Modern seagoing commercial vessels come in all shapes and sizes and are designed to carry a wide variety of cargoes. This article provides a brief overview of the main types of vessels that sail the world’s seas and gives some history on how each has evolved.
For the purposes of this article, cargoes are divided into dry, liquid and specialised, with each of these divided further into subcategories. Dry cargoes include bulk, general and breakbulk, containers, reefer and Ro-Ros. Liquid cargoes are usually oil-based but may also include chemicals and liquefied gasses. Specialised cargoes include passengers, livestock and heavy-lift/project.
Dry Cargo Ships
Historically, dry cargo vessels were the the most common vessels in the world’s merchant fleet. Known as general cargo vessels, they would be equipped with their own cargo loading equipment, usually in the form of a derrick lifting device. Cargo would be stowed in different holds and the speed and effectiveness of the loading/unloading process would depend on the the ship’s crew and the port workers or “Stevedores”. Such ships sometimes operated a regular service as liners between two or more ports, but could also operate in the “tramp trade” where vessels would go wherever they were required.
For dry cargoes with a high weight to cost ratio such as coal, grain and ore, economies of scale produced the modern bulk carrier. These vessels are divided into several separate holds covered by hatches. In port, cargo is loaded by conveyor and spouts, or by crane and grab. Some bulk carriers are geared (usually a crane is located between each hatch) to allow the loading and unloading of cargo at berths without the need for shore equipment.
For unloading, cranes with grabs are the norm, although specialised equipment may be used for certain cargoes. When vessels unload using cranes and grabs, personnel and vehicles are often placed inside the holds to assist the process. Cargo is usually unloaded into hopper barges, then transferred by conveyor to silos or open storage. Smaller vessels can often discharge cargo directly into road vehicles.
General Cargo Vessels
General Cargo, known as breakbulk, is usually on pallets or in bags. There may be specialised handling facilities for such cargo, but usually loading and unloading is carried out using cranes and straps (for boxes) or slings (for bags). These vessels can also carry loose and irregular cargo, in which case the vessel’s crew and port stevedores pack the cargo to minimise damage and maximise space.
Although largely replaced by bulk and container carriers, general cargo vessels still operate throughout the world.
Containers have become the main way of transporting manufactured goods around the world. A container can be transferred between truck, train and ship relatively easily and is a standard size to simplify transportation. Containers can accommodate anything from foodstuffs to electrical equipment to automobiles. They are also used to transport bagged and palletised goods, as well as liquids and refrigerated cargo.
Standard containers are measured as TEUs (Twenty-foot Equivalent Units) and are generally 20 feet (1 TEU) or 40 feet (2 TEUs) long. All standard shipping containers are 8 feet wide and 8 feet 6 inches tall. There are also longer, taller and even shorter standard sizes, but these are less common.
Container ships are made up of several holds, each equipped with “cell guides” which allow the containers to slot into place. Once the first layers of containers have been loaded and the hatches closed, extra layers are loaded on top of the hatches. Each container is then lashed to the vessel but also to each other to provide integrity. Containers are usually loaded by specialised cranes or even general purpose cranes with container lifting attachments. Some small container vessels are geared to allow self-loading and discharging.
Container vessels are used predominantly on liner routes and are some of the biggest vessels afloat. Ultra Large Container Vessels (ULCVs) such as the Emma Maersk (lead ship of the Maersk E-Class vessels) are able to carry approximately 15,000 TEU (depending on container weight). Large container vessels are restricted by their size to certain ports around the world and are also unable to transit certain areas due to draft or, in the case of canals, beam restrictions.
Ships designed to carry refrigerated cargo and perishable goods such as fruit or meat are known as “Reefer Vessels”. Cargo is stowed in holds which are then sealed and temperature controlled. Traditional reefer vessels have been largely replaced by the use of reefer containers which are carried aboard container vessels. Reefer containers need a power source to function and often require inspection during the voyage.
Roll on-Roll off or Ro-Ro vessels come in many forms. They include vehicle ferries and cargo ships carrying truck trailers. The car carrier is the most commonly-used ro-ro vessels. These slab-sided vessels feature multiple vehicle decks comprising parking lanes, linked by internal ramps with access to shore provided by one or more loading ramp. Cargo capacity of such vessels is measured in Car Equivalent Units (CEU) and the largest car carriers afloat today have a capacity of over 6,000 CEU.
Liquid Cargo Ships
These vessels, collectively known as tankers, carry a range of liquid cargoes. Tankers were first developed in the mid-nineteenth century when ships made of iron allowed liquids to be carried in bulk, economically and without leakage. Like bulk carriers, economies of scale have driven up the size of tankers and today the largest examples have a carrying capacity or “deadweight” of over 400,000 tons.
