FORT LAUDERDALE, Fla. — In days of yore, there were only yachts.

Sleek, noble, the 40-foot-long sea chariots served as symbols of opulence, style and refinement in America — the ultimate playthings for the ultimate few.

Until there came superyachts.

Twice as large as their predecessors, with midocean necessities such as spiral staircases, split-level saloons and flybridge hot tubs, these floating mansions did more than eclipse conventional yachts — they made them appear toy-like.

And then, along came megayachts.

These Poseidons, in good truth, possessed more than mere grandeur (nobody dared describe a yacht less than 150 feet as a "mega"). They had crystal elevators, goatskin walls. They had aft-garages that fit fleets of Rolls-Royces and were powered by engines as quiet as those on Cold War submarines. They even came with gyroscopically controlled swimming pools — to keep the water smooth in the most turbulent of seas.

By the late '90s, however, there was talk that the megayacht's reign was coming to an end. It so happens they were right: Ever-bigger yachts are rolling out of shipyards for the world's superrich.

525 feet and counting

The largest privately owned boat now spans 525 feet. And in yachting terminology, "fully loaded" now means boats with on-board helicopters, submarines, full-size movie theaters, missile-detection systems, 18-hole golf courses.

The "grow, grow, grow your boat" craze has shipbuilders, brokers and marine architects in a tizzy, as it does most boat enthusiasts in this seaside city, which bills itself as the yachting capital of the world.

Frank Herhold, executive director of the Marine Industries Association of South Florida, is ecstatic. Still, the expansion of yachts over the past six years has created something of a quandary.

"It would be appropriate to coin a new name for these boats," Herhold says. "We're going to have to do something soon. If you own a yacht that's 400 feet long — well, that's in a class by itself."

On the docks, in the shipyards, a new name is already surfacing.

The gigayacht.

At this year's cruise-industry convention in Miami, Barry Gilmour, chairman of Oceanic Investment Corp., had some news for his fellow shipbuilders: His company had designed, on paper, a 630-foot yacht — a private vessel longer than two football fields.

When eyebrows went up, Gilmour told his colleagues, "Somebody will order one of those, take it from me."

Though accounting for a relatively small slice of yacht orders, yachts of mega and giga proportions are being built like never before — with even bigger ones now on the drawing board. This year, 651 are under construction, up from 507 last year and 482 in 2003 — a nearly $1 billion increase in sales, industry experts say.

Where the money is

A third of the yachts listed on the World's Top 100 by Power & Motoryacht magazine were built after 2000; of the top 40 currently under construction, the average length is 241 feet.

Americans, the world leaders in the purchase and chartering of big yachts, are largely driving that demand, says Jim Eden, who has been brokering yachts for 35 years.

"The bread-and-butter boats used to be 30, 35 feet long," Eden says. "Now, it's hard to get my bosses interested in brokering a yacht under 70."

He attributes the boom to the growing numbers of dot-com and telecom billionaires, and to the Bush administration's tax breaks for America's wealthiest. Equally important, he says, is a sea change in attitude among America's superrich in the wake of Sept. 11.

"The thinking among wealthy people now is, you can die anytime. Nobody can protect you. So you might as well spend your money now and enjoy it."

Wanted: crews

The gigaboom has caught a number of people by surprise, among them Chris Taylor, vice president of International Yachtmaster Training, a school for yachtsmen and crew in Fort Lauderdale.

This year, Taylor estimates a worldwide shortage of crew in the order of 3,000 deckhands, bosuns, mates, captains, engineers, stewardesses and chefs. "We're the biggest yacht-training institution in the world and we cannot find enough qualified crew. It's enough to blow your mind."

In the race to build the world's yachts, the United States presently places second to Italy among the world's Top 10 "yacht-builder nations." (Twenty-two American manufacturers are presently turning out 5.2 miles of yachts.) The Netherlands is running a close third.

German shipbuilders, however, are squarely at the forefront of the gigayacht revolution.

Lurssen of Bremen, Germany, is the quintessential gigabuilder. The average length of its creations is 277 feet — 51 feet longer than its closest competitor, according to the Global Build Report, published by Yachts International Magazine.

It builds boats like Octopus — the $250 million aqua-palace of Paul Allen, co-founder of Microsoft.

At 414 feet, with a crew of 60, which includes former Navy Seals, Octopus became on its maiden voyage in 2003 the world's largest, priciest, privately owned yacht.

Octopus has seven decks. It has two helicopter-landing pads. It has a swimming pool, basketball court, infirmary, a garage for Land Rovers, a movie theater, a concert space for 260 and a recording studio. (It has reportedly become a home away from home for Robbie Robertson, the rock star.)

Octopus also features an underwater-viewing salon — with a glass bottom and stadium-strength lighting — to allow passengers to gaze for hours at sea creatures. (The engineer also suggested adding torpedo launchers, for added security, but Allen considered that excessive.)

All great yachtsmen have great rivals, though, and Allen was soon to be out-giga-d by Larry Ellison, titan of Oracle, the California software firm.

At the time Octopus was launched, Lurssen was secretly crafting Ellison's Rising Sun, a 393-foot behemoth which the owner would later describe as a "sculpture made of metal and glass."

The Sun also rises

However, when it became apparent that Octopus was 21 feet longer than his yacht, Ellison ordered the German shipbuilder to extend Rising Sun to 452 feet.

Upon completion, Ellison's floating skyscraper had five stories, 82 rooms, a wine cellar the size of most beach bungalows and a generator capable of providing enough electricity for a small town.

Rising Sun was outfitted with a $50,000 satellite, radar systems with "intelligent" anti-collision warning features and anchor stabilizers — great fins that trick the waves and keep the monolith from rocking.

Final cost: $377 million.

"I say thank God for people who can afford to do this," says Mike Kelsey, president and CEO of Palmer Johnson, a Fort Lauderdale yacht company. In his opinion, the giga-buyer is doing everyone a great service.

"These boats are absolutely stunning. They're monuments. And they really, really create a better life for anyone who comes in contact with them."