How Do Cruise Ships Float?

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Many people marvel at the sheer size of cruise ships. They secretly wonder how something so large and heavy could move so easily in water or how something that massive could even float in the first place! In this post, we’ll explain how cruise ships float.

The answer to this question boils down to materials, design, water displacement, and buoyancy. So, in short, a combination of design and physical elements.

How Do Cruise Ships Float?

Cruise ships were designed to be massive yet displace the same amount of water that is equivalent to their mass. The ocean’s pressure, at the same time, pushes against the hull of the ship, countering the downward force of the mass of the cruise ship. This downward force in conjunction with the ocean’s upward force work in tandem to make the cruise ship buoyant.

The Science Behind It

This concept is known as Archimedes’ Principle. Archimedes’ Principle teaches that when an item floats, the weight of the displaced water equals the weight of the object. Surrounding fluid will push back with a force that is equal to the displaced amount. When the two forces are equal, the object will float.

In other words, while a cruise ship rests in water, it provides room for its mass by displacing water outward and downward. The opposing forces’ balance is what causes it to float.

Additional Features Supporting Buoyancy

Displacement and buoyancy aren’t the only factors keeping cruise ships floating. There are additional factors in play as well.

Design and Material

To achieve any type of buoyancy, cruise ships must be made of sturdy, lightweight materials that are denser than water, for example, extra-strength steel. Plus, these materials must be used with a design allowing them to displace the weight in water prior to it being submerged.

Most of the design tends to be incorporated into the hull. The hull is the shell or body of the vessel that rests below the main deck. It pushes the water out of the way when the cruise ship is in motion and permits it to float.

Odyssey of the Seas at Shipyard
Photo Via: Meyer Werft

Through years of experimentation, engineers discovered that if they created a wide, rounded, and deep hull, it better disperses the cruise ship’s weight across the body. A large cruise ship will have a hull shaped like a “U.” It permits water to flow away from the cruise ship, creating a smooth ride, dissipating drag, and keeping the cruise ship on schedule.

Other Safety Features and Double Hulls

Cruising smoothly and remaining afloat is not enough for a vessel. The hull design needs to protect guests against reefs, icebergs, and sandbars. These underwater hazards are capable of easily ripping apart a ship’s outer layers, causing damage or sinking it.

Worth Reading: How Much Does a Cruise Ship Weigh?

To protect against this, builders use extra-strength steel. This is used to build vessels containing double hulls (one hull is located inside the other) to provide additional protection. Vessels also have bulkheads helping them float in case the cruise ship receives major damage.

Cruise Ship Hull
Cruise Ship Hull (Photo Credit: Andrew Pridmore / Shutterstock)

The watertight divider is installed through the ship’s interior. It can be closed to prevent water from rushing in if the hull is damaged. Limiting the inflow of water prevents cruise ships from sinking and flooding.

How Does a Ship Stay Standing Up?

With cruise ships testing the limits of height and weight, one must wonder what prevents them from capsizing. The answer to this question rests in the design of the hull. However, to appreciate this, you must first comprehend the differences in a ship’s center of buoyancy and center of gravity.

More Science … Shifting Center of Buoyancy

The center of gravity of a ship (central focus points for gravity’s push downward) can’t be changed. Therefore, a cruise ship’s U-shaped hull is created so the center of buoyancy shifts naturally as the vessel tilts sides. This change in center of buoyancy helps to push the ship back to its upright position, no matter how large it is.

Cruise Ship Buoyancy
Cruise Ship Buoyancy (Image Credit: Fouad A. Saad / Shutterstock)

Center of Gravity

If everything contained on a ship weighed the same, the center of gravity should be in the middle. However, engines, machinery, and fuel weigh much more than things like passenger spaces, cabins, or theaters.

This creates an effect where the center of gravity is placed downward. The center of gravity for a cruise ship is toward the bottom half of the vessel.

Maintaining Centerline

When ships are upright, the force pushing it can swing past the centerline, causing it to tilt to the opposing side. This is known as rolling, which can cause passengers to experience seasickness. To address this issue, cruise lines contain several features limiting the roll of the ship.

Ship Rocking
Photo Credit: J. Helgason / Shutterstock

This includes stabilizing fins located below water and an active ballast tanks/anti-heeling system. An anti-heeling system rapidly pumps the sea water located below the waterline into holding tanks found on one side to the other. This will correct sideways leans.


The stabilizing features of cruise ships are very effective. In fact, these vessels are so well made that it is rare for guests to experience or feel side-to-side motion. Even more so, cruise ships never turn over despite being so huge and heavy.

Read Also: Six Types of Ship Rocking You Should Know

Witnessing a large cruise ship slice gracefully through a calm, blue ocean can be quite fascinating. What is more remarkable is learning how these modern marvels are equipped to float on water. When examining how cruise ships float, the answer truly lies in a mixture of scientific principles and design.

Cruise ships were created with not only physics in mind, but also with how to make the ship more durable. While we learned many years ago that no ship is indestructible (Titanic, anyone?), engineers have learned how best to protect passengers on board. This enables the cruise ship to be safe while gracefully gliding through even the most turbulent waters without capsizing.

While the movement looks effortless, there is a great deal of engineering beneath the surface keeping it afloat.

Haiyan Ma
Haiyan Ma
I absolutely love cruising with my favorite ports of call being in the Caribbean. As a former crew member for Disney Cruise Line, Royal Caribbean, and Carnival Cruise Line I can continue my passion by sharing my experiences with readers. Find out more about us here.


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