Bloomberg reports that delays at naval shipyards mean that nearly 40% of US attack submarines are out of commission for repairs, about double the rate the Navy would like, according to new data released by the service.

As of this year, 18 of the US Navy’s 49 attack submarines — 37% — were out of commission, according to previously undisclosed Navy data published by the Congressional Research Service, raising concerns that this leaves the US at a critical disadvantage against China’s numerically superior fleet.

That’s up from 28% overall in 2017 and 33% in 2022 and below the industry best practice of 20%.

The maintenance backlog has “substantially reduced” the number of nuclear submarines operational at any given moment, cutting the “force’s capacity for meeting day-to-day mission demands and potentially putting increased operational pressure” on submarines that are in service, CRS naval analyst Ronald O’Rourke said in a July 6 report.

CNN notes that the growing number of ships in maintenance stems from a shortage of workers able to carry out the necessary work and a lack of space at the maintenance shipyards, according to the report. In addition, supply chain issues have affected the availability of repair parts, especially since many submarine components are from sole-source supplies.

“The Navy continues to meet all of its operational requirements,” said Commander Jackie Pau, a Navy spokeswoman. “As planned, about one-third of our fast attack submarines are currently in maintenance receiving critical repairs to sustain a mission-ready force. Maintaining a world-class and worldwide deployable Navy is the first line of defense for the United States – Our Submarine force is a critical investment to global security and stability.”