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NAUTICAL KNOW-HOW: The Lowdown on Lines
It's fall, and you're getting ready for last-chance autumn cruising. In looking over your boat's gear, you notice that your dock lines are showing signs of chafe. Not wanting to take any chances, you head to the local chandlery, where you are met with a wall full of rope. What should you know? The information on available options could fill a dissertation, but the following should help you get started.
Materials There was a time when all line was made of cotton or hemp. The problem was that these materials stretched over time, absorbed water and disinte-grated in sunlight. For years boaters combated these prob-lems by coating lines with tar. Then in 1935 a DuPont scientist named Wallace Hume Carothers developed nylon, which brought about a revolution in the industry. Suddenly there was a fiber that was not only stronger, but would not rot, was more resistant to chemicals and didn't disintegrate in the sun. Nylon's drawback was that it melted at relatively low temperatures, so Carothers and his team kept working, eventually developing Dacron (polyethylene terephthalate) in 1941. Since that time the number of synthetics used in ropes has mushroomed, with stronger, higher-tensile materials coming along, including polypropylene, polyesters, high-modulus polyethylene (Dyneema and Spectra), liquid crystal polymer (Vectran), aramids (Twaron and Technora) and PBO (Zylon).
For most recreational uses, nylon and polyester work fine. Nylon is strong, with excellent energy absorption and good UV resistance. It is best for dock lines, anchor rodes and other applications where it must absorb dynamic shock loads. It should be noted, however, that nylon loses some tensile strength when wet. Polyester is also strong and UV-resistant, with better abrasion resistance and less water absorption, but with lower stretch than nylon. In general, polyester is best for running-rigging applications and for lines that primarily handle static loads.
The more exotic materials are mostly used on high-end racing sailboats. They can be very strong, reduce weight aloft and have very little stretch, but they are commensurately expensive. High-modulus polyethylene (HMPE) is stronger and lasts longer than polyester or nylon; it also floats. It is best for high-load applications. Liquid crystal polymer withstands high temperatures, but more importantly for sailors, does not creep, even under very high loads. Aramids have the highest heat resistance and are best for lower load, low-creep applications. Finally, PBO has the highest strength and lowest stretch of any commercially available fiber, but it does not do terribly well in water or sun. This fiber is now used instead of wire or rod for standing rigging in applications where light weight aloft is essential. However, it must be very well protected, either by a coating or a cover.
Polypropylene is light and floats, but it is not terribly strong and, if left untreated, has poor UV resistance. It's usually used where high strength is not necessary, for example, to mark swim areas or as a cover fiber where it's abrasiveness gives it a good hand.
In fact, many ropes are made of a combination of fibers—often a low-stretch core will be surrounded by a more-durable protective cover.
Construction Rope manufacturers are tasked with making fibers into rope. The fibers are extruded and spun into yarns in different directions (S equals clockwise and Z equals counterclockwise twists), then put on spools. This allows the twist of the yarn to align with the twist or braid of the rope. Twisted fibers aligned with the longitudinal direction of the rope will maximize abrasion resistance and tensile strength. The spools are then placed on machines, which wind the strands onto bobbins for rope laying or braiding—all while maintaining uniform strand tension.
In three-strand twisted rope the yarns are wound into strands of varying thickness. These strands are then wound around each other. The direction of this twist is called the lay of the rope. Right-laid is twisted clockwise, and left-laid is twisted counterclockwise. This is the loosest construction method and therefore has the greatest stretch and is the easiest to splice. It is available in soft, medium and hard lay, based on how tightly wound it is. A hard-lay three-strand is best for mooring lines, because it better resists untwisting. Looser lays are better for anchor and dock lines, because they can better absorb shock loads. Twisted rope is less expensive, because it runs much more quickly through the manufacturing process than braids. The drawbacks are that it may not have a great hand, runs less well through blocks and is subject to torque.
The other option is to braid the rope. In this process, the bobbins carrying the yarns weave around each other as on a maypole, creating a braid. This rope generally has less stretch, a better hand, runs better through blocks and is stronger per pound of fiber. Chafe sleeves or coatings may be required to prevent snagging on dock pilings. Braids are also more difficult to splice.
There are several different types of braided rope. The simplest is single-braid, where one set of twisted strands is braided into a hollow construction. Single-braids tend to be most flexible and have a good hand. New England Ropes' Regatta Braid, a popular sheet line, is an example of a 12-strand single-braid. A single-braid core can have a cover braided over it, creating a double-braid, which is probably the most popular construction among recreational boaters. Double-braids tend to be somewhat stiffer, resist flattening and are pleasant to hold. The cover protects the core, making it stronger and more durable.
Another type of line, parallel-core construction, is predicated on the fact that no matter what the fiber, bending it reduces its strength. The core yarns are kept straight or slightly twisted then a cover is braided around them. These constructions often have a very stiff feel and can be difficult to splice.
Paraloc uses a relatively new patented process that braids the cover and the core yarns together, creating a rope that has very little stretch or creep and runs well through blocks and stoppers. However, it is expensive, stiffer than traditional constructions and difficult to splice neatly.
Finishing Techniques Some manufacturers pre-stretch their ropes to minimize stretch and creep under load. Others, like New England Ropes, use heat and pressure to relax the fibers, making them more flexible and less likely to stretch.
Novatec uses UV-stabilized marine-finished polyester and nylon yarns to improve the rope's wet abrasion resistance. For colored ropes, only solution-dyed yarns should be used. Urethane coatings can also be applied to increase abrasion resistance and minimize water absorption.
A Word About Diameters Even before you decide on material and braid, it is a good idea to know what diameter line you are looking for. For some applications this is easy: Choose the diameter that best fits the smallest block through which the line will be running. For others there is more leeway. Dock lines, for example, need to be thick enough that they will hold your boat in a blow and are easy to throw and catch. But they should not be so thick that they don't fit securely on your boat's cleats.
Maintenance A few simple things will greatly extend the usefulness of your lines. First, it is a good idea to occasionally rinse all lines with fresh water. Avoid using detergents, as they could damage the fiber's protective coatings. Second, try to store them, coiled neatly, where they will be exposed to air, but protected from sunlight, water and chemicals.
Back to the Dock-Line Question Based on the information above, you're looking for a relatively inexpensive line made from solution-dyed, UV-stabilized and marine-finished yarns. It should have high energy absorption (recoverable stretch), sufficient tensile strength and twist-aligned cover strands. Nylon three-strand and double braid are the best options for this application. Three-strand is less expensive, has high stretch and, with a medium lay, may be more chafe- and snag-resistant. Double-braid is stronger, more UV-resistant, easier to handle and comes in a variety of colors. Cut the right length, put an eye splice in at least one end and you're ready for the rest of the season—and many more.
About the Author
By Gaelen Adam, contributing writer for Chesapeake Bay Magazine. For more great articles and photos on boating, sailing, fishing, and cruising, visit http://www.ChesapeakeBoating.net
