Checking Rope Strength

Monitoring stretch offers clues to lifespan and ability to absorb shock loads.


We like nylon for docklines, anchor rode, and chain snubbers because it stretches, absorbing jolts that would otherwise be transferred to the boat and ground tackle. But all of that stretching and contracting takes a toll. Fiber wears on fiber. The polymer itself fatigues. Unlike a steel part, which can last basically forever if never stressed beyond its fatigue limit (about 25-35% of the breaking strength), nylon and polyester wear out. Every load cycle has some effect and the severity depends on the load, rate of pull, and range of pull (Does the rope go slack or not?). As a result of the process, the rope becomes permanently elongated, and its elasticity and strength decrease. A certain amount of permanent elongation (4 -6 percent) results from consolidation of the weave, but elongation beyond that indicates fatigue and overloading.

Take the example of a PS reader who reported that his overloaded snubber had stretched 10 percent after a storm. For it to have elongated that far beyond the 4-6 percent range of normal bedding-in, the conventional wisdom says it was very likely loaded beyond its practical working load limit and seriously damaged in the process. Some portion of that 10 percent stretch came at the expense of reduced elasticity. How critical is reduced stretch in anchor rodes? How about snubbers? Can monitoring permanent elongation help identify rope overload damage and predict remaining service life for ropes subjected to moderate overload? We think so.

Any additional permanent elongation after the bedding process is the result of some portion of the fibers being loaded beyond their elastic limit, which may be as low as 35 of the breaking stress. Not all of the fibers share the load equally, and the elastic limit (or yield stress) of nylon fibers is not as well defined as it is for metals. Metals hold up well until very near the elastic limit, beyond which they deform and fail rapidly from fatigue. In nylon rope, this degradation in strength is more gradual, starting at about 25 percent of breaking stress and accelerating rapidly above that (see “Used Nylon Three-Strand Rope Faces The Ultimate Endurance Test,” PS December 2007). More than a few percent excess permanent elongation can indicate that the rope has experienced an extremely high load and its life expectancy has been compromised.

The same can be said of chain. Chain elongates primarily because of corrosion and wear between the links. Though often called chain stretch, this has nothing to do with overload. The extent of stretch due to wear can be determined by measuring a length after a few weeks of service (to smooth off galvanizing roughness), again at a later date, and extrapolating the difference over the length of chain in question. Divide the difference by half the number of links (two contact points), and you will have the average amount of metal lost from each contact point. This can be used to estimate the loss in strength.


To answer some of our questions on elongation, we tested new samples of 3/16-inch Sta-Set polyester double-braid and 3-strand nylon from New England Ropes. We tested different rope weaves because polyester is nearly always used as double-braid, and 3-strand nylon is most common for anchor rode.

We also tested climbing rope in field conditions.


First, we pulled our samples to their recommended working load limits (WLL) and put them through 50-100 cycles. This was to bed the line and to establish a baseline for permanent elongation (PE). We then increased the load factor to overload conditions matching that of a severe storm. (A winter gale can cycle docklines more than 5,500 times at loads far greater than the recommended working load limit.) The polyester line, which has a WLL that is 25 percent of minimum breaking strength (MBS), was pulled to 35 percent of the MBS. The nylon was pulled to about 17 percent of its minimum breaking strength (10% is the recommended WLL). We tested nylon at just 17 percent breaking strength because it has a predicted life of only 630 cycles at 35 percent breaking strength. This is a very short lifespan (see table, “Cycles vs. Stretch”

Also as part of our test, a used climbing rope was marked, used for a season that included many falls, and then re-measured. We do not believe many of the falls in that season exceeded the working load limit.


After about 50-100 cycles at the working load limit, permanent elongation levels out at 3-5 percent, depending on rope construction and fiber. The fibers become more closely fitted, and rope becomes somewhat firmer, although it becomes as limber as new with a little flexing. Elongation is also stable: in nylon rope, there is 2.5 percent elongation at 10 percent breaking strength; in polyester rope, there is 6 percent elongation at 10-percent breaking strength. Little will change in 100,000 cycles at this level. The rope will not get much longer, and the elasticity will not be noticeably reduced. Ropes are expected to last more than 100,000 cycles at the working load limit (more than 1,000,000 for polyester) and will most likely die from UV and chafe before fatigue becomes an issue. Remember that for properly sized ground tackle and snubbers, most of the load cycles while anchored will be far below the WLL. In practice, lines that are not overloaded are known to last a very long time.

