Understanding the various types of ships

Malcolm Latarche
Malcolm Latarche

22 February 2018

There have always been different types of ships but historically the differences would have been with regard to construction or rigging rather than specialisation related to types of cargo. Ships may have been built for specific trades – for example the tea clippers – but there was very little beyond their speed to differentiate such ships from those engaged in other trades.

With the advent of iron and steel hulls and steam and motor propulsion systems, the use of sails was gradually phased out albeit over a very long period of time. Since ships now had to carry fuel as well as cargo, the move towards larger ships was set in motion. That said, the volume of goods and commodities moving around the globe was but a fraction of what it is today and a large ship of the 19th century would be considered very small by today’s standard.

Specialisation by cargo type can probably said to have begun in the second half of the 19th century when the first purpose-built oil tankers were constructed. These were very basic and did not have the segregation or pump systems that would come later. Dry cargo ships were for the most part all built along similar lines with only size differentials marking them out. Colliers - bulk carriers intended for coal - were the first specialised dry cargo ships to appear but the majority of bulk cargoes were carried in sacks or bags in general cargo ships.

Ships and their machinery and equipment have evolved over time but the specialisation that exists today mainly occurred from the second half of the 20th century with developments being driven by various factors particularly the need to accommodate rapidly increasing demand for goods, intermodal traffic and new types of cargoes such as LNG.

Modern ship types can be categorised under five general headings; Dry, wet, passenger, offshore and miscellaneous. Each category has several sub-types and we will cover all of these over the coming weeks.

Wet Cargo Ship Types

The most obvious of these are the tanker family which comprises many niche ship types.

From simple early one or two hold ships, tankers have evolved so that the inner hull is divided into several compartments by way of athwart ship and longitudinal bulkheads. This prevents the free surface effect of liquid cargoes which caused the demise of many of the earlier tankers.

All modern tankers have some form of pumping system that can move cargo between tanks and is used for discharging the cargo. Loading is done using shore pumps. The pipework connecting tanks and pumps culminates at the manifold where shore hoses are connected. Some tankers require heating equipment to keep cargoes from solidifying.

Tankers are mostly thought of as carriers of crude oil and this is the main cargo of the family using several different sizes of ships suited to specific trades. The largest tankers are always used for crude oil carriage.

After the crude carriers come product carriers used for transporting refined and semi-refined oils. There is some cross over at the larger end of the scale where ships may be used for carrying either crude or products. Those that do carry crude and the residuals of refining are referred to as dirty while those that carry the distillates and similar are referred to as clean.

The most numerous tanker type is the chemical tanker used for carrying industrial products such as acids and the like. This segment also contains some ships which can operate in the clean product tanker sector.

In addition to these three main types there are very specialised ships for cargoes such as vegetable oils, molasses, wine, asphalt and bitumen. Chemical tankers can carry some of these cargoes but usually vessels are built for specific trades and operators.

Fruit juice tankers are another specialist type but in many cases these are not true tankers in that the cargo is carried in tanks inserted into the hold rather than the hold being subdivided to create tank space.

A similar arrangement exists for the liquified gas carriers where cargoes must be kept refrigerated to very low temperatures or under high pressures. In some cases a combination of these methods is used to keep the cargoes liquid. Occasionally a chemical tanker may be built with a gas tank installed on the open deck to allow versatility in trading.

LNG carriers and some other gas carriers are powered by steam turbines or dual-fuel engines that use the boil off gases from the cargo as fuel rather than carrying their own fuel as most other ship types do.

Gas carriers are among the newest type of vessels in the world fleet as LNG did not become a cargo type until the late 1960s and for many years the fleet was restricted to a very small number of ships built for specific trades.

Dry Cargo Ship Types

Until the 1970s virtually all dry cargo ships that were not bulk carriers were either designed for general cargoes and even the specialist types that did exist such as reefers for carrying refrigerated cargoes were built to much the same pattern and could be slotted into other trades for economic purposes.

Bulk carriers have a single deck and while some of the smaller sized vessels have box shaped holds, the larger vessels are designed to be self-trimming to reduce the potential for cargo shifting. This means the holds have an octagonal cross section with the cut off corner areas being used for purposes such as ballast.

Purpose built bulk carriers vary in size from small vessels of just a few thousand tonnes deadweight to 400,000dwt ore carriers. The cargoes are typically grains, coal, ores and minerals although those with box-shaped holds will also carry semi-bulk cargoes such as steel or lumber.

The destruction of the merchant fleet during WWII was offset by the huge number of surplus Liberty ships freed up by the end of hostilities. It was these ships and similar newbuildings such as the ubiquitous SD14 that were the mainstay of world shipping for the next decade or so.

The typical general cargo ship would be four or five hatches engines amidships with a shaft tunnel running through the two aft holds. They would be tweendeckers with three or more decks. Only the smaller coasters would be single deckers.

A time of change

What was to follow was a period of great innovation for shipping as a combination of growing world trade interspersed by oil crises led to a search for faster and more flexible fleets. The result was much larger and faster ships but cargo handling was still an obstacle.

Unitisation was seen as the answer with the mix of bags, crates, drums and bales used for general cargo replaced by pallets and eventually containers and ro-ro ships were the metal boxes and roll trailers could be loaded and unloaded with cargo while the ship was at sea leaving port work confined to the more rapid movement of the boxes or trailers on and off the ship.

Containers built upon the old idea of van bodies and are intended to be switched between different modes of transport (ships, truck, rail) without the cargo needing to be removed at any stage.

Initially containers were loaded onto general cargo ships but soon vessels were designed with the standard dimensions of containers as a central factor determining hold and hatch dimensions and tweendecks were phased out. To speed things even further, the holds were fitted with vertical cell guides so that containers could be slotted in to place much more precisely. Although there was initial resistance to containers being loaded on the open weather decks, this was gradually overcome and today a modern container ship is distinguished by its deck load of containers.

General cargo ships did not disappear entirely and they are still around today, although now described as multipurpose or MPP ships rather than as tweendeckers. In many of these, the tweendecks have become movable rather than fixed and are folded away when high or oddly dimensioned cargoes are carried.

Ro-ro vessels are also still very much in evidence although the specialised trailers used for loading the cargo and the tractors needed to manoeuvre them around the quays and onto the ship are generally obsolete with road vehicles or their trailers being used instead.

Ro-pax vessels – almost always used as ferries – are similar in concept but because they are intended for carrying passengers as well, the above deck portion of the ship is designed as a passenger ferry with seating, restaurants and other facilities.

The growing international demand for motor vehicles over the last 50 years has seen the advent of another very specialised ship type the car carrier. Pure car carriers or pure car and truck carriers or PCCs and PCTCs as they are better known are easily recognisable by their slab sided appearance. Inside there are many fixed and movable decks that can be adjusted for height to accommodate the vehicles being carried. The result is effectively a multi-storey car park inside the vessel’s hull. The vehicles are driven on to the ship via a stern ramp and moved around inside using internal ramps.

The very specialist dry cargo ship types include heavy lift vessels, livestock carriers (quite similar in appearance and concept to PCCs) and like those already mentioned will be covered in greater detail.