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OCEAN ENVYScientists look toward marine creatures

to improve watercraft designsBY CARRIE LOCK

Ship propellers have served humankind well formore than a century, enabling vessels to travelin relatively straight lines over great distances.But modern engineers want to design vesselsfor more nuanced tasks. They want vehicles

that can hover at the ocean floor and instantaneouslyrespond to the current to hold their positions. Theywant vessels than can quickly maneuver around small objects or in tight spaces. They wantmachines that can operate in the harsh tur-bulence that would destroy existing craft.And they want all these capabilities in anenergy-efficient package. In all, they wantto reinvent the penguin—or perhaps thewhale or a fish.

Next to any marine animal that uses flip-pers to guide itself through the water, human-made vessels are clumsy. While the gracefulanimals’ flippers cut cleanly through thewater as they move with speed and agility,boats’ propellers and rudders push and steerstiff metal hulls far less efficiently, leavingbehind choppy wakes.

Scientists have long sought to unlock thesecrets of nature’s underwater-locomotionschemes, but they’ve usually met with frus-tration. Since the mid-1930s, when Eng-land’s Sir James Gray declared that dolphinsmove through water so efficiently that engi-neering principles were inadequate toexplain the mechanism, people have soughtto understand marine-animal locomotion.Now, researchers in the field of biomimet-ics—the science of mimicking livingthings—have unlocked some of those secretsand are applying their knowledge to proto-type watercraft.

“Half the jobs biomimetic machines willbe doing in 10 years don’t even have a nameright now,” says Chuck Pell, director of sci-ence and technology for Nekton Research, a Durham, N.C.–basedcompany that’s working to develop the next generation of under-water vehicles. “It’s going to be an amazing transformation.”

ALL DRESSED UP The deceptively simple movement of theswimming penguin is the inspiration for a new propulsion systembeing designed by a group of researchers at the MassachusettsInstitute of Technology (MIT). Penguins “have reached a high levelof performance,” says Franz S. Hover, an MIT ocean engineer. “Wenever miss marveling at them.”

Under the power and guidance of its versatile flippers, a pen-guin can move through the water faster than 10 miles per hour,turn almost instantaneously, and leap out of the water onto an ice-berg. You’ll never see a submarine do that, Hover points out.

To accomplish its feats, the penguin must generate forces thatare huge in proportion to its small body. Although scientists can’tfully explain how the animal does it, it’s clear that for its size, apenguin’s stroke creates forces relatively larger than those of apropeller and does it more efficiently.

The researchers are working to emulate that mechanism in a sub-marine by attaching movable fins to the craft’s hull.

Penguins are more maneuverable than ves-sels because their flippers can make differentkinds of motions than propellers can. Pen-guins’ flippers are attached to their bodies ata single rotation point that’s equivalent tothe human shoulder. The flippers flap up anddown, move forward and back, and twistaround in the joint. Propellers, on the otherhand, just rotate. Although they can turn atdifferent speeds, the orientation of theirmotion is fixed.

Today’s submarines have limited maneu-verability. To turn, the sub’s rudder must guidethe craft to one side, while the propeller sim-ply keeps pushing. The sub’s turn is gradual,making a long arc. Submarines must stopgently too: Slowed by a reversal of the pro-peller, the craft drifts to a stop.

Hover and his team are developing pen-guin-style hydrofoils that may someday driveand instantaneously stop submarines andother vessels. The researchers’ design is moreconstrained than that of the flippers foundin nature. The mechanical fins—made ofwood in test models, so far—can flap up anddown and twist, but they can’t move forwardor backward.

“The fluid mechanics problem is simplerthis 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 cansteer the craft right, left, up, or down and move it forward or back.

“With minor tweaks in the fins’ motion, you can get really highmaneuvering forces really quickly,” says Stephen C. Licht of MIT’socean-engineering department.

In one test, Hover and his colleagues reported in the JanuaryJournal of Fluids and Structures, moving wooden fins with a spe-cific wavelike motion produced a better combination of thrust andefficiency than other types of movements did.

