The Family Cruise Companion

Why Do Cruise Ships Float When They’re Actually Really Massive?

By: Author David Chapman

Posted on Last updated: October 14, 2023

Categories LEARN

What do you think happens if you toss a big rock into the ocean? Typically, it starts to sink right away. But some objects that are many times heavier manage to stay afloat. One such example is a cruise ship, and there is no better example than the largest one in the world, “Wonder of the Seas”. It has a tonnage of 236,857 and is 1,188ft long and 210ft wide, so at that size, why do cruise ships float?

why do cruise ships float

Here’s the short answer: Giant cruise ships can stay afloat because they can displace an amount of water that is equivalent to their mass. While the vessel is propelled forward, it forces the water away, but the water attempts to return, ultimately creating a force that pushes the ship upward. Let’s take a closer look.

How Cruise Ships Float

Cruise ships have a mass that is upwards of 65,000 tonnes (71,500 tons), and not only do they have to keep that weight afloat, but there is also the burden of the crew, passengers, luggage, supplies, and all the various additional amenities aboard the vessel. 

So, why do cruise ships float? It seems utterly illogical that something that weighs that much could possibly float. But just like airplanes can stay in the sky, there is a science behind why cruise ships can float. The way that they can stay afloat has to do with displacement, along with principles pertaining to density and buoyancy.

Engineers will, therefore, not address the mass or weight of the ship when talking about how heavy it is; instead, they will reference displacement. The vessel displaces the equivalent amount of water to their mass when they sit in the ocean, which keeps them buoyant. In theory, it is possible to submerge a cruise ship; plenty of them have sunk (we will get to that in a bit) .

Why Do Cruise Ships Float | photo demonstrating Archimedes Principle -- floating cork vs Physics. A cork floats in water and  a antique weight that sinks.

But ultimately, what they need to do is displace their weight in water before it is submerged. Think of it as though you were trying to sink a bowling ball, as opposed to something like a soccer ball, or football. The latter possess a certain weight but can displace the water, whereas the bowling ball cannot. This is because soccer balls and footballs contain an open, air-filled interior (much like a cruise ship).  

The Concept Of Buoyancy And Cruise Ships

It is not only displacement that aids in the cruise ship’s managing to stay afloat; there is also the science of buoyancy, and we have Archimedes to thank for discovering and explaining the principle. He was a Greek scientist who discovered Archimedes’ Principle (the principle of buoyancy) back in 246 BCE. He stated:

“Any object, totally or partially immersed in a fluid or liquid, is buoyed up by a force equal to the weight of the fluid displaced by the object.”

So why do cruise ships float? Basically, the force, known simply as buoyant force, comes into effect when the ship can displace an equal amount of water as its mass. The force of gravity and the vessel’s mass exerts a downward force, all while the buoyant force pushes upwards against the ship. Then ultimately, if the downward force is less than the buoyant force, the cruise ship will float. 

Why Do Cruise Ships Float

It Has Less To Do With Weight And More So With Empty Volume

Cruise ships of the past have nothing on modern-day cruise ships, especially regarding size and mass. They are literally floating cities, with movie theaters, restaurants, casinos, bars, bowling alleys, swimming pools, and other things to keep their guests entertained. So what else separates them from, say, a solid piece of steel? The answer is empty volume.

The incredible amount of open space is staggering, even considering how much is crammed onto a cruise ship. However, there is a lot more space than you would think there to be in a cruise ship, and if we look at the example of a piece of steel, it is merely a solid object with absolutely no empty volume, and for this reason, it will not float, but the cruise ship will. 

The Shape Of A Cruise Ship Hull Helps It Float

Large ships such as cruise ships use displacement hulls, which aid with displacement , as the name suggests. As mentioned, these hulls push the water out of their way, and it resultantly tries to refill the space, causing the ship to stay afloat. Nowadays, cruise ships are made with the lightest and most buoyant possible material, but it is simultaneously incredibly durable and hardy.  

Cruise ships possess round-bottom displacement hulls that resemble rectangles that have rounded edges. The hulls look much like a giant “U”, and their roundness aids in dissipating drag. The cruise ship also manages to move more fluidly along the water owing to this shape due to there being less force applied to it as it is propelled through the water.

Although they move more fluidly, there is also increased resistance against the water, and they resultantly move far slower than other types of boats and sea vessels. Ultimately, we see that the engine power that propels them forward reaches a point where, regardless of how much drive is exerted by the propellers, the cruise ship will do just that, cruise. 

Although a cruise ship can go fast , there is no need for a cruise ship to have excessive speed potential; instead, a smooth and stable movement across the ocean is preferred; therefore, the “U” shaped hull is a perfect fit for the needs of cruise ships.

What Causes A Cruise Ship To Sink?

There are countermeasures put in place to help prevent a cruise ship from sinking; these are known as bulkheads (these come into play if something penetrates the cruise ship). They are found throughout the vessel’s hull and are vertical, watertight dividers that, if the ship is struck by something or damaged in some way, will enable the ship to stay afloat. 

The way these bulkheads manage to do this is that they will contain any influx of water within a compartment; so that it does not spread throughout the remainder of the hull. So even if the ship takes on water and is going to sink, the bulkheads will buy the crew time to get all the passengers into lifeboats and themselves in ample time.  

The ship’s sinking only occurs because the air in place is forced out and replaced with water. This means that the vessel is no longer buoyant enough to stay afloat and cannot displace its weight. Likely the most famous of these events was the sinking of the RMS Titanic . The ship struck an iceberg that pierced the hull, and water filled the bow, ending in the entire ship sinking.

