The Oasis class is a class of Royal Caribbean International cruise ships which are the world's largest passenger ships. The first two ships in the class, Oasis of the Seas and Allure of the Seas, were delivered respectively in 2009 and 2010 by STX Europe Turku Shipyard, Finland. A third Oasis class vessel, Harmony of the Seas, was delivered in 2016 built by STX France, and a fourth vessel, MS Symphony of the Seas, has just been completed in March 2018. One additional unnamed ship is currently under construction and is expected to be delivered in 2021. The first two ships in the class Oasis of the Seas and Allure of the Seas are slightly exceeded in size by the third ship Harmony of the Seas, while the Symphony of the Seas is the worlds largest cruise ship with the fifth ship planned to be larger than the Symphony of the seas.
The Oasis-class ships surpassed the earlier Freedom-class ships as the world's largest and longest passenger ships. Oasis also is 8.5 metres (28 ft) wider, and with a gross tonnage of 225,282, is almost 45% larger. Oasis-class vessels can carry over 5,400 passengers. Oasis-class ships feature a split structure, with the 5-deck high "Central Park" and "Boardwalk" outdoor areas running down the middle of the ship. These areas feature lush tropical gardens, upscale restaurants, shops, and a working carousel.
To keep the ship stable without increasing the draft excessively, the designers created a wide hull. About 30 feet (9 m) of the ship sits beneath the water, a small percentage of the ship's overall height. Wide, shallow ships such as this tend to be "snappy", meaning that they can snap back upright after a wave has passed, which can be uncomfortable. This effect, however, is mitigated by the vessel's large size. The cruise ship's officers were pleased with the ship's stability and performance during the transatlantic crossing, when the vessel, in order to allow finishing work to go on, slowed and changed course in the face of winds "almost up to hurricane force" and seas in excess of 40 feet (12 m).
The ship's power comes from six medium speed marine diesel generating sets: three 16-cylinder Wärtsilä 16V46D common rail engines producing 18,860 kilowatts (25,290 hp) each and three similar 12-cylinder Wärtsilä 12V46D engines producing 13,860 kilowatts (18,590 hp) each. The fuel consumption of the main engines at full power is 1,377 US gallons (5,210 l; 1,147 imp gal) of fuel oil per engine per hour for the 16-cylinder engines and 1,033 US gallons (3,910 l; 860 imp gal) per engine per hour for the 12-cylinder engines. The total output of these prime movers, some 97,020 kilowatts (130,110 hp), is converted to electricity, used in hotel power for operation of the lights, elevators, electronics, galleys, water treatment plant, and all of the other systems used on the operation of the vessel, as well as propulsion. Propulsion is provided by three 20,000-kilowatt (26,800 hp) Azipods, ABB's brand of electric azimuth thrusters. These pods, suspended under the stern, contain electric motors driving 20-foot (6 m) propellers. Because they are rotatable, no rudders are needed to steer the ship. Docking is assisted by four 5,500-kilowatt (7,380 hp) transverse bow thrusters.
