Gene Sloan

USA TODAY

How will the biggest cruise ship ever built handle on the water? We're about to find out.

Royal Caribbean's much-awaited, 227,000-ton Harmony of the Seas on Thursday headed out to sea for the first time for a series of sea trials in advance of its May debut in Southampton, England.

A sister to Royal Caribbean's giant Oasis of the Seas and Allure of the Seas (the current record holders among cruise ships), Harmony is one of the most-anticipated new vessels in years. It will eclipse its siblings by more than 1,700 tons and hold nearly 100 more passengers at double occupancy. Its total capacity will be well over 6,000 people, not including crew.

Harmony has been under construction at the STX shipyard in St. Nazaire, France for more than a year, and the sea trials are taking place off the coast of France.

Exclusive: First look at Harmony of the Seas, largest cruise ship ever

A major milestone in the construction of a cruise vessel, sea trials involve maneuvers to test propulsion, steering and navigational equipment. They normally take place several months before a ship's debut when only finishing work remains.

Designed for vacationers who love big, bustling mega-resorts, Harmony will feature a giant, multi-deck water slide area -- a first for Royal Caribbean. Harmony also will be the first Oasis class ship with a Bionic Bar where the drinks are served by robot bartenders -- a concept that first debuted in 2014 on Royal Caribbean's Quantum of the Seas.

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Harmony also will have larger cabins than Oasis and Allure in some categories, and windowless "inside" cabins are being outfitted with Royal Caribbean's exclusive Virtual Balconies, which offer a real-time view of the outdoors. The concept first debuted in 2014 on the line's Navigator of the Seas.

Harmony's first voyage with paying passengers is scheduled for May 22. After an initial series of cruises in the Mediterranean, it will re-position to Fort Lauderdale, Fla., to offer alternating, seven-night Eastern and Western Caribbean voyages.

Scroll through the carousel at the top of this story for photos of Harmony setting sail on Thursday and images of its construction over the past year.

USA TODAY Cruise in 2015 was among a handful of U.S. media outlets to get early access to Royal Caribbean's last new vessel, Anthem of the Seas, ahead of its christening in Southampton, England. For our 'first look' tour of the ship, click through the carousels below.

More on Harmony

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Harmony of the Seas to have massive water slide

