The U.S. Navy is facing a temporary SSN (nuclear attack submarine) shortage, and there is no solution that will not involve some pain. The problem is that new Virginia class subs cannot be built quickly enough to replace all the Cold War era Los Angeles class boats that have to retire. Even that will be delayed, at least for 16 Los Angeles class subs, that will get enough refurb to keep them at sea for up to two more years. Meanwhile, many of the shipyards used to build all those Los Angeles class boats, were discarded as part of the Peace Dividend for winning the Cold War.
The shortage will begin in 2022, when the number of SSNs will fall below 48. The bottom will be in 2028, when only 41 SSNs will be available, and the shortage won't end until 2034. While keeping boats at sea more than six months per cruise will insure that all current requirements (that need about ten boats at sea at any given time) are met, the navy wont be able to meet its wartime need for 35 boats. Keep in mind that a certain number of boats are always laid up for upgrades, maintenance or repairs. And some of this work can be speeded up, or even put aside, to get boats to sea in wartime, or a major crises.
Keeping existing boats at sea for longer cruises also comes with a cost. For each additional day (beyond six months) you keep a crew at sea, a certain percentage of them will not stay in the navy. Those long months at sea are hard on the families, and sailors as well. Too much of that, and more of them leave. For submarine crews, the most highly trained, with the highest standards, in the navy, this is no small problem.
There are other ways around the problem. The navy and the shipyards have found ways to built SSNs more quickly. Currently it takes 70 months to build a Virginia. But in the next few years, that will be coming down to 60 months. For the navy, the worst solution is to change war plans, and peacetime use patterns of SSNs, and adapt to a smaller number of attack boats. The navy would rather not think of this, but politicians often do, so the navy must.