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, in British and American shipbuilding traditions the construction is dated from this event. Only the ship's launching is considered more significant in its creation.The word can also be used as a synecdoche to refer to a complete boat, such as a keelboat.