Lurssen, the German mega-yacht company that’s building the yacht, declined any comment on the boat or its owners. Despite its size, Azzam has remained a closely kept secret in the yachting world. Lurssen is known for its near obsessive discretion.

Yet photos taken by SuperyachtTimes.com show the boat to be at least six stories tall, with sweeping windows on the rear deck and several upper decks dedicated to staterooms. The vessel is expected to cost more than $500 million. It is still under construction at a shipyard in Bremen, Germany and is scheduled to be delivered in 2013.

Industry executives and marine engineers say Azzam’s most unusual feature — other than its size — is its propulsion system. Rather than being powered by propellers like most yachts, it will have four giant water jets that will be able to push the ship to speeds of more than 27 to 28 knots.

“We’ve never seen a yacht close to this size with water jets,” said one marine engineer. “This will be a very fast boat.”

The new craft's owner has become the subject of widespread speculation. Yet industry executives say the owner is a member of one of the royal families in the Middle East. The Middle East is striving to become a global yachting center, with a growing number of yacht shows and mega-yacht marinas. Taking the title of world's largest yacht from Russia would further boost its image.

It’s unclear whether Azzam — which means “determination” in Arabic — will have some of the aggressive security systems or secret submarines of many of today’s Russian yachts.

Yet Azzam already ups the ante in the mega-yacht arms race. There is no other yacht currently under construction that would top Azzam, according to industry executives. Roman Abramovich has comfortably held the title since 2010. By most measures, Larry Ellison claimed the title in 2004, with his 454-foot yacht, Rising Sun, which in turn had topped the 414-foot Octopus, owned by Microsoft co-founder Paul Allen.