How Do Cruise Ships Float?
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Cruise Ships are like giant floating cities.
They are as long as the Empire State Building is tall and up to 20 stories high. Today’s cruise ships can accommodate up to 6,000 passengers and 2,500 crew members.
The sheer size of today’s massive cruise ships brings up the question, how do ships float?
Cruise ships float because they can displace an amount of water equal to their mass. Their U-shaped hull, low center of gravity, and hollow interior allow even the largest cruise ships to float.
Additionally, cruise ships are designed with a wide U-shaped hull which helps displace water providing additional buoyancy and smoother sailing.
Now that you know the basics let’s explore some of the science behind how cruise ships float.
As a basic rule, objects float when they have a lower average density than water.
The average density refers to the mass of an object divided by the total volume.
When the density of an object is less than the density of water, the ship floats because the downward force of gravity is less than the upward force exerted by the liquid.
Oil floats because it has a lower density. In contrast, lead sinks because it has a higher density.
Cruise ships can float because they have a lower average density than water, thanks to the large air volume inside the hull.
The principle of buoyancy states that an object immersed in a liquid faces an upward force.
When the upward force is greater than the force of gravity, the object floats. If the gravitational force is greater than the upward force exerted by the liquid, the object sinks.
The easiest way to understand buoyancy is by thinking about a submarine.
Submarines have ballast tanks that fill with water or air. The sub is positively buoyant when the tanks are full of air. Air has a lower density than water allowing the submarine to float.
When the tanks are full of water, the submarine sinks. That’s because the water in the tanks can’t compensate for the density and weight of the steel. An object that sinks has negative buoyancy.
It is possible to fill the ballasts with enough air and water so that the submarine neither rises nor sinks. In this state, the sub has neutral buoyancy.
Boats don’t operate the same as submarines. Ships don’t sink, but they don’t entirely float either.
While most of the ship sits above the waterline, much of the vessel is underwater. How deep a ship sinks depends on the ship’s weight and how much weight it carries – think passengers, crew, cargo, furniture, food, etc.
To ensure a ship floats, naval architects need to calculate how much weight a boat can hold without getting anywhere near the point of negative buoyancy.
We owe this calculation to the Archimedes principle.
According to NASA, “Archimedes Principle states that the buoyant force on a submerged object is equal to the weight of the fluid that is displaced by the object.”
In other words, when an object is resting in water, it feels an upward force equal to the weight of the water it pushes aside (displaces). If the ship is too heavy, the water displaced will never equal the ship’s weight, and it will sink.
When an object is submerged, the buoyant force makes the item lighter than it would be on land.
Cruise ships can float as long as they can displace an amount of water equal to their mass.
The pressure from the ocean pushing upwards against the hull makes the ship float.
Fun Fact: Because freshwater has a lower density than saltwater, ships sit lower in rivers and lakes than in the ocean.
The structure and design of a ship play an important role. Cruise ships are designed to displace an amount of volume equal to their weight.
The most important aspect of the design is the hull. The hull consists of a hollowed-out shell, typically made from steel.
The hollowed-out shells contain enough air to lower the average density of the vessel, making it less dense than water.
Further, while steel is primarily used on the lower decks, lighter materials such as aluminum are used on the upper decks. The choice of materials helps keep the ship lightweight and is a reason why cruise ships don’t tip over.
The shape of the hull also plays a vital role in water displacement. Large cruise ships have U-shaped hulls. The design allows water to flow away from the vessel and provides greater water displacement.
The round-bottom hulls also reduce drag and provide a more stable ride for passengers which helps prevent seasickness.
But that’s not all. Naval architects must also consider the weight of the engines, passengers, crew, cargo, and more. The combination of the ship, and everything it carries, must have a lower density than water to float.
You may have noticed that ships markings close to the waterline. The marking is called the Plimsoll line, named after its inventor Samuel Plimsoll.
According to the National Ocean Service, the “Plimsoll line is a reference mark on a ship’s hull that indicates the maximum depth the vessel can safely immerse when loaded with cargo. “
The depth varies based on the ship’s construction, size, type of cargo, water densities, and time of year.
The principles of buoyancy apply to more than just ships. Here are a few examples of buoyancy at work:
Though many ships have sunk, none are more famous than the Titanic. At the time, the Titanic was the largest and fastest vessel in the world.
So what caused the Titanic to sink?
During the night of April 14, 1912, the Titanic struck an iceberg off the coast of Newfoundland. The gash in the side of the hull caused the ship to take on water. As we know, the water raised the density of the vessel causing it to sink in the North Atlantic.
There are several theories as to why the Titanic struck the iceberg. Some people argue that the captain was traveling too fast to safely navigate the icy water. Another theory claims that Titanic’s radio operator ignored a warning about the ice field from the nearby ship Californian.
Regardless of the circumstances, the ship eventually sank as it took on water. As the ship took on water, its average density rose until it could no longer float.