Until now we have been looking at the loading and discharge of what is known as General Cargo. There has been a large shift away from general cargo vessels with the introduction of containers.
Containers are very large steel boxes with lockable doors at one end. Thr idea is that cargo is loaded into the container by the consignor or by inland depots. It is then only unloaded when it reaches its destination. The containers are loaded onto special lorries/trailers and brought to the container terminal where they are stacked and weighed. They are then loaded by special gantry cranes into the vessel.
In the vessel itself, the holds are specially designed with steel guides, so that slide down into the hold one on top of another. On deck, the containers are also stacked on top of one another. To ensure that they remain in place, they are locked into position by special twist locks that fit into each corner of the container, and into special fittings on the deck or on top of the hatches.
The strength of the container lies in the rectangular frame around its edges and corners where the lock fittings go. It is therefore essential that each container is stacked exactly one above the other at all times.
As with any cargo being loaded or discharged, the stability of the vessel is of the utmost importance at all times. Every piece of cargo that is placed onboard, or into the hold is weighed and this data is used to work out the centre of gravity (CG) of the vessel.
In order to ensure the stability, the cargo is loaded according to a pre-determined plan, that must be adhered to as closely as possible. This takes note of the size, weight, its destination and how long it will take to load/discharge. During the loading/discharge operations, the vessel may well have to shift, pump in or out ballast in order to maintain its stability and trim. If the vessel lists too much, or its trim is too great, then the containers will not be able to slip down the guide rails in the hold or be placed one above the other on deck.
When the cargo plan is made up, each container will have its own special slot aboard the vessel, and a major task is to ensure each one goes into the right spot at the right time. Because of the sheer number of containers, and the lack of time in port and the shortage of officers aboard, the cargo plan is drawn up ashore today. The crew will ensure that the stability remains good, and as the loading proceeds, they will keep the ships stability plan as up to date as possible given the figures they are supplied with by the shore terminal.
Frequently however, the shipper gives the wrong weights, either deliberately or by mistake which leads to problems for the ship. Or, just as dangerous, they wrongly declare the contents so as to avoid various charges, ie they do not declare dangerous/flammable goods. This has lead to disastrous fires onboard, especially when the container cannot be got at because it is in the middle of the stack. Unfortunately, in each case the master is held responsible as he accepted the cargo aboard.
There are many different types of container vessels. The commonest is the type shown in the picture with all the accommodation and machinery spaces amidships. The holds themselves are divided into various bays into which the containers fit. They then also carry stacks of containers on deck up to 6 high. If these containers are not fastened securely, they can collapse causing a great deal of damage. Deck containers can and do get washed overboard regularly in bad weather.
Another type of container vessel is the Ro-Ro, where containers and all sorts of cargo are driven onboard through large hatches/doorways cut into the sides or more usually in the stern. These ships have no holds as such, just large open decks running the full length and width of the ship. This causes great problems if the ship starts taking water or a fire breaks out. They normally have all their accommodation right aft.
Others have their accommodation right forward, with their engines right aft. Yet another type actually has retractable legs which when the ship is alongside, are lowered down to the bottom of the harbour, so s to keep the ship completely steady whilst the cargo is loaded. If there is a lot of movement in the water, it can cause problems for the gantry crane driver trying to drop the container into a very tight spot.
Most ships today are fitted with container spots so that they (the containers) can be stowed anywhere on deck.