1.1 Ship Weight and Volume
Ship Weight and volume
Weight and volume are vital aspects of merchant ships, maritime transport and commerce.
Weight and volume significantly influence ship design, construction, and operation as well as determining carrying capacity, stability, and fuel consumption.
Generally, the tonnage of a vessel indicates an expression of her size, weight or capacity.
For commercial vessels, the calculation of tonnage is defined in The International Convention on Tonnage Measurement of Ships (1969). Adopted by IMO in 1969, this convention was the first successful attempt to introduce a universal tonnage measurement system (UMS).
The following terms characterize a ship's size and load capacity:
Lightweight: the weight of the unloaded ship, ready for sea.
Deadweight: the weight of cargo and stores the ship can carry. In fact, the weight of all the contents the vessel is capable of carrying when loaded to summer mark. The contents of a merchant vessel are cargo, stores, equipment, fresh water, potable water, lubricating oil and fuel.
Gross tonnage: the entire volume of the enclosed spaces of the vessel that can be used for cargo, stores and accommodation.
Net tonnage: the volume dedicated to cargo in the ship. It is calculated by deducting the spaces that are not used for cargo from the gross tonnage. Net tonnage is often used to calculate harbor dues that must be paid to municipal port authorities for the use of all the port facilities.
Cargo carrying capacity: the amount of cargo that a vessel is capable to carry, for example TEU – Twenty Equivalent Units, the standard unit of a twenty-foot (6.1m) transport container.
Cargo Spaces
Bale space - the volume of the cargo holds that can be used to carry general cargo.
Grain space - the volume of the cargo holds that can be used to carry dry bulk cargo.
Oil space - 98% of the total volume of the wet bulk tanks. The remaining 2% are used as ullage. This is the empty space on top of the liquid level that will prevent a tank from overflowing when the oil expands due to heat.
A common way of measuring a vessel’s size is by means of displacement.
According to Archimedes’ Principle, a vessel displaces a weight of water that is equal to its own weight. A vessel will experience an upthrust that is equal to the weight of the displaced mass of water. Displacement is understood the total weight of the vessel and her contents, or the weight of the displacement water mass.