We then increased the load factor to match storm conditions that might produce hidden damage. Immediately, permanent elongation began to increase. In the case of polyester, elongation settled at about 4-5 percent when we reached 10 percent of the number of load cycles predicted for its lifespan. Then came our high-load test of each sample to 35 percent of MBS. After this test, the additional permanent elongation in each polyester sample was barely more than the differences in elongation among the different ropes. As a result, we did not regard the test as yielding useful information. It is worth noting that elasticity clearly declined after the higher load cycles, but this is unimportant in many polyester line applications, and is often desirable.

Nylon, on the other hand, continues to elongate when it is overloaded. Elongation of 6 to 7 percent may be considered normal for a line that has seen long service and only moderate overloads. But elongation of 8 to 10 percent indicates chronic overload and a reduced life expectancy. Elongation greater than 10 percent is cause for considerable concern. Elasticity is also reduced, and in the case of docklines, snubbers, and rodes, this results in accelerated damage, because the forces become greater due to loss in shock absorption.

Ropes that have been chronically overloaded have also been weakened. A rope that shows greater than 10 percent permanent elongation will probably break at 35-50 percent of its rated breaking strength.

Checking Rope Strength
These 7/16-inch nylon rope samples were ruined by shock loading at only 20 percent of breaking strength. The top sample, ruptured at a sharp 90 degree bend through a chock on the fourth try, allowing the core to poke through. The lower sample was secured 4 feet away from a 2-inch radius 90-degree bend, which allowed considerable movement when the line stretched. It actually welded itself to the tubular nylon webbing chafe gear that was used to protect it. With a 90-degree bend and with 4 feet of stretch at low frequency (5 second intervals), external friction was about 20 times greater than internal heating.


Elongation and energy absorption capacity under both high and more moderate loads does not decline as quickly as we expected, but the news is still bad for polyester and worse for nylon. After 1,000 cycles of overload cycling (50 percent of breaking strength) polyester retained 65 percent of its elongation capacity at 10 percent load, but its ability to absorb energy through stretch had been cut by 58 percent. Used as a dock line or anchor rode, that means the load on deck hardware will increase about 50 percent when absorbing a wave or gust. Nylon retained 58 percent of its elasticity and only 33 percent of its energy absorption capacity. Expect loads to double on your hardware and rope, further degrading the rope and increasing strain on the entire ground tackle system. At this point, your snubber isn’t really a snubber anymore.

The used climbing rope elongated less than 0.2 percent during a season of regular use. About 20-30 hard falls (fall factor > 0.25) were taken, and over 100 lesser falls. (The fall factor is the ratio of distance fallen to rope length, with about 1.8 being the practical maximum. A 1.8 fall factor drop begins at a point a full rope length above the anchor point, falls past the anchor point, end finishes at a hanging position nearly a full rope length below the anchor point.) Because the falls were within the working load limit of the rope, no significant fatigue damage was done as indicated by permanent elongation, although there was some abrasion.


Ropes also lose elasticity due to environmental factors. UV damage stiffens the outer layers. Wetting and drying cycles precipitate lime scale inside the rope, increasing friction and damaging fibers. Spinning lubes are washed out. The loss in elasticity we describe in this article is in addition to these factors. Fiber rodes typically suffer less UV and scale stiffening, because the exposed sections (should) be varied each time you anchor and because they are well rinsed in use. Snubbers suffer more because they are in the sun and in the spray range constantly.

Checking Rope Strength
Polyester rope has a long life even when cycled at relatively high load factors. Nylon rapidly weakens when cycled above 20 percent of breaking strength (MBS). However, because of nylon’s elasticity, it takes a high load to reach 20 percent MBS.

Snubbers, fiber rodes, and indeed climbing ropes, must be long enough to absorb the impact. Just as diameter is the main determinant of strength, length x diameter determines the capacity for energy absorption, and length determines stress reduction. There has to be room to stretch. We recommend that snubbers be at least 30 feet long, with more being better up to about 60 feet (see “What is the Ideal Snubber Size,” PS March 2016 and “A Snubber and Hook for All Occasions,” PS May 2020). Likewise, the fiber portion of a chain-rope rode should never be less than 50 feet. In shallow water you must anchor using either all-chain with a long snubber, or let out another 50 feet of rope. We know of failures where only 5-15 feet of rope was deployed, and the rope was overloaded by choppy conditions.

Checking Rope Strength
Nylon rope stretches permanently when severely overloaded, and this measurable permanent elongation increases as the nylon experiences load cycles. To ensure your nylon rope is safely below 25 percent of its maximum fatigue life (measured in load cycles), it’s permanent elongation should not exceed 10 percent. After 1,750 cycles at 17 percent of maximum breaking strength (MBS), permanent elongation in nylon rope reaches 10 percent , and the rope should be replaced. This table predicts nylon failure after about 5,000 load cycles at 17 percent of breaking strength. In polyester rope, permanent elongation levels out at loads just above the working load limit (~20 percent MBS), and would be difficult to measure accurately in the field. See the online version of this report for the corresponding table for polyester rope.


