In an October 2009 speech at the Naval Energy Forum, Secretary of the Navy Ray Mabus, who had taken over the post six months before, unveiled five energy targets he wanted the Navy to hit over the course of the following decade.

"Energy reform is a strategic imperative," he said.

One of the targets involved the deployment, by 2016, of what he called the Great Green Fleet, a carrier strike group "composed of nuclear ships, surface combatants equipped with hybrid electric alternative power systems running biofuel, and aircraft flying only [on] biofuels."

Three years later, following a vicious battle with Republican legislators over the initiative, a 50/50 blend of chicken fat and conventional petroleum successfully powered two destroyers and a cruiser for two days during a month-long warfare exercise in Hawaii. The feat required 450,000 gallons of biofuel at a cost of $12 million; at the time, it was the largest-ever single purchase of biofuel.

But Mabus's vision of the Great Green Fleet — named in homage to the Great White Fleet, which projected America's naval power around the world in the early 20th century — hasn't come to pass. In January, a strike group powered in part by biofuel deployed, but the blend had just 10 percent rendered beef fat compared to 90 percent conventional diesel. Navy planes have used biofuel once, during a test.

This past May, VICE News visited the USS Farragut and USS Bainbridge, Arleigh Burke–class guided missile destroyers that were running on the 10/90 blend. A few weeks later, another destroyer, the USS Mason, took delivery of a fuel blend made up of 94.5 percent conventional diesel and 5.5 percent biodiesel made from palm oil. None of the ships required engine modifications to use the fuel.

'There's more important shit to spend money on in the military, period.'

For 2016, the Navy has purchased just under 80 million gallons of the 10/90 biofuel blend, about 6 percent of the 1.3 billion gallons of fuel the Navy uses annually. The Navy paid $2.05 per gallon, which is roughly in line with the cost of regular marine diesel thanks to robust biofuel subsidies from the US government, though that hasn't satisfied the program's detractors.

"There's more important shit to spend money on in the military, period," says US Representative Duncan Hunter, a Marine Corps veteran and Republican congressman from San Diego, principal homeport of the Navy's Pacific Fleet.

He describes the decision-makers behind the Great Green Fleet as "all the smartest guys that went to the John F. Kennedy School of Smart People," and calls their interest in biofuels "just stupid." The Navy is facing a $7 billion budget reduction in 2017, and the concept of a Great Green Fleet, Hunter says, is simply political posturing by Mabus and the White House.

The Navy would not make Mabus available for comment. But Joe Bryan, the Navy's Deputy Assistant Secretary for Energy, says there's nothing political going on.

"This isn't about the secretary, it's not about the administration," Bryan says. "Our job is to look out into the future, plan for contingencies, and make ourselves as flexible as possible."

The focus on energy, Bryan adds, "is not going to go away. It's too important to the Navy's mission, too important to our sailors and marines, for us to change course."

Whether a blend containing just 5 percent to 10 percent of biofuel can rightly be considered an alternative fuel is up to the Navy, which sets the terms absent any statutory requirements. The relative lack of guidelines can also lead to some seemingly contradictory outcomes. A recent analysis by Transport & Environment, a European conservation group, found that biodiesel made from palm oil leads to three times the overall CO2 emissions of regular petroleum-based diesel. Therefore the biofuel the Navy took on in the Mediterranean earlier this year arguably enlarged its carbon footprint.

But palm oil is hardly the only source of biofuel being explored by the US government.

"The Department of Energy is investing a lot of money into algae-based fuels, which are promising, but often the greenest fuels are also the most expensive ones," says Emily Cassidy, a research analyst at the Washington, DC-based Environmental Working Group.

The Navy has paid as much as $424 a gallon for algae-based biofuel in the past, but expensive alternative fuels are now a non-starter for congressional Republicans, who have blocked the Navy from spending more on a gallon of biofuel than it does on a gallon of regular diesel. Since it costs more to turn seeds, weeds, or beef trimmings into usable fuel than it does to extract fossil fuels from the ground and refine them, it's all but impossible for the fleet to use substantial amounts of biofuels with crude oil prices are as low as they currently are.

That said, it's not necessarily economics that have been holding back the wider use of biofuels, says Professor Chris Somerville, who holds the Philomathia Chair in Alternative Energy at the University of California, Berkeley. The greater problem, he says, has been producing enough of it.

The three military-spec biofuel refineries in the US, which were financed by the US government under the Defense Production Act, are aiming to produce roughly 100 million gallons this year. The Department of Defense burns 4.6 billion gallons of petroleum annually, with the Navy accounting for about a third of that.

Biofuels need to be created from something, and according to a report released last year by the nonpartisan World Resources Institute, a Washington, DC-based research group, meeting 20 percent of global energy demand using plant-based biofuels by 2050 "would require humanity to at least double the world's annual harvest of plant material in all its forms.... Therefore, the quest for bioenergy at a meaningful scale is both unrealistic and unsustainable."

These are a few of the reasons, says Somerville, the Navy will not be able to come close to using a 50/50 biofuel blend on a large scale for a long time, if ever.

The Navy touts other efficiency measures as part of the Great Green Fleet initiative. Some appear significant, like the addition of stern flaps that reduce drag and turbulence, which can boost a ship's fuel economy by 3.4 percent. Others appear less so, like reminders to crew to take water-saving "Navy showers" — a technique that has long been employed by the military — and a switch over to LED lightbulbs.

"We used to save energy if we could," said an officer aboard the Bainbridge. "Now we save energy unless we can't."

There have been no energy savings resulting from the use of biofuel blends.

The military has a long history of subsidizing new technologies, and some of them have ended up making huge impacts, says Richard Heinberg, senior fellow at the Post Carbon Institute, a California think tank focused on sustainability and climate change. But he says biofuels will never join that list of technologies because the physics simply don't work; biofuels can't supply the energy the Navy needs to power its fleet.

Which begs the question: Why does the Navy want to green its fleet with biofuels?

Navy officials say the Great Green Fleet initiative isn't about being green so much as it's about giving ships greater range, the ability to stay on-station longer, and the ability to "deliver more firepower on a tank of gas."

Somerville says that "the Navy is essentially doing, well, I don't really know what they're doing. I suppose they're making a statement that they want renewables."

Hunter's take is that a "green [Navy] sounds great to a large part of the population that votes a specific way."

And Heinberg says it's due to nothing short of an existential crisis.

"[Navy officials] see the future of oil as bleak, and without oil, how do they stay in business? There is no real answer," he says. "They've got to grasp at some straw or another, and this is the one that's nearest.... But just because people need something doesn't mean it exists."