The U.S. Navy is looking at buying more submarines in the coming years, but the most recent Congressional Budget Office (CBO) analysis of the service’s shipbuilding plan says more boats will be needed to address fleet shortfalls.

The Navy’s recently released 30-year shipbuilding plan includes more subs this year than the service had a year ago, CBO notes.

The number of ballistic missile submarines was changed from 12 to a range of 12 to 14, CBO points out, while the number of guided missile submarines was changed from zero to a range of zero to four. Buying subs adds more to the Navy budget bottom line.

“According to this year’s plan, in the near term, building new ships will cost an average of $15.1 billion per year,” CBO says. “In the midterm, replacing the Navy’s current Ohio-class ballistic missile submarines drives up the average cost of new-ship construction to $19.5 billion per year.”

Submarine contracts and contract modifications — excluding the nuclear reactors — ranked fourth among all Navy expenses with about $16.2 billion in transactions between 1999 and 2009, according to an Aviation Week Intelligence Network analysis of contracting data aggregated by the National Institute for Computer-Assisted Reporting.

Despite all the investment, CBO says the Navy sub fleet will still come up short. “The current shipbuilding plan delays buying the first replacement for the Ohio-class ballistic missile submarines [SSBN(X)s] by two years, until 2021; it would then enter the fleet in 2030,” CBO says. “The Navy’s inventory of SSBNs will fall below the stated goal of 12 to 14 between 2029 and 2041.”

Under the 2013 plan, the Navy would purchase 46 attack submarines through 2042, which would not be enough to keep that force up to the stated goal of 48 throughout the next 30 years, the report says. “In 2014, the Navy expects to begin retiring Los Angeles class attack submarines (SSN-688s) — which were generally built at rates of three or four per year during the 1970s and 1980s — as they reach the end of their service life. It would then replace them with Virginia class attack submarines (SSN-774s) and their successors, mostly at rates of one or two per year,” CBO says.

To prevent the ballistic missile submarine force from falling below the inventory goal of at least 12 subs, the Navy could begin purchasing the SSBN(X) in 2019, as scheduled under the 2012 plan, rather than in 2021, as under the current plan.

To prevent the attack submarine force from falling below the inventory goal of approximately 48, CBO says, the Navy could purchase a total of five submarines earlier in the 30-year plan and reduce construction of attack submarines later in the plan. “Specifically, it could purchase five additional attack submarines from 2014 through 2023,” CBO says, “increasing the production rate to three submarines per year for many of those years.”