Ship propellers have served humankind well for more than a century, enabling vessels to travel in relatively straight lines over great distances. But modern engineers want to design vessels for more nuanced tasks. They want vehicles that can hover at the ocean floor and instantaneously respond to the current to hold their positions. They want vessels than can quickly maneuver around small objects or in tight spaces. They want machines that can operate in the harsh turbulence that would destroy existing craft. And they want all these capabilities in an energy-efficient package. In all, they want to reinvent the penguin—or perhaps the whale or a fish.
Next to any marine animal that uses flippers to guide itself through the water, humanmade vessels are clumsy. While the graceful animals’ flippers cut cleanly through the water as they move with speed and agility, boats’ propellers and rudders push and steer stiff metal hulls far less efficiently, leaving behind choppy wakes.
Scientists have long sought to unlock the secrets of nature’s underwater-locomotion schemes, but they’ve usually met with frustration. Since the mid-1930s, when England’s Sir James Gray declared that dolphins move through water so efficiently that engineering principles were inadequate to explain the mechanism, people have sought to understand marine-animal locomotion. Now, researchers in the field of biomimetics—the science of mimicking living things—have unlocked some of those secrets and are applying their knowledge to prototype watercraft.
“Half the jobs biomimetic machines will be doing in 10 years don’t even have a name right now,” says Chuck Pell, director of science and technology for Nekton Research, a Durham, N.C.–based company that’s working to develop the next generation of underwater vehicles. “It’s going to be an amazing transformation.”
All dressed up
The deceptively simple movement of the swimming penguin is the inspiration for a new propulsion system being designed by a group of researchers at the Massachusetts Institute of Technology (MIT). Penguins “have reached a high level of performance,” says Franz S. Hover, an MIT ocean engineer. “We never miss marveling at them.”
Under the power and guidance of its versatile flippers, a penguin can move through the water faster than 10 miles per hour, turn almost instantaneously, and leap out of the water onto an iceberg. You’ll never see a submarine do that, Hover points out.
To accomplish its feats, the penguin must generate forces that are huge in proportion to its small body. Although scientists can’t fully explain how the animal does it, it’s clear that for its size, a penguin’s stroke creates forces relatively larger than those of a propeller and does it more efficiently.
The researchers are working to emulate that mechanism in a submarine by attaching movable fins to the craft’s hull.
Penguins are more maneuverable than vessels because their flippers can make different kinds of motions than propellers can. Penguins’ flippers are attached to their bodies at a single rotation point that’s equivalent to the human shoulder. The flippers flap up and down, move forward and back, and twist around in the joint. Propellers, on the other hand, just rotate. Although they can turn at different speeds, the orientation of their motion is fixed.
Today’s submarines have limited maneuverability. To turn, the sub’s rudder must guide the craft to one side, while the propeller simply keeps pushing. The sub’s turn is gradual, making a long arc. Submarines must stop gently too: Slowed by a reversal of the propeller, the craft drifts to a stop.
Hover and his team are developing penguin-style hydrofoils that may someday drive and instantaneously stop submarines and other vessels. The researchers’ design is more constrained than that of the flippers found in nature. The mechanical fins—made of wood in test models, so far—can flap up and down and twist, but they can’t move forward or backward.
“The fluid mechanics problem is simpler this way,” says Hover.
The fins move a craft by producing high-energy vortices—rings of spinning water. Coordinating the motion correctly creates a jet of water behind a fin. This pushes the fin, and therefore the craft, in the desired direction. Different types of fin movements can steer the craft right, left, up, or down and move it forward or back.
“With minor tweaks in the fins’ motion, you can get really high maneuvering forces really quickly,” says Stephen C. Licht of MIT’s ocean-engineering department.
In one test, Hover and his colleagues reported in the January Journal of Fluids and Structures, moving wooden fins with a specific wavelike motion produced a better combination of thrust and efficiency than other types of movements did.
