Old vessels that once hauled cement and fuel to offshore oil rigs are being reborn as "shadow boats" to ferry Jet Skis, helicopters and luxury cars for the super rich who do not want clutter on their yachts.

A small but growing number of out-of-service oil boats, once relegated to the scrap heap, now carry the baubles of billionaires with big yachts.

The shadow boat conversion industry is one that is expected to grow as the number of new mega-yachts hitting the water increases, said Gary Slatkow, vice-president of sales for Shadow Marine in Fort Lauderdale, Fla.

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As part of the transformation from battered work horses to spiffy vessels, the old boats are completely overhauled and refitted with equipment like large cranes used to lift power boats and Jet Skis, Slatkow said.

New Orleans, La.-based Tidewater Inc. which operates the largest fleet of oil field vessels in the world, has sold six supply vessels to brokers who intend to turn them into shadow boats.

Customers for shadow boats, which range in cost from $7-million to $13-million, are an elite group who are able to afford an extra few million on top a new yacht which might cost more than $30-million.

"Our clients are the mega-, mega-rich," Slatkow said.

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Indeed, listings of the world's largest yachts include a "who's who" of Middle Eastern royalty, along with a few moguls like Microsoft Corp. co-founder Paul Allen.