they can break and when they do , people get cut in half
there is video somewhere of lines breaking, both nylon and hemp , they have different characteristics when they break, but both can be deadly
this is what they taught us in the navy, to be line handlers
As the line stretches, it will reduce its diameter by as much as 30 to 40 percent. Then the tow surges forward towing actually consists of pulling by jerks. Then the cycle starts over. The greatest danger in using synthetic fiber towlines is that if the line should part when under strain, it will snap back its full length like a bull whip. The force of the snapback is tremendous depending on the strain that the line was under at the time it parted. There is no set pattern on how the line will whip back. It may snap back directly on itself or it may whip from side to side. There is no way to tell what it will do. If you see a synthetic fiber line under strain parting or beginning to part--DO NOT RUN--just fall flat down on the deck.
http://www.globalsecurity.org/military/library/policy/army/fm/55-501/chap19.htm
as the throttleman on the submarine, we had to be sure to not spin the prop more than 10 tpm when warming up or maintaining the propulsion turbines before departure. a couple times when i did it , i put way on the ship, meaning i exceeded the 10 rpm limit and made the ship rock against the lines
our boat had One S8G nuclear reactor two geared steam turbines, one shaft, output of 60,000 hp , that is enough to snap mooring lines
this has some analysis
http://www.tensiontech.com/papers/papers/optimoor/optimoor.html
towing situations with tugs was more dangerous than while completely tied up to the pier though