A New Trick to Keep Barnacles From Sticking to Ships

Barnacles may have a small footprint, but their effect on global shipping is large.

When ships’ hulls get coated with barnacles and other creatures, they use more fuel and eventually must be hauled out of water and scraped clean, at an estimated cost of several billion dollars a year. Fuel burned by the shipping industry is a significant contributor to global carbon emissions, too.

To keep barnacles off hulls, boats are coated in anti-fouling paint that kills barnacle larvae. Unfortunately, the paints’ active ingredients also leach into the water and kill other things, like oysters, leading to bans on some formulations and a search for alternatives.

Researchers who study the physics of sticky biological structures at Kiel University in Germany reported last week in the Journal of the Royal Society Interface that one option may involve texture, rather than chemicals. Covering surfaces with microscopic structures shaped like mushrooms, they find, keeps barnacles from getting a firm foothold.

Part of what makes barnacles so difficult to dislodge is that they secrete a liquid glue that makes close contact with the hull’s surface and then hardens into a cement-like substance. If they could develop structures that kept the glue from attaching smoothly, it could compromise the animals’ grip.

Textured coatings that make it difficult for barnacle larvae to settle down already exist. But they only succeed in delaying colonization of ship hulls, said Lars Heepe, a professor of biomechanics. Heepe, Dennis Petersen, a graduate student, and their colleagues decided to test tape made of silicone and covered with forests of either straight pillars or mushroom shapes.

The team tested their samples in the Baltic Sea. Both accumulated barnacles, but the glue had easily seeped between the straight pillars and made a tight seal with the surface.

On the coating with the mushroom shapes, however, there were big gaps beneath the barnacles’ layer of cement. They were perched precariously on top of the mushroom caps.

To see what this meant in practical terms, the researchers lowered samples back into the Baltic and drew them up again every week for more than four months, taking photographs of their surfaces and painstakingly tracking every barnacle that tried to take hold.

For the first seven weeks, barnacles built up on both, but then something interesting happened.

“We were quite surprised by the fluctuation in the number of barnacles,” Heepe said.

Steadily, all the barnacles that had landed on the mushroom surface disappeared, apparently pulled from their insecure seats by the motion of the waves. No new barnacles settled on it for the rest of the experiment, while the other coating continued to attract new colonists.

The team also put a sample of the mushroom coating on a sailboat belonging to the Kieler Yacht Club that subsequently sailed through the Baltic and the North Sea for seven months, covering about 1,800 miles. When it returned, the patch was clear of barnacles.

While this study helps establish the potential of these structures, “in the future, these surfaces can be optimized,” Petersen said. The tape with mushrooms on it is already being manufactured for medical uses — particularly bandages to help burn patients — but it might be possible to shrink the number of mushrooms per square inch and still get a good effect as far as boats are concerned, Heepe said.

The fact that the mushroom structures did well demonstrates the importance of careful testing. Even a relatively simple textural change could have an outsize impact on troubles caused by barnacle hitchhikers.