Can Nylon Anchor Rope be Melted by a Severe Cycling Load ? Myth Busted for Ropes Less Than 1-inch

2014, 2016, Rev. 7-28-2024

It's been reported that nylon sea anchor and ground anchor ropes can fail due to heat build-up when repeatedly cycled to a high percentage of rated breaking strength. True? Let's try a few reasonableness checks. We'll start with climbing ropes, since these are tested for energy absorption and the results are well known.

In a UIAA (the governing authority for climbing gear) test fall 2.5 meters of rope arrests an 80 kg object falling about 5.6 meters, stretch included. This is an impact so severe that the rope is only expected to survive 5-15 cycles; I doubt any anchor rope would do as well, so this is very conservative. if we convert this to US units:

Energy = 176 pounds * 18.3 feet fall/8.2 feet rope = 393 ft-pounds/foot of rope

Expressed as heat. However, only energy dissipated by hysteresis (it takes more energy to stretch a rope than it returns--it is not a perfect spring) is converted into heat. After all, a metal spring does not heat, true? Hysterisis with nylon rope is about generally about 10-20%. We'll assume the worst.

0.20 x 392 ft-pounds/ft = 0.10 BTU/ft

How much heat needs to be lost? If the rope is cycling at more than 50% of this load it won't last long for many reasons, so I will assume 0.05 BTU-cycle as the limit. This corresponds to the SWL being about 20% of the ML, which agrees with typical storm anchor and US Sailing anchor rope calculations. Thisis beyon good design, which holdds that nylon should have a 12:1 safety factor, or WLL = 8% BS.

How many cycles? Assuming we are taking about storm waves, 20 second period seems reasonable, or 180 cycles per hour.

Heat = 180 cycles/hour * 0.05 BTU/cycle = 9 BTU/hr*ft

How much strength does a hot rope lose (PA66 is nylon 6/6)?

About 18% by 80C (176F). Curiously, since this heat will keep the rope dry, it really hasn't lost any strength compared to a wet rope, so we will consider this the no-loss-in-strength temperature. There is a reason clothes don't fall apart in the drier! Long-term, there are oxidation effects, but these take months.


How fast can a dry rope loose heat? Assuming strong winds, about 6-8 BTU/ft2-F, depending on the reference. Assuming there is some spray in the air, we will use the higher number.

Heat loss = 0.11ft2 area/ft * 8 BTU/ft2 * (90-80) = 9 BTU

Clearly the rope won't get that hot. In fact it will top out at about 10F above ambient. Noticeably warm, but not in any danger.

What if the rope were larger (3/4-inch is what the Dashew's reported failing), of a less efficient construction (3-strand), and operated at a higher load factor (30%?)? The surface area to core ratios is triple, the heat generation per cycle is 160% greater, and the rope generates about 20% more heat due to the construction difference. What if the boat were tied to a dock and the period was much shorter? The core temperature gain can reach about 140For 60C--not in the danger zone. Isolated fiber bundles could get hotter, if the load is not evenly carried or if there is significant friction between the fibers in that location.

Below 3/4-inch rope heating due to cycling is probably not an important factor, even in the worst hurricane docking situation; failure will be due to something else. Beginning at 1-inch moving upwards, it can be important, since the larger rope cannot cool as easily. The myth seems plausible. But not in sea anchor applications; the period is too low. The rope would need to be ~ 2 inches in diameter to provide sufficient insulation.

What about heating under chafing gear? 

• If the gear is waterproof, that prevents both water cooling and reduces the internal lubrication that water provides. Bad.
• The gear provides insulation, like an over coat on the line. Thus, a 1/2-inch line is going to heat like a 3/4-inch line, and a 3/4-inch line like a 1 1/4-inch line. Bad.

Thus, any chafing gear that keep the line dry will dramatically weaken any line subject to hard cycling for a long period, such as a hurricane or nor'easter.. Only permeable gear, such as webbing, is acceptable.

Clearly, the larger the boat and the larger the gear, rope heating becomes important, gear sizing becomes important, and good anti-chafe gear--not hose but hollow webbing that will allow the rope to stay wet--becomes important. Boats tied to a dock in a hurricane clearly need to oversize the lines or double them, provide for chafe, and make them as long as possible (lower cycle factor).

But the truth is that nearly all of the failures have NOTHING TO DO  with heating and everything to do with operating way above the fatigue limit. Nylon rope looks melted even when broken at very low speeds and cycles; it is energy release at the moment of failure. When nylon rope operates above its fatigue limit it doesn't last long anyway.

This is very load speed break testing, yet the ends are melted. I think folks just don't understand what they are looking at when they claim mysterious heating. One sailor reported a false observation and it became internet fact.

___________________

Note: I've greatly over-simplified the engineering. Insulation from the rope fibers and the cylindrical coordinates need to be considered. However, the result was similar, about 20% higher. On the other hand, we've assumed that no spray is striking the rope (it remains dry) which seems VERY unlikely in storm conditions.


Note: Ropes that are pulled to failure in once cycle in the laboratory cycle generally appear slightly melted. At the moment of the failure the energy release where the fibers part is great and the ends fuse. This does not mean the rope was overheated in use. This may be in part what people have seen.

An interesting discussion by Steve Dashew. Heating, polyester vs. nylon, and lightweight snubbers. Steve reported rodes breaking at about 45-50% of the rated strength, which seems plausible since the rated strength is not for repeated cycling. I believe that. However, I also wonder how they could have been sure of the load in hurricane conditions! I am not convinced heating was a factor; the rope was probably wet and the cycles were too low to generate heat. What he saw was normal high load rupture melting.

An exhaustive report by the US Coast Guard goes deeply into synthetic moorings. It's a big deal for deep water aids to navigation (ATNs).