Navy Adopts Hybrid-Electric Amphibious Assault Ships

The U.S. Navy is incorporating fuel-efficient hybrid-electric propulsion technology onto several of its next-generation big-deck amphibious assault ships, the service's top civilian said.

The USS America (LHA-6) and the USS Tripoli (LHA-7) are part of what the Navy calls its now-in-development America-class amphibious assault ships designed with, among other things, a larger deck space to increase the ability to transport and utilized air assets, Navy Secretary Ray Mabus told an audience May 21 at the 2013 Energy Efficiency Global Forum, Washington, D.C. The ships are being engineered with a hybrid-drive propulsion system, meaning the ships can use diesel-electric propulsion as well as gas-turbine engines.

When asked about the service's broader initiative to power ships and planes with alternative fuels, known as the "Great Green Fleet," Mabus emphasized that “now is the time to do it.” The effort has drawn criticism from lawmakers such as Sen. Tom Coburn, R-Okla., who say it's not worth the added cost.

Hybrid-electric propulsion systems use a gas turbine engine as well as an electric motor and diesel generator. The electric motors can help propel the ship at speeds up to around 12 knots and the generator can help produce electricity for the ship. When it comes to traveling at speeds greater than 12 knots or so, the ship can then rely upon its gas-turbine engine.  At the same time, the generators can provide on-board power for many of the ships systems such as sensors, weapons and other electronics, Navy officials indicated.

The hybrid-drive allows the ship to propel itself using either electric drives or a traditional gas turbine engine.  Electric propulsion and on-board electrical power generation are both integrated through what’s called a main reduction gear (MRG), a portion of the ship's propulsion system which helps convert energy into the revolutions needed for the propellers to move the ship through the water, according to Navy officials.

“This unique auxiliary propulsion system (APS) is designed with fuel efficiency in mind. The APS uses two induction-type auxiliary propulsion motors (APM) powered from the ship's electrical grid instead of using main propulsion engines to power the ship's shaft. Instead of using its gas turbines which are less efficient at lower speeds, the ship will be able to use its APS for roughly 75 percent of the time the ship is underway,” a Navy official said.

Mabus cited the successful deployment of the USS Makin Island (LHD-8), a 40,000-ton Wasp-class big deck amphibious assault ship able to transport as many as 3,000 sailors and Marines along with massive amounts of equipment, air assets and a Marine Expeditionary Unit (MEU). The Makin Island, commissioned in 2009 and based at Naval Home Port, San Diego, uses a hybrid-electric propulsion system, Mabus explained. As a result, the ship can operate on much less fuel compared to previous ship models of its kind using traditional steam propulsion.

In fact, during its maiden deployment last year, the Makin Island’s hybrid propulsion system helped contribute to as much as $15 million in fuel-cost savings over the course of the excursion, Mabus told the audience.

One analyst said hybrid-electric propulsion technology shows great promise for ships in terms of both propulsion and on-board electrical systems.

“This technology is really coming along with things like the Prius and ground vehicles. We’ve had decades of work of hybrids and now we’re seeing the potential to exploit it to where it is really applicable at the ship level,” said Daniel Goure, vice president of the Lexington Institute,  a Virginia-based think tank.

The on-board power advantage could be substantial when it comes to powering up ship systems such as computer technologies, sensors and weapons such as high-energy lasers, Goure added.

Mabus discussed the Navy’s plans to develop more hybrid-electric propulsion in the context of a broader strategic discussion about the importance of projecting and sustaining Naval power while maximizing energy-efficiency and lowering costs wherever possible.

“The vast majority of power in the Navy comes from fossil fuels. Because we purchase our fuel on the open market, the price of oil has had a dramatic effect on our budget. Every time the price of oil goes up more than a dollar a barrel, it cost the Navy and the Marines S30 million of dollars in additional fuel costs,” Mabus said.

In fact, for fiscal year 2011-2012, the Navy wound up with roughly $1 billion in unfunded fuel costs, he added.

Reducing fuel costs are also a substantial part of the Navy’s “Great Green Fleet” initiative wherein an entire USS Nimitz Carrier group, including both ships and air assets, were successfully powered by a 50-50 blend of biofuels and traditional fuel during last-year’s Rim of the Pacific (RIMPAC) international maritime exercise, Mabus explained

“The ships and aircraft were operating as part of RIMPAC. The 2012 demo was for nearly two days wherein the ships and aircraft operated on biofuels. The intent was to have them sailing on the biofuels – demonstrating a 50-50 blend. Prior to demonstrating great green fleet, we had tested this capability.  This was an opportunity to showcase on a large scale that it was possible to use our existing ships and planes with biofuels,” said Capt. Pamela Kunze, spokeswoman for Navy secretary.

Kunze also explained that the biofuels in development needed to be “drop in” fuels that could work with existing ship or aircraft engines.

However, some critics and lawmakers in Congress have questioned the utility of the Navy’s “Great Green Fleet” initiative on the grounds that alternative fuel sources create vast expenditures in a time of needed fiscal constraint.  Also, Lexington’s Goure added to the chorus of criticisms regarding biofuels, saying the high expense was not likely to pay dividends in the future.

Overall, being able to substantially reduce fuel cost will better enable the Navy to perform missions, maintain a forward presence and achieve its tactical and strategic objectives in an increasingly challenging global environment, Mabus added.

“Being there means presence. Presence matters. When North Korea threatens a missile launch, our ships with missile defense technology are there. When Marines in Afghanistan need air support, Marine pilots from ground based units and pilots from our carriers are there. When the earthquake struck Haiti in 2010 and the tsunami struck Japan in 2011, we had Navy ships helping within hours because they were already there,” Mabus told the crowd.