Tiny Bubbles Under a Ship May Be the Secret to Reducing Fuel Consumption

A technology called air lubrication offers a way to make large ships more efficient

The Norwegian Joy is one of several cruise ships equipped with air lubrication technology. (Patrik Stollarz/AFP via Getty Images)
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Water is not as yielding as you think, says Noah Silberschmidt, founder and CEO of UK-based Silverstream Technologies. For more than a century, gigantic steel vessels have been ploughing the oceans, generating seemingly unavoidable—and surprisingly costly—friction between ship and sea. But this friction can be reduced in an innovative way, says Silberschmidt, with the help of millions of tiny bubbles, each just a millimeter across.

With the push to make shipping more efficient, ship owners are looking for new ways to reduce fuel consumption and emissions. One contender is Silverstream Technology’s eponymous Silverstream System, a device installed in a ship’s hull near the bow that generates a carpet of air bubbles flowing all the way to the ship’s stern.

The concept underpinning the device—air lubrication—is not new, but advancing technology is allowing the company and its competitors to retrofit existing ships with air lubrication systems, or include them in new vessel constructions.

Air is less dense than water, which means that the bubbles reduce the resistance between the ship and the sea around its hull. It’s a bit like gliding your hand through a gently bubbling hot tub versus a still bathtub.

Silberschmidt says that, over time, air lubrication can reduce fuel consumption by five to 10 percent. Fuel savings of a few percent might not sound like much, but Silberschmidt says shipping firms can spend between US $5- and $10-million on fuel for a single average-sized vessel every year.

Cruise lines Norwegian and Carnival have already installed Silverstream’s devices on some of their vessels, and more installations are due to be announced soon, says Silberschmidt.

You can’t blow bubbles for free, though. It requires energy to compress air and position it so that bubbles flow in a steady stream along the hull of a large, flat-bottomed vessel. Silverstream has cut the overall energy needed, however, by filling air release units—small cavities built into the underside of the ship—with air. The bubbles form because of the difference in pressure between the air in these cavities and the seawater below. A phenomenon called Kelvin-Helmholtz instability occurs as the ship moves, which means that the air mixes into the water in the form of little bubbles that then glide rearward below the ship.

Relying as it does on physics, the bubble carpet itself is therefore “Mother Nature–generated” says Silberschmidt. The light, bubble-rich water is akin to the white foamy crests topping waves you might see on a windy day at the beach, he adds.

There are certain conditions that need to be in place, though, for air lubrication to work, says Anthony Molland, professor emeritus of engineering at the University of Southampton in England. The effect of the carpet may be negligible in very rough seas, for instance, and ships have to be traveling quickly to maintain the flow of bubbles.

“If you simply blow the bubbles out and your ship’s not going very fast—we’ve done it with models—the bubbles simply come out the side and don’t do any work at all,” he explains.

But in those scenarios where the technique works, reducing emissions even by a little is worth it, says Silberschmidt: “In this world, we have to do whatever we can.”

This article is from Hakai Magazine, an online publication about science and society in coastal ecosystems. Read more stories like this at hakaimagazine.com.

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