Figure 2.1 Ship types (Eyres, 2007).

• general cargo ships

• container ships

• tankers

• dry bulk carriers

• passenger ships

• tugs.

2.2.1 General cargo ships

The industry distinguishes between break bulk cargo which is packed, loaded and stowed separately and bulk cargo which is carried loose in bulk. The general cargo carrier (Figure 2.2) is a flexible design of vessel which will go anywhere and carry a wide variety of cargo. The cargo may be break bulk or containers. Such vessels have several large clear open cargo-carrying spaces or holds. One or more decks may be present within the holds. These are known as 'tween decks and provide increased flexibility in loading and unloading, permit cargo segregation and improved stability. Access to the holds is by openings in the deck known as hatches.

Hatches are made as large as strength considerations permit in order to reduce the amount of horizontal movement of cargo within the ship. Typically the hatch width is about a third of the ship's beam. Hatch covers are of various types. Pontoon hatches are quite common in ships of up to 10 000 dwt, for the upper deck and 'tween decks, each pontoon weighing up to 25 tonnes. They are opened and closed using a gantry or cranes. In large bulk carriers side rolling hatch covers are often fitted, opening and closing by movement in the transverse direction. Another type of cover is the folding design operated by hydraulics. The coamings of the upper or weather deck hatches are raised above the deck to reduce the risk of flooding in heavy seas. They are liable to distort a little due to movement of the structure during loading and unloading of the ship. This must be allowed for in the design of the securing arrangements. Coamings can provide some compensation for the loss of hull strength due to the deck opening.

A double bottom is fitted along the ship's length, divided into various tanks. These may be used for fuel, lubricating oils, fresh water or ballast water. Fore and aft peak tanks are fitted and may be used to carry ballast and to trim the ship. Deep tanks are often fitted and used to carry liquid cargoes or water ballast. Water ballast tanks can be filled when the ship is only partially loaded in order to provide a sufficient draught for stability, better weight distribution for longitudinal strength and better propeller immersion.

Cranes and derricks are provided for cargo handling. Typically cranes have a lifting capacity of 10–25 tonnes with a reach of 10–20 m, but they can be much larger. General cargo ships can carry cranes or gantries with lifts of up to 150 tonnes. Above this, up to about 500 tonnes lift they are referred to as heavy lift ships.

The machinery spaces are often well aft but there is usually one hold aft of the accommodation and machinery space to improve the trim of the vessel when partially loaded. General cargo ships are generally smaller than the ships devoted to the carriage of bulk cargoes. Typically their speeds range from 12 to 18 knots.

2.2.2 Container ships

Container ships (Figure 2.3) are a good example of an integrated approach to the problem of transporting goods. Once goods are placed in the container at a factory or depot, they can be carried by road, rail or sea, being transferred from one to another at road or rail depots or a port. The container need not be opened until it reaches its destination. This makes the operation more secure. The maritime interest is primarily in the ports and ships but any element of the overall system may impose restrictions on what can be done. Height of container is likely to be dictated by the tunnels and bridges involved in land transport. Weight is likely to be dictated by the wheel loadings of lorries. The handling arrangements at the main terminals and ports are specially designed to handle the containers quickly and accurately. The larger container ships use dedicated container ports and tend not to have their own cargo handling gantries.

The containers themselves are simply reusable boxes made of steel or aluminium. They come in a range of types and sizes. Details can be obtained from the web site of one of the operators. Nominal dimensions are lengths of 20, 40 and 45 ft, width of 8 ft and height 8.5 or 9.5 ft. Internal volumes and weight of goods that can be carried vary with the material. For a 20 ft general-purpose steel container the internal capacity is about 33 m3, weight empty is about 2.3 tonnef and the maximum payload is about 21.7 tonnef. Figures for a 40 ft container would be about 68 m3, 3.8 tonnef and 26.7 tonnef, respectively. Aluminium containers will have about the same volume, weigh less and be able to carry a larger payload. They are used for most general cargoes and liquid carrying.

The cargo-carrying section of the ship is divided into several holds with the containers racked in special frameworks and stacked one upon the other within the hold space. Containers may also be stacked on hatch covers and secured by special lashings. Some modern ships dispense with the hatch covers, pumps dealing with any water that enters the holds. Each container must be of known all up weight and stowage arrangements must ensure the ship's stability is adequate as well as meeting the offloading schedule if more than one port is involved. The ship's deadweight will determine the total number of containers carried.

Cargo holds are separated by a deep web-framed structure to provide the ship with transverse strength. The structure outboard of the container holds is a box-like arrangement of wing tanks providing longitudinal and torsional strength. The wing tanks may be used for water ballast and can be used to counter the heeling of the ship when discharging containers. A double bottom is fitted which adds to the longitudinal strength and provides additional ballast space, see Figure 9.35.

Accommodation and machinery spaces are usually located aft leaving the maximum length of full-bodied ship for container stowage. The overall capacity of a container ship is expressed in terms of the number of standard 20 ft units it can carry, that is, the number of twenty-foot equivalent units (TEU). Thus a 40-foot container is classed as 2 TEU. The container ship is one application where the size of ship seems to be ever increasing to take advantage of the economies of scale. By the turn of the century 6000 TEU ships had become the standard for the main trade routes, and some 80 ships of 8000 TEU were on order plus some of 9200 TEU. Concept work was underway for ships of 14 000 TEU size. Container ships tend to be faster than most general cargo ships, with speeds up to 30 knots. The larger ships can use only the largest ports. Since these are fitted out to unload and load containers the ship itself does not need such handling gear. Smaller ships are used on routes for which the large ships would be uneconomic, and to distribute containers from the large ports to smaller ports. That is, they can be used as feeder ships. Since the smaller ports may not have suitable handling gear the ships can load and offload their own cargos.

Some containers are refrigerated. They may have their own independent cooling plant or be supplied with cooled air from the ship's refrigeration system. Because of the insulation required...