Modern seagoing merchant ships come in all shapes and sizes and are designed to carry a wide variety of cargo. This article gives a brief overview of the main types of ships that sail the world's seas and tells something about their development.
For the purposes of this article, charges will be divided intodry,fluidandspecialized, each of which is further broken down into subcategories. Dry cargo includes bulk, general and breakbulk, containers, reefers and ro-ros. Liquid cargoes are usually oil based but may also contain chemicals and liquefied gases. Special cargo includes passengers, livestock and heavy lift/project.
- dry cargo ships
- bulk carrier
- general cargo ships
- reefer ships
- RoRo ships
- Liquid Cargo Ships
- raw carrier
- product carrier
- chemical carriers
- liquefied gas carrier
- Specialized Cargo Ships
- passenger ships
- Livestock transport
- Heavy lift/project cargo vessels
dry cargo ships
Historically, dry cargo ships have been the most common vessels in the world's merchant fleet. Known as general cargo ships, they were fitted with their own cargo loading equipment, usually in the form of a derrick hoist. Cargo would be stowed in various holds and the speed and effectiveness of the loading/unloading process would depend on the ship's crew and dockers or 'stevedores'. Such ships sometimes operated as ships of the line between two or more ports, but could also operate in the "tramp trade", where the ships went where they were needed.
For dry cargoes with a high weight-to-cost ratio, such as coal, grain and ore, economies of scale gave birth to the modern bulk carrier. These ships are divided into several separate holds covered by hatches. In the port, the cargo is loaded by conveyor belt and spit spouts or by crane and grab. Some bulk carriers are equipped (usually there is a crane between each hatch) to allow loading and unloading of cargo at berths without the need for shore equipment.
For unloading, cranes with grabs are the norm, although specialized equipment may be used for certain loads. When ships are unloaded using cranes and grabs, personnel and vehicles are often placed in the holds to aid in the process. Cargo is usually unloaded into hopper ships and then transported to silos or open storage using conveyor belts. Smaller ships can often unload cargo directly into road vehicles.
general cargo ships
General cargo, known as breakbulk, is typically shipped on pallets or in bags. There may be specialized handling facilities for such cargo, but usually loading and unloading is done with cranes and straps (for boxes) or slings (for bags). These ships can also carry loose and irregular cargo. In this case, the ship's crew and port stevedores pack the cargo to minimize damage and maximize space.
Although largely replaced by bulk carriers and container ships, general cargo ships are still in service around the world.
Containers have become the main mode of transportation for manufactured goods around the world. A container can be transferred between truck, train and ship with relative ease and is of a standard size for ease of transport. Containers can hold anything from groceries to electronics to cars. They are also used to transport goods in sacks and pallets, as well as liquids and refrigerated goods.
Standard containers are measured in TEU (Twenty-foot Equivalent Units) and are generally 20 feet (1 TEU) or 40 feet (2 TEU) long. All standard shipping containers are 8 feet wide and 8 feet 6 inches high. There are longer, taller, and even shorter standard sizes, but these are less common.
Container ships consist of multiple holds, each fitted with “cell guides” that allow the containers to snap into place. Once the first layers of containers are loaded and the hatches are closed, additional layers are loaded onto the hatches. Each container is then strapped to the ship, but also to each other to ensure integrity. Containers are usually loaded by special cranes or even general purpose cranes with container hoists. Some small container ships are designed to be self-loading and unloading.
Container ships are mainly used on scheduled routes and are among the largest ships at sea. Ultra Large Container Vessels (ULCVs) such as the Emma Maersk (lead ship of the Maersk E-Class vessels) can transport approximately 15,000 TEU (depending on container weight). Large container ships are restricted to certain ports around the world due to their size and cannot pass through certain areas due to draft or, in the case of canals, width restrictions.
Ships designed to transport refrigerated cargo and perishable goods such as fruit or meat are referred to as "reefer vessels". Cargo is stowed in holds, which are then sealed and temperature controlled. Traditional reefer vessels have largely been replaced by the use of reefer containers, transported aboard container ships. Reefer containers require a power source to function and must be checked frequently during the voyage.
Roll-on roll-off or ro-ro vessels come in many forms. These include car ferries and cargo ships with truck trailers. The car transporter is the most commonly used RoRo vessel. These slab-sided ships have multiple vehicle decks with parking lanes connected by internal ramps with shore access via one or more loading docks. The loading capacity of such ships is measured in Car Equivalent Units (CEU), and the largest car carriers operating today have a capacity of over 6,000 CEU.
Liquid Cargo Ships
Collectively referred to as tankers, these vessels carry a range of liquid cargoes. Tankers were first developed in the mid-19th century when ships made of iron made it possible to transport liquids in large quantities, economically and without leaks. As with bulk carriers, economies of scale have increased the size of tankers, and today the largest examples have deadweight or "deadweight tonnage" in excess of 400,000 tons.
