Collection: Regenerative Materials | 

Series: Natural Ropes | 

Six fibres, what the chemistry says about them, where the received wisdom holds, and where it quietly falls apart

Choosing natural cordage - Starting with a naming problem

The hemp rope department of one large British firm supplied Atkins and Purser with rope for their 1936 seawater immersion trials. When they tested it, the fibre turned out to be manila. Not a blend. Not mislabelled in good faith by someone who might have been confused. Manila, supplied by a department whose job was hemp rope, from people who presumably knew what they were handling.

Atkins and Purser note it without apparent surprise, as though it confirms something they already suspected about how the rope industry worked. Hemp had been the dominant fibre for so long that everything else acquired the name by association. It matters for their trials because the preservatives they were testing adhere differently to different fibres. If you treat what you believe is hemp using a method calibrated for hemp, and it is actually manila, the results are going to be difficult to interpret.

It is a reasonable place to start any serious engagement with natural rope fibres, because it signals immediately that the category people call "natural rope" contains things with meaningfully different chemistry, different mechanical behaviour, and different responses to preservation treatment. The taxonomy matters. So does the gap between what you are told you have and what is actually in your hands. I have opened bags labelled sisal that contained jute, and handled rope sold as manila that was almost certainly a mixed fibre. This is not always commercial dishonesty. Sometimes it is simply that the supply chain is long and the people at the end of it are not botanists. But it means that any investigation into how natural rope fibres behave has to start with working out what you actually have.

What the chemistry says

The Handbook of Fibre Rope Technology gives a table of chemical composition for the main rope fibres that I keep returning to, because it is the most direct bridge between what a fibre looks like and why it behaves the way it does in water.

Manila — properly abaca, from the leaf stalks of Musa textilis, a relative of the banana grown almost exclusively in the Philippines — runs at around 64% cellulose with roughly 22% combined lignin and extractives. Hemp, extracted from the stem of Cannabis sativa, sits at around 77% cellulose with about 13% lignin. Sisal, from Agave sisalana leaves, is similar to hemp in cellulose content with around 15% lignin fraction. Cotton, a seed fibre, is almost entirely cellulose at around 90%, with very low lignin. Coir, from coconut husk, has the most unusual chemistry in the group — lower cellulose than the others, somewhere around 46%, with a lignin fraction approaching 45%.

The numbers that matter most for marine use are the lignin and extractives fractions, because lignin is hydrophobic. It resists water uptake. It slows the rate at which the bacterial cellulase enzymes responsible for rope decay can reach the cellulose they need to break down. On the basis of the table alone, manila should be the most seawater-resistant of the plant cellulose fibres — not as much as coir, which has the highest lignin fraction of all, but significantly more than hemp or sisal, and dramatically more than cotton.

Manila's reputation for seawater tolerance, then, is not simply received wisdom. There is a chemical basis for it. The question the chemistry cannot answer is how long that advantage actually lasts under real conditions — and that is where the trial results get interesting.

Manila — the reputation and what sits beneath it

Pick up a good length of manila and there is something immediately legible about it. The twist is firm but not harsh. The colour is a warm gold, uneven along the strand, with a slight sheen from the natural oils. It does not have the scratchiness of sisal or the dead weight of jute. In the hand it feels like a rope that knows what it is for.

Manila's large fibre lumen — the hollow centre of each individual fibre cell — makes the finished rope more flexible than sisal for equivalent diameter, and contributes to a handling quality that hemp, despite its superior tensile properties, does not quite match. It floats when dry. It splices cleanly. The Handbook rates it as giving excellent knot retention, better than any synthetic fibre tested. Its reputation is not without foundation.

The limits appear when you ask what happens to that natural oil content over time. The Marlinspike Sailor's description of old manila — soft, limp, colour faded from gold to grey — is an accurate picture of a fibre that has lost its extractives through salt cycling and UV exposure. The hydrophobic protection that the lignin and oil fraction provides is a depleting resource, not a permanent property. Once it goes, the advantages narrow considerably.

