Which mode is characterized as very slow?

Displacement mode is characterized as very slow because it involves a vessel moving through the water by displacing a volume of water equal to its weight. In this mode, the hull remains mostly submerged, leading to a slower speed as the boat is not designed to ride on top of the water like in planing mode. Instead, boats in displacement mode create a bow wave that moves ahead as they move through the water, resulting in lower speeds. This is ideal for larger vessels that require stability and efficiency at slower speeds, commonly seen in cruise ships, cargo ships, and some sailing vessels.

In contrast, planing mode allows vessels to travel at much higher speeds as the hull lifts out of the water, reducing drag. Plowing mode, typically seen in heavy seas, offers a compromise between displacement and planing but is generally not considered slow. Thus, displacement mode uniquely embodies the very slow characteristics distinguished in the question.