What occurs when a boat is at hull speed?

When a boat is operating at hull speed, it has reached a point where the length of the boat's waterline directly affects its speed. At this speed, the boat is effectively "trapped" between the wave patterns it creates, resulting in a situation where it's unable to easily increase its speed without encountering significant resistance from these waves. This phenomenon occurs because the energy expended to overcome wave resistance increases dramatically when a boat attempts to exceed its hull speed, leading to a more sluggish performance.

The other options do not accurately describe hull speed. When a boat is at hull speed, it is not fully out of the water, and it certainly is not at its maximum fuel efficiency—hull speed can often be less efficient in terms of fuel consumption compared to cruising speeds just below it. Lastly, while a boat may be ready to turn at any speed, this is not a defining characteristic of hull speed specifically; rather, it relates more to handling and maneuverability based on the design and hydrodynamics of the vessel.