"Draft" means the number of feet from the waterline to the lowest point of a cruise ship's keel; the depth of water a ship draws; how low the ship sits in the water. The term "air draft" is the number of feet from the waterline to the highest point on the cruise ship.
The nautical term draft is the United States spelling; draught is the British spelling. Interestingly, the two terms draft (USA) and draught (UK) are used to describe beer and also have the same spelling differences in the two countries.
Why the Draft Is Important
The main reason the draft of the cruise ship is very important to the Captain (and all of his crew and passengers) is that the ship will not float in any water less deep than the draft. For example, a ship with a 25-foot draft would hit the bottom if the water is even 24.99 feet deep.
The draft of the ship is determined at the time it is being built. The more decks (or air draft) the ship has above water, the deeper the vessel's draft must be. Ship architects must be sure that the ratio of the vessel draft below the waterline and the air draft above the waterline is within acceptable limits. A ship designer doesn't want his or her ship to be so "top-heavy" that it might tip over. In addition to having a larger draft, these large cruise ship designers make the tall ships with many decks above water wider to stabilize them for a smoother ride. Modern cruise ships also use stabilizers to make the ship ride smoother in rough seas.
These stabilizers are like wings extended out under the water, making the ship "wider".
Since large ships have deeper drafts, they cannot enter shallow harbors as small cruise ships can. However, larger ships with deeper drafts usually handle stormy seas better since more of the ship is underwater and it does not bob up and down as much.
So, guests onboard have a smoother ride. River ships have a very shallow draft, but can still hit the bottom since river channels change frequently.
Classic ocean liners, designed as transportation between Europe and North America, had deeper drafts since the ships were not trying to sail in the shallow water of the Caribbean. For example, the original Queen Mary ocean liner, which was built in 1936, had a draft of almost 40 feet and an air draft of 181 feet. She was 118 feet wide and had a gross tonnage of 81,000 GRT. The Oasis of the Seas, one of the world's largest cruise ships, has a 30-foot draft, an air draft of 213 feet above the water, is 208 feet wide, and has a gross tonnage of 225,000 GRT. Although this newer ship is larger and has more air draft, the Oasis of the Seas has a shallower draft. To compensate for the shallower draft, Oasis is wider and has stabilizers, which add more width if needed in rough waters.
Some people think that modern cruise ships do not have enough draft and might capsize if a huge wave hit during a storm. Although ships do sink, it's a rare occurrence, and it has never been proven that the air draft to vessel draft ratio not adequate and caused a ship to roll over.
The Titanic hit an iceberg, and the Costa Concordia hit a rocky reef. Only on a cruise movie like the Poseidon Adventure has a large cruise ship rolled over due to a wave.