The deadweight is the difference between the displacement and the mass of empty vessel (lightweight) at any given draught. It is a measure of ship’s ability to carry various items: cargo, stores, ballast water, provisions and crew, etc.
SOLAS Convention defines deadweight as follows:
“Deadweight is the difference in tones between the displacement of a ship in water of a specific gravity of 1.025 at the load waterline corresponding to the assigned summerfreeboard and the lightweight of the ship.”
Note that deadweight thus defined is the maximum deadweight of the ship.
Only in the case of heavy loads put at the bottom of the hold, greater deadweight translates automatically into greater amount of cargo. Frequently, a large part of the deadweight is used for water ballast necessary to meet stability requirements.
Example of wording from BNC Technical Specification of a Panamax container ship :
DEADWEIGHT
1. The guaranteed deadweight of vessel at scantling draught of 13.50m, in seawater with a specific weight of 1.025 T/m3, shall be not lower than 63,200t. The guaranteed deadweight is subject to amendments resulting from the alterations requested by Owners or these introduced in consultation with Owners.
2. Deadweight at design draught of 12.00m shall be approximately 51,450t.
3. The deadweight is defined as the difference between an actual displacement and the lightship weight. The lightship weight includes the weight of completely outfitted vessel with inventory according to the List of Inventory, spare parts according to the Class Society requirements and with liquids in engine room systems. In particular lightship weight does not include:
A. Loose container lashing equipment.
B. Spare parts in excess of rule requirements.
C. Provision stores, crew and effects.
D. Fuel oil, diesel oil, lubricating oil, fresh water, ballast water in tanks.
- Cargo deadweight, cargo carrying capacity, payload – The number of tons of cargo which a vessel can carry loaded to her summer freeboard mark. Contrary to the deadweight, which is constant for a given draught, cargo deadweight depends on type of cargo, masse of stores and ship’s stability. Therefore it is not possible to define cargo deadweight just by one figure.