How Does a Cruise Ship Float?

Lisa Maloney
Cruise ship

It's amazing that massive cruise ships don't instantly sink to the ocean's floor. With everything from ice skating rinks and swimming pools to basketball courts, spas, mini malls, and movie theaters onboard, how do these gigantic vessels remain afloat? They do it through a combination of buoyancy, water displacement, materials and design.

How Cruise Ships Stay Afloat

Ships are designed to displace an amount of water equivalent to their own mass. At the same time, the pressure of the ocean pushes up against the ship's hull and counters the downward force of the vessel's mass. The downward force of the ship combined with the upward force of the ocean work together to keep the ship afloat or "buoyant."

This basic idea is often referred to as Archimedes' Principle. According to this principle, an item floats when the weight of the water displaced is equal to the weight of the object. The surrounding fluid pushes back with a force equal to that of the amount displaced; when the two are equal, the object floats.

Here's another way to look at it. When a cruise ship sits in the water, it makes room for itself by displacing water out and down. The water responds by pushing up and in as it tries to take back the space the cruise ship occupies. The balance of these opposing force are what makes the ship float.

Additional Factors Which Support Buoyancy

In addition to buoyancy and displacement, there are several other factors that help cruise ships remain on the water's surface.

Materials and Design

To achieve buoyancy, a ship must be made of lightweight, sturdy materials which are denser than water, such as extra-strength steel. Additionally, those lightweight materials need to be used in a design which allows them to displace their weight in water before they submerge. Most of that design is implemented in the hull which is the body or shell of the ship which sits below the main deck and pushes the water out of the way and allows the vessel to float.

Through years of trial and error, engineers have found making the hull rounded, wide and deep helps disperse the weight of the ship across the body of the ship. Large cruise ship hulls are shaped like the letter "U." This design allows water to flow away from the vessel, dissipates drag, facilitates a smooth ride, and helps keep the vessel on track.

Double Hulls and Other Safety Features

Just staying afloat and cruising smoothly isn't enough; a cruise liner's hull design must also protect the people inside against obstacles like icebergs, reefs and sandbars which could rip apart the ship's outer layers. To prevent a major catastrophe, shipbuilders typically use extra-strength steel and build their ships with double hulls (meaning one hull inside the other) as an extra precaution.

Cruise ships also have bulkheads which can help them stay afloat in case of major damage. These watertight dividers are installed throughout the interior of a ship and can be closed to seal out water rushing in through a damaged hull. Limiting the water inflow can ultimately keep the ship from flooding and sinking.

How Cruise Ships Remain Upright

As of 2016, the biggest cruise ship in the world measures about 210 feet tall, and even the average cruise ships still have impressive height. So what keeps them from tipping over in the water? The answer is, again, in the hull design. First, you must understand the difference between the ship's center of gravity and its center of buoyancy.

Shifting Center of Buoyancy Is Key

According to Engineering Toolbox, a ship's center of gravity (the central focus point for gravity's downward push) cannot be changed. For this reason, a cruise liner's U-shaped hull is designed so its center of buoyancy (the central focus for the water's upward push against the hull) naturally shifts as the ship tilts from one side to the other. This change in the center of buoyancy helps push the ship back to an upright position.

Maintaining a Centerline

When the ship is pushed upright, the force of that push may naturally swing it a bit past the centerline and cause it to tilt to the other side. This is called rolling, and it's what tends to make passengers seasick. To address this problem, cruise liners are equipped with a number of features which limit the ship's roll, including stabilizing fins below the water and active ballast or anti-heeling systems which rapidly pump sea water from below-waterline holding tanks on one side of the ship to the other side. This corrects any sideways lean or "list" the ship may develop.

These stabilizing features are so effective it's rare for cruise passengers to feel any side-to-side motion, and it's almost unheard of for cruise ships to turn over even though they are so tall.

Smooth Sailing

Watching a massive ocean liner glide along on the open sea can be quite thrilling. While the ship's movement may look effortless, there's certainly a lot going on beneath the ocean's surface keeping the vessel upright and afloat. Think about that the next time you take a cruise.

How Does a Cruise Ship Float?