The researchers are also looking at how many fins to put on a craft and where they should go. “We can move fins around or add more to a vehicle,” says Licht. “We can try different combinations and configurations.” He and his team have a futuristic vision of an underwater vehicle with perhaps 50 flapping fins, each moving independently. Licht and his colleagues also plan to develop a feedback system for the flippers, so that a vehicle can sense its environment and appropriately respond to it.
A hurdle to developing commercial applications of this technology is its mechanical complexity. Each fin uses two motors, one for flapping and one for twisting. Accompanying hardware transforms the motors’ energy into the appropriate fin motion. “This is the major thing that scares developers,” says Hover.
The added complexity, however, could pay off in minisubs that have to maneuver nimbly in turbulent waters, Licht says.
Hover predicts that his group’s flapping fins will soon surpass propellers for such applications. “We can generate larger comparable forces, and we can also generate them very fast,” he says. And if the researchers can figure out the finer details of how fin movements affect a body’s overall direction and speed, the maneuverability of a penguin-mimicking craft will far exceed that of a propeller-and-rudder vessel, Hover claims.
“We want to be thinking of how to get the technology out of the labs and into the manmade crafts,” says Hover.
A new kind of fish
Other scientists and engineers are modeling not just the motion of natural fins but also the material. Those teams are using rubber and silicon instead of metal and wood.
“Fish are made out of collagen, snot, and a few mineral salts thrown in,” says Pell. “Traditional manmade things are really rigid,” he says, “but flexible things have many advantages.”
Supple materials can store energy in ways that stiff ones can’t. For instance, a stretched rubber band has potential energy that’s released when it snaps back. Applying some specific forces to flexible materials causes them to oscillate, alternately storing and releasing energy with their movements.
“If you know what you’re doing, you can use this bouncing to your advantage,” as many marine animals do, says Pell.
Imagine a dolphin. When its tail flexes as far as it will bend, energy is stored in the dolphin’s body. When the tail slams down, energy is released, and the dolphin moves forward.
Nekton designed flexible fins for use on an underwater vehicle that makes use of this oscillating motion. The craft, called PilotFish, is more than 3 feet long, weighs 350 pounds, and looks like a giant egg with four fins coming out of its middle. The craft is not designed to travel great distances quickly. Rather, maneuverability is its specialty. In less than a second, it can go from standing still to making two rotations per second around its long axis. Also, unlike any other watercraft, it can stop almost instantaneously by slamming its fins forward.
“The thing looks like it hit a wall. It stops dead,” says Pell. “The only other things that can do that are alive.”
PilotFish is designed to operate in water too turbulent for other craft. For example, it could be used to inspect underwater structures such as bridges and docks. The ever-changing current of a river or shoreline can easily overwhelm, or even carry away, a propeller-driven craft. PilotFish can react to the environment much more quickly says Pell. If a wave rolls it over, the craft can roll back to its original position before the next wave comes. If PilotFish encounters an unexpected object, it can avoid bumping into it.
Each fin has its own, single motor but can move in two different ways. In one motion, the motor twists the fin. This makes the fin undulate like a fish and propels the craft forward. The other motion occurs when the motor jerks the fin in a new direction. “One giant rotation—a single flip or two—can make it stop or change direction violently in a fraction of a second,” says Pell. This movement “moves a huge mass of water in the opposite direction,” he says.
The newfangled fins also generate huge forces. “You have to be careful around it—you could break an arm” if a fin hits it, says Pell, adding that he has himself been hit, with painful but not bone-crushing results.
Nekton is selling its first vehicle this year to a research team studying underwater locomotion, according to Pell. But, he predicts, “the Navy will be our largest customer.”
PilotFish’s fins have advantages beyond endowing a vessel with the ever-important maneuverability. Because each fin is a single piece of rubber, it’s cheaper and less breakable than a hard mechanical fin with multiple parts, says Pell.
