WASHINGTON — Two collisions between Navy destroyers and commercial vessels in the Western Pacific earlier this year were “avoidable” and the result of a string of crew and basic navigational errors, the Navy’s top officer said in reports made public on Wednesday.

In two harrowing reports that told of missed warnings, chains of errors and frantic American sailors fighting to save their fellow shipmates, the Navy determined that both fatal collisions could have been prevented. Seven sailors were killed in June when the destroyer Fitzgerald collided with a container ship near Japan. The collision in August of the John S. McCain — another destroyer, one named after Senator John McCain’s father and grandfather — and an oil tanker while approaching Singapore left 10 sailors dead.

In the case of the Fitzgerald, the Navy determined in its latest reports that the crew and leadership on board failed to plan for safety, to adhere to sound navigation practices, to carry out basic watch practices and to respond effectively in a crisis.

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“Many of the decisions made that led to this incident were the result of poor judgment and decision making of the commanding officer,” the report concluded. “The crew was unprepared for the situation in which they found themselves through a lack of preparation, ineffective command and control, and deficiencies in training and preparations for navigation.”

In the case of the John S. McCain, the investigation concluded that the collision resulted from a “loss of situational awareness” while responding to mistakes in the operation of the ship’s steering and propulsion system while in highly trafficked waters.

“The collisions were avoidable,” Adm. John M. Richardson, the chief of naval operations, said in a summary of the reports, released by the Navy on Wednesday morning.

The release of the reports came a day after the Navy held closed-door briefings for lawmakers on Capitol Hill and sent officers crisscrossing the country to brief family members of the sailors killed. A broader review of the Seventh Fleet’s pace of operations, training, equipment and maintenance is to be released on Thursday.

On Tuesday, Mr. McCain, Republican of Arizona and chairman of the Senate Armed Services Committee, pointed to the automatic budget cuts on the Pentagon since 2013, known as sequestration, as one of the primary culprits behind the combined 17 deaths aboard the two destroyers.

“We’ve deprived them of the funds to do it,” Mr. McCain said of the continuous operations in the Pacific. “We’re putting those men and women in harm’s way to be wounded or killed because we refuse to give them the sufficient training and equipment and readiness. It’s a failure of Congress. It’s on us.”

Already the fallout from the two crashes, as well as two others in the western Pacific this year, has been significant.

The commander of the Navy’s Pacific Fleet took early retirement, while the former commander of the Seventh Fleet, based in Japan and the Navy’s largest overseas, was removed in connection with the accidents.

Several other senior officers as well as the commanding officers of the Fitzgerald and John S. McCain have also been relieved of their duties.

Even before the report, urgent new orders went out in early September for Navy warships.

The directives included more sleep and no more than 100-hour workweeks for sailors. Ships steaming in crowded waters were ordered to broadcast their positions. And ships whose crews lack basic seamanship certification will probably stay in port until the problems are fixed.

These were all seemingly obvious standards, military officials said, except that the Navy rushed the remedies into effect only after the two deadly collisions in two months, despite repeated warnings about the looming problems from congressional watchdogs and the Navy’s own experts dating to 2010.

The narratives of what led to the two collisions are different, but both are rooted in human error.

In the case of the John S. McCain, things began to go wrong at 5:20 a.m. on Aug. 21, as the ship approached the Straits of Malacca, one of the busiest shipping lanes in the world. The moon had set beneath an overcast sky and a three-foot swell rolled under the 505-footlong Arleigh Burke-class guided missile destroyer.

The ship’s commanding officer, Cmdr. Alfredo J. Sanchez, had been on the bridge since 1:15 that morning, and the ship’s second in command had arrived around 4:30. The presence of the McCain’s highest-ranking officers was appropriate for the high volume of merchant ships transiting the straits.

With the sky still black, Commander Sanchez noticed that the sailor steering the ship was having difficulty managing the helm and the complex arrangement of throttles that controlled the power to the McCain’s twin propellers. He ordered that the tasks be divided, one sailor steering at one station, another manning the throttles at another. The move, intended to make operating the ship more manageable, ended up taking away the helmsman’s ability to steer. A secondary and unnoticed effect of the commander’s decision was the inadvertent transfer of steering to the console now designated to control the throttles.

The helmsman, confused and with apparently no control of the ship, said he had lost steering. The ship began turning to the left. As those on watch failed to understand the events unfolding around them, Commander Sanchez ordered the ship to reduce speed. Yet when the sailor operating the throttles tried to slow the destroyer, he managed only to reduce power to one of the propellers, meaning only one reduced speed while the other continued at regular propulsion. The mismatch lasted for more than a minute, causing the McCain to veer left and into the path of the Alnic MC, a 600-foot merchant ship.

The crew eventually managed to synchronize the ship’s steering and throttles, but it was too late. With no attempt from either ship to contact each other and their warning horns silent, the Alnic MC’s bow slammed into the McCain’s left side, punching a 28-footwide hole in the warship that spanned deep under the waterline.

Sailors were thrown to the deck. Those near the point of impact likened the collision to an explosion. The vessels remained melded together for several minutes before breaking free.

The 10 sailors who perished were in a berthing area situated below the McCain’s waterline, near the point of impact. The 15-footwide space, compressed to a third of its normal size, filled with water immediately and was probably completely submerged in under a minute. One sailor, already near the hatch, quickly escaped, while a second was forced to swim through fuel and water to make it out. Those who remained were sealed below in an effort to control the flooding.

The Fitzgerald’s story takes a different course. Less than a day after the Fitzgerald left its home port of Yokosuka, Japan, the ship was within sight of land around 1 a.m. on June 17 when officers on the bridge failed to realize how close their ship had come to a merchant freighter, the Crystal. The Fitzgerald’s captain, Cmdr. Bryce Benson, had left the bridge for his quarters.

In the minutes before the collision, two additional ships came close to the Fitzgerald. But the officers did not change course, then later mistook the Crystal for one of the ships they believed was farther away. By the time they realized the mistake, it was too late.

The Crystal struck the Fitzgerald at 1:30 a.m. Dozens of sailors — awakened in one quickly-flooding berth — raced in the dark to escape, as seawater rushed in. Within 90 seconds, sailors were waist deep, then neck deep, in water. Of the 35 sailors in the berth, seven would not make it. Twenty-seven of the remaining 28 sailors struggled up a ladder, helping one another, and escaped from the port side ladder.

One sailor took a different route. He fought through chest-high water and through furniture. “Someone said, ‘Go, go, go, it’s blocked,” but he was already underwater,” the report said. “He was losing his breath under the water but found a small pocket of air.”

The sailor took a breath and swam. He lost consciousness and does not remember how he got out, but he gradually emerged from the flooding into another berthing area, where he could stand and breathe. He climbed up a ladder and eventually collapsed on the main deck. He was medically evacuated and treated for near drowning, seawater aspiration and traumatic brain injury, before being released on June 19.

The report attributes the collision to failure to maneuver the ship away from the approaching freighter, failure to sound the danger signal and failure to try to contact the Crystal.