Specialized vessels designed to carry one particular type of cargo.
For example:(Bulker, Barge carrier, Roll-on/Roll-off vessel, Container vessel, Tanker)
Barge carrier:
The idea of a barge carrier is the same as that of any unitized goods transporter: to keep cargo in the same package all the way and not to reload it. Large steel barges used as containers may be of different sizes and of different cargo carrying capacity. The only difference between a container and a barge is that the latter is much bigger and can float. Barges can be loaded with principal different cargo — from ore to containers. Several barge carrier designs are available, including the lighter aboard ship (LASH) and the sea barge (Sea Bee) designs.
These designs differ in dimensions and design features but provide similar benefits in that the ship is able to load and unload barges in rivers and estuaries away from docks and quays. Thus they reduce the time in port and avoid the usual problems of port congestion.
The basic difference between the two systems is that the LASH ship is a «lift-up and lift-over» type, using a 500 ton gantry crane, while the Sea Bee ships float barges on to an elevator of 2,000 tons capacity, using a roller system for stowage.
The LASH system has three basic elements, the first of which is the ship itself which has a broad U-shaped stern with structures extending aft of the hull body and forming a docking area. The second element is the barge. The third element is the massive 500 ton gantry crane which can drive along rails fitted to the upper deck and lower a barge into the desired location on the ship and discharge the barges at their delivery points.
The vessel is of the single deck type with the superstructure forward to give maximum clear working space. The holds are covered with pontoon covers capable of sustaining two tiers of barges on deck. The total load comprises 83 barges made up of 49 barges below deck — stowed athwart ships — and 34 on deck. Each barge has a bale capacity of 555 cu.m, a maximum deadweight of 375 tons and a large hatch with pontoon covers in three sections.
The Sea Bee System. — The Sea Bee ship has three continuous cargo decks, there are no hatch openings on the upper deck as a horizontal loading system is adopted. The submersible platform which has a lifting capacity of 2000 tons is arranged at the stern and can be lowered into the water so that one or two barges can be floated on to it. The lift is raised on the level of the deck required where a transporter is rolled underneath the barge, by remote control, and moves it forward to the desired position on that particular deck.
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The average time taken to lower the elevator, float two barges on and raise it is 29 minutes, and further 9 minutes are required to transfer the barges from the elevator by transporter.
Both LASH and Sea Bee ships can be operated as container ships. The advantages of the barge carrying system:
1). rapid loading and unloading;
2). the barge carrier can anchor and moor clear of quays and thus avoid port congestion;
3). the life of barge is 2.5 times that of a container and the depreciation of a is about half that of the equivalent number of containers;
4). the organization of the barges is simpler than that involved with the much larger equivalent number of containers.
The disadvantages of the barge carrying system:
1). distribution is limited, since the barge is restricted to a navigable inland waterway suited to the draught of the barge;
2). ice on waterways can restrict barge transport;
3). inland waterway transport is relatively slow; this is important in relation to perishable products;
4). the LASH type ship is more expensive than the equivalent container ship.
One/single purpose vessels designed to transport one particular kind of cargo.
For example:
(Cellular type full container ship, PCTC=pure car-truck carriers, Sheep-carriers, Cattle carriers=trot on/trot off vessels)
The cellular type full container ship is equipped to carry containers in the holds and on the hatch covers. Within the holds of the vessel there is a cellular structure of angle bars forming
container guides into which the containers are stowed, one on lop of another. The containers carried on deck are carefully secured to ensure that they will not shift, the only movement of the container within the ship is vertical one, and thus loading and discharging is carried out by vertical movement only. Heights of container stacks vary and may be six high in the holds and three high on deck. In general, cells are arranged fore and aft in groups, their number depending on the ship's breach. The distinguishing features of container ships are very large hatch opening and elimination of tweendecks. The fore and aft groups of cells are separated by heavy web frames, or similar structure, to act as support to the cells and also give rigidity to the vessel. Each cell is for one container stack and extends vertically from the coaming level to the tank-top. The basic functions of the cell-guide system are:
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1. to facilitate the lowering and positioning of containers;
2. to distribute the lateral loads from the containers (arising from the rolling motion of the ship) to the adjacent hull structure.
Cellular type full container ships are serviced by shore-based or shipboard gantry cranes. Cellular ships, operating between loadcentres equipped with shore-based cranes, have the advantage of greater deadweight capacity as compared to self-sustaining ships fitted with shipboard mounted gantry cranes.
In many areas of the world on short interport voyage service feeder ships of similar configuration but of reduced capacity are in operation. By fully mechanizing the handling of cargo and using shore based travelling gantry cranes the typical feeder ship can be turned around in about five hours, a practical loading-unloading cycle being two to three minutes per container.
There are also roll-on/roll-off container ships on which the containers are mounted on the wheels or trailers and are then rolled on the ship and stowed complete with their undercarriage. Such ships are particularly advantageous for short voyages, but they can carry fewer containers as compared to cellular full container ships or feeder ships of equivalent size because of considerable space occupied by trailers and access ramps from deck to deck. This difference may be as much as 35% or even more.
Like all other specialized ships designed to carry unitized cargo container ships have a high service speed ranging from 16 to 26 knots, and even up to 30 knots, in dependence of the ship's deadweight.
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