Rope Structure and Construction
There are many different ways of making ropes. Depending on the type of construction, different characteristics can be introduced to the finished rope
Twisted or Laid Rope
Traditionally ropes were twisted or laid. A rope maker would have a rope walk sometimes twice as long as the finished rope was to be. The fibres would be laid out the length off the walk and twisted. Each strand would be twisted tightly then joined and the twist of the strands would coil the rope. Rope can be made of 2 strands but we mostly find 3 strand ropes. Rope length was limited to the length of the rope walk and larger or longer ropes were difficult and bulky to make. Fibres were first made into threads or yarn and then these would be further twisted into strands. These strands made up the rope.
On board ship, old rope that was damaged but not rotten was picked apart and the fibres reused. Various finishes were applied to rope to preserve it and keep it from sun, rot, and water. Old tarring recipes abound.
Machines were developed first to replicated the manual process, limiting the length to the length of the rope walk. Later machines were developed that could make laid rope continuously. Still later braiding machines were developed.
Laid rope is a stretchy material with a hard bumpy finish. Laid rope does not flow through block and hold in cleats as well as braided ropes. It can also be harder on the hands.
In old days, the grooves in laid ropes were sometimes filled with smaller ropes. This made a smoother better running rope that could be finished and waterproofed more easily.
Because mooring and anchor lines benefit from the increased stretch, laid line is still available. Sometimes only the core is laid and the outside layer is braided, this is sometimes found in climbing ropes where some bounce is safer. The outside is often polyester. Braided lines have replaced laid line in rigging and sheets except in traditional styled boats where the look of the laid rope is desirable. Even there the material is often synthetic.
Braided ropes of the same weight and material as laid ropes are usually softer to the hand and more flexible while benefiting from reduced stretch.
Inexpensive polypropylene is sometimes braided with no core but most ropes have a core.
By combining different materials inside and out a rope can be made to take advantage of the properties of different materials. For example polyester is a stable UV resistant material that holds knots well and is abrasion resistant. It is often used as a covering braid for other materials such as Ultra High Molecular Weight Poly Ethylene UHMWPE which is very strong and light, but is notoriously slippery, and prone to creeping. By using a combination UHMWPE core and polyester covering the best of both materials is incorporated to produce a rope that is strong and light, but knots better and is less prone to creeping than if the UHMWPE is alone
Similarly Kevlar and Twaron's poor UV resistance can be improved and its amazing strength and low stretch use, (although rarely used on boats except as standing rigging in high end racers because of its sensitivity to shock and poor flex/fatigue resistance)
When kevlar ropes are used, the blocks are larger diameter to minimize flex. The kevlar is also braided more loosely to minimize the difference between outside of the rope and inside, and thus reduce strain on the rope.
Sometimes a coating will be applied between the outside cover and inside core to improve the cohesiveness of the rope and prevent twisting or bunching of the core. This happens in really slippery cores. Sometimes an intermediate core is inserted between the inner and outer layers for the same reason.
Ropes can be braided in many different ways and the size of the core can be larger or smaller. In this way the features can be adjusted. It is also a way that rope makers can offer a rope with a specific content and save by making the expensive inner core content smaller. Note that the strength of a rope can vary tremendously depending on the relative size of the core to the outer covering. 2 ropes of exactly the same outer size and both having a polyester and UHMWPE content can be very different in strength if the UHMWPE core is reduced in the core of one of the ropes.
When buying a rope it's important to look at the core as well as the cover. Cheaper ropes will have cheaper cores even if they look on the outside like more expensive ropes.
The number of strands used in braiding has a direct effect on how the rope feels. 8 strands is stiffer than the more common 16.
Parallel Strand Kevlar Rope
In order to reduce stretching and to benefit from the full strength of Kevlar, ropes are sometimes made of long roving with no twisting or braiding.
To keep the rope together it is then coated by protective material. This is the case of Kevlar ropes designed to be high strength stays or guy wires.
How strong does a rope need to be?
It is very complicated to calculate how strong a rope has to be, and much research has been devoted to this. The regular loads are not so hard. The sudden extra loads are trickier. How strong does a rope have to be to stop a falling climber, an unexpected jibe or a boom dragging in the water? How about racing boat stays? What about mooring lines in a storm, shock loads? How about for ropes that are older? or been exposed to chemical attack, sweat, salt water?
One way is to look at the hardware of your boat and choose ropes that have a similar break rating. Keep in mind that ropes are weakened by knots and by misuse and age. Sometimes it's worth taking a calculated risk such as in the case of high end racing where every ounce matters. Most of the time it's not worth risking your safety and your boat and you should stay well within the safety strength recommended by the designers.
Here is an article on the effect of knots on the breaking strength of ropes. It is sobering. David Smidwriting.com It's interesting that splices are much stronger then knots.
email me if you find mistakes, I'll fix them and we'll all benefit: Christine
This information is for general knowledge. If you have a critical application check with supplier. Different manufacturing processes change the feel, appearance, and strength of ropes.