Complex sentence: 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, with only the ship's launching considered more significant in its creation.
Split into simpler sentences: 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 considered more significant in its creation.