What I have not done is run a controlled comparison of new and weathered manila against equivalent hemp under the same preservation treatment to see whether the starting advantage produces a measurable difference in treated rope longevity. The trials did not run that comparison either — they tested untreated rope, which tells you about the bare fibre but not about the fibre once you have done something to it. That gap in the evidence is worth naming.

Hemp — the tradition and why it holds up

Hemp fibre is finer than manila, extracted from the bast of Cannabis sativa stems in a process closer to linen production than to the leaf-fibre extraction that produces manila and sisal. Run a length of good hemp rope through your fingers and the difference is there — a denser, more uniform surface, less of the individual fibre character that makes manila feel almost agricultural. It is a more resolved material.

It is also stronger per unit weight than manila, with lower stretch under load. The Handbook's characterisation of hemp and flax as "the most suitable natural fibres for strong, fine cordage" is borne out by the tensile numbers. Lower stretch made it the historical choice for standing rigging, where a shroud that gives under load is not providing the support the rig needs.

Hemp absorbs moisture faster than manila on initial contact, which is the source of its reputation as inferior in seawater. The Atkins and Purser results complicate this. In heavily contaminated water, untreated hemp and manila both reached zero retained strength within twelve months. The difference in initial water uptake is real. The difference in long-term survival under severe conditions appears negligible. What the bare-fibre comparison misses is that hemp was never used untreated for serious marine applications — it was tarred, and it takes Stockholm tar better than any other fibre in this list. The fine bast structure absorbs tar deeply and evenly. The treatment and the fibre work together in a way that the seawater immersion data for untreated rope simply does not capture.

This leaves me genuinely uncertain about how to read the manila-versus-hemp question. The chemistry favours manila for untreated seawater resistance. The practical tradition favours hemp for treated standing rigging. The trials test untreated rope and find them equivalent in failure. I do not think any of these positions is wrong. I think they are measuring different things, and the question you should be asking is which one is relevant to your specific situation.

Sisal — strong, inconvenient, and poorly understood

Sisal sinks immediately in water. This is the first thing worth knowing about it for marine use, and it is not a defect so much as a consequence of density — the fibre is heavier than water and goes straight down. A line that sinks is a line that can foul a propeller, and on a boat under engine this is not a theoretical risk.

The Atkins and Purser trials found sisal the most durable of the untreated rope types in their tests at 18% retained strength after twelve months, against zero for hemp and manila. This result is sometimes cited as evidence of sisal's superiority for marine applications. Eighteen percent retained strength is not a working rope. The result means sisal degraded more slowly, not that it survived usefully. The trials also noted widely varying duplicate results on sisal — two samples of identically treated sisal showing very different outcomes — which the authors attributed to either wave action fatigue or inconsistent preservative penetration. The coarser fibre structure makes deep impregnation harder to achieve reliably than with hemp.

Properly treated sisal performed significantly better: copper resinate in coal tar held it at 80–97% retained strength over twelve months. So the picture is similar to the other fibres — the bare material's properties are less important than what has been done to it. Where sisal earns its place in my practice is in yard work, temporary lashings, and any application where high strength, low cost, and genuine biodegradability matter more than seawater tolerance or the ability to sink a propeller.

Cotton — the honest case

Cotton is close to pure cellulose, which means it has essentially no inherent rot resistance. The near-complete absence of lignin gives the bacteria responsible for rope decay fewer obstacles than any other fibre in this list. Atkins' 1928 fishing nets trials found cotton and hemp reaching comparable states of unserviceability on similar timescales — two months in contaminated water in summer. The fibre is genuinely vulnerable.

It is also genuinely pleasant to handle. There is a softness to good cotton rope that none of the others quite match — something yielding and almost textile about it, closer to clothing than cordage. This quality is not incidental. It is why cotton belongs on signal halyards, bell ropes, lanyards, and decorative ropework rather than on anything structural. The stretch that makes it unsuitable for a halyard — it gives significantly under load — works in its favour in a fender lashing or a snubber, where absorbing dynamic load is exactly what you want.

