A boat's hull speed is the maximum speed a boat can go efficiently.    

If you'd like to know how fast you can do a 50 yard Freestyle, efficiently, then you'll need a little math:  150 / Hull speed (in ft/second).   I am 5'9", which means my hull speed is 5.43 ft/s.   150/5.43 gives me a 50 FR time of 27.61s, or a 100y FR time of 55.21s.   This does not take into account turns, but the turn actually makes it possible to go FASTER than you're hull speed because you're pushing off something solid (a wall) and you're underwater!