A deadline and the bottom line shaped the new, electric propulsion system for the Colchester-South Hero bike ferry last week.

The changes will be subtle.

Don't look for a wind turbine tower or a solar array along the Island Line causeway -- not this year. But Local Motion's 30-foot pontoon boat that plies the 200-foot gap for summer trail traffic will lose its gas outboard motor and gain energy efficiency.

Faced with a $10,000 budget and a looming graduation from the University of Vermont's College of Engineering and Mathematical Sciences, five seniors confirmed a conservative and conservation-minded design. It features a hybrid power train field-tested by modern locomotives: a diesel generator coupled to electric motors.

The pontoon ferry's 400-pound generator will run on biodiesel, a renewable, plant-derived fuel that the students defined as an interim solution toward even cleaner technologies.

"It's a plug-and-play system," said Justin White, 22, of Danville, at Thursday's meeting of his Senior Experience in Engineering Design (SEED) team. "When something better comes along, we'll lose the generator."

The boat's payload will include a bank of batteries, storing and regulating the onboard generator's output.

Marco Maffeo-Robinson, 21, of Marblehead, Mass., said the system, far from elegant, was nonetheless "future-proof" against advances in power generation.

"You can charge batteries made in the '60s with anything we've got now, or what turns up years from now," he said. "We really want this to be part of a broader education about energy. We want people to talk it up and pay it forward."

The SEED team's design challenge began as a purely academic exercise. Their notes and drawings document brain-storming sessions where nothing was ruled out: a submersible bridge, a tunnel, an elevated cable car, a lighter-than-air balloon.

They even imagined kiosks at each side of the Cut, where tourists could mount pedal-powered generators, burn off some extra steam, and trickle-charge another bank of batteries.

Fanciful ideas should be preserved on the back burner, said Brian Costello, a Local Motion planner for the Island Line Trail who advises the SEED team. He even suggested embellishments to the pedal-power kiosks: a video-game-style interface for the cyclists, one that would display record-breaking sessions at the dynamo.

Friday, that brainstorm edged closer to reality. Costello announced that Windstream Power of Charlotte had donated a human-powered generator to the ferry project, as well as a small wind turbine to mount on the ferry.

Unlike SEED projects sponsored by corporate powerhouses, the bike ferry challenge depends on contributions from small businesses and agencies. Local Motion is contributing $4,000 in cash and staff time out of its operating budget.

Cost-cutting might lead to innovation, and converting the red, wheeled generator to run straight biodiesel will almost certainly void its warranty, said professor Michael Rosen, the team's faculty mentor.

"It shouldn't be a problem," he said. "Engineers routinely violate warranties to push the boundaries of design."

The students seemed content with the compromise they'd reached between endless possibilities and stark spending guidelines. Utility had trumped whimsy.

"Everybody wants to save the Earth, but nobody can afford to do it all at once," Maffeo-Robinson said. "Sure, people can construct a super-efficient house. But what are they willing to do to make it happen -- take out a loan?"

He suggested the team get in touch before the next meeting to discuss further design refinements. They scheduled a conference call.

Contact Joel Banner Baird at 660-1843 or joelbaird@bfp.burlingtonfreepress.com