The Navy needs to grow its fleet from 308 to 355 ships to meet its strategic requirements amid mounting threats from China and Russia, according to a new document released Friday.

The new number roughly matches President-elect Trump's calls on the campaign trail for a 350-ship Navy, but comes from a 2016 Force Structure Assessment that began in January.

The fleet currently has 273 ships. The new force structure calls for 12 aircraft carriers, 104 large surface combatants, 52 small surface combatants, 38 amphibious ships and 66 submarines. Most of the growth comes in large warships and attack submarines, as well as an additional aircraft carrier.

The Gerald R. Ford-class carriers are being built by Newport News Shipbuilding, a subsidiary of Huntington Ingalls Industries; the Virginia-class submarines are being built through a partnership between Newport News Shipbuilding and General Dynamics' Electric Boat; and General Dynamics Bath Iron Works is building the first of the Flight III Arleigh Burke-class destroyers.

The plan could stress U.S. shipyards. Bryan McGrath, an expert at the Hudson Institute, said it's doable, but is "going to be tight."

"Very skilled people like nuclear welders, they don't grow on trees," he said.

Once the ships are built, they'll need to be maintained, which will continue to strain the industrial base. Because many repair shops have been shuttered, "the maintenance and upkeep of a 355-ship Navy is going to stretch the existing industrial repair base."

Beci Brenton, a spokeswoman for Huntington Ingalls Industries, said the company is ready to leverage its "hot production lines" to support the Navy.

"Although we haven't seen the new shipbuilding plan based on this force structure assessment, we would hope that it accelerates the construction of warships to take advantage of the current momentum in our shipyards so we can continue to build our nation's fleet in the most efficient and cost effective manner," she said.

The plan could also cost billions of dollars, Mcgrath said, estimating that shipbuilding costs alone could rise to $10-13 billion a year and manning, training and equipping a 355-ship fleet could cost about $40 billion a year.

"A 355-ship Navy is a significantly more expensive undertaking than a 272-ship one," he said.

McGrath said the Navy got the new force assessment "mostly right" and said he agreed with the increase in aircraft carriers and attack subs, but argued that the service could "get more Navy for the money" by increasing the number of small surface combatants instead of large surface combatants, which the Navy said it needs to increase ballistic missile defense.

"The money that is spent on those ships I think would be better put to a new class of frigate to replace the [littoral combat ship,]" he said. "I think the ballistic missile defense requirement that the Navy is standing up to drives a force structure that skews toward large surface combatants when Navy really needs to be thinking more about sea control."

McGrath also argued that the service would need more logistics ships such as oilers and tenders in a time of war, so should take that higher number into account for ship planning instead of relying on peace-time requirements.

An executive summary of the new plan stresses that this is not the fleet size the Navy would want if it was under no budget constraints. When regional commanders were asked for their "unconstrained desire," the resulting numbers would have required the Navy's annual budget to double.

"Rather, this is the level that balances an acceptable level of warfighting risk to our equipment and personnel against available resources and achieves a force size that can reasonably achieve success," the summary says.

The force structure was last updated in 2014.

"To continue to protect America and defend our strategic interests around the world, all while continuing the counterterrorism fight and appropriately competing with a growing China and resurgent Russia, our Navy must continue to grow," Navy Secretary Ray Mabus said in a statement. "All of the analysis done to date, inside and outside of the Navy, recognizes, as we have for nearly the last eight years, the need for a larger fleet."

Byron Callan, an analyst with Capital Alpha Partners, said a dramatic improvement in U.S.-Russia relations under Trump's administration could lead the Navy to rethink this force structure again.

Mabus recently publicly clashed with Defense Secretary Ash Carter over his refusal to cut the number of ships in the fiscal 2018 budget request, which the Obama administration is preparing before leaving office.