Closing Thoughts

Although weighing many tons, a cruise ship will float on the ocean with ease. Principles of basic physics explain why cruise ships float. It comes down to how their weight is distributed, and how they are able to displace water in their path.

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How Do Cruise Ships Float?

Hint: Magic has nothing to do with it.

Melanie Lieberman is the senior travel editor at The Points Guy and was an editor at Travel + Leisure.

why do cruise ships float

Not everything that's heavy sinks right to the bottom of the ocean floor, and there's no greater proof than Royal Caribbean's Wonder of the Seas, the biggest cruise ship on Earth.

The world's largest cruise ship boasts 21 restaurants, 14 bars, three water slides, a mini golf course, and more, all contributing to the 236,857 tons it weighs. And that's without the 7,000 passengers it can hold.

We know that giant ocean liners don't sink, even with their rock climbing walls, nightclubs, and full-blown waterparks. But have you ever wondered why?

It's not magic or flying unicorns that keep the Wonder of the Seas plying the open waters. It's all about buoyancy: the power of a liquid to keep things afloat.

Colossal vessels stay above water by displacing an amount of water equal to their mass (the wide, U-shaped hull helps with this). As the ship moves forward and pushes water away, the water is ceaselessly trying to return to fill the space, with an energy that forces the ship upward.

And it's not just about total weight. A solid bar of steel dropped from a cruise ship balcony will undoubtedly sink until it reaches the bottom of the sea. But a boat actually has a lot of open space. No matter how many restaurants, bars, swimming pools, and casinos they cram into these floating cities, there's still an awful lot of empty volume.

Engineers are careful to keep the average density of a ship (considering both the physical weight of the vessel as well as all the air) less than the average density of the water. After all, the ocean is massive — and extremely dense. To the seas, a cruise ship is nothing more than a leaf drifting effortlessly along the surface.

So don't spend any time stressing about this one. Your ship is an engineering (and entertainment) marvel. Just sit back and enjoy the cruise .

why do cruise ships float

How do Cruise Ships Float? The Science Behind Cruising

15th February 2024

Have you ever wondered how cruise ships can float so effortlessly on the water despite their huge size and stature? With a combination of simple scientific theory and clever design techniques, cruise lines around the world have perfected their ability to launch ships of all sizes that can float and sail across seas, oceans and even rivers.

When you really think about it, though, it can seem unfathomable that ships as massive as the Royal Caribbean’s Icon of the Seas, weighing over a quarter of a million tonnes and taking the crown as the largest cruise ship in the world, can navigate through waterways without any problems. So how do they make it possible?

It’s no surprise that many people are left in awe of these feats of engineering. If you’ve ever wondered how cruise ships can offer amazing activities like rock climbing and ice skating, along with other amazing facilities, we’ve got you covered with our informative guide explaining everything from ship hulls to buoyancy. Whether you’re curious about how these massive ships stay afloat or any other aspect of their operations, our guide is here to help you out.

How do Cruise Ships Float? Buoyancy Explained

how do cruise ships float diagram

The reason that large cruise ships carrying hundreds of thousands of tonnes can float on water is largely down to physics. Put simply, cruise ships float because their mass is low in relation to the volume of water they’re sailing on. This creates an effect called buoyancy, which keeps the ship upright and afloat.

Cruise ships must be able to withstand the weight of the ship, the crew, luggage, supplies and passengers. This mass can total anywhere upwards of 65,000 tonnes. While this might seem like a lot, it’s nothing compared to the mass of water in the world. Cruise ships can displace the equivalent amount of water to their mass when they put pressure on the ocean, which pushes the ship upwards.

To keep afloat, the cruise ship must be able to displace its weight in water before being submerged. That’s a lot easier to do if the cruise ship is constructed in a way so that it’s less dense than the water below it. So how do cruise ship engineers achieve buoyancy during construction? The solution is simple – they choose lightweight, sturdy materials and disperse the weight of the ship across the hull, which we’ll explain in more detail later.

The History of Archimedes Principle

Archimedes is to thank for our understanding of buoyancy. Archimedes’ Principle states that any object totally or partially immersed in a fluid or liquid is buoyed up by a force equal to the weight of the fluid displaced by the object. So how did he find this out?

Archimedes came up with a unique idea to measure the volume of his body accurately against a body of water. As he sank into the water, it rose up and became displaced. From this experiment, he realised that if he could measure the amount of displaced water, he would be able to measure the volume of his body. The story goes that he also used this principle to find out if a goldsmith was cutting his gold with a less-dense metal like silver, but today his principle can be used to understand buoyancy.

Nassau, Bahamas- December 28,2021: The drone aerial view of Paradise Island and Nassau port, Bahamas.

For any object to float, the downward force of the object on the water should be less than the upward force of the water pushing back against the object. According to NASA, Archimedes’ principle states that the buoyant force, which is the difference in the upward and downward forces on an object in water, is equal to the weight of the water that the object displaces.

The easiest way to understand how it works is to imagine a toy boat in a bucket or bath filled with water. The volume of water being displaced by the object is tiny in comparison to the mass of water in the container, which allows it to float. Cruise ships work using this physics-based theory – only on a much larger scale.

Does this Work for Huge Cruise Ships?

Theoretically and practically, Archimedes’ Principle applies to huge cruise ships. While it’s true that the bigger the cruise ship, the more design and build considerations are needed to check that it will be buoyant when launched into the sea. But with the incredible engineers and technology available from shipbuilders, it isn’t a difficult task for them to create huge cruise ships with multiple restaurants, hundreds of staterooms, onboard theme parks, giant swimming pools and anything else they can think of.

How Much Water does a Cruise Ship need to Float?