So it turns out our reader was correct: 10 percent stretch is evidence of serious overload and incremental line damage. His intuition to replace the stretched-out snubber was dead-on.

We think the main application of permanent elongation monitoring is fiber rodes, snubbers, and docklines. Fiber rodes don’t take the hard hits a snubber does, and they are sized for very long fatigue lives. They are expensive and we don’t like to replace them unnecessarily. But the consequence of failure is severe and a conservative outlook is required. We like to trim the first 20 feet every few years, because of corrosion under the rope-to-chain splice and because of potential for wear on the bottom (remove a few links of corroded chain at the same time). Every three years is not excessive for the occasional cruiser, and more often if you anchor out frequently. Because failure is catastrophic, anything over 8 percent permanent elongation is grounds for retirement, even if it looks good.

Snubbers are intended to stretch, saving the rest of your ground tackle, and ultimately, should be seen as disposable wear items. A fat, strong snubber will protect the windlass and last a long time, but won’t reduce forces on your boat or anchor. A long, thinner, stretchy snubber can reduce rode tension more than half in certain conditions (steep chop or shallow water), but will wear in the process. By monitoring permanent elongation, you can replace it before is snaps in the field. And like any wear item, you should have a spare ready to go, so that replacement is easy and failure is a minor thing. Anything over 10 percent permanent elongation is clear grounds for retirement. Sooner if a big storm is in the offing.

Docklines? Stretch can tell you a lot about how hard the lines were working in a big storm when you weren’t there. If they are stretched more than 10 percent you may want to change your tie-up strategy to reduce forces, and buy new lines at the same time.

We hope this helps with your rope budget and with peace of mind.


1/2-INCH20 degrees13
1/2-INCH45 degrees38
1/2-INCH90 degrees820
1/2-INCH180 degrees1860
Heating ratio is the ratio of external fiber temperatures to internal fiber temperatures taken from “The Dynamic Behavior of Nylon and Polyester Rope under Simulated Towing Conditions,” May 6, 1988. Captain Christopher Toomey, USN.
Observing Rope Condition


Every time a rope rubs against something there is wear, and a diligent sailor is constantly inspecting his lines for chafe. A rough surface is obvious enough, but even smooth surfaces can do damage over time if there is enough pressure. Reduce the turning angle by improving alignment and reducing bends. Reduce movement using low-stretch lines and by moving the anchor point closer to the bend. Make sure tackles are not twisted and lines don’t rub. Mooring lines should not cross, even right at the cleat. Rope clutches and jammers are common sources of abrasion. Don’t release jammers under load—take the pressure off the line with a winch first. Smooth any rough edges and protect ropes with anti-abrasion coatings (“Chafe Protection for Fiber Rodes,” November 2019) or chafe sleeves (“UV, Chafe Protection,” March 2015). Some authorities say the loss in strength is roughly proportional to the cross sectional area lost, but PS testing has shown that is only true for overall wear, not localized damage. Polyester ropes can actually get fatter as they grow fuzzy, and high modulus ropes can lose strength three times faster than the loss in area suggests, depending on the pattern and how many strands are affected; the slippery, non-stretch fibers do not share loads well.

Checking Rope Strength


Double braid ropes can fail inside, hidden from view. Rock climbers attentively slide their fingers along the rope every time they coil it, feeling for the telltale dents and bumps that signal internal damage. Hard loading over a sharp bend, such as a chock or knot, are common causes.


Any rope that seems considerably stiffer than normal, specifically in isolated areas, should be suspected of severe overload or excessive external friction, which resulted in fibers fusing together. The exception is UHMWPE (Amsteel), which can become stiff due to bedding. If the rope does not become supple with a little gentle handling, it is damaged. Alternatively, extreme stiffening could be due to severe UV damage and lime scale accumulation inside the core—same result, the rope is done in.


Is it dirt or mildew? These are generally harmless with synthetics, but wash it off to be sure. Exposed to battery acid? Nylon is totally ruined, since the damage is probably worse on the inside where it lingered. A few drops of paint or a brush mark? Generally harmless, though we mark lines with water-based paint to reduce the risk. Sharpie has been documented to have some minor effect on nylon, but any relationship to actual rope failure is doubted by most testers.


Look for fibers that shed when rubbed with your fingernail. Although the conventional wisdom is that the cover bears the brunt of the damage, we’ve found that with lighter colors the damage can go clear through a 3/8-inch line. The portion of a halyard that lives inside the mast is likely fine, while the bit near the masthead may be severely degraded by UV and chafe.


