A 17-year-old US mariner piloted his battered sailboat into a Southern California harbour to complete a gruelling 13-month voyage and become the youngest person to sail around the world alone.

Sandy-haired teenager Zac Sunderland arrived in Marina Del Rey aboard his 11-metre sloop Intrepid on Thursday (local time).

He becomes the first person under 18 to circumnavigate the globe by sea alone, and the youngest to date. The previous record-holder was Australian David Dicks, who completed his voyage in November 1996 at age 18.

During his 52,000-kilometre journey Mr Sunderland braved storms, equipment failures, close calls with freighters and a run-in with suspected pirates.

"It's been a crazy 13 months and ... yeah, it's awesome to finally get back here," Mr Sunderland, of Thousand Oaks, California, said after he was welcomed home by a flotilla of well-wishers.

Fewer than 250 people have sailed solo around the globe, with three times that many scaling the top of Mount Everest, according to the American Sailing Association, which certified Mr Sunderland's feat.

He left Marina Del Rey, just south of Los Angeles, on June 14, 2008, and celebrated his 17th birthday at sea while off Africa's Cape of Good Hope.

His return was delayed near the end of his voyage when his single-masted boat sustained a broken bulkhead in rough seas off Mexico, forcing him to stop at Puerto Vallarta long enough for his father, Laurence, a British-born sailor and shipwright, to fly in to make repairs.

'Load your gun up'

An earlier storm in the Indian Ocean kept Mr Sunderland up for four days and nights, struggling to keep his vessel on course in gale-force winds that heavily damaged the boat's rigging.

Another frightening moment occurred in the Indian Ocean off Indonesia when Intrepid was stalked by a suspected pirate boat as Mr Sunderland repeatedly took evasive action.

The intruders fled after he climbed onto the deck of his vessel with a loaded pistol and a flare gun to ward them off.

Laurence Sunderland said he gave his son precise instructions in a tense discussion between the two by satellite telephone.

"I said load your flare gun up, load your gun up. ... and when the boat gets close enough, fire two warning shots over the top of the boat with the flare gun, and if it keeps on coming, get ready to go with your real gun," the father recounted. "And I said, 'You shoot to kill, stay out of the line of fire and be very purposeful in what you do.'"

The teen sailor never had to fire a shot and later dumped the gun before entering Mexican waters, where it is illegal to carry private firearms.

The oldest of seven children, Mr Sunderland, who plans to write a book about his adventure, was home-schooled and partly raised on a boat.

Mr Sunderland's record as the youngest is likely to be broken later this year by Mike Perham, 17, a British sailor who is a couple of months younger and began his round-the-globe voyage in a larger, more modern boat on November 15, 2008.

Mr Perham is on track to finish is voyage in less than a year, Nobles said.

- Reuters