How Do Cruise Ships Not Tip Over?
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When looking at an incredibly tall cruise ship it’s easy to wonder “How do cruise ships not tip over?”
If you’ve taken a cruise recently, you might have noticed that modern cruise ships look particularly top-heavy. These mega-ships are like floating cities, carrying up to 6000 passengers and 2000 crew members.
It’s easy to wonder how cruise ships stay upright.
Cruise ships can avoid tipping over thanks to the combination of the ship’s wide hull, low center of gravity, ballast tanks, and stabilizers.
Naval architects take special care to ensure ships remain stable and seaworthy even in bad weather conditions.
As part of the never-ending competition between cruise lines, ships continually push the boundaries of size – growing taller, wider, and longer.
Today’s cruise ships resemble tiered cakes stacked a few tiers too high. It’s easy to wonder how cruise ships can remain seaworthy when they have a top-heavy appearance.
According to naval architect Rick Spilman, “one cannot calculate a ship’s stability simply by the ship’s appearance. Stability depends on many factors – the ship’s vertical center of gravity, the moment of inertia of the waterplane, the area under the righting arm curve, windage, free surface, and so on.”
The photo below shows Royal Caribbean’s Wonder of the Seas during a dry dock. In the image, it’s clear that less than 20% of the ship is submerged during sailing.
While the ship in the photo looks top-heavy, cruise ships are incredibly stable.
Cruise ships are designed with the heaviest equipment on the lowest decks and a wide curved hull for stability. Further, advanced technology such as stabilizers and ballast tanks help counter rough seas and provide smooth sailing for happy passengers.
According to Nasa, “The center of gravity is the average location of the weight of an object.”
The lower the center of gravity, the less likely an object will tip over.
Cruise ships store much of the heaviest equipment on the lowest decks of the vessel. Placing a cruise ship’s large engines, machinery, cargo storage, ballasts, and fuel stores at the bottom of the ship lowers the center of gravity.
The cabins, theatre, and restaurants on the upper decks are mostly just air.
According to Captain Christopher Turner of Holland America’s Zuiderdam: “If you look around the ship, most of it is just empty space. All our weight, our engines, our machinery, our freshwater tanks, our fuel tanks, are all down below. Although we have about 8 meters under the sea and 50 meters above the sea, the center of gravity is right down at the bottom. Even burning the fuel does not make too much difference. It does not bring the center of gravity up too much.”
Ships have a center of gravity that is below the water line. With such a low center of gravity, it’s difficult for an object of any size to topple over.
If you’ve read our article on how cruise ships float, you know that even a large mass, such as a cruise ship, can stay afloat thanks to the principle of buoyancy.
The buoyancy of an object depends on its average density. If the object’s density is less than the density of water, it will float. But, if it has a higher density than water, it will sink.
Objects placed in water displace a volume of water equal to its weight. The water exerts an upward force on the object equal to the weight of the displaced water, keeping it from sinking completely.
The heavier the ship, the lower it will sit in the water.
Ship hulls are constructed from lightweight and sturdy materials.
Cruise ships are designed with a U-shaped hull for stability, known as a displacement hull. The hull is wide with a deep bottom, which promotes stability.
You’ll often notice round edges where the ship meets the water. The rounded edges increase ship stability by minimizing drag, preventing the vessel from rolling, and providing a smoother ride.
As an additional benefit, the smooth ride helps to prevent motion sickness on a cruise ship.
Additionally, cruise ships are designed with a lighter structure on the upper parts of the boat. The hull is thicker near the water line than at the upper levels. And while the lower hull is made from steel, the upper levels are constructed out of lighter materials, such as aluminum.
Modern cruise ships are equipped with ballast tanks, which help keep the ship upright. Ship officers can control the water in the ballast tanks to raise or lower the ship’s center of gravity, controlling buoyancy and keeping it stable.
The ballast tanks on a ship also allow the captain to correct trim or list (tilt).
In cases of emergency or rough seas, the ballast tanks can be used to counter rough waves or high winds and minimize rocking.
Under the surface, cruise ships are equipped with wing-like stabilizers. These stabilizers help reduce the roll (sideways motion) of the ship.
The stabilizers are shaped like airplane wings and work under the same principles that planes use to generate lift.
The ship’s officers can control them independently with the option to deploy one or more stabilizers to suit the sea conditions.
When the seas are calm or the ship is in port, the stabilizers fold back into compartments along the ship’s hull.
Sensors detect the ocean currents and roll of the cruise ship.
As water flows over a stabilizer, the shape of the wing creates drive or lift. The stabilizers automatically pivot to exert pressure in the opposite direction and counter the ship’s orientation.
Captain William Wright of Royal Caribbean says that the stabilizers “eliminate about 85% of the vessel’s roll.”
Most cruise ships are equipped with two stabilizers. However, huge cruise ships like Royal Caribbean’s Oasis-class ships have four, two on each side.
Despite the safety features and design, cruise ships have a top-heavy appearance.
Cruise ships rise over a hundred feet into the air, yet only 25 feet extend below the surface.
But cruise ships are not top-heavy. Cruise ships have a low center of gravity, wide hull, ballast tanks, and stabilizers that prevent them from tipping over.
In fact, cruise ships can actually list 60 degrees and recover.
Cruise ships are unlikely to tip even in rough seas or encounters with rogue waves.
Rough weather is the most dangerous threat to the stability of a cruise ship.
Large waves and high winds can make for a bumpy ride. Fortunately, passenger’s vessels are designed to withstand 50-foot waves (15 meters), which are extremely rare.
In an interview with BBC, Harry Bolton, retired California Maritime Academy captain, claimed that a modern cruise ship could theoretically withstand a 70 to 100-foot wave under the right condition.
Fortunately, severe storms are rarely a concern for cruise passengers. “I guarantee you’re never going to be in those kinds of waves anyway,” Bolton said. “[Cruise ships] avoid bad weather like the plague. They don’t want the passengers in peril, they don’t want to risk any injury or accidents.”