The hull speed is simply the maximum speed that a displacement hull can move through the water without needing and extaordinary amount of power to go faster.
When guests ask this question when sailing, for a laugh, a friend and I invariably repeat in unison “The hull speed in knots equals 1.34 times the square root of the length in feet at the water line”.
Simply, to understand it, put these two facts together...
1. the larger a wave is, the faster it travels through water, and
2. the longer the hull is, the larger the wave it can make.
So therefore, to make a boat faster, make it longer at the water line, so it can make a larger wave, so it can move faster through the water with that wave. The largest wave it makes is the sum of the two waves that come, one from the bow, and the other from the stern.
If you wanted to double your speed from hull speed, you would need 2^6 power, or 64 times more power. For example, if you needed 50 hp at hull speed, you would need 3200 hp to double your speed! (Actually more, in practice).
As Russ points out, this all changes once you start to plain on the surface, as our boats do.
If that is still not clear, just ask.
-Brian.
Note that this only happens at a water/air interface and is peculiar to displacement-hull boats – so you can't make a submarine, or airplane faster by making them longer.
dealer even with proper line, but no manual whatsoever how to rig and even to raise it. Shall I install a new halyard?). 
