Boats and ships are defined by their ability to float on water. They achieve their buoyancy by displacing a volume of water equivalent to their own weight. Because of its hollow construction, a ship weighing thousands of kilograms can displace its weight's worth of water and thus float high on the sea, while a compact one-kilogram stone quickly sinks to the bottom.

The hull, or main body, of a boat or ship drastically increases its buoyancy by distributing the craft's mass over a large surface area. The more surface area a hull has the more water it displaces and the more mass it can support. Even so, not all boats are designed to carry a maximum amount of cargo. Indeed, boats and ships serve a wide range of functions. For example, some are designed for speed, while others are built for comfort or for maximum stability.

A boat's design — in particular its hull shape — reflects its function. An extremely broad, shallow hull enables ships called barges to carry extremely heavy loads and to move along rivers or across oceans with the least possible power input for the weight they carry. Hulls that extend more deeply beneath the water's surface are typical of sailboats, yachts, and ocean liners. These hulls cut through waves more readily than the flat hulls of barges or skiffs and thus provide a more comfortable ride and greater maneuverability in open water.

In general, the slimmer a hull is, the less drag it will experience as it passes through the water, and the faster and more efficient the boat will be. However, as the FETCH!™ cast members quickly learn in this video segment, the narrower a boat is, the less stable it will be as well. Kayaks, for example, slice through the water very efficiently, but they are also notoriously easy to tip over. One way to increase stability while maintaining much of a watercraft's efficiency and speed is to add an outrigger or two. These slim beams are attached to and run parallel to the hull of a narrow boat. This expansion of the boat's surface area with buoyant materials increases stability and keeps the narrow hull from capsizing. The FETCH! cast members solve their instability problem by adding an outrigger on each side of their boat. These additions help the boat stay upright and enable the team to achieve its mission.

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