Don't get too caught up in the theoretical hull speed concept as a performance standard.
Why not?
Keep in mind though, Hull Speed is a term with a very specific meaning. It expresses an idea for optimal speed
through the water for energy put into the system based upon waterline length. It is not intended to express a limit or even be particularly accurate except as a base-standard.
When sailing, different boat designs do different things to that waterline length. For example: boats with a lot of overhang often experience a lengthening of their waterline as they heel. This effectively changes the optimal hull speed while reaching. A boat with a waterline length of, say, 36 feet will have a hull speed of 8.04 knots (HS = 1.34 x √36). However, if the overall length is 42', the effective waterline length when reaching could be as much as 40'. Then, the effective hull speed is about 8.45 knots.
This is the wave speed of the wake generated by the hull when the wave's length is 36 feet or 40 feet respectively. As you push the hull faster than this speed, the wave length increases such that you are trying to sail up the bow wave with no counter balance from the stern wave aiding in lifting the stern. Larger boats are often built with a bulb on the bow below the waterline. This has the effect of pushing a column of water ahead and can artificially lengthen the waterline length thus improving on the hull speed number.
What's important is to remember this standard refers to speed through the water, not speed over the ground. However, it works quite well as a baseline for comparing the efficient performance of your sail trim.
-Will (Dragonfly)