What is the defining feature of a semi-displacement hull?

A semi-displacement hull is characterized by its design that incorporates features from both planing and displacement hulls. This unique combination allows the vessel to operate efficiently at various speeds. At lower speeds, it behaves more like a displacement hull, moving through the water and displacing it rather than riding on top. As the speed increases, it begins to gain some planing capabilities, allowing it to partially rise out of the water and reduce drag. This versatility makes semi-displacement hulls suitable for a wide range of operations, particularly useful in situations where speed must be balanced with stability and comfort.

The other options don't accurately describe the nature of a semi-displacement hull. For instance, saying it operates exclusively underwater isn't accurate, as it is designed for performance on the water surface. Claiming that it can truly plane on the water suggests a full planing capability, which doesn't fully encompass the semi-displacement nature. Lastly, suggesting that it is used solely for racing overlooks its practical applications for various maritime purposes beyond just competition.