The body or frame of a ship or boat. It is a central concept in water vessels. The hull is essentially what keeps the water from entering the boat and acts as the walls and floor of the vessel.

Nearly all watercraft, from small boats to the largest ships adhere to one general class of hull shapes that serve the needs of stability and efficient propulsion, featuring

* horizontal cross-sections that have narrow, usually pointed, fronts (at the bow),

* smooth widening from the bow until roughly the middle (the beam), and often narrowing smoothly but usually significantly to the extreme end (the stern), whose width may range from a large to an insignificant fraction of the beam width),

* characteristic vertical cross-sections perpendicular to the beam.

Such a cross section will usually feature

* an open top on a small boat (kayaks being the most familar exceptions), or a level deck (with various superstructures) on large boats or on ships,

* below that level, possibly widening and/or narrowing to some extent, smoothly, down the relatively sharp bend called the 'knees',

* below the knees, either having a relatively flat bottom or narrowing smoothly to an angled seam at the center, and

* usually featuring either a keel or retractable centerboard at that centerline, or retractable sideboards roughly vertical and close to the most vertical portion of the hull.