During a foreign policy speech at the Virginia Military Institute on Oct. 8, Republican presidential candidate Mitt Romney added a new item to the military wish list — three submarines per year.

Romney already pledged to spend 4% of U.S. GDP on defense, an idea that defense analysts view as an aspiration. Under that umbrella, Romney has said he would buy 15 ships per year over the current nine, bring back the F-22 Raptor and prevent reductions in troop levels.

Under the Obama administration’s budget plan for fiscal 2013, the Navy would buy 46 attack submarines through 2042, causing the number of submarines to slip below the service’s goal of 48 over the 30-year period, according to the Congressional Budget Office. To maintain the size of the submarine fleet, the Navy could buy three subs per year between 2014 and 2023 and allow that number to drop in subsequent years, the budget office suggested in a report released this summer.

The ongoing problem, however, is that even the current plan for Navy shipbuilding is unaffordable. “The total costs of carrying out the 2013 plan — an average of about $22 billion per year in 2012 dollars over the next 30 years — would be much higher than the funding amounts that the Navy has received in recent years and higher than the costs for the 2012 plan,” CBO says.

If the Navy budgeted $21.9 billion per year for Navy shipbuilding, that would represent a 37% increase over the average Navy shipbuilding spending for the last 30 years, CBO adds.