AGILE EGG — PilotFish, NektonResearch's unmanned underwater vehicle (top), is tested in a pool in Durham,N.C. Each of its flexible fins (bottom) isconnected to a motor inside the craft.

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The researchers are also looking at how many fins to put on acraft and where they should go. “We can move fins around or addmore to a vehicle,” says Licht. “We can try different combinationsand configurations.” He and his team have a futuristic vision of anunderwater vehicle with perhaps 50 flapping fins, each movingindependently. Licht and his colleagues also plan to develop a feed-back system for the flippers, so that a vehicle can sense its envi-ronment and appropriately respond to it.

A hurdle to developing commercial applications of this tech-nology is its mechanical complexity. Each fin uses two motors,one for flapping and one for twisting. Accompanying hardwaretransforms the motors’ energy intothe appropriate fin motion. “This isthe major thing that scares devel-opers,” says Hover.

The added complexity, however,could pay off in minisubs that haveto maneuver nimbly in turbulentwaters, Licht says.

Hover predicts that his group’sflapping fins will soon surpass pro-pellers for such applications. “Wecan generate larger comparableforces, and we can also generatethem very fast,” he says. And if theresearchers can figure out the finerdetails of how fin movements affecta body’s overall direction and speed,the maneuverability of a penguin-mimicking craft will far exceed thatof a propeller-and-rudder vessel,Hover claims.

“We want to be thinking of how to get the technology out of thelabs and into the manmade crafts,” says Hover.

A NEW KIND OF FISH Other scientists and engineers aremodeling not just the motion of natural fins but also the mate-rial. Those teams are using rubber and silicon instead of metaland wood.

“Fish are made out of collagen, snot, and a few mineral saltsthrown 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’sreleased when it snaps back. Applying some specific forces to flex-ible materials causes them to oscillate, alternately storing andreleasing energy with their movements.

“If you know what you’re doing, you can use this bouncing to youradvantage,” 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 vehiclethat makes use of this oscillating motion. The craft, called Pilot-Fish, is more than 3 feet long, weighs 350 pounds, and looks likea giant egg with four fins coming out of its middle. The craft is notdesigned to travel great distances quickly. Rather, maneuverabil-ity is its specialty. In less than a second, it can go from standingstill to making two rotations per second around its long axis. Also,unlike any other watercraft, it can stop almost instantaneously byslamming its fins forward.

“The thing looks like it hit a wall. It stops dead,” says Pell. “Theonly other things that can do that are alive.”

PilotFish is designed to operate in water too turbulent forother craft. For example, it could be used to inspect underwaterstructures such as bridges and docks. The ever-changing cur-rent of a river or shoreline can easily overwhelm, or even carryaway, a propeller-driven craft. PilotFish can react to the envi-

ronment much more quickly says Pell. If a wave rolls it over, thecraft can roll back to its original position before the next wavecomes. If PilotFish encounters an unexpected object, it can avoidbumping into it.

Each fin has its own, single motor but can move in two differentways. In one motion, the motor twists the fin. This makes the finundulate like a fish and propels the craft forward. The othermotion occurs when the motor jerks the fin in a new direction.“One giant rotation—a single flip or two—can make it stop orchange direction violently in a fraction of a second,” says Pell. Thismovement “moves a huge mass of water in the opposite direc-

tion,” he says.The newfangled fins also gener-

ate huge forces. “You have to becareful around it—you could breakan arm” if a fin hits it, says Pell,adding that he has himself beenhit, with painful but not bone-crushing results.

Nekton is selling its first vehiclethis year to a research team study-ing underwater locomotion,according to Pell. But, he predicts,“the Navy will be our largest cus-tomer.”

PilotFish’s fins have advantagesbeyond endowing a vessel with theever-important maneuverability.Because each fin is a single piece ofrubber, it’s cheaper and less break-able than a hard mechanical fin with

multiple parts, says Pell.

