Collection: Field Notes - Regenerative Materials | 

Series: Natural Ropes | 

Why tar natural cordage? The four mechanisms that destroy natural rope, how they interact, and why understanding them changes what you do about it

What you are actually watching

A rope does not simply wear out. That framing — gradual, even, predictable — describes something that is not quite what happens. What happens instead is that several distinct processes run simultaneously, each accelerating the others, and the result looks like gradual deterioration from the outside while something considerably less orderly is occurring within the fibre bundle.

I keep coming back to a detail in the Atkins and Purser 1936 Plymouth Sound trials. Untreated hemp and manila rope, immersed under a pier in contaminated seawater, reached zero retained strength within twelve months. Not weakened — gone. And the site, while severe, was a real harbour environment rather than a laboratory acceleration tank. The ropes were not boiled or chemically attacked. They were just wet, in the sea, for a year.

Understanding the mechanisms is not academic tidiness. It changes what you do. A rope treated only against bacteria but left to work dry is still cutting itself from the inside. A rope well lubricated but left salt-damp between uses is still feeding the organisms that will destroy it. The mechanisms interact, and a response that addresses one while ignoring the others is only partially effective. This is why no single treatment approach is ever quite enough on its own.

Bacterial decay

This is the dominant failure mode for natural rope in any wet marine environment, and it is worth understanding at the level of what is actually happening rather than simply noting that rope rots.

Natural rope fibres — hemp, manila, sisal, cotton, coir — are built from cellulose, the structural polymer of plant cell walls. Cellulose is the primary food source for a substantial community of water and soil bacteria, particularly Cytophaga and Sporocytophaga species, along with various cellulose-degrading fungi. These organisms produce cellulase enzymes that break the β-1,4-glycosidic bonds holding the cellulose chains together. They work from outside the fibre inward, progressively reducing tensile strength without producing obvious surface damage until the process is well advanced. A rope can look sound while being structurally compromised throughout.

The conditions these organisms require are moisture, moderate temperature, and substrate. Marine rope in use provides all three without interruption. Salt water does not inhibit the bacteria — this runs counter to a persistent intuition that salt is somehow preservative, but the evidence from the trials does not support it. Atkins' earlier fishing nets work, published in 1928, found cotton and hemp netting becoming unserviceable in under two months in summer in contaminated aquarium tank water. Two months. The contamination elevated the bacterial load, but the principle held across conditions. Clean water was less severe — the cleaner tidal fish pond at Cawsand produced slower degradation than the Plymouth Pier site — but still destructive. For anyone keeping a boat in a busy marina rather than a clean anchorage, that distinction matters.

The critical variable is free moisture within the fibre structure. Bacterial cellulase activity requires liquid water, not vapour, not surface damp — water inside the fibre bundle. A rope that is genuinely dry stops bacterial action entirely for as long as it stays dry. This sounds straightforward and is considerably less so in practice, for a reason the trials identified and that I have since observed myself. Once a rope has been wetted by seawater and partially dried without rinsing, it does not reach the same dryness as a rope that was rinsed first. The salt deposited in the fibre draws moisture back continuously from the surrounding air. I have stored identically treated ropes side by side over winter — one rinsed before stowing, one not — and opened them in spring to find a measurable difference in condition. The unrinsed rope was soft in places where the rinsed rope was not. I did not run tensile tests, so I cannot put numbers to it. But it was not subtle. This is why the sequence of rinsing with fresh water before drying and storage matters as much as it does. Skip the rinse, and drying is partly cosmetic.

The preservative response to bacterial decay works by depositing compounds within the fibre structure that are toxic to cellulase-producing organisms. In the trials, copper soaps were consistently among the highest performers — copper oleate and copper naphthenate maintained retained strength above 70% at ten months where untreated controls were at zero. Coal tar and Stockholm tar work partly through physical exclusion of water and partly through the toxicity of their phenolic fractions to the relevant bacteria. The ecological implications of copper in marine environments are a real and separate concern. The mechanism of action, at least, is well established.

Internal abrasion

A rope under load is not a static structure. The fibres within each yarn and the yarns within each strand move against one another with every change in tension, every pass over a sheave, every cycle of loading and unloading. In a dry rope this movement generates friction but limited cutting action. In a wet rope the geometry changes.

When natural fibre rope becomes saturated, individual fibres swell transversely. Hemp and cotton swell more than manila; coir swells least. This swelling increases internal pressure within the rope structure, tightening contact between fibres and yarns. The increased contact pressure, combined with the continuous movement of a working rope, produces accelerated internal abrasion — fibres cutting into one another at contact points, generating fine debris that appears as a powdery residue when an old rope is opened.

I was not expecting this the first time I saw it. I had cut open a length of rope that looked externally reasonable — not pristine, but not obviously condemned — and found the interior fibres reduced to something closer to dust than to fibre. The outer strands were holding their form. Inside, the yarn structure was largely gone. The rope would have failed under any serious load, and nothing visible on the outside would have warned me. That is the inspection problem with internal abrasion. It does not announce itself on the surface, and by the time the surface shows it the interior is well past the point of safe use. Opening rope at intervals — not just looking at it — is the only way to actually know what is there. The inspection post covers what to look for in detail.

Salt crystals compound the abrasion. As a salt-wetted rope dries, sodium chloride crystallises throughout the fibre bundle in cubic form, with edges sharp enough at the microscopic scale to cut fibres as the rope moves under load. This is additive to the direct fibre-on-fibre abrasion, and shares the same trigger — salt cycling.

