AT SEA ABOARD THE USS NIMITZ » U.S. Navy ships and planes operated on a biofuel blend off Hawaii during Rim of the Pacific war games Wednesday as part of a renewable-fuel push by Navy leaders — the biggest test of its kind and one that has become politicized and opposed by some in Congress because of its costs.

The sea service’s top brass — Navy Secretary Ray Mabus and Chief of Naval Operations Adm. Jonathan Greenert — flew out to the aircraft carrier Nimitz in helicopters running on the biofuel mix to lend their support to the effort, visiting the Pearl Harbor-based destroyer USS Chafee and other ships in the carrier strike group that has been dubbed the "Great Green Fleet."

"This is a great day for the United States Navy, but it’s a great day for America, too," Mabus told thousands of Nimitz sailors in the hangar bay of the carrier. "It shows that we can make big strides toward energy security. It shows that we can make big strides toward energy independence. It shows that we can reduce the vulnerability that we currently have because of our dependence on foreign sources of oil."

The problem has been the cost.

The Pentagon in December announced the largest government purchase of biofuel in history — 450,000 gallons for $12 million — to fuel RIMPAC ships and aircraft in the first large-scale test of the cleaner-burning alternative to petroleum products.

Mabus acknowledged at the time the price of the biofuel was about $26 per gallon, and about $15 a gallon blended with petroleum for the RIMPAC demonstration. The Navy currently pays less than $4 a gallon for its fuel.

The House Armed Services Committee rebelled in May, proposing in the 2013 defense authorization bill that the Defense Department be prohibited from paying more than traditional fossil fuel costs.

"Now, look, I love green energy, so I am not against it. It is a matter of priorities," the Stars and Stripes newspaper quoted U.S. Rep. Randy Forbes, R-Va., as telling Mabus during a hearing in February. "You are not the secretary of energy. You are the secretary of the Navy."

Dynamic Fuels LLC, a joint venture between Tyson Foods Inc. and Syntroleum Corp., was awarded the contract to supply the Navy with the renewable fuel for RIMPAC. Solazyme Inc. helped Dynamic Fuels fulfill the contract with biofuel that is algae-based, although making up a small percentage of the biofuel. Dynamic Fuels’ plant in Geismar, La., manufactured most of the fuel from used restaurant cooking oil.

Mabus previously said the biofuel purchase "furthers President Obama’s goal to achieve more energy security by finding ways to lessen our dependence on foreign oil and fossil fuels."

He said Wednesday that the Navy still "has a lot of support in Congress" for the renewable energy push.

"If you look at the reasons we’re doing it, we’re not doing it to be faddish, we’re not doing it to be ‘green,’ we’re not doing it for any other reason except it takes care of a military vulnerability that we have," Mabus said during a news conference tied into the meeting Wednesday with sailors.

The Navy has to figure out a way to get "homegrown" fuel that is "stable in price, that is competitive with oil, that we can use to compete with oil," Mabus said.

He added that the Navy has long been an innovator in powering its ships.

"The Navy has done this throughout its history, went from sail to coal, we went from coal to oil, we pioneered nuclear, and once again, this is what the Navy does," he said.

The longer-term goal is to deploy in 2016 a carrier strike group on a normal multi-month deployment using 50 percent biofuel for the surface ships and aircraft. Navy officials said they expect biofuels to be competitively priced with petroleum by then — with a jump-start in demand from the Navy.

The Nimitz, with its clean-burning nuclear reactor, along with its air wing, is part of the several-day Great Green Fleet demonstration. The destroyers Chafee and Chung-Hoon, both based at Pearl Harbor, as well as the cruiser Princeton and oiler Henry J. Kaiser, also are taking part.

The 900,000 gallons of the 50/50 biofuel and petroleum-based marine diesel or aviation fuel blend is expected to be used over several days during RIMPAC. As the Nimitz operated about 115 miles northeast of Oahu on Wednesday, an Australian helicopter also gassed up with biofuel on the carrier in a show of support for the effort.

Lt. Cmdr. Jason Fox was waiting to test the biofuel blend in his E-2C Hawkeye early-warning radar aircraft.

"We’re all excited about it, actually," he said. "We’re looking forward to going out and showing that this can be done in an operational sense."

He added that "everything that came out of testing says there’s no difference."