Once "on top," the __________ hull skims along the __________ of the water.

The correct answer relates to the characteristics of a planing hull, which is designed to rise and glide over the surface of the water at higher speeds. When a boat with a planing hull reaches a certain speed, it generates enough lift to overcome its weight and starts to "plane" or skim along the water's surface. This design allows for more efficient movement through the water and leads to less drag, making it ideal for speed and agility.

In contrast, the other options reflect different types of hull designs that do not perform in the same manner under similar conditions. A displacement hull, for example, moves through the water rather than skimming and is designed to carry loads at lower speeds effectively. The semi-displacement hull has characteristics of both displacement and planing hulls but does not achieve the same high-speed gliding effect. Lastly, the reference to a heavy hull skimming along the edge is not accurate, as heavy hulls typically are not constructed to achieve planing speeds. Thus, the description of a planing hull accurately captures the action and performance of that hull type when it is in motion.