Essentially, cruise ships can stay above the water as long as they can displace an equal amount of water to their mass. This means that most cruise ships are best suited to sailing around seas and oceans where there is more than enough water. Obviously, a cruise ship wouldn’t be able to sail on a small body of water that wouldn’t be able to displace the weight of a ship, which is why you won’t see any floating on smaller lakes.

This is why smaller cruise lines such as Fred. Olsen are perfect for meandering along rivers and reaching narrow port openings, while larger ships like much of Royal Caribbean’s fleet are best suited for transatlantic sailings. Their size is much more relative to the body of water they’re floating on, which means they can be propelled upwards and remain steady as they sail.

Small cruise ship in the waters of andaman sea with rocky island on background. At sunny day

Does the Design of a Cruise Ship Help?

Engineers and shipbuilders use their incredible knowledge of cruise ship design to make sure the ship can float and glide through the water with ease while also providing passengers with wonderful onboard experiences. So which design features do they pay the closest attention to?

The Ship’s Hull

The hull of a ship is not only important for stability, but it also provides protection for the entire ship. Fibreglass, composite materials, and even steel can be torn apart by reefs, sandbars, and icebergs as we all famously know from the Titanic. To prevent such catastrophic damage, shipbuilders make use of super-strength steel and insert double hulls (a hull inside a hull, technically speaking). This way, the hull can offer even more protection and give everybody peace of mind that the ship can remain stable and secure no matter what happens. Passengers on ships with these hull types rarely feel any rocking or side-to-side movement.

Shipbuilders will often construct a round-bottom displacement hull which has rounded edges to minimise the force of the water against the hull. This allows larger and heavier ships to sail smoothly. Unlike other hull types such as the v-hull which rises out of the water and skirts the waves, round-bottom hulls push through the water, making them extremely stable and seaworthy.

On the rare occasion that an unforeseen accident does happen, vertical watertight dividers called bulkheads are installed throughout the interior of the hull. These dividers keep damaged ships afloat by containing incoming water in one or two compartments, which means the cruise ship can continue its voyage without flooding and also with little to no effect on crew and passengers.

Ship Layout Design

The ship layout can also have a big impact on the centre of gravity, water displacement and mass. This is why you’ll find that cruise ships are designed to be bottom-heavy since this keeps the centre of gravity low and allows just enough of the ship to sit underwater and displace its mass accordingly through the seas and oceans. Storage facilities, water tanks, engine rooms and anything else requiring heavy materials will always sit on the lowest decks of a cruise ship, and these are often inaccessible to passengers because of health and safety.

Anything else – including staterooms, pools, restaurants, entertainment facilities and more – will be found on the higher decks of the ship. These are built with lightweight materials which helps with the ship’s centre of gravity. Plus, it means that the facilities are quicker and easier for passengers to access since they’re all on relatively similar decks, making it a win-win situation.

So there you have it – we hope you enjoyed our guide explaining the ins and outs of how cruise ships float. And if all this talk of cruise ships has left you yearning for a holiday at sea, be sure to check out our cruise deals .

Discover Secrets of How Do Cruise Ships Float And Not Tip Over?

If you’re like me who overthink about whatever they see or wherever they are, then you must have been concerned about how do cruise ships float and not tip over.

While coming back from my last cruise, I promised myself to solve this mystery and here I am ready to share all my findings with you. You will be amazed to know that cruising is one of the safest travel on this earth. Yes, it is.

I have done some research work and talked to a few cruise engineers to base my hypothesis on solid grounds. So, what did I conclude? Actually, it’s all about science, design, and smart building.

cruise ship floats on the sea

Yes, these massive ships stay upright and safe for everyone on board due to some cool tricks. There are a lot of amazing facts that I found out during my research journey.

Now, I am ready to share what I learned. So, in this blog, we’ll look at how these big ships stay on top of the water and why they don’t tip over.

Let’s get a little deeper into the amazing world of cruise ships together and discover how they do it!

Table of Contents

How Does A Cruise Ship Float?

A ship floats because of the buoyant force which is a crucial factor that counters the weight of the ship. It also allows it to remain floating on the water. Let’s discuss all the counting factors that play this phenomenon.

Cruise Ship Float

1- Bulkheads: Understand How Ships Stay Upright

Bulkheads are essential to ensure a cruise ship’s stability. Compartmentalizing the ship into separate sections not only contributes to the structural integrity but also keeps the cruise ship upright.

This division helps manage the ship’s stability effectively and ensures that it can withstand various sea conditions.

2- Watertight Compartments: The Key To Buoyancy And Stability

Watertight Compartments serve as the foundation for a ship’s buoyancy and stability. The principle here is straightforward: a ship stays upright because the weight of the water it displaces is less than the total weight of the ship which makes it denser than water.

These compartments ensure that even if water breaches one section, the entire vessel doesn’t become compromised.

3- Sounding Tubes: How They Maintain The Center Of Buoyancy

Sounding Tubes are critical in maintaining the ship’s center of buoyancy. They allow the crew to monitor and manage the water levels within the ship. It ultimately ensures that the center of buoyancy moves appropriately to keep the ship balanced and prevent it from tipping.

4- Trim and List: Ship Stability Mechanics

The concepts of Trim and List are central to understanding the ship stability mechanics. Trim refers to the longitudinal tilt of the ship, while list is the lateral tilt.

If you want to keep the vessel upright, managing these aspects is crucial. It ensures that cruise ships stay balanced and can return to an upright position after being disturbed by wave action.

Ship Stability Mechanics

5- Free Surface Effect: Cruise Ship’s Center Of Buoyancy

The Free Surface Effect poses a challenge to cruise ship stability, as it involves the movement of liquid within the ship. This affects the ship’s center of gravity.

