LONDON — The end of summer is a nervous time for superyacht designers, and not because they fear that the owners of their latest creations may be disappointed with the first outings in the Mediterranean.

The worry is about the designers’ next vessels, because this is the time of year when clients whose boats are still in production come back from holidays with a wish list of new features — usually, based on what they saw on their friends’ yachts or at the Monaco Yacht Show, which ended Sept. 28.

“Right now we are quite far down the line in completing a big yacht in northern Europe for one client who has just spent time on a friend’s boat, which is not necessarily helpful,” said Dickie Bannenberg, the head of one of the world’s best-known superyacht design houses, Bannenberg & Rowell. He was in his London studio, an airy two-story space lined with sleek models of its creations.

“The delivery date is in the first half of next year, and that is sooner than it might seem,” Mr. Bannenberg said. “It’s fine when it’s superficial — let’s say they liked the plates or towels on their friend’s yacht — but if you’re not careful it can verge on, ‘Oh, my friend’s gym was like this, can we have something similar?’ or, ‘I would really like to add a submersible vessel.’”

The complex production schedules of these vessels mean shipyards will resist significant changes. “Re-engineering or rebuilding is going to cost a lot of money,” Mr. Bannenberg, 58, said.

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That end-of-summer tension illustrates some inescapable truths about life dealing in the world’s most expensive consumer products and ultimate discretionary purchases. One vessel alone can cost $5 million to $500 million, with annual operating costs of perhaps 10 percent of that.

This is an industry in which problems include protecting the owner’s Picasso collection from salt air, clumsy crew members and faulty sprinklers.

Or maybe you have to decide whether to build one 330-foot vessel (100 meters) or join a trend of the last few years by opting for a “smaller” 200-foot yacht with a 165-foot support vessel to carry a submarine, helicopter, speedboats and other toys. Aviva, a 320-foot yacht launched in 2017, was the first in the world to include a full-size indoor paddle tennis court.

William Mathieson, the editorial and intelligence director of the Superyacht Group, the leading analyst of the industry, said there are about 3,500 active vessels in the world that meet the loose definition of a superyacht by measuring more than 100 feet; just 55 top 330 feet.

Mr. Bannenberg’s father, Jon Bannenberg, who died in 2002, used to say that nobody in the world needs a superyacht, so it was the designer’s task to make them want one.

Jon, a charismatic Australian, is widely credited with inventing the profession of superyacht designer. In the 1960s, he brought together interior and exterior design skills with an understanding of marine engineering to replace what had previously been relatively simple structures sitting on top of hulls designed by naval architects.

He had waves of clients, starting with Greek shipping tycoons in the 1960s. Then came Middle Eastern royals in the 1970s, German and American industrialists in the 1980s, tech titans from the United States in the 1990s and wealthy Russians.

After Jon’s death, Dickie, who had worked as his father’s project manager for 15 years, brought in Simon Rowell, a hotel designer, as the studio’s creative director.

A short walk from Wandsworth Bridge on the River Thames, the studio holds 15 people, who manipulate detailed computer images of planned vessels, pore over design drawings and phone Italy to order marble fittings.

There are usually six or seven projects at various stages of a construction process that takes four to five years, and that often extends to designing stationery and a logo for crew uniforms, as well as commissioning sculptures to go on board. Jon Bannenberg liked to design the cutlery and crockery, flower vases, the light fittings and door handles.

He ran his practice like a Renaissance artist, training a stream of apprentices who now run some of the world’s top studios, and relying on wealthy patrons for commissions.

CreditTom Jamieson for The New York Times
CreditTom Jamieson for The New York Times

Those patrons included J. Paul Getty, Malcolm Forbes and Larry Ellison. Projects were discussed with Fidel Castro and the Shah of Iran that never made it to the water.

Almost inevitably, many people rich enough to spend tens or hundreds of millions of dollars on a yacht have proved to be controversial. The Australian billionaire Alan Bond was a Bannenberg customer before being jailed for fraud, and so was the Saudi arms dealer Adnan Khashoggi, who commissioned a 280-foot ship called Nabila. Donald Trump bought that one (and renamed it Trump Princess) in 1987 for a reported $30 million, with a running cost of $2.5 million a year, justifying the expenditure by saying it was “the ultimate toy” and that he hoped it would make other yacht owners feel inferior.

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A string of Bannenberg yachts were built for the British businessman Gerald Ronson, who also did jail time for fraud, and the American magnate Bennett LeBow was forced to repay millions of dollars to companies he controlled for loans that were spent on his yachts.

The body of Robert Maxwell, the publisher and fraudster, was found floating off the back of his Bannenberg yacht, the Lady Ghislaine, which was named after his daughter who is now in the headlines over her involvement with Jeffrey Epstein, the financier charged with child sex trafficking.

