"Hull speed" is the theoretical maximum speed of a boat hull and it corelates strongly to the water line length of the hull but is also influenced by other factors such as the length to beam ratio (the reason multihulls can often exceed hull speed by large amounts) among other factors.
In reality, there is not a true hull speed as a boat will keep going faster the more power you apply. The problem is horse power needs increase with the cube of speed so every time you double the speed it takes many times the horse power. Eventually you need to add massive amounts of horse power to get even a tenth of a knot
more speed. Theoretical hull speed usually correlates to the point where adding more power starts to become silly as you are buring lots more fuel
for very little gain.
For example our 34' boat will happily chug along at 3kts with just our 5hp dingy motor
at half throttle. With the 25hp primary outboard
, she cruises easily at 6kts at half throttle. Give her full throttle and we only get up to around 7.5kts. Before we replaced the old outboard
, we had 40hp and could make 8kts wide open. Do you see the pattern? This is why when we repowered, we dropped to 25hp as hull speed was the limit.