What is the primary structural element that runs along the bottom center of a ship from bow to stern, acting as its backbone?

The primary structural element that runs along the bottom center of a ship from bow to stern, acting as its backbone, is the keel. The keel is the fundamental and strongest structural member of a ship's hull, providing its main longitudinal strength. Longitudinal strength refers to the ship's ability to resist bending forces that act along its length, such as those caused by waves or uneven cargo distribution, preventing the vessel from hogging (bending upwards in the middle) or sagging (bending downwards in the middle). The bow is the forwardmost part of the ship, and the stern is the rearmost part. The keel serves as the central spine of the ship, connecting all the major structural components like frames (transverse ribs) and hull plating (the outer skin), distributing stresses uniformly, and forming the base from which the rest of the ship's structure is built. For most modern vessels, this takes the form of a flat plate keel, which is a thick, continuous plate running along the centerline of the ship's bottom.