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July 2019

The fascinating tale of a what a weed can teach us about reducing drag on ships

On trips to southern Brazil and to the southern states here in the US, I have seen firsthand the devastating effects of an aquatic weed known as the Giant Salvinia.

Giant Salvinia can grow extremely fast - doubling in weight every 2 to 3 days in optimal conditions. The floating weed can invade ponds and lakes producing a mat as thick as 2 feet (60 cm) though typically it's less than half that. Since this can kill fish and plants below the surface, it's considered a threat to biodiversity.

Despite its potentially negative impact on the eco system, Giant Salvinia may have a few redeeming qualities. A study done in the Philippines shows that the weed could be used to treat blackwater effluent with the capacity to remove nearly half of fecal coliform.1 Additionally, researchers in Texas (where there is a lot of Giant Salvinia) have discovered that the weed can be used to stop the growth of human cancer cells.2 

However, perhaps the most amazing and potentially useful discovery is what's referred to as the Salvinia Effect, a term coined by researchers at the University of Bonn.3 The plants leaves are covered with small hair-like structures which are extremely hydrophobic. In some species of the plant, the structures - under magnification - look like inverted eggbeaters. See SEM image below. The tips of the structures, however, are hydrophilic. When the leaf is submerged, tiny air pockets are formed by the hydrophobic structures. But it's the hydrophilic tips that pin the air pockets and keep them from rolling off. The end result is a thin layer of air between the leaf and the water.

Researchers, in the spirit of biomimicry, are attempting to emulate this structure in order to create surfaces that permit a thin layer of air between the submerged surface and water. Tests have shown that a Salvinia-inspired micropatterned surface greatly reduces drag.5 The application that appears to potentially benefit the most from this discovery is transoceanic shipping. Preliminary research suggests that friction can be reduced in excess of 30%. In addition, the thin layer of air between the hull and the water not only reduces energy, but is also environmentally superior to conventional anti-fouling methods that rely on toxic metals.

A container vessel, like the massive 1300 ft (396m) long Emma Maersk, shown above, can carry 15,200 shipping containers and clock along at 25.5 knots (29 mph).6 The equally massive 14-cylinder turbo engine, the largest in the world, drinks 380 tons of fuel daily.

If you extend your vista to the 100,000 or so ships that ply the world's ocean waters, that translates to an astonishing 2.5 billion barrels of bunker and diesel fuels consumed annually. Even if a Salvinia-inspired surface could reduce friction by a modest 25%, the annual savings for the world's fleets would be in excess of $30 billion USD at today's fuel prices. In addition to cost savings, there would be enormous reductions in emissions, and the use of toxic anti-fouling hull paints could be eliminated helping make the planet less polluted. Now you know how one of Mother Nature's weeds can teach us about reducing drag on ocean bound vessels while saving billions of dollars in fuel and reducing emissions and pollution in our seaways.

1 See this study summary from Xavier University - Ateneo de Cagayan.
2 See this study from Stephen F. Austin State University.
3 See the excellent summary in this article and a more detailed article here.
4 Issempa, Salvinia zoom, cropped out third image only, CC BY-SA 3.0.
5 See this paper for details on 3D Micropatterned Surfaces Inspired by Salvinia Molesta
6 More on Emma Maersk here.

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