Stern gate

A linkspan or link-span is a type of drawbridge used mainly in the operation of moving vehicles on and off a 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 (or a combination of any) 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.

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.

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these types of linkspans were less well designed for the various conditions of the tide, wave and current and so were superseded by underwater tank linkspans that through compressed air can be adjusted for ferry ramp height and often need no adjustment for tidal height.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 the roughly 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.

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Wikipedia