How Is Rope Made? Friction Doing the Work of Glue

Traditional rope is built in stages, each twisted in the opposite direction to the last. Fibres are twisted one way into a yarn. Yarns are twisted the other way into a strand. Strands are twisted back the first way into the rope. That alternation is the essential trick: each level is trying to untwist, and because it is locked inside a layer twisted the opposite way, the tendency of each to unwind is exactly what holds the next together. The result is stable under tension and will unlay itself if the tension is released and the end is free, which is why rope ends are whipped or sealed. This construction is called laid or twisted rope, and a right-hand laid rope, with the strands spiralling to the right, is by far the most common, which is why sailors traditionally coil rope clockwise, since coiling against the lay puts twist into it. The same principle at a smaller scale is what makes spun thread possible and is the basis of essentially all textile yarn.

Before machinery, making long rope required a long building, because the fibres were laid out at full length and twisted along their whole length at once. A ropewalk could be a quarter of a mile long, and the surviving examples are among the longest buildings of their period. Workers walked backwards paying out hemp fibre from bundles around their waists while a wheel at one end applied twist, and the finished rope was formed by combining strands under controlled tension using a wooden top that moved down the walk as the lay formed. Naval demand drove the scale of this, since a single large sailing warship required tens of miles of cordage of many sizes and needed it replaced regularly, which made rope-making a strategic industry and hemp a strategic crop. Government dockyards ran their own ropewalks precisely because the supply could not be left to chance, and the quality control mark of a yarn run through the rope is the origin of the practice of identifying official cordage.

Synthetic fibres displaced natural ones for most purposes after the 1950s, and each has a distinct profile that decides where it is used:

• •Manila and sisal, natural fibres that grip well and are pleasant to handle, and which rot, lose strength when wet and have been largely superseded
• •Nylon, very strong and notably elastic, which absorbs shock and makes it right for anchoring and climbing and wrong where stretch is dangerous
• •Polyester, strong with much less stretch, resistant to sunlight, which makes it the standard for rigging and general marine use
• •Polypropylene, cheap and buoyant, which floats and is therefore used for rescue lines, and which degrades in ultraviolet light faster than the others
• •High modulus polyethylene and aramid fibres, which are extraordinarily strong for their weight and barely stretch at all, used where steel wire once was
• •Braided constructions, particularly a braided core inside a braided sheath, which do not rotate under load and separate the strength function from the handling and abrasion function

Rope rarely breaks at its rated strength in service, because that figure applies to a new rope pulled straight, and real use is nothing like that. Knots reduce strength substantially, typically by a quarter to a half, because the fibres on the outside of a tight bend carry far more load than those on the inside, and the differences between knots in this respect are well measured and matter. Bending over a small radius does the same thing, which is why a pulley or thimble has a minimum diameter relative to the rope. Abrasion cuts individual fibres, and internal abrasion from grit worked into the rope is invisible from the outside. Ultraviolet light degrades synthetic fibres progressively. Shock loading can generate forces far above the static load. And chemical damage, particularly from acids on nylon, can leave a rope looking sound while being worthless. That combination is why climbing and lifting ropes are retired on age and history rather than on appearance.

Twisting presses short fibres together hard enough that friction stops them sliding, so the load itself is what holds the rope together. Each stage is twisted opposite to the last, so every layer's urge to unwind locks the one inside it. Ropewalks a quarter of a mile long existed because fibres were twisted at full length, and naval demand made the trade strategic. A knot costs a quarter to a half of the rated strength.