Tankers are divided into separate tanks into which the cargo is pumped via a pipeline system. Modern tankers have large and separate ballast tanks to allow them to sit "empty" lower in the water on the return trip to improve stability. Many tankers also have systems for adding an inert gas to the tanks to reduce the risk of fire and explosion.
The largest ships at sea are the Very Large Crude Carriers (VLCCs) and the Ultra Large Crude Carriers (ULCCs). These ships are designed to load and transport crude oil to refineries around the world where it can be processed into petroleum products. The largest crude oil carriers often load and unload at offshore buoys and terminals as they are too large to enter most ports.
These vessels, generally smaller than crude oil carriers, transport the refined products from larger terminals to smaller ports around the world. Products transported include petroleum, kerosene, diesel, asphalt, lubricating oil and tar. Smaller tankers are also used to transport non-petroleum bulk liquids such as molasses and palm oil.
These ships typically have a deadweight tonnage of 5,000 to 40,000 tons and often have specialized cargo systems suited to the type of cargo being carried. These systems may include heating or cooling equipment and advanced cleaning systems to ensure cargo retains its purity when loaded into a general purpose tank.
liquefied gas carrier
These ships started out as converted oil tankers but have evolved into highly specialized, purpose-built ships. Cargo tanks are designed to carry liquefied petroleum gas (LPG) or liquefied natural gas (LNG) under pressure and are generally spherical in shape for strength. LNG carriers are usually larger than LPG carriers, the largest LNG carriers are the "Q-Flex" vessels with a gas capacity of up to 266,000 cubic meters.
Specialized Cargo Ships
Most types of cargo can be considered specialized because of the special loading, unloading or stowage arrangements required. However, many of these cargoes are moved with such regularity and ease that the term "specialized" takes on new meaning. For the purposes of this article, it refers to cargoes that are either difficult to classify as dry or liquid, or cargoes that are relatively difficult to handle.
This category includes everything from 10-person foot ferries to cruise ships that can carry over 6,000 passengers. Perhaps the most specialized cargo of all, the needs and desires of passengers have driven the design of modern ferries and cruise ships.
Ferries, once seen by most as a 'means to an end', are now replete with lounges, restaurants, shops and entertainment - especially when the ferry travels a relatively long route. The ships have also gotten bigger. For example, the Ulysses, which operates between Holyhead and Dublin, can carry over 1,300 cars and 2,000 passengers.
The first example of ships embarking on a public "cruise" can be traced back to the 19th century, but cruising gained mass popularity in the late 20th century. Many cruise ships were originally liners that were sent to warmer climates on their regular routes during seasonally inclement weather. The last of the liners is arguably the Queen Mary 2, which still operates a regular transatlantic service.
Today, cruise passengers expect a wide range of facilities including casinos, gyms, shops, theaters, cinemas, swimming pools, restaurants and bars. The largest cruise ships are up to 360 m long. long and 60 m. wide. Popular sailing areas are the Mediterranean, the Caribbean and Scandinavia.
Often converted from other ship types, these ships are equipped with bays for large numbers of animals. The main considerations when transporting livestock are adequate ventilation, feed and water. It is also important that the ports that receive these vessels have facilities to handle the animals. Some cattle trucks can transport up to 120,000 sheep. A common route for livestock trucks is through Australia and New Zealand to the Middle East.
Heavy lift/project cargo vessels
These ships are often purpose built to specialize in transporting extremely heavy or bulky objects such as other ships and large industrial components. These ships are used, for example, to transport offshore platforms from their construction sites to drilling sites. Some heavy lift vessels are equipped with heavy lift cranes to load in ports with no heavy lift capacity. Other types are semi-submersible, allowing the cargo to be placed in position before the heavy lift vessel de-ballasts to lift the cargo out of the water.
Common project cargoes are wind turbine blades and towers, quay cranes and industrial machinery. Project cargo ships are often adapted to their role. For example, "jack up" ships can discard "legs" to lift themselves out of the water. This is commonly used by vessels installing offshore wind farms where stability is required while the turbine towers are erected.
Despite the advent of highly maneuverable vessels, the tug is still vital to the maritime industry. With a pulling power of over 100 tons, modern tractors are very manoeuvrable! Harbor tugs are common in ports around the world and are generally less powerful. These ships assist in the docking, undocking, and moving of large or inescapable ships within the port boundaries. Tugs are also used to assist ships in inclement weather or when transporting dangerous or polluting cargo. Port tugs are also used to move barges, floating cranes and personnel in ports. Larger units are kept on standby at strategic locations to act as ocean rescue and salvage tugs.
Tugs are also used to tow barges from port to port and to move large structures such as offshore platforms and floating storage units. Some tugboats can push barges; This is particularly common on rivers where the tug can apply more torque to the tow rope. There are also tugboats designed to be "plugged" into a barge or hull. Once secured, this composite unit behaves and is treated like a normal powered ship. These composite units are common in North American river and coastal trade.