Strategies to mitigate this effect include compartmentalization and the use of anti-roll tanks to reduce the impact on the ship’s stability.

6- Metacentric Height (GM): The Center Of Gravity In Cruise Ship Stability

Metacentric Height (GM) is a key parameter that ensures the ship’s ability to resist tipping. It represents the distance between the center of gravity and the metacenter, where a higher GM indicates greater stability.

Hence helps to keep the ship from tipping over in rough seas.

7- Ballast Tanks: Help Cruise Ships Float And Stay Upright

Ballast Tanks are ingeniously used to help cruise float and stay upright. The ship can alter its center of gravity by adjusting the amount of water in these tanks. This creates an upward force that counteracts the downward force of the ship’s weight and effectively keeps the ship stable.

8- Gross Tonnage (GT): Keeps Cruise Ship Stable

While cruising on the Icon of the Sea, the largest ship of the Royal Caribbean , I kept thinking about how these giants keep floating. And that’s when I found out about Gross Tonnage (GT). It’s a measure that shows the overall internal space or volume of a ship.

Gross Tonnage affects the distribution of mass and volume of the large ships for optimal stability. So, the reason why these large ships move with stability is basically their internal space and not their overall weight.

9- Dynamic Stability: Prevents The Ship From Tipping

Dynamic stability is the ability of the ship that make it stable and prevent it from tipping under external forces. This concept is crucial to ensure that cruise ships can safely navigate through different sea conditions.

10- Stability Criteria And Regulations

Adherence to Stability Criteria and Regulations set forth by the cruise industry ensures that cruise ships are designed and operated to meet stringent safety and stability standards.

These criteria are vital to prevent accidents and ensure the passengers’ crew’s safety.

What Causes Cruise Ships To Sink?

why cruise ship sinks

It’s not just one but many factors that cause a ship to sink. Most of the time, it’s the water that gets inside the ship and makes it less buoyant. This happens if the ship hits something big like an iceberg or gets a massive tear in its hull.

Do you remember the legend Titanic? That’s what happened with this historical giant of the sea. Titanic, as compared to modern cruise ships , wasn’t supported by various factors we discussed earlier in this blog. So, simply by hitting an iceberg, the water rushed in, and the Titanic went down within hours.

Unlike Titanic, modern ships are designed super smartly to avoid these problems. They have special sections and alarms to keep water out and prevent ships from sinking.

So, even though there might be some risks today’s cruise ships are much safer because of advanced engineering and strict safety rules.

How Likely A Cruise Ship Can Tip?

Now, how often do you think a cruise ship might tip over? I know it sounds scary, but actually, it’s very rare.

Today’s engineers design ships very carefully and plan the routes even more meticulously to avoid any mishap in the seas. So, it’s very unlikely for a  cruise ship to sink . Experts say that the technology and design of these ships keep them stable, even in rough water.

After the Titanic, in the last 100 years, fewer than 25 cruise ships have tipped in the ocean.

So, when you’re cruising, you’re on one of the safest ways to travel. The chance that a cruise ship would tip is so low that it’s not something I worry about when planning my next cruise adventure.

How Much Of A Cruise Ship Is Underwater?

Cruise Ship Is Underwater

It’s all about the ship’s draft. The draft is the part of the ship below the waterline – basically, the bottom of the ship that you can’t see when it’s floating.

A major portion of the ship (depending upon its size) is submerged to make sure that it’s stable and buoyant. This balance keeps the ship steady and helps it glide smoothly across the water.

So, when you’re looking out at the sea from one side of the ship, remember, there’s a whole underwater section keeping you afloat.

Do Cruise Ships Have Flat Bottoms?

Well, not all cruises have flat bottoms. But surely, this shape can be beneficial in certain conditions and provide a larger surface area for stability. However, many cruise ships have a variety of hull shapes. These huls are designed to balance efficiently through the water. It proves that there’s a lot of science behind that smooth sail.

Let me make it more clear to you. To do so, we will have a look at the chemistry of hull design. There is a phenomenon known as Archimedes’ principle, which will help you understand all this.

According to this principle, the buoyant force acting on a submerged object is the same as the fluid’s weight that the object displaces. So, cruise ships are designed for optimal stability and buoyancy.

What Is The Most Stable Part Of A Cruise Ship?

As far as I explored, it turns out that stability depends upon the design of the ship, especially the water plane area. This is the heaviest and widest part of the ship that’s usually found at the bottom or near the center.

This area is very important to keep the ship balanced, especially in rough seas. And if you have seasickness, it’s better to stay in the lower and central parts of the ship. Because these areas tend to feel less movement.

So, next time when you’re aboard, find a spot closer to the water plane area to make your cruise even more enjoyable.

Are Large Cruise Ships More Stable?

Yes, that’s true! When it comes to stability, the size of a large cruise ship does play a role. That’s because the world’s largest cruise ships, or giant cruise vessels, are designed in such a way that they stay stable. Actually, their massive structure and cruise ship stabilizers help them stay more stable in rough waters.

When Was the Last Time a Cruise Ship Tipped Over?

The last instance when a cruise ship tipped over was on 13 Jan 2012. Plus, in the previous ten decades, just 22 ships have been tipped so far.

This number shows the importance of the advanced systems onboard. For example, the ability to pump water from one side to another. This technology is key in maintaining the balance of the ship and prevents it from tipping.

So, there are just a few incidents in the past where a ship actually tipped. That’s why it’s evident that such matters are exceptionally rare. Why? Due to the cruise industry’s stringent safety measures and design innovations.​

How Shallow Can Cruise Ships Go?

