downwind are pratically limited to their hull speed, other that can go easely 2 or 3k over hull speed (allmost all modern designs) and there are some new generation fast light cruising boats that can sail safely downwind at almost two times their hull speed.