Throughout history ship design has been dictated by the demands of trade and local conditions and available materials. Wood and sail have been the dominant choices through most of history until the 19th century when iron and steel replaced wood and mechanical power began to displace sail.
Specialisation in ship design really began in the late 19th century as before then any ship would carry every type of cargo that was available. Liquid cargoes were mostly carried in barrels and the same method would also be used for many dry goods. Tun, an old word for barrel used throughout much of Europe, is the origin of the word ton or tonne which is still used today to describe the capacity of ships whether in volume (gross and nett tonnage) or weight (deadweight tonne).
In 1852 the first dedicated bulk carrier appeared but dry goods were still mostly carried in general purpose vessels fitted with tween decks to allow for separation and to avoid damage to cargo at the bottom of stacks. In 1878 Zoroaster began the first successful sea going purpose built oil tanker.
Through the 20th century specialisation of vessel types began to accelerate especially after WWII when new ships were needed to replace the many destroyed during the war. This specialisation and a desire to speed up cargo handling led to the introduction of new ship types with pallet carriers and ro-ros being among the first followed by container ships in the 1960s and 1970s. The first gas carrier appeared around this time as well.
In the late 1960s and 1970s, exploitation of offshore oil and gas reserves began. At that time this required the use of offshore platforms and drilling rigs, towed into place, and anchored using newly devised anchor handling tugs. Initially servicing of these rigs was done by fishing vessels but over time a whole new area of specialisation was developed leading to PSVs, OSVs and later OCvs and even drill ships being devised. The era of offshore wind has led to a number of new specialised vessel types.
Over time, the search for greater speed, and larger capacity has seen ships grow to sizes that would once have been unimaginable. Today, speed is seen as something to be discouraged and efficiency is the name of the game. Size though is still increasing albeit at a slightly slower pace in the very recent past than in the fifty years from the late 1960s.
Through the Second World War from 1939 to 1945 a very large proportion of the world merchant fleet was destroyed. Fortunately the Liberty and Freedom class ships built in incredible numbers by US shipyards provided a buffer supply of ships until newbuilding capacity recovered. In the 1970s the various oil crises forced a massive size increase in ships to gain economies of scale. It was in this era that the first Capesize bulkers and VLCC tankers appeared.
Also in the 1960s/70s attempts to speed up cargo handling resulted in new methods of packing cargo either on pallets, in containers or on trailers. This led to new ship types being developed to handle these and eventually on the liner trades they displaced the conventional ‘tweendeckers. The latter type do survive to the present but now they are referred to as multipurpose or MPP types and have evolved to be better suited to project and heavy lift cargoes that are unsuited to containerisation. There are also other specialist ship types in the dry trades including cement carriers, livestock carriers, car carriers and more.
Bulk Carriers
Within SOLAS, the IMO defines bulk carriers as ships intended to carry cargoes in bulk. While it is possible for bulk cargoes to be carried in almost any dry cargo ship, the cargo handling involved can become complicated, so dedicated bulk carriers are favoured for most bulk trades.
Bulk carriers are the work horses of the shipping industry moving the raw commodities and semi-finished products around the globe. It is a sector plagued by overcapacity but solving that problem is not easy except in specific size or trade segments. By almost all measures, the bulk sector is the largest in shipping.
Bulk carriers are the simplest ships in terms of construction and specialist equipment. Most consist of a single skin hull with machinery and accommodation aft and a series of holds ranging from one on so-called mini bulkers through to nine on the largest ore carriers. Most of the Handymax sizes will have five cargo holds and Panamaxes seven.
For a long time the bulk sector comprised six basic segments by size; Mini (upto 9000dwt) , Handy (10-30,000dwt) , Handymax (30-60,000dwt), Panamax (60-80,000dwt), Capesize 80 – 200,000dwt) and Very Large Ore Carriers (VLOCs) above 200,000dwt. More recently Supramax/Ultramax and New or NeoPanamax have been added.
At the lower end of the scale and through to Panamax, the ships may be equipped with their own cargo handling gear. The larger ships are invariably gearless and loaded using shore gear.
Multipurpose ships (MPPs)
The natural evolution of the tween decker but usually with fewer intermediate decks than ships of the 1960s and 1970s. These ships can carry bulk cargoes, containerised cargo, semi bulks such as steel and lumber and project cargo. They are multi-hold ships and frequently have one hold that is significantly longer than the others to be able to accommodate oversized pieces of project cargoes. Some MPPs also have vehicle ramps to allow cargo to be driven inside.
They are usually geared with many having some heavy lifting ability and cranes able to operate in tandem to load large and heavy items of cargo. MPPS are rarely above 25,000dwt which means that their trading range is rarely limited by port dimensions.
Ro-ro and car carriers
These ships are equipped with ramps that allow vehicles to be pulled or driven directly into the ship. Inside there are many different deck levels that in the Pure Car and Truck Carriers (PCTCs) can be hoisted to different levels allowing vehicles of different heights to be accommodated. Some ro-ros may also be equipped with cargo hatches allowing them to accommodate containers and these are usually referred to as Con-ros.