The depth a cruise ship can travel in water depends on its density. Ships are designed to be less dense than water which allows them to float. The ship’s draft, which is the portion of the cruise ship underwater , must be considered along with water density to avoid grounding.

In short, the ability of the ship to navigate shallow areas is limited by these factors which makes sure that it remains afloat without touching the sea bottom in less dense water environments.

Bottom Line

So far in this blog, I’ve uncovered how these floating giants stay upright and navigate the seas. Cruise ships are designed with safety and stability in mind.

The blend of technology and design ensures that these vessels can face the vast oceans with confidence. This peek into their workings not only satisfies curiosity but also reassures us about the safety and ingenuity behind cruise travel.

Can a storm tip a cruise ship?

Cruise ships are designed to withstand storms thanks to advanced engineering and the rigorous safety standards of cruise lines, making tipping highly unlikely.

Can a cruise ship capsize in rough seas?

Modern cruise ships, like the Wonder of the Seas, are built to stay upright in rough seas, significantly reducing the risk of capsizing.

Could an iceberg sink a modern cruise ship?

The density of modern ships and their advanced navigation systems make it highly unlikely for an iceberg to sink a ship like today’s cruise vessels.

Can a whale tip over a cruise ship?

Considering the weight of the cargo and the massive size of cruise ships, a whale cannot tip over a cruise ship as the ship moves through the water.

Do cruise ships stop in the middle of the ocean?

Cruise ships may pause in the middle of the ocean if the volume of water or displaced water is equal to the weight of the ship, creating an opposing upward force, but they usually keep moving to their destination.

I am Zoe Grace, a passionate enthusiast of cruise ships. With a decade of firsthand experience in the cruising industry, I have developed a deep understanding of the intricacies and wonders that these majestic vessels hold.

Now, I am excited to embark on a new journey as an author, sharing my knowledge and insights with readers who share my fascination for the world of cruising.

Join me as we explore the captivating world of cruise ships together.

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How ships float, avoid icebergs, & other questions from our not-so-scientifically-inclined writers.

As a Sailor on one of our cruises, there’s some stuff we don’t want you to worry about. Cooking for yourself, for example. Stressing about work. And we certainly don’t want our Sailors to be preoccupied wondering, “how do cruise ships work?” or “no seriously, how do they stay afloat like that?” Our engineers worry about these things so you don’t have to. But even if you’re not contemplating the Archimedes Principle of positive buoyancy while you’re soaking up rays on the pool deck, you might have always wanted to know how cruise ships stay afloat, or how they avoid obstacles at night. These are interesting questions, even for the non-mechanically inclined.

Since we’re in the cruising business, we find this stuff fascinating, too. That’s why we interviewed Chief Engineer Rhea Wilson and her team to get the inside scoop on everything related to ship mechanics and operations. They break down how cruise engines work, where the water you’re drinking comes from, how cruise ships detect icebergs, how they avoid icebergs, and — yeah, a lot of other iceberg-related questions. If you knew nothing about ship mechanics before, that’s about to change. This one’s for all the nerds out there.

How cruise ships work: An interview with our Chief Engineer

Maybe a stupid question, but how does such a massive boat stay afloat, anyway?

Engineers : Science. Archimedes Principle to be exact, positive buoyancy. Basically the bit of the ship in the water weighs less than the weight of water it replaces (lots of tanks to create buoyancy). Why a tree trunk will float but a brick will sink despite the tree trunk weighing more.

Where is the engine located, and how are cruise ships powered? 

Engineers : Cruise ship engines are located near the bottom of the ship as they are heavy, and they are usually towards the aft of the ship. Cruise ships use Diesel-Electric propulsion systems. The main engines are generators which produce electricity which then powers the azipods. VV ships have 4 engines and  2 azipods. There are 3 bow thrusters to provide maneuverability when arriving/departing a port.

Azipods are a motor with a built in propeller and rudder. The electrical power created by the engines drives this big azipod motor which turns the propellers, and pulls the ship forward. Basically think of how a jet engine pushes air across a plane wing — that's how an azipod and propeller pushes water across the rudder and drive/steer the ship. They can turn 360 degrees for maneuverability, and the rpm of the propeller varies depending on the required ship speed.

People tend to imagine engine rooms looking like the one on Titanic, with sweaty laborers shoveling coal into the engine. What does a cruise ship engine room actually look like?

Engineers : Well it can be a hot working environment, there is a lot of heat generating machinery in the engine rooms. However a modern engine room is bright and clean and nothing like Titanic. The engines are a large part of the engine room, but the whole lower part of the ship is the engine rooms and compartments with auxiliary equipment used to supply a whole ship. So fresh water generators, a wastewater treatment plant, stabilizers for minimizing ship movement etc, not only the engines like you see in the movies.

Speaking of Titanic, what technology is in place to keep cruise ships from hitting icebergs?

Engineers : Much better equipment for detecting objects in the water. Be it small craft, other large ships, or icebergs. That and proper binoculars for the lookouts.

Cruise ships are pretty huge. How quickly could it change direction and navigate around an iceberg, if one was detected?

Engineers : As above, the azipods make a large ship really maneuverable for it's size, much more so than a ship with conventional propellers and rudders. 

If the ship did hit an iceberg, and started taking on water, what would happen? 

Engineers : There are 5 main vertical zones along the ship, and split further into compartments. Every one of these compartments are capable of being fully isolated in case water came in and the ship would still be operational. There are regulations in place for essential services to stay functional in case certain sections were lost to fire or flooding.

Okay, enough about icebergs. We don’t even sail near them. People are more aware now than ever of their carbon footprint. What are cruise ships doing to reduce emissions ?