WHALE WATCH Focusing on a different aspect of fins, anotherteam is studying the scalloped edge of the side flippers of hump-back whales. Frank E. Fish, a biologist at West Chester Universityin Pennsylvania, and his colleagues are asking why the flippershave this leading edge.

For their size, humpbacks are surprisingly maneuverable. A50-foot, 30-ton animal can swim in a tight corkscrew pattern,sometimes less than 10 feet across. The whales do this to cap-ture a meal. They blow bubbles as they swim in a circle, thuscreating a rising barrier around a vertical cylinder of water. Thewhale simply swims up through the cylinder and feasts onshrimp and small fish trapped within.

Scientists long wondered how the humpbacks could accom-plish this feat of agility. In the May Physics of Fluids, Fish, work-ing with engineers at Duke University in Durham, N.C., andthe U.S. Naval Academy in Annapolis, Md., showed a connec-tion between the whales’ capacity to swim in tight circles andthe scalloped flippers.

The researchers built two plastic, 2-foot-long whale flippers,about one-eighth the maximum length of an adult’s flipper. Oneartificial flipper had scallops, or tubercles, like the humpback’s,and the other was smooth, like all other whales’ flippers. Thescientists then mounted each fin on a table, hooked up electronicmeasuring devices, and turned on a stream of air. They adjustedthe air’s speed so that the airflow approximated the propertiesof water rushing over a humpback’s side flipper. The scientiststhen measured the forces created by the air stream and the flip-per at various angles.

The tubercles significantly altered the flipper’s perform-ance in the fluid flow. Lift, comparable to the upward forceon an airplane wing, was 8 percent greater on the scallopedflipper than on the smooth one. Drag, the counterbalancingforce to lift, was as much as 32 percent less on the scallopedflipper than on the smooth one. The extra lift and reduceddrag on the flipper turns a humpback’s body more sharply

FARTHER, FASTER — The two underwater fins of a HobieCat kayak are powered by a person pedaling while sittingon the craft and are more efficient than hand-held oars.

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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 fluidclosest to the fin’s top surface. This adds extra energy to the layer,keeping it flowing alongthe whale’s flipper ratherthan detaching into a dis-organized jumble.

When fluid flowdetaches from a wing orflipper during a turn,whatever is being pow-ered—whale, boat, or air-plane—falls off the curvedpath. Without tubercles,the tightly circling whalewould jump the track likea race car taking a turn attoo great a speed.

Tubercles could beused in a variety of seavessels or in entirely dif-ferent craft, such as airplanes, says Fish. “This is a case where thedesign of animals can be integrated into engineered devices,” hesays.

UNIQUE APPLICATION Biomimetic fins are already in com-mercial use, although in applications that earlier researchersmight not have expected.

In 1997, James T. Czarnowski, then a graduate student at MIT’socean-engineering program, launched Proteus the Penguin Boat,a 12-foot craft propelled by two penguinlike fins, down Boston’sCharles River. At the same time, Gregory Ketterman at the Ocean-side, 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 pad-dling, the user pedals with his or her feet to power two fins under

the kayak. The fins can move

larger volumes of waterthan a traditional oar canand do so with far lessenergy expenditure. Thisenables kayakers to go far-ther and faster before get-ting tired. “It works sur-prisingly well,” saysCzarnowski.

Hobie Cat’s kayaks usea system slightly differentfrom that of the MIT team.For one thing, humanmuscles, not motors,power the fins. Foranother, 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 antici-pate that other commercial applications and some military oneswill soon follow.

Once these vessels have made a splash, the field of biomimet-ics will face other challenges, not least of which is improving onnature’s blueprints.

“It’s a trap you don’t want to fall into: Thinking this is the wayit’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 restric-tions on animals.” ■ W

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TIGHT TURNS — A humpback whale shows off its side flipper with its distinctive scalloped edge. Researchers built models of a humpback whale’s flipper (left) and another whale’s flipper (right) to compare their performances.

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