“I’m a huge imperial stout fan, and I’ve been hearing a lot about Kate being one of the best,’’ said Jeff Gosselin, 37, of Arundel, Maine, who took the day off from work so he could drink the beer. He arrived at 10 a.m., at which point the line - more than 400 people deep - snaked around the block. Gosselin was hoping to get in the door by early afternoon. “Everybody I know thinks I’m crazy for doing this.’’

That first batch of Kate at the Portsmouth Brewery lasted for months. But then Beer Advocate magazine proclaimed it the best beer in America, and the craze was on. After that, the lines started forming, and they have gotten longer by the year, as the craft beer boom keeps expanding. Users of such sites as RateBeer.com and BeerAdvocate.com - it gets a perfect score of 100 on both - feed the frenzy.

Tod Mott, head brewer at the Portsmouth Brewery, had been brewing variations of his imperial stout recipe since 1996, when he was at the Back Bay Brewing Co. in Boston. In 2006 he christened it Kate the Great, in honor of Catherine II, the 18th-century Russian empress who was said to have a fondness for stouts.

The Kate the Great phenomenon started six years ago, the result not of clever marketing but of something more organic and viral.

About half of the beer is sold on draft - $8 for an 8-ounce pour - and half is bottled. Acquiring a bottle of Kate is perhaps more challenging than getting it on draft. In order to buy one of the 2,000 bottles - $8 for 11.2 ounces - one first had to have bought a winning scratch ticket that conveyed the right to purchase a bottle. The brewery sold all 15,000 tickets, which cost $2 apiece (all of that money went to charity), in less than 24 hours.

In Portsmouth, the first Monday in March has become known as Kate the Great Day, and it is the only day of the year that this particular beer is available. Hundreds of people wait in line for hours for it, praying it doesn’t run out before they get in. They come from all over New England, a few from other parts of the country. The demand for Kate the Great is so intense that all 420 gallons of it are drained in one day.

They arrived at 2:45 a.m., ensuring themselves the first spots in line. By the time the brewery opened eight hours later, 500 people were waiting to get in.

“It’s a great beer, but it’s more about the experience,’’ said Johnson, a 37-year-old Internet company executive who flew in from San Francisco so he and Hering, who lives in Boston, could drink a few glasses of the Russian imperial stout called Kate the Great and make a Web video of themselves doing so.

PORTSMOUTH, N.H. - By the time the doors to the Portsmouth Brewery opened at 10:45 a.m., Ron Johnson and his friend Dave Hering had been waiting outside on the sidewalk, in subfreezing temperatures, for eight hours.

The mania confounds even those who brew the beer. Ask them why this particular stout inspires such adoration, and they’re not sure.

“I have no idea,’’ said Mott. “There are so many great Russian imperial stouts. I don’t know if Kate is any better than the other ones. It’s a good beer. Let the hoo-hah continue.’’

Pitch black and viscous, Kate the Great is a thick, hearty beer suited for sipping. Six different kinds of malt - “tons of malt,’’ Mott said - and seven varieties of hops go into its creation. At 10 percent alcohol, it’s twice as strong as a typical beer. Tastes come through in waves: roasted malt with chocolate and coffee overtones, hints of plums and figs, followed by a pleasantly bitter finish.

“It’s the most delicious tasting beer I’ve ever tried,’’ said Bill O’Connor IV, 25, who drove up from Haverhill, got in line at 3 a.m., and snagged a prime spot on a stool at the bar. “Plus it’s a great experience.’’

When the doors opened, customers streamed in, excited but orderly, arms raised in triumph, as though they had completed a marathon. By 11:30 the place, which holds 325 people on two floors, was full.

For the next 7 hours and 15 minutes, the bartenders would pull the Kate tap handle almost nonstop. Often they just left the tap open, sliding one tulip glass after another under the flow of what resembled used motor oil.

“I cannot tell you how blessed we all feel to have such great friends in the world of beer,’’ owner Peter Egelston said as he toasted the crowd.

Johnson and Hering, the first guys in line, sat on the same side of a booth and set up a webcam across from them to record their tasting.

“It’s so incredibly smooth,’’ Johnson said after taking his first sip. “Four words: It was worth it.’’

By 2 p.m., most of those who had been in line when the doors opened had gotten in, but the line had barely shrunk, because new customers kept arriving. Most had either taken the day off or, they admitted, called in sick.

Despite all the alcohol consumed, the crowd inside was relaxed, almost subdued. People sipped their beers patiently and chatted. Nobody was rowdy. No one seemed drunk. People had a beer or two - in rare cases three - and left. The staff didn’t have to shut anyone off or impose time limits.

Peter Papadopoulos, 55, of Dover, N.H., looked almost sleepy as he nursed a tulip glass of Kate by himself. By the time he scored his seat late in the afternoon, he’d stood in line for 2 1/2 half hours. He said he didn’t mind the wait, nor did he regret buying 10 scratch tickets last month without a winner among them.

“I appreciate it when you have to pay your dues and earn your way for your bottle,’’ he said.

But why not go to the store down the road and buy an imperial stout that you can have any day?

“The ones I can have aren’t as good as this one,’’ he said.

Hollie Chadwick, doing Kate the Great Day for the fourth time, said its attraction is as much about the convivial atmosphere as the actual beer.

“Everybody comes together,’’ said Chadwick, 37, who lives in Manchester, Maine, and writes for Yankee Brew News, a regional beer publication. “You get to see some of the same people every year. It’s kind of like a festival.’’

In all, the brewpub served 2,780 glasses of Kate over the course of the day. At $8 a glass, that’s $22,240 worth of Kate - a total that doesn’t include other beers, mixed drinks, or food that customers bought. Though local charities wound up with $30,000 from the scratch tickets, Kate the Great Day is a boon for the Portsmouth Brewery, which otherwise would do far less business on a Monday in late winter.

This raises an obvious question: If the demand for Kate is so great, why not brew it more often?

For one thing, the brewers said they don’t have the capacity. It takes five months to make Kate - tying up a fermentation tank for one month and a conditioning tank for another four. But there’s another reason Kate comes only once a year.

“Then the specialness goes away,’’ said brewery co-owner Joanne Francis. “Psychologically we all want something rare.’’

As the sun sank, the line on the sidewalk finally wound down. At 6:25 p.m., the last customer waiting outside was in the door, beer in hand. At 6:45, foam started flowing from the Kate tap. “That’s it,’’ one of the bartenders said. No announcement, no fanfare - Kate was gone for another year.

Steve Greenlee can be reached at greenlee@globe.com. Follow him on Twitter @SteveGreenlee.

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