Engineers : There are stringent rules and regulations on emissions globally and locally to the ports the ships visit. There are advanced gas cleaning systems always in use to comply with these rules.

If someone fell overboard and the ship needed to stop, how long would that take? No one will, but I just have to ask.

Engineers : The requirement for a complete stop from full speed is 15 ship lengths, but the Virgin Voyages ships take less than half that for a stop from full speed.

Where do the ship engineers (and other essential ship operating staff) sleep when they’re not working? 

Engineers : The majority of ship engineers/essential staff have accommodations close to the Engine Control Room, so they are close to their workplace. Most engine officers have an alarm fitted in their cabins which is activated from the ECR in case of technical emergency. Usually the deck below the main passenger decks, so with a porthole for daylight :)

Is there a giant 12,000,000,000 lb anchor like you see in movies?

Engineers : Not so much. There are two anchors by the bow, but they're not comically large. An anchor's “weight” to keep a ship in place is in the chain, not the anchor itself.

Where does the drinking water come from on a cruise ship (please tell us it’s not toilet water)?

Engineers : There are large Reverse Osmosis plants which take in heavily filtered sea water, and then after many processes produce fresh water. This fresh water is then treated to public health requirements and standards. So no, it's not from the toilet water. That water never goes anywhere near the potable fresh water.

Sometimes I’ll see the ship captain dining in a restaurant, or wandering around the boat. When he’s gone, who’s keeping the ship away from icebergs?

Engineers : Well there is a whole team for deck and engine departments, and the Captain can't be navigating the whole time. There is a watchkeeping team – consisting of deck officers on the bridge and technical officers in the ECR – 24/7 keeping the ship away from icebergs and the lights on while the Captain is out having dinner.

Wow, we certainly learned a lot about cruise ships!

Take that curiosity a step further and check out our Ships page to see more about our lady ships.

Related Articles

Come aboard our adults-only cruises for a kids-free getaway at sea., we’re all about the best “ships,” from friendship to partnerships, to cruise ships and beyond., cruises vs. all inclusive resorts: why one is the clear victor….

Maritime Page

How Do Cruise Ships Float? The Science Behind Their Buoyancy

Cruise ships are massive vessels that can weigh hundreds of thousands of tons, yet they are able to float effortlessly on the water. Have you ever wondered how do cruise ships float?

The answer lies in the principle of buoyancy, which is the ability of an object to float in a fluid due to the upward force exerted by the fluid.

Cruise ships are designed to displace a large amount of water, which creates a buoyant force that supports their weight. The shape of the ship’s hull is also critical in ensuring that it stays afloat.

The hull is designed to be wider at the bottom, which helps to distribute the weight of the ship more evenly and increase its stability. Additionally, the hull is curved upwards at the front and the back, which helps to reduce drag and improve the ship’s speed .

Overall, the buoyancy principle and the design of the ship’s hull are the key factors that enable cruise ships to float. As you delve deeper into the fascinating world of cruise ships, you will discover many more interesting facts about these impressive vessels.

How Do Cruise Ships Float

Understanding Buoyancy

Buoyancy is the force that allows objects to float in a fluid. This force is created by the difference in pressure between the top and bottom of an object submerged in a fluid. The concept of buoyancy was first discovered by the ancient Greek mathematician and physicist Archimedes, who realized that the buoyant force acting on an object is equal to the weight of the fluid displaced by the object.

The buoyant force is what keeps a cruise ship afloat. When a cruise ship is placed in water, it displaces an amount of water equal to its own weight. This displaced water creates an upward force that counteracts the downward force of the ship’s weight , allowing the ship to float.

The amount of water displaced by a ship is determined by its shape and size, as well as the density of the water. The density of water is affected by temperature and salinity, which can affect a ship’s buoyancy.

The buoyant force acting on a ship is equal to the weight of the water displaced by the ship. This force is always directed upwards, towards the surface of the water. If the weight of the ship is greater than the buoyant force acting on it, the ship will sink. If the weight of the ship is less than the buoyant force acting on it, the ship will float.

In summary, a cruise ship floats because of the principle of buoyancy. The weight of the water displaced by the ship creates an upward force that counteracts the downward force of the ship’s weight, allowing the ship to float. This principle is based on Archimedes’ principle, which states that the buoyant force acting on an object is equal to the weight of the fluid displaced by the object.

Cruise Ship Design and Structure

Hull design.

The hull of a cruise ship is designed to be streamlined and efficient, allowing the ship to move through the water with minimal resistance. The most common hull shape for cruise ships is the u-shaped hull, which provides stability and buoyancy. The hull is typically made of steel, which is strong and durable enough to withstand the rigors of the open sea.

Double Hulls

Many modern cruise ships have double hulls, which provide an additional layer of protection against accidents and collisions. In the event of a breach in the outer hull, the inner hull can help prevent water from flooding the ship and causing it to sink. The use of double hulls has become increasingly common in recent years, as cruise lines seek to enhance the safety of their vessels.

Deck Layout

The deck layout of a cruise ship is carefully designed to maximize space and provide passengers with a range of amenities and activities. Most ships have multiple decks, with public areas located on the lower levels and cabins and suites on the upper levels. The deck layout typically includes restaurants, bars, lounges, pools, and other recreational facilities.

Safety Features

Cruise ships are equipped with a range of safety features to ensure the safety and well-being of passengers and crew. These features may include lifeboats, life rafts, life preservers, and emergency evacuation systems. In addition, ships are equipped with sophisticated navigation and communication systems to help avoid accidents and respond quickly in the event of an emergency.

