Choosing the line with the amount of elasticity you need
All rope stretches under load on board a boat, but some materials stretch more than others, so you have to be careful to choose the right line for the job.
 |
| Approximate stretch in rope under 30 percent of its breaking load. |
The three most commonly encountered synthetic materials found on boats are nylon, Dacron, and polypropylene. Their uses are as follows:Nylon stretches about 10 to 15 percent under a load equal to 30 percent its breaking strength; therefore, it is used for anchor lines, docklines, and towing lines, where its great strength and stretchability are advantages.Dacron, a polyester fiber, stretches only about 3.5 to 5 percent under the same conditions, so it is used for sheets, halyards, and other applications where stretch would be undesirable. Dacron is known outside North America as Terylene, Duron, Fortrel, A.C.E., and Kodel.Polypropylene has little elasticity, tends not to retain knots, and has only about 60 percent the strength of Dacron, but it’s light and inexpensive and it floats. It’s used for heaving lines, dinghy painters, and water-skiers’ tow-lines.The more exotic ropes made of Kevlar and Spectra stretch about 1 to 2 percent, and even less in some cases. Kevlar is several times stronger than steel, but it’s not durable when bent sharply, so it is not suitable for halyards. Spectra is even stronger than Kevlar and accepts bending. It’s also good at resisting chafe. Its Achilles’ heel is elongation—it tends to “creep” slightly under a heavy sustained load.See also
Dacron; Halyards; Nylon Line; Polypropylene; Rope Strength; Sheet Sizes.
