Types of Cargo Ships

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Types of cargo ships

General Cargo Ship

The general cargo ship is generally confined to smaller ships. The cargo, as the name implies, is of many different kinds and varieties. They can range from canned food, sawn timber, steel bars, grain sacks, manufactured consumer goods packed in cardboard boxes, to liquid cargo like latex and vegetable oils. Containers can also be loaded and secured on deck.

The general cargo ship has their own cargo loading gear. This may be in the form of cranes or winches-derrick-boom. Cargo loading is comparatively slow and labor intensive. Stevedores have to physically secure the lifting hooks, nets, wire slings, and sort the cargo. Cargo can be loaded from barges, or from the wharf. 

Ships using derrick have steel cables running all around the cargo gear. During unloading, by using a system of winches and pulleys, the cargo is lifted up, moved horizontally, swung out over the sides of the ship and then lowered down. Sometimes, loose cargoes fall off, the nets and are damaged. 

In this type of ship, it is quite common to see the ship leaning to one side at one moment, and then leaning to another side the next. The ship may also lean permanently to one side. Because of the variety of cargo, and the use of stevedores, this type of ship tends to be quite messy and dirty. In some ports, the stevedores even bring their families to do their cooking while they work overnight on the cargo.

For the seaman working on such a ship, it is quite good life, if he can ignore the mess all around. Normally the ship's stay in port will be quite long. This means more leisure time on shore. When cargo is damaged, it becomes finders-keepers.



Tanker Ship

Tankers are ships that carry liquid cargo, normally oil. Some are built for petroleum oil, while some are for vegetable oils and chemicals like caustic soda. When used to carry oil, the freeboard (the portion above the water level) of the ship is usually less than other ships. This is because the oil cargo is lighter than water and can contribute to the buoyancy of the ship if the ship's hull is damaged. For the control of oil pollution, the oil tanks are sometimes filled by double containment. This means there is a distance between the outside hull of the ship and the oil tank. Should the hull be damaged, there is another tank barrier which can prevent the oil from flowing out and polluting the environment.

The deck on these ships will be full of pipelines, because tankers carry their own cargo pumps. Unloading of the cargo is by pumping from the ship to shore.

Those working on crude oil vessels, have to be vigilant not to cause any electrical or friction spark while on the deck or in any explosive zones. This means no chipping of paints on deck using steel hammers. All the devices installed in these areas are explosion proof or intrinsically safe. 

The atmosphere in the crude oil tanks itself is inerted with carbon dioxide gas. During loading and unloading of the cargo oil, it is especially important to maintain the inert nature of the tanks. The carbon dioxide is usually taken from the exhaust gas of the boilers. The percentage of the oxygen content in the exhaust gas has to be maintained low before it can be sent to the cargo oil tanks. 

The cargo loading and unloading is usually very fast. The stay in port is thus very short. In addition, engineers usually have to maintain the operation of the cargo oil pump turbine and their associated systems. Boilers are run to supply steam to the pump turbines. Usually they will work on watches just like when the ship is underway.

During cargo pumping operations, it is very important to keep the cargo flowing in the pipes especially with freezing temperatures of the winter months. Although the cargo oil or caustic soda is heated while it is in the tank, the liquid can freeze up if the flow is interrupted. It is very difficult to make the cargo flow again if it freezes in the pipes. 

Bulk Carrier

These ships carry cargo in bulk. Examples of these cargoes are grain, wood chips, coal, and iron ore. Usually these ships are not fitted with their own cargo handling gear like cranes. All the loading and unloading are done by the shore facilities. Light objects like grain or woodchip, are blown into the ship's cargo hold using flexible ducting and fan. Some shore installations have conveyors that drop the cargo into the cargo hold. When one cargo hold is full, loading is continued on the next hold, but not necessarily in sequential order. The ship's deck officers will load, keeping the ship's stability in mind. If the shore installation is not movable, then the ship will have to move to accommodate the next cargo hold location. Usually, manipulating the mooring ropes at the bow and stern of the ship does this. Sometimes, the engines have to be started to assist in the movement of the ship.

The bulk carrier's stay in port can be quite long, perhaps about 4 to 5 days. The location of the cargo loading port is usually remote, because the bulk cargo is usually taken from mining, forested, or farming areas. Some of these areas can be very beautiful scenic spots, unspoiled, and away from the regular tourist destinations.

The deck of the bulk carrier is normally free of obstructions and good for jogging in fine weather. This type of ship is easy to maintain and life on board is comparatively simple.

Many of these ships operate on tramping charter. This means the ship goes wherever there is cargo, as opposed to the liners that ply on a fixed route. The seaman working on a bulker can expect to travel to many different unplanned destinations during his service on board.

Container Ship

These ships carry containers as cargo. The containers are preloaded with goods for export. In this way the goods can be locked and sealed before they are loaded onto the ship. With the use of shore based independent moving gantry cranes, the loading and unloading work is extremely fast. 

All the cargo holds contain guides for the containers, so that it is easy to slide them in place. The containers are made so that the corners can be locked in place very easily. Because the containers are lowered in place precisely and the corners are matched for interlocking, it is important to keep the ship at even keel during the cargo work. For this purpose, container ships have remotely controlled ballast pumps and valves that can be controlled by deck officers.

In line with the fast cargo handling work, container ships are usually built for speed, so that cargo can arrive at their destinations fast. Some ships are fitted with bow-thrusters to shorten the maneuvering time.
Since the cargo is not messy, the container ship remains clean even when in port. 

The ports are usually located in big cities so going ashore is easy. However, seamen working on container ships have little time to spend ashore. Sometimes the stay in port may be just a few hours. Whatever activities on shore have to be carried out quickly. 

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