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

Zac Sunderland arrived in Marina Del Rey aboard his 36-foot sloop Intrepid at about 10am local time. During his 28,000-nautical-mile journey, he braved storms, equipment failures, close calls with freighters and a run-in with suspected pirates.

"It's awesome to be back." 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 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 to fly in to make repairs.

Another tense moment occurred in the Indian Ocean off Indonesia, where the Intrepid was approached by a suspected pirate boat.

The intruders fled after a plane flew overhead and Sunderland climbed onto his deck with a gun to ward them off, said Charlie Nobles, executive director of the association.

Sunderland, who remained in satellite telephone contact with his father during the voyage, dumped the gun before entering Mexican waters, where it is illegal to carry private firearms.

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

(Agencies)