PCTCs vary in size and are easily recognisable by their slab sided appearance. This can make them prone to the effect of side winds so great care is need in properly stowing and operating them. Car carriers usually operate on regular services from major car producing countries. In some cases this allows for a service akin to a liner operation but the number of shippers is typically far fewer than on a liner vessel and may in some cases be confined to a single shipper.
Small ro-ros are often employed on ferry services carrying trucks or unaccompanied trailers. If they accept more than twelve drivers to accompany their vehicles, they technically become passenger ships and much more stringent regulations are applied to them.
Container Ships
The container ship is very much a product of the logistics industry carrying standardised boxes that can be switched between almost all modes of transport with relative ease. The standard ISO container is typically 8 feet 6 inches square in cross section and 20 or 40 feet long. The use of non-metric measurements is a result of their USA heritage. However there are now many other common sizes in use reflecting the growth in road transport vehicle size.
Early container ships had a capacity of around 600-700teus (twenty feet equivalent units) but by the turn of the century vessels of Panamax size able to carry 4-5000teu were common. Today the size has mushroomed, and the largest ships afloat can carry 24,000teus.
Although many containers are carried under the deck, the vast majority of cargo on a container ship will be carried as deck cargo. The containers are locked together using twistlocks and lashing rods but it is not unusual for containers to be lost at sea in heavy weather conditions.
Other Dry Cargo ships
As well as the types mentioned above, there are some other ship types operating in the dry cargo sector. Cement carriers are specialised bulk carriers but the cargo is pumped in and out rather than loaded using cranes and grabs. This is because the cargo comprises such small particles that it behaves more like a liquid.
Livestock carriers are another specialist ship type used for the carriage of live animals. This is a trade that is losing popularity but which still exists. A livestock carrier may resemble a PCTC in general appearance with high sides and several decks. Special care is needed to keep animals fed and watered and for removal of waste material.
Refrigerated cargo ships or reefers are generally small ships resembling MPPs but with refrigeration equipment to keep holds chilled. They are used for carriage of meta, fish, fruit and vegetables. Often they will have fittings suited to carriage of refrigerated containers as well as the chilled holds. Some have side ramps for loading by forklift trucks with ramps or lifts between deck levels. Decks are usually limited in height so as to increase the insulation effect. Newsprint carriers are similar to reefers but do not need the refrigeration systems.
Tankers & Gas Carriers
Tankers and Gas Carriers along with the related FPSOs FSOs FSRUs and FSUs are often lumped together but the two ship types are very different in construction and complexity.
Tankers are similar to bulk carriers in general construction but instead of holds the hull is divided into separate tanks. Unlike dry cargo vessels where the hold is across the full width of the ship, on a tanker there can be numerous subdivisions. There are no hatches and cargo is pumped in using a manifold and piping system. There is usually one or two hose handling cranes that lift the hoses from the shore to the deck where they are secured to the manifold. When loading cargo pumping is done from shore but when unloading the ship’s pumps are used.
Tankers can be used to carry a single commodity (crude oil tankers and wine tankers) or multiple different products (product carriers and chemical tankers) Those designed for single commodities usually have a simple interconnected pumping and piping system but for multiple commodities the cargo handling system allows for segregation of the commodities to avoid contamination.
Each tank is usually fitted with a cleaning gun system which allows for cleaning chemicals to be directed into all parts of the tank. In a crude tanker, fresh cargo is used for cleaning.
Tankers range in size from very small ships below 1000dwt to ULCCs of over 300,000dwt. Chemical and wine tankers are the smallest types while Product carriers and Crude carriers cover the larger segments.
Gas carriers will carry LNG, LPG and related cargoes such as Ethylene of ammonia. To make gases liquid they must either be carried at very low temperatures or under high pressure. LNG carriers invariably use the temperature option while LPG carriers may choose either or a combination of both.
Unlike a tanker where the tanks are formed from division of the hull, in a gas carrier the tanks are separate constructions built inside the hull. In the cryogenic ships, the tanks have high levels of insulation to prevent the cargo boiling. In both LNG and LPG carriers, the boil off cargo can be used as fuel for the ship’s propulsion system. Some LNG carriers are equipped with reliquefaction plants that can convert boil off gas back to liquid.
Bunker barges are smaller version of tankers and share the same characteristics.
Passenger Ships
From small ferries to luxury cruise liners, passenger ships represent the most diverse segment of the world fleet. Although most are equipped with diesel engines, a few of the most recent are pure electric and rely on a battery and electric motors for propulsion. Others are hybrid ships which combine the battery with an engine.
Passenger ships are subject to many more safety regulations than cargo ships and must carry an appropriate level of lifeboats, life rafts and other equipment.
Offshore Ships
This is another very diverse sector with ships ranging from platform service vessels to carry equipment and cargo to rigs to construction vessels and drill ships which build and take the place of rigs and other offshore facilities respectively. Also included are crew transfer vessels and accommodation ships which carry and house the many workers needed to operate and build offshore oil, gas and wind facilities.
Another specialist ship type is the Anchor Handling Tig and Supply ship (AHTS) which is sued to tow rigs around the globe and to secure them in place at their operational destination.
Because many offshore ships need to perform tasks at fixed locations in the open sea, they will eb equipped with dynamic positioning systems that allow them to hold station using all of the ships propulsion and thrusters systems while taking into account, currents and wind.