Overall, the design and structure of a cruise ship is carefully planned and executed to provide passengers with a safe, comfortable, and enjoyable experience at sea. The use of advanced materials, such as steel and double hulls, and the incorporation of a range of safety features help ensure that passengers can relax and enjoy their time on board without worrying about the ship’s ability to float.

The Science of Stability

Weight and balance.

Cruise ships are massive floating structures that can weigh up to hundreds of thousands of gross tons. To ensure that they remain afloat, it is essential to maintain a proper weight and balance. The weight of a cruise ship must be distributed evenly to prevent it from tipping over. This is achieved by placing heavy items like engines and fuel tanks at the bottom of the ship and lighter items like cabins and restaurants at the top.

Center of Gravity

The center of gravity is a crucial factor in determining a cruise ship’s stability. It is the point at which the weight of the ship is evenly distributed. A ship’s center of gravity must be below its center of buoyancy, which is the point at which the water’s upward force equals the ship’s weight. If the center of gravity is above the center of buoyancy, the ship will become unstable and capsize.

Stabilizing Systems

To maintain stability, cruise ships are equipped with stabilizing systems that counteract the rolling motion caused by waves. Stabilizing fins are one such system that extends from the ship’s hull to reduce rolling. Anti-heeling systems are another type of stabilizing system that uses water tanks to counteract the ship’s heel caused by waves.

Overall, the science of stability is a crucial aspect of cruise ship design and operation. By maintaining proper weight and balance, ensuring the center of gravity is below the center of buoyancy, and using stabilizing systems, cruise ships can remain stable and afloat even in rough seas.

Managing Ship’s Weight

Cruise ships are massive vessels that need to be able to float in the water. One of the key factors that determine a ship’s ability to float is its weight. Managing a ship’s weight is crucial to ensure it is balanced and can remain afloat. Here are some of the ways in which cruise ships manage their weight:

Ballast Tanks

Ballast tanks are compartments within the ship that are used to control its stability and balance. These tanks are filled with water to help the ship maintain its equilibrium. When the ship needs to be stabilized, water is pumped into the ballast tanks . Conversely, when the ship needs to be lightened, the water is pumped out of the tanks. This process helps to ensure that the ship remains stable and balanced, even in rough waters.

Fuel is a significant weight factor on a cruise ship. The ship needs fuel to power its engines and generate electricity. The amount of fuel required for a voyage can be significant, and it is essential to manage the weight of the fuel to ensure that the ship remains balanced. Fuel tanks are located in the lower part of the ship to help maintain the ship’s center of gravity.

Water Tanks

Water is another significant weight factor on a cruise ship. The ship needs fresh water for drinking, cooking, and cleaning. Water is also used to generate steam for the ship’s engines. Water tanks are located in the lower part of the ship, and their weight is managed to ensure that the ship remains balanced.

In conclusion, managing a ship’s weight is critical to ensure that it remains stable and balanced. Ballast tanks, fuel tanks, and water tanks are all used to manage the ship’s weight. By carefully managing the weight of these entities, cruise ships can remain afloat and provide a safe and comfortable voyage for their passengers.

Cruise Ships in Motion

Cruise ships are designed to float, but they also need to move through the water. The motion of a cruise ship can be affected by a number of factors, including the ocean itself, the open ocean, and ocean waves.

When a cruise ship is in motion, it is propelled forward by its engines. These engines work by burning fuel, which generates heat. The heat is then used to create steam, which drives turbines that turn the ship’s propellers . The propellers push against the water, which propels the ship forward.

The motion of a cruise ship can also be affected by the ocean itself. The ocean is constantly moving, and this movement can cause the ship to rock back and forth. This motion is known as pitch and roll. Pitch is the up-and-down motion of the ship, while roll is the side-to-side motion.

When a cruise ship is in the open ocean, it is subject to larger waves than it would be in calmer waters. These waves can cause the ship to pitch and roll more than usual. To counteract this motion, cruise ships are equipped with stabilizers. Stabilizers are fins that extend from the sides of the ship. They work by creating resistance against the water, which helps to reduce the ship’s motion.

In summary, cruise ships are designed to move through the water by using engines and propellers. The motion of the ship can be affected by the ocean itself, the open ocean, and ocean waves. To counteract this motion, cruise ships are equipped with stabilizers that help to reduce the ship’s motion.

Famous Cruise Ships and Companies

Royal caribbean international.

Royal Caribbean International is a popular cruise line that has been operating since 1968. It is one of the largest cruise lines in the world and has a fleet of 24 ships. The ships are known for their innovative designs and features, such as rock climbing walls, ice skating rinks, and surf simulators. Royal Caribbean International is also known for its commitment to sustainability, with a focus on reducing its environmental impact.

The Titanic is one of the most famous cruise ships in history. It was built in 1912 and was considered one of the most luxurious and advanced ships of its time. However, it’s tragic sinking on its maiden voyage has made it an infamous ship. The Titanic’s sinking was caused by hitting an iceberg, which caused the ship to sink and resulted in the loss of over 1,500 lives.

Wonder of the Seas

The Wonder of the Seas is a cruise ship that is set to debut in 2022. It is being built by Royal Caribbean International and is expected to be one of the largest cruise ships in the world . The ship will have a capacity of over 6,000 passengers and will feature a range of amenities, including a water park, a casino, and a spa.

Overall, these famous cruise ships and companies have played a significant role in the cruise industry. From the innovative designs of Royal Caribbean International to the tragic sinking of the Titanic, these ships have left a lasting impact on the industry and continue to shape its future.

Frequently Asked Questions

How do cruise ships stay afloat with so much weight.

