PUERTO VALLARTA, Mexico — Zac Sunderland is wedged in his small bunk, reading, as his 36-foot sailboat ascends and careens down mountainous, shifting peaks.

Just ahead on this late June morning is Mexico’s first seasonal tropical depression, whose winds have roiled the Pacific Ocean. To the south, churning up the coast: a larger storm building into a hurricane.

Sunderland, 17, is more than 100 miles offshore on the final leg of a 13-month, around-the-world odyssey. He holds course but is interrupted by a jarring thud and what sounds like a gunshot.

His boat, Intrepid, has launched from a 10-foot wave and its port-side bulkhead has buckled on impact. The deck flexes and chain plates with lines supporting the mast have ripped loose. Wind hisses menacingly.

He must change course and try to reach the nearest refuge, Puerto Vallarta.

Sunderland has grown accustomed to adversity since he embarked from Los Angeles on June 14, 2008, on a mission to become the youngest sailor to circumnavigate the globe alone. He was 16 and didn’t even have a driver’s license.

The idea had been in his mind since he read “The Dove” as a child. The book chronicles a five-year circumnavigation by Robin Lee Graham, whose voyage ended in 1970, when he was 20.

Sunderland, a shipwright’s son and an experienced sailor, planned the journey himself. He would cross the Pacific and Indian oceans before rounding Africa’s Cape of Good Hope, then cross the Atlantic, pass through the Panama Canal and sail north along the Central American and Mexican coasts before returning home.

He would subsist on freeze-dried and canned food when fresh provisions ran out, and he would desalinate his drinking water with an on-board kit.

What Sunderland, due to return to Los Angeles around July 14, could not foresee were the dangers and difficulties.

Notable was the pirate scare. In October, he was 150 miles beyond Indonesia, on a course from Australia to the Cokos Keeling Islands, when he encountered a mysterious boat. The 60-foot wooden vessel did not appear on his radar screen. He tried to raise its crew on the radio. He changed direction; it changed direction.

Winds were light and he could not escape, so he clutched his satellite phone — his lifeline — and dialed his home in Thousand Oaks, Calif.

A sister answered. Laurence Sunderland heard his son’s panicked voice, grabbed the phone and rushed into his office. Zac’s heart raced as he digested the instructions: Load your pistol and flare gun, then issue a radio security alert with your position.

Fire a warning shot if necessary, but at the first sign of aggression, shoot to kill because they’ll try to kill you.

Laurence recalls: “For two hours we’re sitting here not knowing what the situation was or whether Zac could handle it.”

The decrepit craft swept directly into Intrepid’s wake, its crew still hidden, as Sunderland placed his emergency call. Then it motored away. “For 30 minutes I was living on the edge out there, not knowing what to do,” he says.

Yet this was not the most harrowing experience for a long-haired adventurer who rarely expresses emotion while recounting his adventure.

“The whole trip was scary,” he says, almost dismissively, during an interview last week in Puerto Vallarta, where he had stopped for repairs. “Broken forestay … broken boom … broken tiller … the rogue wave off Grenada that broke over the back of the boat at 2 a.m. and took out all the electronics. …”

Laurence can vividly recall “four specific times that we’ve been put to our knees in prayer.” But neither parent expresses regret. On the contrary, Laurence says, “This has meant something far bigger than words can express.

“I don’t think Marianne or myself have realized until now how much of us has been out there with him. But it’ll be a big day when he gets back and we’ll be very proud, because Zac will have achieved something that very few people will ever be able to achieve, and it has changed him and it’ll be with him the rest of his life.”