SOONER or later even a billionaire runs out of space.

Paul Allen, co-founder of Microsoft and reputed to be the world’s richest bachelor, has sent agents to shop for a new luxury boat. But the job of the new boat will be simply to trail his 413ft mega-yacht, the Octopus, and carry its luggage.

Allen, 53, whose on-board parties are famous for A-list guests and lavish hospitality, has run out of storage after cramming Octopus with a recording studio, a basketball court and parking space for two helicopters.

His move will raise the target for the aspiring wealthy yacht owner — the new breed of “shadow boats”, also known as “toy haulers”, can be as big as superyachts of a decade ago.

One company, Shadow Marine in Fort Lauderdale, Florida, is now dedicated to producing shadow boats and has already taken three orders.

Allen’s agents are understood to be focusing on the Zein, a motor yacht which, under the name Deo Juvante II, was given by Aristotle Onassis, the shipping tycoon, to Prince Rainier of Monaco and Princess Grace for their honeymoon in 1956.

The 147ft ship was built in 1928 and saw service with the Royal Navy in the second world war. It may be cramped by modern billionaire standards, but its £367,000 price tag is a bargain compared with the multi-million-pound cost of a new luxury cruiser.

Neither Allen nor the Mexican broker Baja Yachts, which is showing buyers around the Zein at its Florida mooring, returned calls last week. But Allen’s advisers have been seen at the mooring in Fort Lauderdale. “They are interested in turning the Zein into a shadow boat, which might seem like a dreadful insult to a classic beauty but it’s all the rage among the super-super rich,” said one veteran skipper.

Orders for new yachts grew by 28% last year, according to ShowBoats International magazine, which calculates that the total length of luxury yachts under construction currently amounts to 14 miles.

There are nearly 6,000 80ft-plus “mega-yachts” sailing the seas, twice as many as a decade ago. Shadow boats, however, are for billionaires who want to step up from captain to admiral of their own fleet.

Last year the Paladin Shadow, a former offshore drilling supply ship, was unveiled as the first designated shadow ship at the Fort Lauderdale Boat Show in Florida. Several others are under construction.

John Divine, a spokesman for Shadow Marine, said the vessel could carry two smaller 40ft boats, a mini-submarine, jet-skis, motorbikes and a Humvee 4x4 and still have room on the top deck for two helicopters.

They are designed to follow their mother ship and offer not only extra luggage room but also video-wired cabins for children who can be kept separate from their partying parents.

Manufacturers and outfitters are targeting only the world’s wealthiest nautical enthusiasts, such as Roman Abramovich, the British-based Russian oligarch and owner of Chelsea Football Club.

Abramovich already has his own fleet, including the 375ft Pelorus, and two other magayachts, but is now believed to be drawing up blueprints for a new ship. This would outsize the 525ft Platinum, owned by the Dubai royal family and currently the world’s longest.

Abramovich is a relative newcomer to high–priced yachting. Three years ago he spent £50m on his first mega-yacht, the 300ft Le Grand Bleu, taking it off the hands of Paul Allen who had traded up to the Octopus.

Allen, who left Microsoft in 1983 to battle blood cancer only to return a few years later with a bolder, more hedonistic outlook, loves his toys.

The three-year-old Octopus may now be only the fourth-longest private yacht in the world, but it offers all the latest in fun technologies including a detachable yellow submarine.

It takes 60 crew, including a troop of former US navy security specialists.

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