Introduction to the Ro/Ro Ships
The introduction of the roll-on/roll-off system to major trade routes of the world has added new dimensions to the modern cargo handling techniques offered to shippers. The roll-on/roll-off system was first introduced about the same time as the containership and has only recently gained wide acceptance. While there has been a rapid buildup of terminal areas, berths, and handling equipment for containerization at many ports, the roll-on/roll-off system has proved that it complements and supplements rather than competes with the containership and container handling methods.
More important for developing trade, it is a system that does not require massive specialized terminal facilities and shore-based equipment. Many operators conclude that the го/го method combines the best features of containerization, unitization, and breakbulk techniques. However, these ships also have unsatisfactory features such as wasted space and lashing problems. Recent ro/ro ships have been designed as almost full containerships where the containers are loaded and unloaded ro/ro fashion with forklift trucks.
Examples of cargo that have been literally rolled aboard ro/ro ships are heavy earth-moving machinery, automobiles farm equipment, large pieces of lumber, wood pulp, newsprint, sheet steel, piping, and other similar commodities. Rolling stock is ready for delivery upon arrival at the discharge port, and loading, stowing, and discharge operations are simplified.
The ro/ro ship is usually capable of discharging a great deal of cargo in a very short time. One of the first full ro/ro ships in the U.S., the Comet, loaded 298 vehicles representing 7,971 measurement tons, where one measurement ton equals 1.13 m3 (40 ft3), in 4 hr 55 min. This same cargo was discharged in 2 hr 23 min. The ro/ro ship is volume-limited, rather than weight-limited, due to the space required for ramps, access, and underneath the wheeled cargo itself. It is, therefore, usual to discuss cargo stowage in terms of cubic capacity instead of tons.
The straight car carrier is a special adaptation of the ro/ro, where the deck height is reduced to that required for an automobile. Access to decks can be by ramp or elevating mechanism, and, since the loads are lighter, most equipment moves faster than on the ro/ro ship. Car carriers with capacities up to 4.000 cars can discharge in as little as 20 hours, utilizing two to three ramps. Fig. 36 shows hoistable car decks in place and stowed in a bulk carrier.
In order to prevent confusion, this chapter has tried to discuss only single-purpose ships, but, of course, any combination of modes of cargo handling is possible. Some of the more successful combinations are:
• Containers on deck with ro/ro below deck — This is a particularly good mix as the cargo functions are separated on the pier with the ro/ro going off a stern ramp and the containers being worked amidships by shore cranes.
• Bulk carrier/car carrier — This arrangement requires refitting the ship at each end. The bulk trade is certainly not overly compatible with car decks, but the mix does provide two-way trade on some runs.
• Pallet/elevator below deck, containers on deck — This too is a good combination for some runs, but in this case the two modes do interfere with each other.
To many owners, the "everything" ship appears to allow them unlimited flexibility, when in reality each additional mode compromises the limits and prime cargo handling modes. The designer should carefully analyze the actual requirements of each trade and work toward an efficient cargo handling system if he is best to meet the needs of the owner.
The next article will describe different types of ro/ro vessels.
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