If you've ever been around small boats much, you may have used mooring lines. "Mooring lines" are, to landlubbers "the ropes what you use to tie the boat up." When you go into a marine supply store, you may see single-braided nylon lines and double-braided nylon line, up to maybe 5/8" or so in diameter.
The mooring lines on ships are 5" lines or larger and are usually double-braided. Those lines are phenomenally strong. Six lines, doubled up, will hold a 8,000 ton ship to the pier against the winds and tidal current (unless the winds are howling). But exert enough force on them and they will part.
Usually what happens is that the bow-most line is passed over to the pier, the linehandler on the pier drops the loop at the end of the line over a bollard and the linehanders on the ship make the line fast to the bits on the forecastle. Then either the wind catches the ship wrong or the tugs pull the ship or the Conning Officer rings up an astern bell; all the thousands of horsepower that the tugs or the main engines can exert pull on that line. It stretches waay out and then it snaps.
If you have ever had a small rope snap on you, you may have noted that it tends to whip back along the line of force exerted on the rope. If you were to break a 5" mooring line, it comes back with unbelievable speed and force. Generally, but not always, the line comes back low to the deck and if you are in the way, you can forget about walking on your own legs, as the line will smash both of your legs to the point that if the line didn't amputate them, the doctors will have to.
This happened on one ship almost 30 years ago; the XO of the ship was up by the bullnose of the ship, yelling at someone on the pier, when the mooring line running through the bullnose (#1 line) parted. Both of his legs were shattered and his blood was spattered all over the forecastle.
He survived and later appeared in a training film on snapback. That film had a scene were a number of mannekins were placed by a mooring line that was deliberately parted; the dummies went flying every which way.
Mooring lines were just another item on ships that people worked with and around all the time, but if you didn't respect them, they could kill you.
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