By BEN WIDDICOMBE

House Greyjoy is a clan of oceangoing raiders in “Game of Thrones,” a storm-soaked, humorless bunch even by the grim standards of the hit HBO fantasy series.

Alfie Allen, 29, plays Theon Greyjoy, a fan-favorite character who has suffered more than most in the course of the show, currently in its sixth season. He has been kidnapped and castrated, has switched sides and is currently on the way back to his ancestral castle, where, judging by the teaser for Sunday’s episode, a showdown with his sister awaits.

But Greyjoy seafaring skills weren’t much in evidence last Friday, when Mr. Allen, a lifelong Londoner with a winking laddish charm, was in New York to participate in practice races for next year’s America’s Cup.

“I’m not an experienced sailor,” he said after briefly joining the British team, officially known as Land Rover Ben Ainslie Racing. Mr. Allen was the sixth man, a nonfunctional position that may just as well be called “celebrity ballast.”

“They need the weight of a person sitting on that spot, the back center,” he said later, in the restaurant of the Conrad Hotel in Battery Park City, near the team’s mooring at the North Cove Marina.

Mr. Allen is a slightly built man with fair hair and piercing blue eyes, who weighs, he said, “10½ stone.” That is 147 pounds, or two-thirds of the 100 kilograms (about 220 pounds) the sixth man is expected to provide. The rest was made up by weights.

“I wasn’t really doing much,” he said. “You sort of lie down in the back and watch them do their thing.” Ben Ainslie, the English competitive sailor, “was lovely,” Mr. Allen said. “You’re not supposed to talk to the team when you’re on the boat, but they were talking to me. And you can’t swear.”

He added, “I swore.”

Friday was cold and wet, conditions that are familiar to Mr. Allen from filming in Northern Ireland.

“I’d say just shooting ‘Game of Thrones’ prepared me for that sort of thing, because HBO likes to keep it real,” he said. “When you see me sleeping with the dogs, I’m really in there with the dogs, in the dirt. It definitely can get pretty freezing.”

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Mr. Allen is sometimes portrayed by British newspapers as a party boy, but on Friday evening, he was a model of rectitude, nursing a hot tea even as his retinue of three handlers (some of whom had been on duty in the elements for 12 hours) moved on to adult drinks.

“I got you the Malbec,” he told his manager, solicitously, as she motioned to order another glass of whatever it was she was drinking.

“I would love a Malbec if she’s having a Malbec,” a weary publicist added.

Mr. Allen, who was smartly dressed in a John Varvatos collarless jacket, immaculate white Nike Air Force 1 low-tops and a T-shirt from the artist Mike Kelley’s posthumous exhibition at the Los Angeles Museum of Contemporary Art, merely cupped his tea, a slightly forced study in boyish innocence.

Mr. Allen grew up among artists, musicians and filmmakers. His father is an actor, his mother a film producer, and his sister the pop singer Lily Allen.

“For years, my dad’s friend Joe” — Strummer — “was just my dad’s friend,” he said. “And it was only when I was 12 or 13 I learned that he was the lead singer of a band called the Clash.”

Mr. Allen’s popular cultural debut came 10 years ago, as the subject of a single from his sister’s debut album. The song, “Alfie,” portrayed him as a hopeless teenage stoner, in need of an intervention. “I just can’t sit back and watch you waste your life away/You need to get a job because the bills need to get paid,” Ms. Allen sang in her cheerful, upbeat style.

“The song is born out of love for me,” he said, adding that he was “in Canada getting qualifications to teach kids how to snowboard” when it was released. “My sister, I guess, she was missing me. I didn’t really expect it to be what it was.”

That all seems very long ago, six seasons into the world’s most successful TV show. Now, Mr. Allen has his pick of projects. This week, he began rehearsing “The Spoils,” a play written by and co-starring Jesse Eisenberg, which debuted in New York last year and is having a London run this summer.

As for Theon: His father, the Lord Reaper of Pyke, was recently hurled to his death, creating a power vacuum in the Iron Islands. It seems very likely that the young lord will have to battle his ambitious sister to become the new head of House Greyjoy.

Right, Mr. Allen?

“Maybe something close to?” he offered, stroking his blond whiskers. “But it’s not. So you’ll just have to wait and see.”