A line that is heavily loaded while running across even a smooth bend can be externally heated enough by friction to melt on the surface, losing much of its strength on that side. This is common with docklines, occasionally seen on snubbers (see photo page 16). Even rodes can do this, particularly if the anchor roller is jammed. Internal damage is also common, sometimes observable as a lumpiness or stiffness. The broken Kevlar core of the rope in the adjacent photo is indicated by the necking and pliability at the break.

Checking Rope Strength


Machine washing a rope in the first 1-2 years for cosmetic reasons commonly does more harm than good. Instead, wash it on the deck with a brush or in a bucket when you wash the boat. Water will remove the sea salt. Over the years, however, ropes can become stiff from loss of spinning lubes, internal wear, and accumulation of lime scale within the core. The sea salt evaporates leaving behind lime, which rain has difficulty removing. Laundry detergents have chelating agents that help remove lime build up, as does the steady kneading motion, but you can restoring internal lubricants (see “What’s the Best Way to Clean Marine Rope?” PS July 2011 and “Aftermarket Cordage Treatments,” PS December 2011).


Renew splices when they appear worn, or when the thimbles or chain they connect to appear corroded. Trimming the ends back liberally as needed; remove the first 5-10 feet every few years is common, so allow for that when you buy the rode. End-for-ending is another common practice.

Docklines are constantly cycling during use, so they are often much weaker than we think. The ropes broken during our test of old lines broke at 1/3 their rated strength.

Checking Rope Strength

The Arborists Rope Manual produced by Samson Ropes offers helpful guidance on judging the condition of rope. You can find it at this link:

Tracking Rope Stretch

Ropes elongate in three basic ways. The first is elastic elongation, which is divided into a portion that is recovered within seconds, and the remainder which may take several hours to recover. The second is construction elongation. A rope will grow about 4-6 percent as a result of the weave consolidating during the break-in period. About 20-100 cycles to near the working load limit of 10-12 percent are required to fully bed the weave, after which the length becomes stable.

Finally, there is permanent elongation. Over time, the rope will increase in length, as the fibers wear and the polymer itself degrades and stiffens. This process is very, very slow for ropes that are kept within their working load limit, and much more rapid for ropes that are overloaded.

Both construction elongation and permanent elongation reduce elasticity. As the weave becomes set, friction between fibers increases, and the polymer itself becomes harder, the result of molecules and crystals within the fiber becoming more aligned. Because more force is required to stretch the rope the same amount, greater force is generated when absorbing impacts. Shock loads increase and deterioration accelerates.


Mark new docklines, anchor rodes, and snubbers with a pair of hashes or whippings exactly one meter (or some other equivalent known distance—100 cm just makes the math easy). You may want to mark anchor rodes in several places; the first 50 feet of a rode may become “stretched out” while the remaining length is good. We recommend marking at about 10 feet, the midpoint, and the last 10 feet (for when you reverse the rope end-to-end). Mark the rope after just a few nights of use to get a baseline. Measure again after few dozen breezy nights (but not a major storm), after the splices and construction stretch has settled in. Record this as the permanent elongation. It should be about 4-6 percent for nylon rope. Always tension the rope to about 10 pounds during all measurements, just enough to get it straight and eliminate kinks.

How much rope rode do you deploy? If you have minimal chain and typically deploy more than 50 feet of rope, wear should be fairly even. The first 10 feet may scuff the bottom a bit more, but there is enough rope deployed to easily absorb gusts and waves. On the other hand, if you have 50-100 feet of chain and sometimes deploy only a minimal length of rope, less than 20 feet, this short length of rope will be working as a chain snubber and will soon become stretched out and possibly damaged. There isn’t enough chain catenary to absorb shocks and there isn’t enough rope length to absorb the shocks. Our advice is to either anchor on all-chain with a long snubber, or deploy more than 30 feet of rope. We know of folks that have snapped too-short rope anchor rodes when anchored in shallow water and wind and chop came up. It wasn’t cutting on coral or rubbing on the roller. It was the combination of steep chop, too little chain to maintain a catenary, and not enough rope to absorb the shock without overloading.

Drew Frye
Drew Frye, Practical Sailor’s technical editor, has used his background in chemistry and engineering to help guide Practical Sailor toward some of the most important topics covered during the past 10 years. His in-depth reporting on everything from anchors to safety tethers to fuel additives have netted multiple awards from Boating Writers International. With more than three decades of experience as a refinery engineer and a sailor, he has a knack for discovering money-saving “home-brew” products or “hacks” that make boating affordable for almost anyone. He has conducted dozens of tests for Practical Sailor and published over 200 articles on sailing equipment. His rigorous testing has prompted the improvement and introduction of several marine products that might not exist without his input. His book “Rigging Modern Anchors” has won wide praise for introducing the use of modern materials and novel techniques to solve an array of anchoring challenges.