Every knot you tie makes your rope weaker
Tie a knot in a rope and pull until something gives: the break almost always comes at the knot or right beside it. The bends and pinches that hold a knot together stress the fibres unevenly, so common knots keep only 40 to 80 percent of the rope's strength, and critical ropes are rated with safety factors as high as 15 to 1.
A knot is simply a deliberate complication in a cord, practical or decorative or both. Knot tyers sort the useful ones by job. A hitch ties a rope to something else, such as a post or ring. A bend joins two rope ends. A loop knot makes a closed circle, and a splice weaves strands together. In the narrowest sense, a knot is just a stopper, a knob at the end that keeps a rope from sliding through a hole. Many are ancient: the bowline and the clove hitch have been tied for millennia, while the sheepshank dates from 1627 and the Western Union splice arrived with the telegraph.
Measuring how much a knot weakens rope is surprisingly hard. The result, called knot efficiency, shifts with the fibre, the rope's construction and thickness, whether it is wet, how neatly the knot was dressed, and how fast or often it is loaded. Researchers are even developing fibres that change colour under strain to watch the stresses build. Splices can keep nearly all of a rope's strength, but ordinary knots are far quicker, so careful users simply build in a big safety margin.
A rope that does not snap can still let you down. Under load, the working end can creep back through the knot until it unravels, especially if the load keeps coming and going. Some knots capsize, flipping into a different shape that may hold far less; others are deliberately capsized as a quick way of tying them. At the other extreme, a water knot can jam so tightly that it is miserable to untie, while a Zeppelin bend comes apart easily after heavy loading.
That is why knots are passed on hands-on by sailors, climbers, cavers, arborists, rescuers, stagehands and surgeons. A trucker's hitch gives mechanical advantage for lashing a load, a bowline can serve as a rescue loop, and in mountains the right knots let a team lower a rescuer into a ravine and haul two people back out. Macramé builds entire fabrics from nothing but knots, and Chinese decorative knotting has been a folk art since the Tang and Song dynasties.
Source: Knot
