What’s deadweight tonnage?

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Deadweight tonnage (DWT) measures a ship’s total carrying capacity, including cargo, fuel, ballast, crew, potable water, and stores. Cargo DWT is the actual payload weight, and volume measurements are favored in modern maritime regulations. DWT is an adaptation from the early days of merchant ships, where wine was often shipped in barrels or jars.

Deadweight tonnage (DWT) is a measure of a ship’s total carrying capacity. Traditionally, this has been measured in long tons, where one long ton equals 2,240 pounds (1,016 kg). The ton, or metric ton, of 1,000 kg (2,204.6 lb) is now also commonly used. Fuel, ballast, crew, potable water and crew stores, as well as cargo, are included in the total.

Cargo deadweight tonnage is the actual payload weight of the ship. Since the cargo portion of the weight is by far the most significant component, deadweight tonnage is often used to refer to a ship’s total carrying capacity. The term can also sometimes be used to refer to the actual weight of a ship’s cargo when carrying less than a maximum payload.

Measuring a ship’s cargo is sometimes a matter of weight, volume, or displacement. This in turn is related to the safe handling of the vessel, as well as the fees charged for shipping, port use, and channel passage. The carrying capacity is equivalent to the earning capacity for a merchant ship. This may be limited by the ship’s maximum safe displacement and the volume of cargo space available.

If a ship were to take 400 tons (about 363 metric tons) of cargo, it would also displace an equal amount of water, as indicated by a rise in the waterline. When the waterline rises to the Plimsoll line, the ship’s maximum safe operating displacement is reached. At this level of displacement, the ship’s deadweight tonnage has also been reached.

Modern maritime regulations have favored the use of volume measurements. The gross tonnage (GT) of a ship is the volume of all its enclosed spaces. Its net tonnage (NT) is the volume of all cargo spaces. The cost-effective design of the ship would imply that filling the available cargo space with a typical payload mix would approach the maximum deadweight tonnage for safe operation.

Merchant ships are designed to enhance the cargo component of deadweight tonnage. A warship and a merchantman could have a similar DWT. However, the merchant ship could carry a larger cargo payload. Using the volume measurement for actual cargo space recognizes the disparity due to design that strict reliance on deadweight tonnage does not. It also promotes a uniform standard for enacting regulations, rates, and fees for merchants.

DWT is an adaptation from the early days of merchant ships. Wine was often shipped in barrels or jars, holding 252 gallons (approximately 945 liters) and weighing approximately 2,240 pounds (1,016 kg). The number of tunnels on board could be quickly estimated for tax purposes by looking at the displacement caused by weight. This was indicated with reference to the standard load lines painted on the ship’s hull.

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