Three years ago, mechanical engineer Megan Leftwich of George Washington University was at the zoo with her kids and noticed something strange about the sea lions jetting about their tank. Other marine mammals like whales and dolphins, and almost all fish, power themselves around with their tails. Sea lions, though, are mavericks. They pull themselves through the water with their powerful front flippers.

And the result is magical: Sea lions are unparalleled in their maneuverability, dancing about in pursuit of prey (or while fleeing their enemies). So what is it about the sea lion’s flipper, Leftwich wondered, that makes this possible? She’s now unraveling the answer by—what else—3-D printing her own robotic flipper replicas, with the mind that one day engineers could use the sea lion’s secrets to create super-maneuverable, super-stealthy vehicles that’ll better navigate dangers like underwater minefields.

To build a robo-flipper, Leftwich first had to understand the sea lion’s movement. She already knew the basics: The sea lion drags its large, powerful front flippers from its nose down to its belly, “and that creates a jet,” says Leftwich. “The jet goes in one direction and the sea lion goes in another. It’s sort of like a breast stroke for a human.” And thanks to its streamlined shape, the sea lion doesn’t need to constantly paddle, instead flapping and coasting, flapping and coasting.

But things get less straightforward when you look closer at the trailing edge of the sea lion’s flipper. It’s not straight, but undulating. Marine mammals like humpback whales have these structures on their flippers as well, but, Leftwich says, “I haven’t seen many that are as regular and pronounced as the sea lion.” These, she reckons, may be helping direct the flow of water uniformly over the end of the flipper to optimize thrust.

To figure out the mechanism at work, Leftwich and her team looked closely at 21 different spots on the limb, identifying almost 21 different textures on its surface. “The leading edge has these thick, scaly patches of skin and not much hair,” she says. “And the trailing edge has no hair at all, and all of the grooves go toward the end. They’re very fine wrinkles in one direction.”

FigsInProgress.pptxScanning electron microscope images of the various textures on a sea lion’s flipper. Notice the grooved texture on the trailing edge and the bumpier texture at the leading edge. Megan Leftwich

So what’s going on here? Leftwich thinks that the scaly leading edge of the flipper generates turbulence, which helps keep the flow of water “attached” to the sea lion—that is, running in lines parallel to the surface. This would help the sea lion get a “grip” on the water, improving maneuverability. Back at the trailing edge, those grooves may help funnel the water in a consistent direction, as opposed to flowing off the flipper willy-nilly.

But still, that’s all theory. To really find out what makes the sea lion’s flipper work, Leftwich had to build her own.

Flipper, Flipper, Faster Than Lightning

The sea lion’s first development timeline was rather brutal—evolution killed off lots of imperfect designs before settling on this particular flipper. Leftwich’s approach is rather more gentle. “We have very detailed laser scans, we know exactly what it should look like, and we can print a robo-flipper that looks just like that,” she says. The final product is made of silicone, packed with robot guts like wiring. Additionally, Leftwich and her team have done motion-tracking studies on swimming sea lions to determine how their robo-flipper should be flipping.

Prototype in hand, the team can get to the important stuff: testing and optimization. Placing the robo-flipper in a large tank with flowing water, they can measure things like thrust and drag as the structure claps. Then, the scientists can make slight modifications—leaving out the characteristic undulations at the trailing end of the flipper, for instance—and observe changes in the structure’s performance. “We can test the difference in thrust production, the drag coefficient, of these different shapes to tell us possibly how it might have an effect,” Leftwich says, noting that they should have some solid data by the end of the year.

The idea is to use the findings to one day build underwater vehicles that mimic the way a sea lion swims. “One application certainly with any animal this size that you’re studying, you’re thinking mid-sized aquatic vehicles,” Leftwich says. “So we’re not talking about little remote-control sailboats, but we’re also not talking about full-scale submarines.”

Sophisticated autonomous underwater vehicles—which are often shaped like torpedoes with wings and tails—already roam the oceans. But AUVs are propeller-powered, and about as maneuverable as sedated manatees.

flipper_424 Jared Soares for WIRED

“But what if you need to be more agile, what if they need to send an underwater vehicle through a field to disarm old bombs that were put down there in World War II?” Leftwich asks. “Using your arms to push, you can be much more agile, you can turn much more quickly if you all of a sudden see something coming up.”

Propellers also produce a telltale wake structure, making the vehicles trackable for someone who may not appreciate their presence. Dolphins and sharks produce characteristic vortices too, but not the sea lion.

“The wake of the sea lion appears to be just a strong jet with some not-terribly-coherent vortices surrounding the jet,” Leftwich says. Perhaps by exploiting the sea lion’s strategy, engineers could create AUVs that don’t put out a characteristic wake, making them “hydrodynamically quiet,” as she puts it.

As for AUVs that can also balance a ball on their noses, that might take a bit more work. But it never hurts to dream.

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Sea Lion Flippers Could Inspire Super-Stealthy Submersibles