Collection: Field Notes - Regenerative Materials | 

Series: Natural Ropes | 

Placing the rope preservative trial results in their ecological context — what the best-performing treatments cost the water they are used in

The uncomfortable inversion of natural rope preservation

The Atkins and Purser trials are the most useful evidence base I have found for natural rope preservation. They are also, read from an ecological perspective, a ranking of how much damage you can do to a marine environment in exchange for how well your rope holds up. The treatments that performed best in Plymouth Sound in 1935 are, with very few exceptions, the treatments that are most harmful to the organisms living in Plymouth Sound today.

This is not a coincidence. The mechanism that makes copper soap so effective at preserving rope — copper ions are toxic to the cellulose-degrading bacteria responsible for rope decay — is the same mechanism that makes copper toxic to marine organisms at low concentrations. A treatment that kills bacteria in rope fibre does not distinguish between those bacteria and the algae, molluscs, crustaceans, and juvenile fish in the water around the rope. The biocide does not know it is supposed to stay in the rope.

I kept this tension at the back of the preservation posts throughout the series, noting ecological concerns where they arose and pointing to this post for the fuller treatment. What follows is an attempt to work through the options honestly — not to manufacture a comfortable conclusion, but to understand the actual trade-offs between preservation performance and ecological impact, and to identify where the least bad options sit.

How to think about the ecological question

The ecological impact of a rope treatment in marine use depends on three things: the toxicity of the active compounds to marine organisms, the persistence of those compounds in the marine environment once leached from the rope, and the concentration at which they are present in the water around the boat.

Toxicity varies enormously across the compounds in the trial results. Hexavalent chromium — the bichromate in Cunningham's method — is a human carcinogen and acutely toxic to all aquatic life. Copper is a potent marine biocide, toxic to algae, molluscs, and crustaceans at concentrations measured in parts per billion. Polycyclic aromatic hydrocarbons from coal tar are persistent, bioaccumulative, and several are carcinogenic. Stockholm tar's phenolic fraction is biocidal but breaks down faster and does not bioaccumulate in the way PAHs do. Linseed oil is essentially non-toxic in the marine environment. Cutch tannin is biodegradable and of low toxicity.

Persistence matters as much as acute toxicity for the marine environment. A compound that is highly toxic but breaks down quickly in seawater produces a local, temporary effect. A compound with lower acute toxicity that persists in sediments for years and bioaccumulates through the food chain may cause more total harm. Coal tar PAHs are the exemplar of the second type — the benzo[a]pyrene and related compounds in coal tar have been found in marine sediments around traditional boat storage and maintenance areas at concentrations that affect benthic organisms, even decades after the tarring has stopped.

Concentration at point of use is harder to assess. A single rope in open water leaches negligible amounts of any treatment into an effectively infinite dilution. A fleet of traditionally rigged vessels concentrated in a marina, all using copper soap treatments, with their anchor rodes and docklines creating a distributed source of copper in an enclosed basin, is a different situation. The ecological concern about copper antifouling in marinas is well documented — copper concentrations in marina sediments near ports with high density of treated vessels can exceed levels toxic to sensitive organisms. Rope treatment contributes less than antifouling paint, but it is the same compound and the same mechanism.

Working through the options

Untreated rope. Zero ecological impact. Zero preservation. The trials are unambiguous on this — untreated rope in bacterially active seawater has a working life measured in months. It is not a viable option for any application where the rope matters.

Cutch alone. Derived from acacia bark, biodegradable, of low toxicity to marine organisms. The trials found it nearly useless as a preservation treatment — 17% retained strength after ten months against 13% for the untreated control. The marginal improvement does not justify treating cutch as a preservation system. As a pre-treatment that may improve uptake of subsequent treatments, it has a limited role. As a primary preservative in seawater, the evidence is clear.

Linseed oil. Non-toxic in the marine environment. The oxidation products of linseed in seawater are not harmful at the concentrations involved. As a lubricant and moisture barrier it is useful, and heat-bodied linseed avoids the metallic drier concerns of boiled linseed. As a standalone biocidal treatment it is insufficient. The ecological position is good. The preservation performance is limited.

Beeswax and tallow. Essentially non-toxic, biodegradable, functionally useful as lubricants and moisture barriers for rope above the waterline. For rope in sustained immersion, inadequate as standalone treatments. The ecological case is clean and the performance case is limited to specific applications.

Stockholm tar. This is where the analysis becomes more nuanced. Stockholm tar contains phenols and cresols that are biocidal against the relevant bacteria, and these compounds do leach into the surrounding water over time. Phenols are toxic to marine organisms, but they are also relatively biodegradable — they break down in the marine environment rather than persisting or bioaccumulating. The polycyclic aromatic hydrocarbons in Stockholm tar are present at much lower concentrations than in coal tar, and the composition is different — dominated by the diterpene resin acids rather than the naphthalene-series compounds that drive coal tar's toxicity and persistence.

The EU Biocidal Products Regulation does not list Stockholm tar as a banned or severely restricted substance for marine use in the way it does coal tar and creosote. Traditional uses of Stockholm tar on wooden boats and rope have continued without the regulatory pressure applied to coal tar derivatives. That is not a clean bill of ecological health — it means the evidence of harm is insufficient for regulatory action, not that there is no harm at all. My working position is that Stockholm tar is the most defensible of the effective treatments, while being honest that it is not neutral.

