Its skimpy beaches do not suggest Nantucket or the Vineyard, and a sudden fog bank can drop a dark curtain on an afternoon's sail, but in every other way, little Sandhamn and the Stockholm Archipelago will charm lovers of islands and tranquility.

Sandhamn, easternmost of the archipelago that sprawls eastward from Stockholm, has been called the Swedish Newport. But the handle doesn't quite fit. Sandhamn has never pretended to be so grand as Newport, and it is much more remote. It has an understated stylishness, an innocence that for one weekend a year, in late August, surrenders to a ritual frenzy even the laid-back Swedes find remarkable.

The occasion is a yacht race called Sandhamns Jachtclubben that winds 25 nautical miles through the archipelago and finishes at the gates of Sandhamn's baronial old yacht club. More than just a yacht race, the weekend is an excuse to celebrate the crayfish, an endangered crustacean the Swedes consume with gusto the last two weeks of August. It is also a kind of farewell to summer.

Of course you don't have to be a Swede or a sailor to get in the act. You don't even have to go in August. September and early October in the islands may well bring warm, gentle days, yellowing birches and playful seals. This is the Swedish version of Indian summer, known as brittsommar (or British summer). Whatever the season, you can get a taste of the archipelago by anchoring on the very fringes of Stockholm. Last summer, just off a plane and reeling from jet lag, I chose to stay in the seaside suburb of Saltsjobaden before moving out to Sandhamn.

Saltsjobaden, a half-hour drive or 20-minute train ride from the city, has the feel of a faded fin de sie`cle health spa, and no wonder: The Grand Hotel Saltsjobaden, a noble black-domed mansion that stands above the harbor, was the cornerstone of a fashionable resort built in the 1890s by the industrialist Knut Wallenberg (a relative of the mysterious World War II humanitarian, Kurt Wallenberg).

Quietly revived in recent years, Saltsjobaden can keep visitors busy through the protracted summer light with boating, water-skiing, windsurfing and fishing. Below the hotel terrace, out on a point beyond rampant rose beds, are two weathered wooden bathhouses -- one for women swimmers, one for men. At water's edge is a lovely trio of red-clay tennis courts frequented by Ulf Schmidt and Jan Lundquist, retired Swedish Davis Cuppers who live in the area. Schmidt, an old trouper who should know, says the only prettier courts in Europe are a pair in Monaco.

One of my main missions at Saltsjobaden, besides defeating jet lag, was to explore a wondrous maze of islands that begins just beyond the hotel's awninged balconies. This I did by hiring a motorboat (and a pilot) one sultry afternoon. We puttered for miles through narrow cuts and channels, passing the neat clapboard summer houses of Stockholm's elite, with their snug red sauna cabins and boathouses tucked up to the shore.

By rough count there are 24,000 skerries in the archipelago, about 150 of them inhabited. Some are as bare as a whale's back; others are fringed with pine and birch. At the back of a broad sloping lawn I saw an impressive house with a red-tile roof and a white dome. There were a tennis court and a boathouse holding a racy little speedboat called Thunder Can. They were the playthings of a youthful retiree named Bjorn Borg.

In the midst of a shining afternoon, the sun suddenly vanished and a gray chill overtook us. We were immersed in a thick fog bank. We turned about and headed toward Saltsjobaden, and in minutes the bright Swedish summer was back.

There are ferries from Stockholm and Saltsjobaden out to Sandhamn, but it's important to know that the timetable is sharply cut back in mid-August. In Sweden it isn't weather or holidays or migrating geese that dictate the seasons: It's kids. With the reopening of school on Aug. 15, steamer service is curtailed, most families pack up and leave their summer cottages, and yacht traffic thins out to weekending Stockholmers not ready to give up their precious Baltic retreat.

In other words, it's a great time to see Sandhamn. The days are still long and mild; the roses are in bloom; the cafes, seafood restaurants and bakeries are open; the beaches on the eastern tip -- lovely little boulder-backed coves -- are all but empty, and there is room at the island's two inns -- the 16-room Sandhamn Hotel and the more rustic 40-room Seglar's Hotel.

Imagine my surprise, on a quiet Friday afternoon preceding the boating bacchanal (which is taking place this weekend this year), to look out on an unroiled harbor and see a pretty sloop flying the Stars and Stripes. And to hear a blond boy leaping onto the dock with, "Mom, I want my boogie board."

The boat held six Southern Californians, out on a laid-back Baltic cruise and blithely ignorant of the Jachtclubben race.

"It's our floating summer house," said Billy Graham, the skipper, a Malibu filmmaker. "Frankly, it's been an experiment. We wanted to do something to improve the quality of our lives, to go places in the summer you can't get to by motorcar but at the same time bring everything with us. And to meet the people. Mostly we meet other yachtsmen. We go to each other's 'homes' for drinks and they mark our charts for us."

By Friday evening the weekenders were beginning to arrive by private boat and ferries from the city's eastern reaches, but Sandhamn (population 150) still belonged to the Sandhamners. At a cozy harborside restaurant, Sandhamns Vardshus, where the lowly herring is raised to sublime heights, I walked in on an ancient Swedish rite. A dozen women were toasting a friend with a bridal dinner called a mohippa, or maiden party. Ann Graahed, the bride to be, looked like a creature cast up from the deep. Her friends had blindfolded her, dressed her in a doughnut-shaped life preserver and a headdress of leeks, then spirited her to an uninhabited island -- all part of the mohippa hazing. Now they were singing, "For she's a jolly good maiden."