Tankers are divided into separate tanks into which the cargo is pumped via a pipeline system. Modern tankers have large and segregated ballast tanks to allow them to sit lower in the water on the return ’empty’ journey to improve stability. Many tankers also feature systems to add an inert gas to the tanks to reduce the risk of fire and explosion.
The largest ships afloat are the Very Large Crude Carriers (VLCCs) and the Ultra Large Crude Carriers (ULCCs). These ships are designed to load crude oil and transport it to refineries around the world where it can be processed into petroleum products. The largest crude carriers often load and unload at offshore buoys and terminals as they are too large to enter most ports.
These vessels, which are generally smaller than crude carriers, transport the refined products from larger terminals to smaller ports around the world. Products carried include petroleum, jet fuel, 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 usually have a deadweight of 5,000-40,000 tons and often have specialised cargo systems suited to the type of cargo carried. These systems can include heating or cooling apparatus and advanced cleaning systems to ensure the cargo maintains its purity when loaded into a multi-purpose tank.
Liquefied Gas Carriers
These ships began as converted oil tankers but have evolved into highly specialised purpose-built vessels. Designed to carry Liquefied Petroleum Gas (LPG) or Liquefied Natural Gas (LNG) under pressure, the cargo tanks are generally spherical for strength. LNG carriers are usually larger than those carrying LPG, the largest LNG carriers are the ‘Q-Flex’ vessels with a gas capacity of up to 266,000 cubic metres.
Specialised Cargo Ships
Most types of cargo could be considered as specialised due to the specific loading, unloading or stowage arrangements required. Many such cargoes are however, moved with such regularity and ease that the term ‘specialised’ takes on a new meaning. For the purpose of this article, it refers to cargoes that are either difficult to categorise as dry or liquid, or to cargoes that are relatively difficult to handle.
This category includes everything from 10-person foot ferries up to cruise ships able to carry over 6,000 passengers. Perhaps the most specialised cargo of all, the needs and desires of passengers have driven the design of the modern ferries and cruise vessels.
Ferries, once seen as ‘a means to an end’ for most, are now lavishly equipped with lounges, restaurants, shops and entertainment facilities – particularly when the ferry is on a relatively long route. The ships have got larger too. The Ulysses, for example, which runs between Holyhead and Dublin, is able to carry over 1,300 cars and 2,000 passengers.
The first example of ships undertaking a public ‘cruise’ can be traced back to the nineteenth century but cruising gained mass popularity in the latter twentieth century. Many cruise vessels were originally liners which were sent to warmer climates during seasonal bad weather on their regular routes. Arguably, the last of the liners is the Queen Mary 2, which still operates a regular transatlantic service.
Today, cruise passengers expect a wide range of facilities including casinos, gymnasiums, shops, theatres, cinemas, pools, restaurants and bars. The largest cruise vessels are up to 360 m. long and 60 m. wide. Popular cruising areas are the Mediterranean, Caribbean and Scandinavia.
These ships are often converted from other types of vessel and are equipped with pens for large numbers of animals. The main considerations during the transport of livestock are adequate ventilation, food, and water. It is also important that ports that receive these vessels have facilities to handle the animals. Some livestock carriers can transport up to 120, 000 sheep. A common route for livestock carriers is Australia and New Zealand to the Middle East.
Heavy-lift/Project Cargo Vessels
These vessels are often purpose-built to specialise in the transport of extremely heavy or bulky objects such as other ships and large industrial components. For instance, these vessels are used to transport offshore platforms from their construction site to drilling sites. Some heavy-lift vessels are equipped with high capacity cranes to load at ports without a heavy-lift capability. Other types are semi submergible, allowing cargo to be floated into 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 vessels are often adapted to suit their role. ‘Jack up’ vessels for example are able to put down ‘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 put in place.
Even with the advent of highly manoeuvrable vessels, the tug is still vitally important to the maritime industry. Modern tugs are highly manoeuvrable with pulling power that can exceed 100 tonnes! Harbour tugs are very common at ports around the world, and generally less powerful. These vessels assist in berthing, unberthing and moving large or unyieldy vessels within port limits. Tugs are also used to assist vessels during bad weather or when carrying dangerous or polluting cargoes. Harbour tugs are also employed to move barges, floating cranes and personnel around ports. Larger units are kept on standby in strategic locations to act as deep-sea rescue and salvage tugs.
Tugs are also used to tow barges from port to port and move large structures such as offshore platforms and floating storage units. Some tugs can push barges; this is particularly common on rivers where the tug is able to exert more turning force on the tow. There are also tugs that are designed to ‘slot’ into a barge or hull. Once secured, this composite unit behaves and is treated like a standard powered vessel. These composite units are common on North American river and coastal trade.