Lightship
Lightship (or lightweight) is the weight of a ship as built — the hull, machinery and permanent equipment — with no cargo, fuel, ballast, fresh water, stores, crew or their effects on board. It is effectively the empty weight of the vessel.
Lightship is the reference point for ship weight accounting. Deadweight (DWT) is the difference between the loaded displacement at a given draught and the lightship weight — in other words, everything the ship can carry. Lightship is established at completion from the inclining experiment and stays broadly constant through life, apart from modifications.
Because deadweight — the earning capacity of the ship — is displacement minus lightship, every tonne of extra lightship weight is a tonne of lost cargo. That is why yards control build weight carefully, and why the inclining experiment that fixes lightship is a closely witnessed newbuilding test.
What is the difference between lightship and deadweight?
Lightship is the empty weight of the ship as built. Deadweight is everything it can then carry — cargo, fuel, water, stores and crew. Loaded displacement equals lightship plus deadweight.