Ships come in a variety of sizes. The size of a ship is measured by its weight carrying capacity (deadweight) and by its volume carrying capacity (gross tonnage). Cargo with low weight per unit of volume fills the ship’s volume before it reaches its weight capacity. Deadweight (DWT) is the weight carrying capacity of a ship in metric tons. That includes the weight of the cargo, as well as the weight of fuels, lubricating oils, supplies, and anything else on the ship. Gross Tonnage (GT) is the volume of the enclosed spaces of the ship.
Ships come also in a variety of types. Tankers are designed to carry liquids in bulk. The larger ones carry crude oil while the smaller ones usually carry oil products, chemicals, fruit juices and other liquids. Bulk carriers carry dry bulk commodities such as iron ore, coal, grain, bauxite, alumina, phosphate and other minerals. Some of the bulk carriers are self-discharging. They carry their own unloading equipment and are not dependent on port equipment for unloading their cargo. Liquefied Natural Gas (LNG) carriers carry refrigerated natural gas under very low temperatures usually around -162ºC.
Container ships carry standardized containers in which packaged goods are stowed. General cargo vessels carry in their holds and above deck all types of goods, usually packaged ones. These vessels often have multiple decks or floors. Since handling general cargo is labour intensive and time consuming, general cargo has been containerized during the past decades, thus reducing the time that these ships spend on port cargo operations from days to hours.
Refrigerated vessels or reefers are designed to carry cargos that require refrigeration or temperature-controlled cargos like fish, meat, fruit, eggs, flowers, etc. but can also carry general cargo. Roll-on–Roll-off (Ro–Ro) vessels have ramps for trucks and cars to drive on and off the vessel. Other types of vessels are ferries, passenger ships, fishing vessels, service/supply vessels, barges, research ships, dredgers, naval vessels and other special purpose vessels. Some ships are designed as combination of the above types, e.g., ore-bulk-oil, general cargo with refrigerated compartments, passenger and Ro–Ro vessels and so on (MariEMS 2017).