The Handbook of Fibre Rope Technology discusses yarn-to-yarn friction as a determinant of how efficiently a rope's component fibres contribute to overall load-bearing. The implication is that a rope abraded from within is losing effective cross-section at a rate its external appearance cannot reveal. The traditional response is lubrication — introducing a substance into the fibre structure that reduces friction between fibres and inhibits crystal formation at the contact points. This is the source of the US Government rope specifications Atkins and Purser cite, which required lubricant content of 8–12% by weight of rope as sold. Not preservative content. Lubricant content, specified independently, because the two functions are distinct and both necessary. A treatment that addresses only the biocidal side while leaving the rope dry and stiff has solved half the problem.

Salt crystallisation damage

The crystallisation mechanism does more than act as an abrasive. As sodium chloride crystals grow within a confined fibre bundle, they exert expansive pressure on surrounding fibres. The force at any individual crystal site is small. Across thousands of sites in a working length of rope under repeated wet-dry cycling, the cumulative mechanical stress produces fibre fatigue at the crystal contact points — not the cutting action of abrasion, but a slower weakening of the fibres at the sites of crystal growth.

The interaction with bacterial decay is direct. Mechanically fatigued fibre offers easier access to cellulase enzymes than intact fibre — the disruption creates sites where bacterial attack can establish faster than it would on an undamaged surface. And the presence of crystals within the bundle increases the severity of abrasive damage when the rope is loaded. The mechanisms do not simply add. They each worsen the conditions for the others.

The difference between rinsed and unrinsed rope I mentioned earlier is directly about this. The unrinsed rope, soft in places after a winter in storage, was not soft from biological decay alone — the timescale was too short for that to be the complete explanation. It was soft partly from the mechanical damage that occurs when salt crystals are present throughout a fibre bundle that has been repeatedly stressed by handling, coiling, and thermal cycling over the months of storage. Rinsing removes the salt before any of that begins. It is the simplest intervention with the widest effect.

Ultraviolet degradation

The UV story is the one I am least certain about from direct experience, and it is worth naming that uncertainty honestly.

What the sources say is that cellulose itself is not particularly sensitive to UV radiation in the way synthetic polymers are — polypropylene embrittles and fails under UV exposure in a way that hemp does not. The mechanism of UV damage in natural rope is indirect: the lignin fraction of the fibre is photochemically active, and UV radiation degrades it progressively through photo-oxidation. Since lignin is the hydrophobic component that gives fibres like hemp and manila some inherent resistance to moisture uptake and bacterial access, its degradation matters. A bleached rope, the argument goes, is a rope with diminished natural defences.

What I have not done is compare sun-exposed and stored rope of the same age and treatment history in a way that would let me say whether the lignin degradation produces a measurable difference in service life under realistic conditions. The surface bleaching is visibly real — rope left in direct sun for a season looks noticeably different from rope kept below. Whether that visual change corresponds to a meaningful structural or biological vulnerability, and over what timescale, I cannot say from my own practice. The trials literature does not address it directly either, because the Plymouth Sound experiments were about immersion rather than aerial exposure.

What I do observe is that rope which looks grey and dry rather than dark and slightly resinous is also rope where a maintenance dressing is clearly overdue. The appearance tells you the surface treatment has depleted, which is the actionable information regardless of whether the underlying UV mechanism is the primary cause or a contributing one. The practical response — retreat before the surface dries out, store out of direct sun where possible — is the same either way.

Why the combination is the real problem

Each mechanism has a response. Bacterial decay is addressed by biocidal preservatives and genuine drying. Internal abrasion is reduced by lubrication and by avoiding salt cycling. Salt crystallisation is interrupted by fresh water rinsing. UV degradation is slowed by surface coatings and appropriate stowage.

The problem is that these mechanisms do not wait their turn. They run simultaneously, and the interactions between them mean the combined rate of deterioration is faster than any individual mechanism would produce alone. Salt-saturated fibre that cannot dry provides continuous conditions for bacterial activity even in periods between use. Bacteria-weakened fibre fails earlier under abrasive stress. UV-degraded lignin allows faster moisture penetration, accelerating the salt cycle and bacterial access. The rope that looks sound from the outside while failing internally has usually arrived at that state through these interactions rather than through any single cause.

This is why the Atkins and Purser finding of zero retained strength at twelve months is not simply a story about bacteria, or about salt, or about abrasion. It is a story about what happens when all four mechanisms run unchecked together. And it is why no single intervention is sufficient. A rope that is tarred but never dried properly, or dried carefully but stored in a sealed damp locker, or treated with a biocide that has no lubricant function — each of these is better than doing nothing, but none of them addresses the full picture.

Once you understand what is actually happening inside a rope in service, the maintenance decisions become considerably less arbitrary.

Sources: 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). H.A. McKenna, J.W.S. Hearle and N. O'Hear, Handbook of Fibre Rope Technology (Woodhead Publishing, 2004). Charles Bushell, The Rigger's Guide and Seaman's Assistant (Griffin & Co., 1874). Hervey Garrett Smith, The Marlinspike Sailor (International Marine, 1971). 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, standing rigging on traditionally rigged designs is specified in tarred hemp as standard — the chemistry has not changed, even if the boats are smaller than they used to be.

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