Antifouling coating is a specialized paint or surface treatment applied to structures submerged in water, primarily ship hulls, to prevent marine organisms from attaching and accumulating. It works by either releasing chemicals that deter or kill settling organisms, or by creating a surface so slippery that organisms cannot gain a foothold. Without these coatings, barnacles, algae, mussels, tube worms, and slime-forming bacteria colonize underwater surfaces within days to weeks, creating a rough biological crust that drags against the water and drives up fuel consumption dramatically.
Why Fouling Is Such a Problem
Any surface left in seawater will begin collecting life almost immediately. Within hours, bacteria and single-celled algae form a thin biofilm sometimes called “slime.” Within weeks, larger organisms like barnacles, mussels, and seaweed begin anchoring themselves. On a commercial ship, that growth creates a rough, textured hull that has to push through much more water resistance than a clean one. The fuel penalty is severe: even light calcareous fouling (think scattered barnacles and small shells) can increase a vessel’s greenhouse gas emissions by roughly 30 to 50 percent, while heavy fouling can more than double the power needed to maintain cruising speed.1Applied Ocean Research. Underestimated penalty of hull fouling: A scenario-based analysis of GHG emissions from global shipping Computational studies have shown that barnacle fouling alone can increase a ship’s frictional resistance by up to 93 percent.2Ocean Engineering. An investigation into the effect of biofouling on the ship hydrodynamic characteristics using CFD
For the global shipping fleet, which moves about 80 percent of world trade by volume, that fuel penalty translates into billions of dollars in added cost and enormous volumes of extra carbon dioxide, sulfur oxides, and nitrogen oxides pumped into the atmosphere each year. Antifouling coatings exist to prevent that. They are not optional accessories; they are fundamental to making modern shipping economically and environmentally viable.
How Biocidal Coatings Work
The oldest and still most common strategy is straightforward: embed a toxic substance in the paint and let it leach out slowly, poisoning organisms that try to settle. For most of the twentieth century, the toxin of choice was tributyltin (TBT), an organotin compound that was devastatingly effective. TBT killed virtually everything that came near a hull. The problem was that it also killed organisms nowhere near a hull. TBT accumulated in harbor sediments, entered the food chain, and disrupted the reproduction and growth of shellfish, fish, and other marine life far from any ship.3PubMed. Tributyltin antifoulings: a threat to the Hong Kong marine environment The damage was so extensive that the International Maritime Organization adopted the AFS Convention in 2001, which banned TBT-based antifouling paints on ships worldwide.4PubMed. The legal design of the international and European Union ban on tributyltin antifouling paint: direct and indirect effects
Since the TBT ban took effect, the industry shifted overwhelmingly to copper-based biocides. Cuprous oxide or metallic copper particles are mixed into paint, and as the paint slowly wears or dissolves, copper ions leach into the surrounding water. Copper is toxic to most fouling organisms at relatively low concentrations. Research into release rates shows that copper levels of about 2 to 7 micrograms per square centimeter per day are generally enough to deter barnacles, algae, and other macrofouling organisms in most coastal waters.5Progress in Organic Coatings. Investigation of critical copper release rates for dose optimization of antifouling coatings Some formulations also include zinc compounds or organic co-biocides like zinc pyrithione to broaden the spectrum of organisms they repel.
Self-Polishing Paints and Controlled Release
Simply packing a paint with biocide and letting it dissolve haphazardly creates problems: the release rate starts high when the paint is fresh and tapers off as the biocide depletes from the outer layer, leaving a fouling-prone surface underneath. Self-polishing copolymer (SPC) coatings solve this by engineering the paint’s polymer backbone to hydrolyze (break down chemically) at a steady rate in seawater. As the outermost layer dissolves, it exposes a fresh layer of biocide beneath it, maintaining a consistent release rate and a perpetually smooth surface. The “polishing” effect also sloughs off any organisms that do manage to attach.
Researchers are now exploring greener versions of this chemistry. One approach swaps synthetic biocides for naturally occurring compounds. A recent study demonstrated a self-polishing coating built around eugenol, the compound that gives cloves their distinctive smell. The polymer releases eugenol as it hydrolyzes, creating a non-toxic antifouling effect. Release rates stabilized at around 0.14 micrograms per square centimeter per day after the initial immersion period.6Progress in Organic Coatings. Eco-friendly self-polishing antifouling coating via eugenol ester hydrolysis That rate is orders of magnitude lower than copper-based coatings require, which hints at the potency of botanical antifoulants but also raises questions about whether such low doses can handle heavy fouling pressure in warm tropical waters.
Foul-Release Coatings
A fundamentally different philosophy asks: what if, instead of killing organisms, you just made the surface so slick they could not stick? Foul-release coatings take this approach. They are typically based on silicone (polydimethylsiloxane, or PDMS) and work by creating a surface with very low energy and high elasticity. Organisms may land on the coating, but they adhere only weakly, and the shear force of water flowing past the hull as the ship moves peels them off.7PubMed. Silicone-Based Fouling-Release Coatings for Marine Antifouling
The trade-off is that foul-release coatings work best on vessels that move frequently and at reasonable speed. A ship sitting idle in a warm harbor for weeks will still accumulate fouling because there is no water shear to strip organisms away. That makes them well-suited for fast-moving commercial vessels like container ships and naval frigates, but less practical for slow-moving barges, floating platforms, or boats that spend long stretches at the dock. Foul-release coatings are also softer and more easily damaged by scraping or impact than hard biocidal paints, so they require careful handling during mooring and maintenance.
