“There is no rope on a boat, only line.” This is a fun phrase often heard by new sailors but it is only partly true. Rope is cordage that has not yet been given a job. Once you rig it to do something, it is considered line and usually given a specific name. Any rigging that is easily and often adjusted is called running rigging.
There are many different constructions of rope and they are suited to very different tasks. The first consideration is the use of the line. Are you going to use it for a halyard or a dock line? A jib sheet or a traveler control line? Different uses require different characteristics and often different material and construction.
3-Strand Line is exactly what it sounds like. 3 strands of line, twisted together to make a larger, stronger line. This construction of line usually has a particular “hand” that it lays (direction to coil it) based on the direction of the twist used to construct it.
Single Braid line is one type of line braided together. Most often there are 12 strands braided together to form one larger, stronger line. There is no cover over the braid. What you are seeing is the load-bearing part of the line.
Double Braid line is probably the most common thing you see on your average sailing vessel. It is basically a single braid with a braided cover over it to protect the load bearing part of the line. The cover is often constructed of a different material that is better suited to abrasion resistance and UV protection. That allows the core to be made of materials specifically suited for strength. Double braid line will develop a twist over time if not handled correctly. This is due to the interaction between the cover and the core.
The materials that cordage is built from have many different names and characteristics. The characteristics we are usually most concerned with are stretch, abrasion resistance, UV resistance, and overall load-bearing strength. The use of the line will dictate what characteristics are most important.
Nylon was one of the first synthetic fibers available. It stretches under load, which makes it good for dock lines and tow lines. It also floats.
Polyester is often seen in the cover of double braid lines. It has good abrasion resistance as well as good UV resistance. It streches more than some of the newest exotic lines, but for general cruising applications, you see a polyester double braid for sheets and other lines that are frequently adjusted.
Polypropylene is light, inexpensive stretchy line that is most often used to increase the weight and handling characteristics of a stronger line that could do the same job at a lower diameter. It saves on cost and is often seen in “performance cruising” lines.
High Modulus Polyethylene – This material has several names it goes by depending on the manufacturer and the patent. Most commonly Dyneema or Spectra, this type of line has the lowest stretch and highest strength-to-weight ratio. Because it doesn’t hold knots well, it is often the core, or at least a component of the core, of a double braid line.
Liquid Crystal Polymers like Vectran are now venturing into the highest levels of performance materials. With almost no stretch, Vectran is ideal for upwind sailing halyards and other highly loaded systems. It is most often found in the core, as it’s UV and abrasion resistance is only moderate.
Technora and PBO are exotic materials that are almost solely found in Grand Prix applications. They have amazing strenght characteristics, but often require special handling as water absorption and UV can cause rapid failure.