If ship hulls were coated with special bo-inspired high-tech air trapping materials, up to 1% of global CO₂ emissions could be avoided according to a new study by researchers from the University of Bonn together with colleagues from St. Augustin and Rostock. According to the study, ships could save up to 20% of fuel as a result of reduced drag. If antifouling effects are also considered—such as the reduced growth of organisms on the hull—the reduction can even be doubled. The open-access study appears in the journal Philosophical Transactions A.

Ships are among the worst fuel guzzlers in the world. Together, they burn an estimated 250 million tonnes per year and emit around one billion tonnes of carbon dioxide into the air—about the same amount as the whole of Germany emits over the same period. The main reason for this is the high degree of drag between hull and water, which constantly slows the ship down. Depending on the type of ship, drag accounts for up to 90% of energy consumption. This also makes it a huge economic factor: After all, fuel consumption is responsible for half of transport costs.

Drag can be significantly reduced using technical tricks. For example, “microbubbles technology” actively pumps air bubbles under the hull. The ship then travels over a bubble carpet, which reduces drag. However, the production of the bubbles consumes so much energy that the total savings effect is very small.

Novel high-tech coatings may offer a solution. They are able to hold air for long periods of even weeks.

Around ten years ago, we were already able to demonstrate on a prototype that in principle it is possible to reduce drag by up to ten percent. Our partners at Rostock University later achieved a 30-percent reduction with another material developed by us.

—co-author Dr. Matthias Mail, Nees Institute for Biodiversity of Plants at the University of Bonn

Schematic of hull drag reduction by air layers. (a) Flow profile on a conventional ship hull. The water velocity at the surface boundary layer is zero. (b) An air layer functions as slip agent, the water velocity at the interface is larger than zero, drag is reduced. Busch et al.

Since then, various working groups have taken up the principle and developed it further. The technology is not yet mature enough for practical use. Nevertheless, the authors forecast a fuel-saving potential of at least five percent in the medium term, but more likely even 20 percent.

In their paper, they calculated the economic and ecological advantages this would bring. For example, a commercial container ship on its way from Baltimore (USA) to Bremerhaven could reduce its fuel costs by up to $160,000. Worldwide, emissions of the greenhouse gas carbon dioxide would be reduced by a maximum of 130 million tonnes.

Taking into account the reduced growth of barnacles and other aquatic organisms, which causes enormous additional drag loss, this quantity even rises to almost 300 million tonnes. This corresponds to almost one percent of global CO₂ emissions.

Of course, these figures are optimistic. But they show how much potential this technology has.

—Matthias Mail

The high-tech layers are based on models from nature, such as the floating fern Salvinia molesta. This is extremely hydrophobic: When submerged and pulled out again, the liquid rolls off it immediately. After that, the plant is completely dry. Or to be more accurate: it was never really wet in the first place. Underwater, the fern wraps itself in an extremely thin dress of air. This prevents the plant from coming into contact with liquid—even during a many weeks-long dive. Scientists call this behavior “superhydrophobic”.

Scanning electron microscope image of a surface that is modeled on that of Salvinia. © Prof. Dr. Wilhelm Barthlott/University of Bonn

Salvinia has tiny egg-beater-like hairs on the surface of its leaves. These are water-repellent at their base, but hydrophilic at their tip. With these hair-tips, the aquatic fern firmly “pins” a water layer around itself. Its little dress of trapped air is kept in place by the water layer.

Resources

• J. Busch, W. Barthlott, M. Brede, W. Terlau, M. Mail (2018) “Bionics and green technology in maritime shipping: an assessment of the effect of Salvinia air-layer hull coatings for drag and fuel reduction.” Phil. Trans. R. Soc. A 377 doi: 10.1098/rsta.2018.0263