Iron naphthenate / iron oleate. Iron is an essential marine micronutrient rather than a biocide at the concentrations involved in rope treatment leaching. The iron soap preparation post covers this in detail — the ecological case for iron over copper is clear, and the performance data from the trials, while not directly comparable for iron versus copper soap combinations, is encouraging. The Stockholm tar carrier used in the preparation I am working with has the ecological profile described above. The combination is probably the best available option for rope in sustained seawater contact in terms of balancing preservation performance against ecological impact. Whether the performance actually matches the copper-tar results from the trials remains to be established from my own testing.

Zinc naphthenate (colourless Cuprinol). Zinc accumulates in marina sediments and is toxic to aquatic invertebrates and algae at the concentrations found in enclosed harbours. It combined mediocre performance in the trials with a real ecological concern. There is no compelling reason to choose it.

Copper naphthenate / copper oleate. Effective. The trials data is clear. The ecological cost is also clear — copper is a potent marine biocide, the microplastic and antifouling post covers the broader context of copper in marina environments, and the true environmental impact of boating post addresses the regulatory direction of travel. The copper antifouling regulatory tightening across Europe is moving toward restriction rather than accommodation. Copper rope treatment is a smaller source than antifouling paint, but it is the same compound accumulating in the same sediments around the same harbours. My position is that copper treatments are not appropriate for a natural materials system that is trying to reduce rather than replicate the ecological footprint of conventional boating. Others will draw the line differently.

Coal tar and coal tar distillates including creosote. The PAH content — naphthalene, pyrene, benzo[a]pyrene and related compounds — is persistent, bioaccumulative, and well documented as harmful to marine benthic communities. Creosote is banned for most marine uses across the EU. Coal tar for rope treatment occupies a grey area — it is not sold as a biocidal product in the regulatory sense and therefore sits outside the Biocidal Products Regulation's direct reach — but using it on rope in contact with seawater is difficult to defend on ecological grounds regardless of the regulatory position. The 21% performance advantage over Stockholm tar in the trials does not, in my view, justify the environmental cost.

Cunningham's bichromate method. Hexavalent chromium Cr(VI) is a human carcinogen, mutagenic, and acutely toxic to all aquatic life. It is banned under REACH and equivalent frameworks globally. It also performed poorly in the trials — 26–32% retained strength — managing to be both ecologically indefensible and inadequately effective. It appears here only because it appeared in the trials, and because the same method or variants of it are occasionally encountered in older fishing and net preservation literature as though they were acceptable options.

The hierarchy as I currently hold it

From most to least ecologically defensible, cross-referenced against what the performance evidence actually supports:

Stockholm tar with iron oleate carrier is the best combination I have found that is both meaningfully effective and ecologically justifiable. The iron soap adds biocidal performance to Stockholm tar's hydrophobic and lubricating properties without introducing a marine biocide of the copper class. The performance relative to copper-tar combinations is unknown from my own testing — that is what the current season's exposure testing is trying to establish.

Stockholm tar alone is effective, ecologically manageable, and practically superior to coal tar for working rope because it dries. It is the default for most of my rope treatment work and the benchmark against which the iron soap addition is being measured.

Linseed oil with Stockholm tar, used as a maintenance dressing between annual tar treatments, adds lubricant function without adding ecological burden. The combination addresses the lubricant requirement that the US Government specifications identify as separate from and equal to the biocidal requirement.

Cutch as a pre-treatment on cotton rope before tar application, where the tannin pre-treatment may improve treatment uptake. Not as a primary preservative.

Everything else in the trials either performs inadequately, carries unacceptable ecological cost, or both.

The honest gap

The gap I keep returning to is between what the trials established and what I would need to establish to be confident in the iron soap alternative. The trials showed that copper-tar combinations significantly outperformed Stockholm tar alone. They did not test iron-tar combinations under equivalent conditions. The chemistry suggests iron oleate in Stockholm tar carrier should perform better than Stockholm tar alone — the iron soap adds biocidal activity that the tar alone does not fully provide. Whether it performs comparably to copper-tar is the question, and the answer requires time in the water rather than reasoning from chemistry.

The current exposure testing will not give me the full twelve-month comparative data that the Atkins and Purser trials provide. What it will give me, if the season goes to plan, is a preliminary indication of whether the iron soap addition produces a measurable improvement over Stockholm tar alone under the specific conditions of the test site. That is a narrower claim than the trials support, and I am trying to hold it to that narrowness rather than extrapolating beyond what the evidence actually shows.

The fibreglass disposal crisis and the broader environmental context of conventional boating are the backdrop to all of this. A natural rope system using Stockholm tar and iron soap, maintained carefully and composted at end of life, is not a zero-impact system. It is a considerably lower-impact system than the conventional alternatives, and the direction of the trade-offs is knowable even where the precise numbers are not. That is about as far as the current evidence takes me, and it is where this investigation currently rests.

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). 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). H.A. McKenna, J.W.S. Hearle and N. O'Hear, Handbook of Fibre Rope Technology (Woodhead Publishing, 2004).

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