Mr. Epstein, who died in jail in August, represented the retail tycoon Leslie Wexner during the construction of his 300-foot yacht Limitless, another Jon Bannenberg project.

Five imposing models of Limitless still sit on the walls of the studio. Dickie Bannenberg said he never dealt with Mr. Epstein, though Ms. Maxwell “may have come to a design meeting but I have never met her.”

“It’s a tricky one,” Mr. Bannenberg said. “Legally in any industry you have a requirement to know as best you can the source of your client’s money, so in our contracts our lawyers require us to find the beneficial owner behind the project.”

“The shipyard asks the same questions,” he said. “They won’t just build for Mysterious Corporation of Grand Cayman, they need to know who is behind it.” The ownership of some yachts is a tight secret, with the owner’s passion for privacy and security often extending to teams of private guards in every port.

Adam Ramlugon, a lawyer who specializes in superyachts, said the legal obligations to avoid “dirty money” fall on regulated professions rather than on designers and builders themselves.

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“It is the designer’s bank and lawyers who are required to know the source of funds, but any company should be very careful because their bank might decide to stop acting for them if they don’t know the source of some money sloshing around in their bank account,” Mr. Ramlugon said.

Mr. Bannenberg said that “in real life, there is a limit to what we can do.”

He recalled being hired by a Moscow shipyard to do design work for a client whom he and Mr. Rowell met “once or twice including one memorably uncomfortable meeting” in a Majorca villa.

One sign that something was odd was that the meeting was held in what felt like a “safe room.” Mr. Bannenberg said that “a much bigger sign came three years later.”

“After the yacht had been delivered, Simon was a bit terrified to notice a newspaper photo of the client being led away in handcuffs by two Spanish police officers wearing balaclavas. He was allegedly the head of an organized crime gang. How could we know that?”

Mr. Rowell, 50, said that “once or twice” the firm has made its own inquiries and decided to stay away from a potential client, but a lot of these problems, especially white-collar crime, “only become obvious with hindsight.”

The types of buyers and their demands keep changing. More than a decade of heavy spending by Russian and East European clients began drying up after the Russian annexation of Crimea — “we lost one job half an hour after that,” Mr. Bannenberg said — as Western sanctions on Russian oligarchs have continued to bite.

The rising number of billionaires in mainland China has not yet translated into new buyers, and Mr. Bannenberg believes the Chinese face political and cultural restraints “on being so upfront with your wealth.”

More promisingly, there has recently been a pickup in buying from the United States, Mr. Bannenberg said, “because America still has the most high-net-worth individuals.”

The Trump tax cuts have fueled demand for superyachts, according to industry analysts, and shipyard order books are solid. Notably, this is despite recent softness in top-end sales of art, cars and real estate, amid broader fears of an economic slowdown.

Research by the Superyacht Group shows that after peaking in 2008 and then slumping after the financial crisis, the production of luxury yachts has been stable in recent years, with an annual output close to 150 new vessels.

While Americans remain the biggest buyers, the United States’ own yacht output has shrunk, with the global industry consolidating into fewer shipyards. The Italians now make the most vessels, and Dutch and German builders dominate the top of the market.

The most striking change in the industry is a shift in what the boats are actually for, as a new generation of owners want to do more than show off while anchored off Sardinia.

“The clients that approach us nowadays don’t really want a floating palace,” Mr. Rowell said. “They want a boat they are going to live on and even work on, and use for more than two weeks a year.”

The Microsoft billionaire Paul Allen, who died in 2018, is often cited as an example of a more active owner, as he used his yachts for ocean research and roaming the world.

A 600-foot-long monster called REV that emerged from a Romanian shipyard in August took that trend even further: Its Norwegian owner had it designed to double as a marine research vessel capable of supporting 60 scientists. The world’s largest yacht, REV (short for Research Expedition Vessel) can sail around the world without refueling.

“Owners today do realize that these are extraordinary bits of equipment that can go to pretty exciting places that are really difficult to reach, and that changes the way you design the yacht,” Mr. Rowell said. Modern owners sail everywhere from the Northwest Passage to Antarctica.

There is “still a minority of attention seekers, status seekers, whatever you want to call them, who really are happy sitting off St.-Tropez and Cala di Volpe and the Amalfi Coast,” Mr. Bannenberg noted.

There is a movement, he said, “towards a much greater sense of connection between the yacht and the immediate sea, by which I mean swim platforms, ‘beach clubs,’ folding terraces and hull doors that open up to the sea.”

A growing sense of environmental issues is also having an impact, Mr. Bannenberg said. “There are a few yacht-based movements and marine foundations, which are sometimes labeled as a yacht-owner’s guilt trip, that are part of the whole environmental conversation going on at the moment,” he added.

“It all adds up to a much bigger desire to actually interact with the ocean rather than sitting in a glitzy apartment that happens to be floating.”