The Navy’s No. 2 officer said Thursday that the service must look at how it deploys ship crews “from the ground up,” after two deadly collisions drew attention to gaps in training that had quietly persisted in vessels that do not return to the United States for years at a time.

Adm. William F. Moran, the vice chief of naval operations, told a House Armed Services subcommittee on military readiness that the demands on ships that do not return to the United States on a regular basis will be scrutinized. The review comes after seven sailors aboard the destroyer USS Fitzgerald were killed on June 17 and 10 sailors aboard the destroyer USS John S. McCain were killed Aug. 21.

Both ships were assigned to the Navy’s 7th Fleet, which has headquarters in Yokosuka, Japan, and includes between 50 and 70 ships at a time.

“Forward-deployed” naval forces go years at a time operating from ports abroad and were seen by the Navy as a way to keep up with increasing demand for U.S. military presence around the world. But the ships are sometimes deployed without sailors keeping up on training and ships maintaining certifications that show they are ready for all operations, the admiral acknowledged.

“All of these things culminate with this notion that we aren’t big enough to do everything that we are being tasked to do,” Moran said. “Our culture is that we will get it done, because that’s what the Navy is all about. But sometimes our culture works against us.”

The hearing was called as Congress begins to more fully investigate three collisions and a grounding at sea this year, all in the 7th Fleet. Moran testified alongside Rear Adm. Ronald A. Boxall, a two-star officer who oversees surface warfare at the Pentagon, and a member of the Government Accountability Office, which released a new report Thursday that said 37 percent of all cruiser and destroyer crews based in Japan are operating without certifications granted by the Navy.

John H. Pendleton, the GAO’s director of defense capabilities and management, testified Thursday that he does not yet know what caused the fatal collisions, but he found that the Navy is caught between “unrelenting operational demands and a limited supply of ships.” However, the way in which the Navy is managing the situation has caused problems in training and the maintenance and manning of ships, he said.

“Their aggressive deployment schedule gave the Navy more presence, it’s true,” Pendleton said. “But it came at cost, including detrimental effects on ships’ readiness.”

Pendleton said that Navy crews working out of Japan disclosed to GAO that they “trained on the margins,” squeezing in training around other assignments rather than setting aside time to make sure a ship’s crew was ready to deploy.

Moran, asked how ships could be allowed to operate without certifications, said that there is a process under which waivers can be granted if a ship’s commanding officer requests it. A task force in Japan and, ultimately, the 7th Fleet commander approves those requests along with a “risk mitigation plan,” Moran said. The Navy removed the commander of the 7th Fleet, Vice Adm. Joseph P. Aucoin, two days after the collision of the McCain, citing a loss of confidence in his ability to command.

Moran said GAO was correct to point out that the trend in the number of waivers that ship captains were seeking to keep operating without certifications had been rising “at an alarming rate, one that should give us all pause at just how hard we are driving the crews in 7th Fleet.”

Boxall said the units that certify ships as ready to deploy also are not fully staffed. In an attempt to improve the situation, the Navy increased the combined size of two certification staffs in Japan from 120 sailors to 180, but they are still 30 to 40 people short. Nonetheless, the evaluators must travel to meet ships while they are deployed because the crews aboard have so little time.

“The reality that we are seeing is that because of these compressed timelines, they have to train in smaller and smaller periods, meaning we have to send those evaluators to different places to catch up with the ship,” Boxall said. “That’s a very inefficient model, and it further exacerbates a challenging certification process.”