Six months submerged in the waters off Sweden, Denmark, and France, painted test panels told a surprising story: the most effective shield against the slimy buildup that weighs down boats is not a paint laced with copper, nor is it the one marketed as environmentally safe. It is, counterintuitively, a slippery silicone coating that contains no biocides at all.
When algae, barnacles, and other marine organisms cake a boat hull, the vessel becomes heavier and slower, burning more fuel and frustrating owners. This is why boat owners regularly paint with antifouling paints—coatings that release toxic substances into the water to prevent growth. The market has long been dominated by coprous oxide paints, which work well but carry a steep environmental cost: copper contaminates water and sediments, harming organisms far beyond the boat hull.
Maria Lagerström and her team at Chalmers University of Technology in Sweden set out to test whether the environmental footprint of these paints could shrink without sacrificing their protective power. Working with colleagues from Danish and French universities, they tested seven different paints across three coastal sites: Tjärnö in Sweden, Hundested in Denmark, and Arcachon Bay in France. Five of the paints contained cuprous oxide at concentrations ranging from 6 to 32 weight percent. One was a biocide-free silicone paint designed to be so slippery that organisms cannot grip it. The seventh contained tralopyril and zinc pyrithione instead of copper—a formulation marketed as environmentally friendly in several European countries, including Denmark and France, though not sold in Sweden.
The results defied conventional wisdom. After six months in the water, the silicone paint proved most effective at resisting fouling, followed closely by the tralopyril paint. Among the five copper paints, copper content made no meaningful difference; a paint with 6 percent copper performed essentially as well as one with 32 percent. "The copper content in the paint was not a deciding factor," the researchers found, despite the wide range of concentrations tested.
But the story darkened when Lagerström's team assessed the environmental impact of each paint. Using models to predict biocide release into seawater based on laboratory measurements, they discovered that none of the biocidal paints would meet current environmental risk criteria—except the silicone, which contains no biocides to measure. One paint stood apart from the rest: the tralopyril formulation. The amount of biocide it released into the water was several thousand times higher than acceptable levels, a result that shocked even researchers accustomed to environmental risk modeling. "I've never seen such high values," Lagerström said.
In a second test, researchers exposed four different marine species to water containing leachate from each painted panel. Again, the tralopyril paint performed worst, followed by the copper paint with the highest concentration of the metal.
The implications are clear and striking. The paint industry's assumption that lower copper is better misses a broader truth: copper content, within tested ranges, matters less than the type of biocide used. The real winner is the silicone paint—effective, benign, and ready to reduce both fouling and environmental harm at the same time. As boat owners worldwide seek to lighten their ecological footprint, this laboratory result from three European seas points toward a simpler, slipperier path forward.