What's my line? Depends on the job
5/25/2004 5:55:59 PM by
A proper knot isn’t much good if it’s tied in a line that won’t handle the job at hand. Polypropylene lines have a lot going for them: They’re among the least expensive available; they have low stretch characteristics and tremendous strength for their weight; they show virtually no deterioration after long periods in salt water or exposure to gasoline or diesel fuel; and they float.
But there’s a flip side: they bind on a winch and won’t ease out smoothly over the metal winch drum; they’re hard to coil smoothly; they tend to hackle (form small, sharp bends) and jam in blocks (pulleys); and their surface gets rough after prolonged use and will take the skin off hands better than sandpaper. These drawbacks make polypropylene virtually useless for jib sheets. But the positive aspects make this line the best water-skiing tow rope you can find. The floating property makes it less likely the line will be fouled in the propeller while maneuvering around a downed skier, and because the boat operator and skier don’t have to handle the line under a heavy load, it won’t harm their hands.
Nylon is the line of choice for anchor and permanent mooring lines and tow lines. It has a lot of elasticity, which lets the line take up a lot of the shock when the boat tugs at its anchor in a strong sea or a tow line comes taut. Aone-inch nylon line, the size on which we’ll base all comparisons, has a breaking strain of about 30,000 pounds, but all lines use a working strength of 20 percent of the breaking strain. Thus, a one-inch nylon line has a working load of about 6,000 pounds when dry. But nylon loses about 10 percent of its strength when wet, so a safe working load for that line would be about 5,000 pounds. Because it stretches so much, it isn’t much good for jib sheets and other rigging lines on which steady tension must be maintained, and its smooth surface gets softer with use, making it relatively easy on hands. A100-foot coil of one-inch nylon line weighs about 25 pounds.
Polypropylene and the similar polyethylene lines have a breaking strain of about 13,000 pounds and a working load of 2,000. The strength of polyethylene increases when it is wet. Both types are rough on hands, but because they float, they make good lines for man-overboard floats and life rings, and for towing dinghies behind boats. These are also the lightest lines available, a 100-foot coil of one-inch line weighs less than 20 pounds.
Dacron lines, often called polyester, have a breaking strength of 22,000 pounds and a working load of 4,000. They are probably the most commonly used lines for jib and main sheets, halyards and other control lines, because they, too, become soft with use and are very easy on hands. They also are extremely strong, with a breaking strain strength of 30,000 pounds. But because it does not lose strength when wet, a polyester line has a working load of 6,000 pounds. It has an elasticity of less than 5 percent, another plus for uses such as running rigging, and is almost impervious to chemicals, sunlight and marine organisms. It weighs about 32 pounds per 100 feet.
The newest line to reach the yachting market is Kevlar, the man-made fiber that is used in sheet form to make sails, bulletproof vests and radial tires. It is enormously strong, but also enormously expensive, about four times as expensive as Dacron. Its strength makes it the line of choice for halyards and sheets on many ocean racers. Aone-inch Kevlar line has a breaking strength of 70,000 pounds, 50 percent greater than steel, and it has a working load of 14,000 pounds. Its stretch rate is almost zero, and while a one-inch Kevlar line weighs about the same per 100 feet as Dacron, its strength allows much smaller diameters to be used for the same job.
Its major drawback, in addition to its cost, is its tendency to wear out quickly.
Natural fiber lines, such as manila, were standard until after
World War II. They’re still available, but their drawbacks are so great
they generally aren’t worth using. Manila costs about one-third the
price of nylon, but it has only one-third the breaking strain (although
it does gain about 20 percent in strength when wet.) This means much
thicker lines must be used to the same application, effectively
negating any cost saving.
In addition, manila breaks down so quickly because of rot and marine organisms, and is so vulnerable to deterioration from exposure to common chemicals such as gasoline, that it must be replaced two or three times faster than synthetics.