Amphiphilic and Zwitterionic Surfaces
Marine organisms are diverse, and their adhesion strategies vary. Barnacles use a protein-based cement. Diatoms (microscopic algae) secrete sticky polysaccharides. Bacteria form biofilms with complex chemical architectures. A surface optimized to repel one type of organism may be vulnerable to another. Amphiphilic coatings attempt to address this by combining both water-attracting and water-repelling chemistry on the same surface. The mix of chemical domains creates a confusing landscape for settling organisms, none of whose adhesive strategies are well-matched to the surface.
One version of this approach grafts zwitterionic segments, which carry both positive and negative charges within the same molecule, onto a silicone backbone. The zwitterionic portions bind water molecules extremely tightly, forming a hydration layer that physically blocks proteins and cells from reaching the surface. In lab tests, a silicone coating modified with sulfobetaine zwitterionic groups reduced protein adhesion by about 88 percent, bacteria adhesion by roughly 99 percent, and algae adhesion by over 99 percent compared to plain silicone.8Langmuir. Amphiphilic Marine Antifouling Coatings Based on Zwitterion-Modified Silicone Polymers Another design uses hydrophilic polymer chains that migrate to the surface when submerged, reconstructing the coating’s outer chemistry in contact with water and forming dense hydrogen-bonded hydration layers.9PubMed Central. Surface Reconstruction of Silicone-Based Amphiphilic Polymers for Mitigating Marine Biofouling
Zwitterionic coatings are not limited to marine use. The same principle has been applied to medical implants, biosensors, and water-treatment membranes, anywhere unwanted biological adhesion is a problem. The core mechanism, a tightly bound water layer that acts as a physical barrier, has been directly observed at the molecular level through spectroscopy research, confirming why these materials resist protein adsorption and organism attachment so effectively.10PubMed. Surface Hydration and Antifouling Activity of Zwitterionic Polymers Biomimetic zwitterionic polymers anchored using catechol chemistry, inspired by the adhesive proteins mussels use to grip rocks, have shown high resistance to both protein fouling and bacterial biofilm formation over multi-day tests.11PubMed. Ultra low fouling zwitterionic polymers with a biomimetic adhesive group
Shark Skin and Other Biomimetic Textures
Sharks stay remarkably clean despite spending their lives submerged. Their skin is covered in tiny tooth-like structures called denticles, arranged in overlapping patterns that reduce drag and discourage organisms from settling. Researchers have replicated these patterns on synthetic surfaces with promising results. Shark-skin-patterned surfaces combined with photocatalytic titanium dioxide nanoparticles reduced bacterial attachment by about 70 percent compared to smooth surfaces of the same chemical composition.12ACS Applied Materials & Interfaces. Bioinspired Photocatalytic Shark-Skin Surfaces with Antibacterial and Antifouling Activity via Nanoimprint Lithography
The fine geometry matters more than you might expect. In real shark skin, the denticles are tilted at roughly 60 degrees rather than standing straight up. Computational fluid dynamics simulations of membranes with sharkskin-inspired patterns found that the natural 60-degree tilt produced the lowest fouling and the best water flow performance among all angles tested, confirming that evolution had already landed on the optimal design.13Journal of Membrane Science. Bioinspired anti-fouling membranes featuring novel tilted sharkskin patterns Lab experiments on bacterial biofilm development offer an interesting nuance: the microscale texture of shark skin may actually promote initial bacterial attachment, but it prevents those bacteria from organizing into the mature, sticky biofilms that cause real fouling problems.14PubMed. Inhibition of biofilm formation by rough shark skin-patterned surfaces The distinction between individual cells landing on a surface and those cells building a cooperative structure is key; it is the biofilm, not the first arrivals, that causes fouling.
Enzymatic Coatings
Another biologically inspired approach embeds active enzymes into the coating itself. Fouling organisms rely on biological adhesives, proteins, polysaccharides, and lipids, to glue themselves to surfaces. If the surface actively digests those adhesives, attachment fails. Researchers have incorporated cellulase, protease, and lipase enzymes into water-based polymer coatings and found that the enzyme-loaded films significantly suppressed the accumulation of diatoms and green algal spores compared to enzyme-free controls and controls where the enzymes had been deliberately deactivated by heat.15Advanced Materials Interfaces. Testing of Marine Fouling‐Inhibiting Enzymes in a Water‐Based Methacrylate Polymer Matrix The deactivated-enzyme control is an important detail: it confirms the enzymes are actively working, not just changing the surface’s physical properties.
Enzymatic coatings face practical hurdles. Enzymes are proteins, and proteins degrade in seawater. Maintaining enzymatic activity over the three-to-five-year lifespan expected of a commercial antifouling coating is a major engineering challenge. Most demonstrations so far are lab-scale or short-term field trials, and no enzymatic coating has reached widespread commercial use on ships. But the concept is attractive because it avoids releasing biocides altogether.
