Busted! - Sail Magazine

Busted!

Sailors have been tying knots for millennia, and no doubt innovators have been trying just as long to invent both stronger rope and better knots. Yet the basic problem still remains: Every rope is weakened when its fibers are bent. Loading a knot with a large amount of weight creates a sheer force on the fibers; given enough force, the fibers break and the rope fails. Today’s
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Sailors have been tying knots for millennia, and no doubt innovators have been trying just as long to invent both stronger rope and better knots. Yet the basic problem still remains: Every rope is weakened when its fibers are bent. Loading a knot with a large amount of weight creates a sheer force on the fibers; given enough force, the fibers break and the rope fails.

Today’s double-braid polyester and nylon ropes tolerate sheer forces well, but ropes made of modern high-modulus fibers do not. While these fibers are stronger than steel (pound for pound), they cannot handle the forces created when a knot constricts on itself. With knots tied in them, high-modulus ropes are prone to break at loads far below their rated pull strength. Thus, unless these ropes are spliced instead of knotted, they have difficulty handling the loads generated by boats with powerful sailplans.

We tied five commonly used knots (the bowline, the figure 8, the round turn and two half hitches, the clove hitch with two half hitches, and the double fisherman’s knot) in three types of rope (1/2-inch double-braid nylon; 1/2-inch Sta-Set double-braid polyester; and 1/2-inch Endura Braid, a line with a 100 percent Spectra core and a polyester sheath) to determine exactly how much each rope’s strength is depleted by these knots. Pull tests were conducted over a course of two days at New England Ropes’s facility in Fall River, Massachusetts. We used a vertical-tensile tester, with constant rate of travel of one foot per minute and a maximum pulling force of 100,000 pounds. One end of the rope was captured by wrapping it around a 9-inch-diameter drum and then cleating it off, while the other end was tied to the machine’s business end with one of the test knots. None of the ropes were precycled (prestretched) prior to the pull test, as our intention was to see exactly how much force was required either for the knot to slip or for the rope to fail. Each knot was tested three times on each of the three ropes; the averages of the results are reported here. I tied each knot to reduce the statistical possibility that a different pair of hands might introduce a bias into the test.

Since the goal was to determine how much of a rope’s strength is lost with each type of knot tied in it, we recorded how much force was required for the knot to slip or for the rope to fail and then calculated what percentage of the rope’s strength this represented. We also tested the ropes using splices instead of knots to determine how much of each rope’s strength is preserved by splicing rather than knotting it. (Note: The ends of the spliced ropes were not whipped or heat-sealed.) New England Ropes supplied the actual total strength of each rope (our control group) based on pull tests they conducted previously; in these tests both ends of the ropes were wound around drums, secured with cleats, and pulled by the vertical-tensile tester until they failed. Here’s what we found.

1/2-inch double-braid nylon is a dynamic rope (it elongates under load) and is commonly used as docking line or for towing a dinghy. Its rated breaking strength is 10,800 pounds.

Bowline: 5,983 pounds

Percentage of rope’s strength lost: 44.6%

Clove hitch with two half hitches: 7,021 pounds

Percentage of rope’s strength lost: 35%

Round turn and two half hitches: 5,148 pounds

NOTE: This knot rolled open in all three tests before the rope snapped.

Percentage lost before the knot slipped: 52.33%

Figure 8: 6,227 pounds

Percentage of rope’s strength lost: 42.33%

Double fisherman’s knot: 5,820 pounds

Percentage of rope’s strength lost: 46.1%

Double-braid splice: 8,384 pounds

Percentage of rope’s strength lost:

Sta-Set (double-braid polyester) is a static rope that is commonly used for sheets and halyards and as general-purpose cord; 1/2-inch Sta-Set’s rated breaking strength is 10,200 pounds.

Bowline: 5,617 pounds

Percentage of rope’s strength lost: 44.9%

Clove hitch with two half hitches: 6,471 pounds

Percentage of rope’s strength lost: 36.6%

Round turn and two half hitches: 6,492 pounds

NOTE: This knot rolled open in all three tests before the rope snapped.

Percentage lost before the knot slipped: 36.37%

Figure 8: 5,352 pounds

Percentage of rope’s strength lost: 47.5%

Double fisherman’s knot: 5,250 pounds

Percentage of rope’s strength lost: 48.5%

Double-braid splice: 8,934 pounds

Percentage of rope’s strength lost: 12.4%

Endura braid (rope with a 100-percent Spectra core and a polyester cover) is a static rope used for sheets and halyards; 1/2-inch Endura Braid’s rated breaking strength is 19,300 pounds.

Bowline: 7,855 pounds

Percentage of rope’s strength lost: 59.33%

Clove hitch with two half hitches: 7,448 pounds

Percentage of rope’s strength lost: 61.44%

Round turn and two half hitches: 6,166 pounds

NOTE: This knot rolled open in all three tests before the rope snapped.
Percentage lost before the knot slipped: 68%

Figure 8: 7,713 pounds

Percentage of rope’s strength lost: 60%

Double fisherman’s knot: 6,268 pounds

Percentage of rope’s strength lost: 67.5%

Core-to-core splice: 20,594 pounds

Percentage of rope’s strength lost: 0%

Conclusions

As expected, tying a knot in any fiber substantially reduces its strength, so knowing how much of a rope’s strength is preserved by a particular knot is an important factor in selecting both a knot and rope for a particular job. The percentage of total strength preserved by each knot is similar when comparing the double-braid nylon and the Sta-Set (for example, a bowline tied in double-braid nylon preserved 55.4 percent of the rope’s strength, while a bowline tied in Sta-Set preserved 55.1 percent), but was significantly different for the Endura Braid (a bowline tied in Endura Braid preserved only 40.67 percent). Interestingly, each of the five knots had consistent strength-preservation percentages across the range of rope types, with the bowline, figure 8, and clove hitch with two half hitches emerging as the three strongest knots in this evaluation.

Conversely, splices made in the double-braid nylon and Sta-Set preserved 77.63 percent and 87.6 percent of total breaking strength, respectively, while spliced Endura braid preserved a whopping 106.73 percent of its total strength. As these tests show, it is imperative to splice, rather than knot, exotic-fiber ropes, regardless of the application. For instance, if a genoa sheet breaks, you can count on a prompt repeat failure if you replace the busted sheet and attach the new sheet with a bowline. While this might work in a light-air emergency, understand that the sheet, which is rated to 19,300 pounds, will fail at loads that are roughly 60 percent less than the brochure states.

Many thanks to Jeff Guiggey and Dave Goncalves of New England Ropes for helping conduct these tests. And special thanks to New England Ropes for use of their facility and for donating a generous number of ropes, all of which are now broken.

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