In boats and ships,
keel can refer to either of two parts: a structural element that sometimes resembles a fin and protrudes below a boat along the central line, 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.