Lightweight Tonnage
Abbreviation: LWT
Ship's mass when launched, empty: hull, machinery, equipment, no fuel, water, ballast, stores, or cargo.
In practice
Quoted on every certificate but only consulted at scrap-sale time and at major dry-dock work. The master rarely uses LWT directly; the loading instrument uses displacement and DWT.
Regulatory detail & full definition
Lightweight tonnage (LWT) is the mass of a ship as built and empty - the hull, machinery, equipment and permanent fittings - with no fuel, fresh water, ballast, stores or cargo on board. It is established from the inclining experiment carried out after launch and is the constant term in the relationship displacement = LWT + deadweight (DWT). LWT is also the basis for scrap valuation (LWT multiplied by the scrap-steel price), and some yards deliberately over-build a little so the ship still meets its contract DWT if the actual LWT comes out heavier than calculated.
Authoritative source: Tonnage Convention 1969 ↗
Related
Source: Tonnage Convention 1969; standard naval arch text
High confidence
Editorial confidence: how we grade.