Ultrasonic Antifouling
Not all antifouling solutions involve coatings. Ultrasonic systems mount transducers on the hull or inside the vessel and pulse high-frequency sound into the surrounding water. The sound creates tiny cavitation bubbles and pressure fluctuations at the hull surface that discourage organisms from settling. A four-month sea trial using 23 kHz ultrasound projectors on one side of a vessel showed that the treated side remained fairly clean while the untreated side accumulated heavy fouling.16PubMed. Sea-trial verification of ultrasonic antifouling control
More recent work has pinned down the intensity thresholds needed: about 0.5 watts per square centimeter at 20 kHz deters barnacle larvae, while green microalgae require about 1.4 watts per square centimeter at 40 kHz.17Ultrasonics. Mechanistic insights into ultrasonic vibration-induced avoidance behavior of fouling organisms on Ti alloy surfaces Ultrasonic systems are already commercially available for pleasure boats and smaller vessels, typically as compact units that bolt inside the hull. Scaling them to large commercial ships is harder because the sound field must cover vast areas of hull plating, and energy consumption rises accordingly. They are most commonly used as a supplement to coatings rather than a standalone replacement.
Environmental Costs of Current Coatings
Copper is far less toxic than TBT, but it is not harmless. In enclosed harbors and marinas where many coated hulls sit together and water circulation is poor, copper concentrations can climb to levels that harm marine life. A study of marinas in the San Diego region found that 86 percent of the marina area exceeded California’s water quality objectives for dissolved copper, and about a fifth of the marina area showed actual toxicity to test organisms, with copper identified as the primary cause.18PubMed. Extent and magnitude of copper contamination in marinas of the San Diego region, California, USA Sediment studies in marina basins have also found elevated copper and residual TBT concentrations localized within enclosed harbor areas, where limited water flushing traps contaminants.19PubMed. Sediment contamination and biological effects in a Chesapeake Bay marina
In-water hull cleaning, where divers or robotic systems scrub fouling off a hull while the ship sits in port, creates its own pollution stream. The scrubbed material releases metals from the coating into the water column. Research exposing juvenile rockfish to in-water cleaning discharge found that zinc was the most abundant metal released, reaching concentrations of roughly 190 micrograms per liter at the highest levels tested. Even though no fish died during the seven-day exposure, the discharge triggered oxidative stress, immune disruption, and nervous system disturbances at the genetic level.20PubMed. The enzymatic and transcriptional adverse effects of hull in-water cleaning discharge on juvenile rockfish (Sebastes schlegeli) These sub-lethal effects are easy to overlook but can accumulate in coastal ecosystems where cleaning happens frequently.
Antifouling Beyond Ship Hulls
Ships get most of the attention, but any structure in seawater faces fouling. Aquaculture nets are a major example. Fish farm nets that clog with algae and invertebrates restrict water flow, reduce oxygen levels inside the pen, and stress the fish. Cleaning the nets is labor-intensive and disrupts farm operations. Silicone-based foul-release treatments have been adapted for net panels. In a 46-week immersion trial in the Baltic Sea, nets impregnated with diluted silicone (PDMS) experienced only about 20 percent mesh occlusion, compared to 60 percent for untreated nets.21ScienceDirect (Aquaculture Reports). Core-shell structured nets for biofouling control in aquaculture The treatment also kept the nets flexible by adding only about 50 percent extra weight, whereas a thick undiluted silicone coating tripled the net’s weight and made it stiff and impractical.
Other applications include offshore wind turbine foundations, subsea pipelines, oceanographic sensors, and desalination plant intake structures. Each setting brings different constraints. A sensor needs optical clarity, not just biological cleanliness. A desalination membrane needs to resist fouling without releasing substances that would contaminate the drinking water. The antifouling field is increasingly specialized, with different chemistries and surface treatments tailored to each use case rather than a single universal paint.
Fouling and Invasive Species
There is a less obvious reason antifouling matters beyond fuel savings: biosecurity. Ships travel between oceans, and organisms hitching a ride on their hulls can establish themselves in waters where they have no natural predators. Hull fouling is now recognized as one of the major pathways for transferring non-native marine species around the globe. Alarmingly, some invasive fouling organisms continue to be transported even on hulls treated with copper-based antifouling paints, likely by settling in niche areas like sea chests, propeller shafts, and other recesses where coating coverage is incomplete or water flow is too low to maintain effective biocide concentrations.22PubMed. The influence of antifouling practices on marine invasions
Countries like Australia and New Zealand have adopted strict biofouling management requirements for arriving vessels, mandating clean hulls and documentation of antifouling coating type and maintenance history. The intersection of antifouling performance and invasive species risk is pushing regulatory frameworks to demand not just that coatings work on the main hull panels, but that entire underwater surfaces, including hard-to-reach areas, be managed. That pressure, in turn, is driving interest in complementary technologies like ultrasonic systems and robotic cleaning that can address the gaps left by conventional coatings.