A linkspan or link-span is a type of drawbridge used mainly in the operation of moving vehicles on and off a roll-on/roll-off (RO-RO) vessel or ferry, particularly to allow for tidal changes in water level. Linkspans are usually found at ferry terminals where a vessel uses a combination of ramps either at the stern, bow or side to load or unload cars, vans, trucks and buses onto the shore, or alternately at the stern and/or the bow to load or unload railroad cars.
History The first linkspans appeared at the end of the 19th century when train ferries came into operation. Each rail ferry berth has to be specifically designed to make sure that it fitted one class of ship. In most of these vessels it was also possible to carry some road vehicles. By the mid 20th century with the rise of road transport, general purpose Ro Ro ferries started to come into service. Most could use the rail ferry berths but generally they were fitted with stern ramps that had the dual function of giving a watertight closure to the ship's stern access door and also acting as a drawbridge to the quay which allowed vehicles to drive on and off the vessel. Using the ramp for access has limitations in that if there is any significant tidal range; gradients on this ramp become too steep to be manageable. The operation of these vessels was initially limited to areas such as the Baltic and Mediterranean seas. Very soon there was a demand for these ferries to be used in tidal waters. Ship's ramps were also developed in size, as was forward access through a bow door closed by a drawbridge ramp inside a visor. These features are now common to most Ro Ro drive through ships.
Operation Initially a linkspan was a ramp that was attached to the pier at one end and was suspended above the water at the other. The height above the water was controlled either by hydraulic rams or cables, these types of linkspans were less well designed for the various conditions of the tide, wave and current and so were superseded by underwater tank linkspans that through compressed air can be adjusted for ferry ramp height and often need no adjustment for tidal height. The aim of all this is to have the linkspan at roughly the same height above the water as that of the car deck on whichever ferry happens to be docking at the time. All that is then needed is for a ramp (usually on the vessel) to be lowered, bridging the gap between the ferry and the linkspan. In ports such as Dover a Marine Development "double deck" linkspan can be found where two decks of a large ferry can be loaded simultaneously. Linkspans can also be used for passenger walkways.
Variants
Train ferry
To ensure that the rail tracks on the train ferry or car float and the linkspan align precisely it is necessary for the ship to have a ledge at its stern onto which the linkspan is rested. To be certain that the rail tracks do not have a step at the junction of ship and linkspan, this ledge or shelf must be of a depth the same as that of the end of the linkspan. It is also fitted with a locating pin that ensures the linkspan is in the exact athwart ships (sideways) position. To protect the linkspan from impact as the ship makes its final approach, stern fenders are positioned in front of it. These absorb the energy of the ferry's impact, guide its stern and hold it from moving sideways when finally berthed. These guide fenders also prevent excessive loads being transferred to the locating pin. As the trains roll onto or off the ship its freeboard and trim will change significantly. The linkspan moving with the ship provides acceptable gradients which for railway traffic should not exceed 1:25 (4%). This relatively shallow gradient limited the areas where train ferries could operate. Where the tide is only 2 meters (6.56 ft) for example the linkspan must have a length of at least 50 meters (164 ft). For any greater tide, the linkspan must be very long; other problems also arise which can be very costly to solve. Rail linkspans are generally supported at their outer end by counterweights. This means that when the linkspan is lowered onto the ship's ledge only a small proportion of its weight rests there. However half of the weight of the train on the linkspan is transferred to the ledge. When it becomes necessary to make longer linkspans to accommodate a greater tide range the train loads become proportionately higher until a critical reaction is reached. Before this point is reached, it is usual to create a second span with this inner span being adjusted at its outer end, where it is hinged to the outer span. Rail ferries must not only have the correct rail alignment, but their stern configuration and beam must be an exact fit for the berth it is to use.
General purpose Those linkspans designed originally for train ferries were therefore very restricting for the new general-purpose ferries. Dover, which was one of the earliest tidal rail ferry ports, continued to adopt the “precise fit” approach so that road vehicular ferries had to have the exact beam to fit a berth. Their bow and stern configuration also had to conform to fit with the guide fenders to allow the vessel to “nest” into them. At the bow it was necessary to fit a “moustache” which is a steel structure projecting from the stem. Such ships have neither a support ledge nor drawbridge ramps: the link across the gap between ship and linkspan is bridged by flaps about 2–2.5 m (6.6–8.2 ft) long. When stowed these flaps stow vertically to the end of the linkspan and in so doing prevent a ramped vessel lowering its ramp. Most of the other tidal rail-ferry ports initially adopted this arrangement in the English Channel, North Sea and Irish Sea routes but have now moved away to the more flexible arrangement described below. Dover/Calais route, one of the busiest in the world, still require that vessels using these ports are configured to suit the restraints of each berth, in doing so this limits them from being used in service elsewhere.
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