How Does a Cruise Ship Float

If you’ve ever seen a massive oceanliner gliding effortlessly on the open sea, you might have wondered, "How does a cruise ship float?"

This massive cruise ship floats thanks to buoyancy.

You aren’t the first person to ask, "How does a cruise ship float?" Nor will you be the last. For most people it is almost unfathomable that massive cruise ships, which weigh upwards of 71,000 tons, do not instantly sink to the ocean’s floor. So how do gigantic cruise ships, which feature everything from ice skating rinks and swimming pools to basketball courts, spas, mini malls, and movie theaters, remain afloat? The answer may surprise you.

Despite what you may think, there is no technical wizardry involved in keeping a cruise ship from sinking. Rather, the answer to the question: “How does a cruise ship float?” is rather simple. A colossal size cruise ship remains afloat thanks to buoyancy and displacement.

Ships are designed to displace the amount of water equivalent to their own mass. The ocean meanwhile pushes up and keeps the ship afloat, or buoyant. In other words, the pressure of the water pushing up on the bottom (or hull) of a cruise ship counters the downward force of the vessel's gravity, thereby helping it float on the water’s surface.

This basic idea is often referred to as Archimedes' Principle. According to this principle, an item floats when it displaces more than its own weight of the liquid that it's immersed in. The surrounding fluid pushes back with a force equal to that of the amount displaced, so when the two are equal the object floats.

In addition to buoyancy and displacement there are a number of other factors that help cruise ships like the massive Queen Mary II, which can carry more than 3,000 passengers, remain on the water’s surface.

To achieve buoyancy a ship must be made of lightweight, sturdy materials that are denser than water, such as extra-strength steel. In addition, those lightweight materials need to be used in a design that allows them to displace their weight in water before they are submerged. Common cruise ship construction includes hulls, which push water out of the way and help the vessel stay afloat.

A ship’s hull is the body of the ship, which sits below the main deck. Through years of trial and error engineers have found that 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." The design allows water to flow away from the vessel, dissipates drag, and facilitates a smooth ride.

In addition to providing passengers with a fluid ride, hulls help protect a cruise ship so that it doesn’t sink. Icebergs, reefs, sandbars, and other large, sharp objects can rip apart a ship’s outer layers. To prevent a major catastrophe shipbuilders typically use extra-strength steel and insert double hulls--or a hull inside a hull--as an extra precaution.

Mega cruise ships also stay afloat thanks to bulkheads. These vertical watertight dividers are installed throughout the interior of the hull and help keep damaged ships afloat by containing incoming water into compartments. By containing the gushing water, the ship won’t flood and sink.

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