On Jan. 19, Mr. Savin posted his last diary entry on Facebook.

“I’ve run into some problems in the last 10 or so days,” he wrote. “The solar collector (which should recharge the battery of my water desalination system) has stopped working. I have been forced to use my manual desalinator, but that takes all my physical energy.”

“Rest assured,” he added, “I’m not in danger!”

He was heading to the Azores, the Portuguese archipelago, he said.

“There’s a beautiful marina with an airport next door,” he wrote. “Everything I need is there.”

“Despite the current difficulties of strong swells and winds, it has become easier as the wind pushes me toward the archipelago,” he wrote, adding, “Despite it all, I will absolutely not give up!”

Mr. Savin, who turned 75 during this voyage, hailed from the oyster farming town of Arès, in southwestern France. He was a former military parachutist, pilot and park ranger in Africa who did not settle quietly into his golden years.

By the time he was 71, Mr. Savin had already sailed solo across the Atlantic four times.

In 2018, he crossed the Atlantic again, this time in an orange barrel-shaped capsule that he said he had built himself. A New York Times article described the capsule, which was about 10 feet long and 6 feet 8 inches wide, as “smaller than a pickup and held upright by a concrete ballast.”

He completed the journey in 127 days. In an interview afterward, Mr. Savin described his time at sea as “complete freedom.”

“It’s hard to convey,” he said. “No one tells you what to do. There are no rules. It’s freedom.”

Though he was skilled, and was equipped with sophisticated navigation and communication tools, that voyage was not without its challenges, he conceded. “Twice, I almost collided with large ships,” he said.