What characterizes the semi-displacement speed in planing boats?

The semi-displacement speed in planing boats is characterized by the hull being unstable and generally associated with higher fuel consumption. At this speed, the boat begins to transition from being fully submerged in the water to rising partially out of the water, which creates a mixed state of hull interaction with both water and air. This state can lead to a more challenging operation since the hull does not have a steady or stable flow over it, making it less efficient in terms of fuel consumption compared to cruising at a fully planing or fully displacement speed.

The other characteristics described in the alternatives, such as smooth riding over water, perfect visibility for the operator, and high fuel efficiency, are not typical of semi-displacement speeds. While a planing hull can have good visibility and may occasionally be fuel-efficient at higher speeds, the instability and fuel consumption issues at semi-displacement speed are defining traits of this specific performance range.