In boats and ships, keel can refer to either of two parts: a structural element, or a hydrodynamic element. These parts overlap. As the laying down of the keel is the initial step in construction of a ship, the construction is dated from this event, with only the ship's launching considered more significant in its creation.
A structural keel is a large beam around which the hull of a ship is built. The keel runs in the middle of the ship, from the bow to the stern, and serves as the foundation or spine of the structure, providing the major source of structural strength of the hull. The keel is generally the first part of a ship's hull to be constructed, and laying the keel, or placing the keel in the cradle in which the ship will be built, is often a momentous event in a ship's construction—so much so that the event is often marked with a ceremony, and the term lay the keel has entered the language as a phrase meaning the beginning of any significant undertaking. Modern ships are now largely built in a series of pre-fabricated, complete hull sections rather than being built around a single keel, so the start of the shipbuilding process is now considered to be when the first sheet of steel is cut.