Keel

The keel is the backbone of a ship, the main structural member that runs along the very bottom of the hull on its centreline from the bow right aft to the stern. Just as a spine gives a body its length-wise strength, the keel gives the hull the longitudinal strength it needs to resist the bending forces imposed by the sea and by the distribution of cargo and buoyancy along the ship's length. All the other structure (the frames, floors, bottom plating and girders) is built up from and around the keel, which is why laying the keel has traditionally been regarded as the symbolic beginning of a ship's construction, and the date on which it is laid is an important reference in a ship's history and in the application of regulations.

In modern steel ships the keel usually takes the form of a flat plate keel, a strengthened strip of bottom plating along the centreline, often with a vertical centre girder above it, forming part of the ship's double bottom. Older and smaller vessels may have a bar keel, a solid bar projecting below the hull. Many ships also carry bilge keels, which are quite different in purpose: these are long, narrow fins fitted along the turn of the bilge on each side of the hull, not for structural strength but to damp the ship's rolling motion and make it steadier and more comfortable in a seaway.

The keel also gives rise to everyday references in ship operation. Draught is measured from the waterline down to the bottom of the keel, and under-keel clearance (the gap between the keel and the seabed) is the critical margin a navigator watches in shallow water. To be "on an even keel" means to float upright and level. As the foundation on which the whole hull is built and the reference from which depth beneath the ship is measured, the keel ranks among the most fundamental parts of any vessel.