Cutch pre-treatment extends cotton's life meaningfully according to the trials, and aluminium stearate adds useful water repellency without stiffening the fibre. Neither makes it a marine rope. Together they make it a rope that can get wet occasionally without immediate consequences, which is a more accurate description of what cotton rope in practice is asked to do.

Coir — the exception

Coir is strange enough in its combination of properties to resist easy summary. It has the lowest tensile strength of any fibre in this list. It has the highest lignin content. It floats with a buoyancy that surprises you the first time you coil a length of it over water and watch it sit there — not reluctantly, the way manila does when dry, but with apparent ease, as though the water is doing it a favour. And it stretches, considerably and springily, before it breaks.

These properties together — buoyancy, elasticity, low cost — make it the right material for specific applications that nothing else serves as well. For fenders, it is close to ideal: the buoyancy keeps it positioned, the elasticity absorbs impact, the texture grips the hull side. A well-made coir rope fender outlasts a cheap foam fender with reasonable maintenance, and when it finally fails it does not shed microplastic particles into the water.

The weakness that matters most for coir is not its strength — it is the invisibility of its decay. Atkins and Purser note that coir towing ropes in service had often deteriorated well below their rated strength without external signs. The rope looks sound and is not. Inspect coir working rope regularly and open it along the length rather than relying on surface condition. The consequences of not doing so on a towing hawser are disproportionate to the effort.

Jute — briefly, honestly

Jute comes from Corchorus species stems, grown mainly in Bangladesh and India. The Handbook classifies it as a lower-quality fibre, and the classification is accurate. It weakens significantly when wet, bristles unpleasantly in the hand when dry, and degrades quickly in marine conditions. It appears here because it is sometimes sold alongside better fibres in chandleries and garden centres without clear labelling. Brown, slightly scratchy rope of uncertain provenance has a reasonable chance of being jute. Fine for lashing down a tarpaulin ashore. I would not take it to sea.

The rope strength table

Strength figures are indicative for comparable construction and diameter. Actual values vary by manufacturer, construction, and condition. For any structural application, use manufacturer data.

Working conclusions — held provisionally

For running rigging: manila is where I start. Good hand, floats when dry, responds well to dressing. Hemp is stronger and lower-stretch but absorbs moisture faster on first contact — an argument for manila in situations where the rope might occasionally be neglected, and against it where the rope will be properly treated and maintained from the beginning.

For standing rigging on a traditionally rigged vessel: hemp, treated and wormed, parcelled, and served. The historical record and the mechanical properties point the same direction. Bushell specifies hemp throughout his 1874 rigging guide without discussion, the way you specify oak for a frame — as a starting assumption rather than a considered choice. I think that confidence is earned, though I hold it with more caveats than Bushell needed to.

For fenders, snubbers, and towing: coir. The elasticity is the point and nothing else serves it as well.

For decorative work, lanyards, and rope used mainly dry: cotton with appropriate treatment.

For temporary lashings ashore: sisal. Do not leave it over winter expecting it to be usable in spring.

Jute: not at sea.

The caveat that sits underneath all of this is the one the hemp department fiasco introduces: you need to know what you actually have. Buying from a supplier who can identify their fibre source and tell you where it came from is not a minor detail. It is the prerequisite for everything else, including buying rope that is worth preserving in the first place.

Sources: H.A. McKenna, J.W.S. Hearle and N. O'Hear, Handbook of Fibre Rope Technology (Woodhead Publishing, 2004). Hervey Garrett Smith, The Marlinspike Sailor (International Marine, 1971). Charles Bushell, The Rigger's Guide and Seaman's Assistant (Griffin & Co., 1874). W.R.G. Atkins and J. Purser, The Preservation of Fibre Ropes for Use in Sea-Water, Journal of the Marine Biological Association of the United Kingdom (1936) (1936). W.R.G. Atkins,The Preservation of Fishing Nets by Treatment with Copper Soaps and Other Substances, Journal of the Marine Biological Association of the United Kingdom (1928).

At VAKA I design regenerative sailing craft built throughout from natural materials — the rope choices aboard follow the same logic as the hull choices: honest assessment of what each material does well, and where its limits are.

Plans at VAKA Boatplans | Full knowledge base at Field Notes