Hull speed, otherwise referred to as displacement speed, is the maximum efficient speed of a displacement, or non-planing hull. Based on the standard shape of a boat hull, the hull speed is the speed at which further increase would take a disproportionately greater amount of power or energy. Going faster than the approximated hull speed requires a lot more power and makes for a less pleasant ride. There's a standard equation for calculating hull speed.
Measure the craft's waterline length--otherwise referred to as the LWL--in feet. You should be able to find your craft's waterline length by referring to the manual or manufacturer's guidelines. Otherwise try searching for this information online. You can also simply take a tape measure and measure the waterline length yourself.
Take the square root of the LWL.
Multiply the square root of the LWL by 1.34. This number is the hull speed for your vessel as measured in knots.
Double check your answer. The equation written out looks like:
v = 1.34 (square root of LWL)
Where v is the speed of the vessel in knots, and LWL is the length of the craft's waterline in feet.
Triple-check your answer by plugging in the information on a hull speed calculator, such as the one on sailing USA's website: http://www.sailingusa.info/cal__hull_speed.htm.
The hull speed is a rule of thumb, or approximation, of hull speed, since the measurement of the craft's waterline length is actually just a substitution for wavelength, which is much harder to accurately measure. Also, other factors effect hull speed, including weight, narrowness, and fullness of the bow and stern. There are sources available to add further variables to this formula to get an even more precise hull speed calculation.