"It's important to hug her a lot," said Sonja Naslund. "When you embrace her you can feel if she is a product of Sandhamn. I have lived here seven years, but you can't be a real Sandhamner unless you are born here. My father-in-law came in the '30s and he is doubtful."

By 10 o'clock Saturday morning, as 90 sailboats were dashing away from the starting line at Saltsjobaden, Sandhamn was beginning to put on its race face. Down at the public boat landing, a small inter-island ferry let off a score of young people lugging musical instruments. Without so much as an introduction, and with no bandleader in sight, the Gustavsberg Symphonic Band struck up a series of marches. Had they come to play for the race? No, a band member told me, the group was making its Saturday morning rounds and was ready now to hop the next ferry for home -- ready, that is, until a Royal Swedish Yacht Club (K.S.S.S.) member, promising ice cream for all, persuaded them to move down the harbor front and set up for a final flourish in front of the yacht club.

The high-spired, barn-red yacht club, Sandhamn's most impressive structure, looms like a cathedral above the waterfront. "The club moved into this building around the turn of the century, but we're much older than that," said Gunnar Ekdahl, a vice commodore of the K.S.S.S. and a Stockholm business executive, who stood tall, blond and erect in his dark blazer and white ducks. "The K.S.S.S. started in Stockholm in 1830, which makes us one of the five or six oldest yacht clubs in the world."

By 2 p.m. the first boat, the 50-foot Bla Carat, was across the finish line and preparing to tie up. It was clear that people weren't paying much attention to places or times. A big happy party was taking shape as boats of all kinds -- some part of the race, most of them not -- poured into the marina.

All along the curving, tree-lined harbor front, boaters and boat watchers were filling up the cafes and restaurants. Cafe Strindbergs Garden specializes in waffles and coffee and alfresco chatter. The garden is studded with curios like old copper pots, 18th-century wood shovels, a blackened oak anchor. The cafe was named for the playwright Johan August Strindberg, who came to Sandhamn as a young man and, finding no hotels on the island, took a room in the little red house beside the garden -- now the owner's cottage.

In the still-broad daylight of 8 p.m., scores of sailors and their friends materialized for an awards ceremony, having jettisoned their jeans and sweat shirts for blazers and white slacks, dresses and heels. When the last tiny trophy had been handed out and the harbor master had fired a toy cannon to signal the day's end, hundreds of people filed into the old clubhouse to undertake the real order of business: crayfish consumption.

The increasingly scarce and protected crustaceans are harvested in Swedish waters for just two weeks in late August. Swedes celebrate the crayfish season with something like pagan zeal, feasting on mounds of the spiny little critters while wearing silly paper hats, singing rollicking songs and downing glasses of chilled aquavit. Never mind that the crayfish you're wrestling with may have been shipped in frozen from Louisiana or California or that it takes the dexterity of a surgeon to extract the tiny quantity of edible meat: It's a sure way to get old friends together for a last hurrah each summer.

Looking back, I marvel now that the 90-year-old clubhouse stood up to another crayfish banquet and that only one partygoer had to be carried out. Around midnight, I stood on the docks amid a group of chattering yachtsmen who were performing a post-mortem on the race, the party, the whole season. On one side of us loomed hundreds of swaying, arc-lit masts; on the other, the rocking clubhouse with its ground-floor disco. It was easy at that moment to feel a slight loss of equilibrium, and to understand why the Sandhamn Jachtclubben is to some the last and the best of the Swedish summer.

David Butwin is a writer in Leonia, N.J. WAYS & MEANS

GETTING AROUND:Even after the tourist season slows down in mid-August and public transportation among the islands is cut back, you can still cover a lot of the rambling Stockholm Archipelago with a combination of ferries, trains and buses. In fact the string of 24,000 islets and skerries often shows its best face in the mellow, yellowing days of September and early October.

In the height of summer there are frequent ferries from downtown Stockholm out to Sandhamn, the easternmost island and headquarters of the Royal Swedish Yacht Club (K.S.S.S.). After mid-August the boat Cinderella makes the trip on weekends. You can also get to Sandhamn in the slack season by catching a bus to Stavsnas, a waterside suburb, and making an hour's ferry ride. Another way to Sandhamn by boat is from Vaxholm, an interesting archipelago stop in itself with a fortress and museum looming above the harbor. Vaxholm is a half-hour drive or bus ride from Stockholm. WHERE TO STAY: On Sandhamn there are two places to stay, the 16-room Sandhamn Hotel, a pretty clapboard house on a hill above the harbor (about $75 for a double room) or the yacht club's spare but clean 40-room Seglar's Hotel ($65). Full Swedish breakfasts, from eggs to herring, are included in the price of both hotels.

There also are cottages available for rent throughout the Stockholm Archipelago.

If you stop off at Saltsjobaden, a historic and briny suburb of Stockholm, the Grand Hotel Saltsjobaden will put you up in grander style than anything the little Sandhamn hotels can offer. Rooms run about $55 a person including breakfast.

INFORMATION: For more information, contact the Swedish Tourist Board, 655 Third Ave., New York, N.Y. 10017, (212) 949-2333.

-- David Butwin