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What are those disks sometimes seen on mooring lines of big ships?

Posted August 28, 2018

If you ever pass by a sea port, take a look at the ships. They are fixed to the port structures with thick mooring lines. Even if these ropes don‘t look fit for the task, they are very strong. Sometimes you may notice big metals disks on the mooring lines, but can you guess what is their function? Why ships for several hundred years are using this strange invention that is seemingly useless?

Usually rat guards are spinning freely on the mooring lines, but not always. Image credit: U.S. Navy photo via Wikimedia

These disks can be noticed on military and civilian ships, on cruise ships and container ships, on small tug boats and huge tankers. These disks are installed immediately after the ship is moored – you cannot postpone this work. They usually spin freely around the rope and have smooth edged to prevent cuts. If the crew of the ship would forget to install these devices, the entire ship could be in danger and the risk of huge problems would be greatly enhanced. So what are they?

A big central hole or inadequate positioning allows rats to pass the rat guard. Image credit: Dornum72 via Wikimedia (CC BY-SA 3.0)

Those disks are rat guards. Sea ports still have a rodent problem, which has a history of several hundred years. Rats and mice are attracted by heated building, smell of fish, gran and other food. Of course, every port has to take measures to control the rodent population. They are poisoned, hunted and some other methods are used to prevent them from settling down in the territory of the port. However, rats on board of the ship are even more dangerous. The crew has limited resources and they cannot be sacrificed to satisfy the hunger of rats. Furthermore, rats may be interested in chewing on some wires or hoses, which could also be a big safety issue. And then there are hygiene problems and risk of infectious diseases. To sum it up, rats are not welcomed on-board. But how do you prevent them from coming?

Not all rat guards are shaped like a disk. Image credit: U.S. Navy photo via Wikimedia

Well, mooring lines are the only thing spanning the distance between the port territory and the actual ship. Sure, rats can swim, but there is virtually no way for a rat to climb up a steep, wet steel hull. But mooring lines provide enough grip and are quite a comfortable way for rats to climb into the ship. That is why these rat guards are used. These disks typically spin freely around the rope, which means that rat will fall off attempting to climb over them. Sometimes additional measures, such as slippery plastic sleeves, are used. The most important features are a small central hole, no wide slots and proper positioning. Rats can jump, so you don’t want to fit your rat guards too close to the moor or the hull.

Of course, sometimes rats still find their way into the ship and must be exterminated. That can be done using traps when the ship is in the sea. But once it reaches a port, rodent control companies can be hired to deal with the rat problem.

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