Whale watch
Focusing on a different aspect of fins, another team is studying the scalloped edge of the side flippers of humpback whales. Frank E. Fish, a biologist at West Chester University in Pennsylvania, and his colleagues are asking why the flippers have this leading edge.
For their size, humpbacks are surprisingly maneuverable. A 50-foot, 30-ton animal can swim in a tight corkscrew pattern, sometimes less than 10 feet across. The whales do this to capture a meal. They blow bubbles as they swim in a circle, thus creating a rising barrier around a vertical cylinder of water. The whale simply swims up through the cylinder and feasts on shrimp and small fish trapped within.
Scientists long wondered how the humpbacks could accomplish this feat of agility. In the May Physics of Fluids, Fish, working with engineers at Duke University in Durham, N.C., and the U.S. Naval Academy in Annapolis, Md., showed a connection between the whales’ capacity to swim in tight circles and the scalloped flippers.
The researchers built two plastic, 2-foot-long whale flippers, about one-eighth the maximum length of an adult’s flipper. One artificial flipper had scallops, or tubercles, like the humpback’s, and the other was smooth, like all other whales’ flippers. The scientists then mounted each fin on a table, hooked up electronic measuring devices, and turned on a stream of air. They adjusted the air’s speed so that the airflow approximated the properties of water rushing over a humpback’s side flipper. The scientists then measured the forces created by the air stream and the flipper at various angles.
The tubercles significantly altered the flipper’s performance in the fluid flow. Lift, comparable to the upward force on an airplane wing, was 8 percent greater on the scalloped flipper than on the smooth one. Drag, the counterbalancing force to lift, was as much as 32 percent less on the scalloped flipper than on the smooth one. The extra lift and reduced drag on the flipper turns a humpback’s body more sharply than a smooth fin could.
The key, says Fish, is that tubercles disrupt the flow of water over the humpback’s flipper, causing vortices in the layer of fluid closest to the fin’s top surface. This adds extra energy to the layer, keeping it flowing along the whale’s flipper rather than detaching into a disorganized jumble.
When fluid flow detaches from a wing or flipper during a turn, whatever is being powered—whale, boat, or airplane—falls off the curved path. Without tubercles, the tightly circling whale would jump the track like a race car taking a turn at too great a speed.
Tubercles could be used in a variety of sea vessels or in entirely different craft, such as airplanes, says Fish. “This is a case where the design of animals can be integrated into engineered devices,” he says.
Unique application
Biomimetic fins are already in commercial use, although in applications that earlier researchers might not have expected.
In 1997, James T. Czarnowski, then a graduate student at MIT’s ocean-engineering program, launched Proteus the Penguin Boat, a 12-foot craft propelled by two penguinlike fins, down Boston’s Charles River. At the same time, Gregory Ketterman at the Oceanside, Calif., company Hobie Cat, which manufactures catamarans, sailboats, and kayaks, was trying to develop similar technology.
Today, Czarnowski is a design engineer at Hobie Cat, working on the company’s Mirage Drive system for kayaks. Instead of paddling, the user pedals with his or her feet to power two fins under the kayak.
The fins can move larger volumes of water than a traditional oar can and do so with far less energy expenditure. This enables kayakers to go farther and faster before getting tired. “It works surprisingly well,” says Czarnowski.
Hobie Cat’s kayaks use a system slightly different from that of the MIT team. For one thing, human muscles, not motors, power the fins. For another, the flexible fins twist on their own.
These pedaled kayaks, already on the market, are leading the way in applications of biomimetic-flipper design. Researchers anticipate that other commercial applications and some military ones will soon follow.
Once these vessels have made a splash, the field of biomimetics will face other challenges, not least of which is improving on nature’s blueprints.
“It’s a trap you don’t want to fall into: Thinking this is the way it’s done in nature, so this is the way we have to do it,” says Licht. “We want to learn as much as we can, so we can bypass the restrictions on animals.”
