Zipping through the water, a penguin uses two flippers to propel its rigid body quickly and efficiently. MIT engineers have applied that "technology" to a man-made vehicle that recently took its maiden voyage down a short stretch of the Charles River.
Proteus the Penguin Boat is a 12-foot craft with two oscillating foils, or flippers, attached to its stern. Named after the son of the sea god Poseidon, Proteus could lead to full-scale ships that move more efficiently--and consume less fuel--than those with traditional propellers. An experimental model, Proteus is too narrow to fit even a single passenger.
The boat is the first of its kind. In the past, researchers have performed theoretical computations for propelling a boat with flippers, but until now, no one had built one.
Preliminary tests are promising. When tested in the laboratory, the new propulsion system achieved up to 87 percent efficiency. "The system as implemented on the boat is still being evaluated, since many of the components are novel and require further development," said Professor Michael S. Triantafyllou of the Department of Ocean Engineering, who leads the research team. Professor Triantafyllou noted that the average efficiency of existing ships is at or below about 70 percent.
According to 1992 US Fuel and Shipping Statistics, "converting only 3 percent of the United States shipping fleet to a propulsor with 10 percent higher efficiency would mean an [annual] monetary savings of $15 million, and 120 million fewer liters of petroleum fuel being burned," wrote James T. Czarnowski, a graduate student in the Departments of Ocean Engineering and Mechanical Engineering, in a paper that he and two MIT undergraduates will be presenting on the work at the 1997 International Society of Offshore and Polar Engineers (ISOPE) conference next month.
FISH-LIKE PREDECESSOR
Proteus is a direct descendant of "robotuna," another biologically inspired MIT creation. This four-foot-long robotic fish, patterned after a bluefin tuna, was designed to test the efficiency of a single oscillating foil--a fish's tail. That efficiency proved to be about 85 percent, under conditions that developed the amount of force required normally by marine vehicles, and in 1995 MIT was awarded a patent on the propulsion mechanism.
"The experimental work on the robotuna foil showed high efficiency and high promise. So the next focus was to give it a real-life application," said Professor Triantafyllou.
But ships patterned after the robotuna itself, whose entire body swishes back and forth as it moves through the water, aren't practical. "We thought that if we could remove the body and just keep the tail, we could take a lot of the good from the tuna without the need to have the undulating body," Mr. Czarnowski said.
As with the robotuna, nature helped the engineers with the new design. "Penguins and sea turtles are the biological analogies to what we were looking for," Mr. Czarnowski explained. "Both have rigid bodies like a boat, and propulsion is achieved through oscillation of pectoral flippers."
After videotaping penguins at the New England Aquarium and conducting a feasibility study, Mr. Czarnowski spent six months building a six-foot prototype. The device showed enough promise that Professor Triantafyllou gave the go-ahead to develop a more advanced version.
Among other things, when the engineers studied the wake of the prototype, they found that it closely resembled the wakes of the robotuna and of living tropical fish (Danios). "That was important because it showed us that with two flippers, we were producing the same hydrodynamics that a fish tail produces, and therefore we should see similar efficiencies," Mr. Czarnowski said.
A little over a year later, Mr. Czarnowski, Professor Triantafyllou and colleagues produced Proteus. The multidisciplinary team included three undergraduates: Timothy R. Cleary, a senior in aeronautics and astronautics; William R. Kreamer, a junior in ocean engineering; and Michael C. Murphy, a senior in electrical engineering and computer science. Czarnowski, Cleary and Kreamer will be presenting the work at the ISOPE conference.
Proteus lets the researchers study a wider range of flipper motions than the prototype was capable of, and also allows them to more accurately measure the propulsion mechanism's efficiency. "The prototype was entirely mechanical and was built using 19th-century technology, whereas Proteus uses state-of-the-art robotics," Mr. Czarnowski explained. "It's also twice as big, so we can measure larger forces and have a wake that's easier to visualize."
The boat, which is about 1.5 feet wide by 12 feet long, is a scale model of a fast ship. Packed inside are two car batteries, a 486 desktop computer, a power-sensing circuit, and a voltage converter that changes the battery power into the power used by the computer and by the four motors that control the motion of the flippers. Two large motors allow the flippers to move toward and away from each other; two smaller motors allow them to twist slightly as they do so.
Before the researchers send the boat for a run down the Charles River, they program it for a specific "flapping motion" of the flippers. These commands are relayed to the onboard computer via a monitor and keyboard that are then disconnected from the computer and remain on shore. Once Proteus begins moving through the water, it is operated by a remote control. The same remote control also allows the researchers to start recording (via the onboard computer) how much power the motors are using. Those data, in turn, allow them to calculate the efficiency of each run.
"Once we determine the most efficient flapping motion on the Charles, we want to bring the boat back to the MIT testing tank to study the wake so we know what an efficient wake looks like," said Mr. Czarnowski.
Future work will include designing a flipper system that allows for maneuvering as well as propulsion. Currently Proteus can only move in a straight line. Consequently, for tests on the Charles, the boat is attached via two guides on its left to a long string of fishing line that in turn is strung between two piers. Once it has completed a run from one pier to the other, one of the researchers hauls it back to the "start" with a fishing pole.
A LOVE FOR BOATS
Jim Czarnowski has always loved boats. "I've been building strange little boats like this since I was eight years old," he said. "My first one was a papier mache paddle-wheel boat with a small electric motor that powered a popsicle-stick paddle wheel. Now that I'm at MIT, I can build more advanced models."
What's been the best part of the Proteus project? "Finally seeing it work, and having everyone who's worked so hard on it enjoying that success as well," Mr. Czarnowski said.
He also likes sharing the excitement with others. Every test on the Charles has drawn an audience, even though those tests at the MIT Sailing Pavilion begin at about 5:30am to take advantage of the calm conditions on the river. "Seeing their curiosity and explaining to them what we are doing is very rewarding," he said.
The work is supported by the Office of Naval Research and the MIT Sea Grant College Program.
A version of this article appeared in MIT Tech Talk on April 3, 1997.