Cruise ships are designed to displace a large amount of water, which creates an upward force called buoyancy. The weight of the ship is distributed evenly across the hull, which is shaped to maximize buoyancy. This allows the ship to float even with a large amount of weight on board.

How do cruise ships avoid sinking?

Cruise ships are equipped with a variety of safety features to prevent sinking. These include watertight compartments that can be sealed off in the event of a breach, multiple redundant systems for propulsion and steering, and regular inspections and maintenance to ensure the integrity of the hull and other critical components.

How do cruise ships maintain balance?

Cruise ships use a combination of ballast tanks and stabilizers to maintain balance. Ballast tanks can be filled or emptied with water to adjust the ship’s weight distribution, while stabilizers are used to counteract the effects of waves and wind.

How do cruise ships stabilize in rough seas?

Cruise ships use a combination of stabilizers and advanced navigation systems to maintain stability in rough seas. Stabilizers are designed to reduce the rolling motion of the ship, while navigation systems can help the ship avoid the worst of the weather.

How do cruise ships generate electricity?

Cruise ships generate electricity using a combination of diesel generators and other power sources such as solar panels and wind turbines. The electricity is used to power the ship’s propulsion systems, lighting, air conditioning, and other onboard systems.

How do cruise ships recycle waste?

Cruise ships are equipped with advanced waste management systems that allow them to recycle a significant portion of their waste. This includes systems for treating and purifying wastewater, as well as for recycling materials such as paper, plastic, and glass.

About the author

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I worked as an officer in the deck department on various types of vessels, including oil and chemical tankers, LPG carriers, and even reefer and TSHD in the early years. Currently employed as Marine Surveyor carrying cargo, draft, bunker, and warranty survey.

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How Do Cruise Ships Float?

How A Cruise Ship Floats

One of the most common questions in the cruise industry is how a cruise ship floats in water. Read the article below to find out why and what factors affect how a cruise ship floats.

Why do cruise ships float?

The answer to this question has been debated for years. The most common theory states that they are buoyant because of their large size. But why would you want your ship to be big if it floats on water? Why not just make them smaller so they can fit more people inside? Well, there’s no need to worry about that anymore! Royal Caribbean Cruise Lines have come up with an ingenious solution to this problem – floating dry docks!

How Cruise Ships Stay Afloat?

The first thing you need to know about floating on water is that there are two types: Buoyant or Displacement. “ A ship displaces its own weight in water by creating an area of low pressure at the front end of the vessel which pushes out against the surrounding water. This creates more surface area for the hull to push up against, thus increasing the amount of force pushing down into the water below.”

A buoyant ship does exactly what a name suggests; it floats due to air trapped within the structure of the ship itself. These ships use ballast tanks filled with seawater and/or sand to keep themselves afloat. When these ballast tanks fill up with water, the ship sinks until the tank empties again. If you’ve ever seen a boat bobbing around in the ocean, chances are it was a buoyant ship.

So now that we know how cruise ships float, let’s get back to our original question. There are several reasons why cruise ships float. First off, they are very large compared to regular boats. Because of this, they require a lot less energy than traditional boats to remain upright. Another reason is that they are built specifically to withstand rough weather conditions. Finally, they are designed to sit above the waves rather than ride along side of them. All three factors contribute to their ability to float.

The first thing you need to know about floating on water is that if something isn’t heavy enough to stay in place when submerged under water, then it will float away. This means that anything made out of wood or metal sinks like a rock; however, most modern cruise ships are built with steel frames covered by an outer skin of aluminum alloy which makes them more resistant to sinking than their wooden counterparts.

The second factor contributing to the stability of a cruise ship is its shape. Most cruise ships have sharp angles where the hull meets the deck so that there is no flat surface for things to rest against. Also, because the decks are angled downward towards the center of the vessel, any weight placed on top of the ship has nowhere else to go but down. Lastly, since the entire structure sits higher than the surrounding waters, the pressure exerted by the water pushes upwards rather than downwards. Together all of these elements help make cruising safer than other types of transportation.

Floating Dry Docks

Now we’re getting closer to understanding how cruise ships stay afloat. In order to understand this concept better, let’s take a look at some examples of different kinds of boats.

Example 1 – Airplane

An airplane doesn’t really “float” as much as it flies through the air. It uses wings to create lift which allows it to fly without sinking. However, when landing, the plane needs to sink back into the ground to stop from crashing.

Example 2 – Boat

Boats don’t actually float either. They rely on displacement instead. Like airplanes, boats also use sails to move forward. Unlike planes though, boats usually only sail forwards. So even though they aren’t technically floating, they still depend on the same principle behind being able to float.

Floating Dry Dock

Now that we’ve covered both forms of boats, let’s talk about floating drydock. A floating dry dock is basically a giant version of a boat. Instead of using sails like normal boats, however, they use huge propellers called thrusters. Propeller thrusters work similarly to jet engines. As long as the engine runs smoothly, the blades spin quickly enough to generate thrust. Thrusters allow vessels to travel across land while remaining stationary.

What is buoyancy?

Buoyancy is the force that keeps a body floating. It’s also called Archimedes’ Principle, named after Greek mathematician and physicist Archimedes of Syracuse. He was one of the first people to discover this principle. In his book On Floating Bodies he wrote: “If you place an object on water it will float because its weight is less than the amount of fluid displaced by its volume.” This means if there’s more air inside your lungs than outside, you’ll be able to breathe easier. If you’re swimming underwater, you can’t see what’s happening at the bottom of the ocean, but you still feel buoyant because the water around you weighs more than the air inside your lungs. Buoyancy helps keep us afloat even though we don’t weigh much.

Why do some objects sink?

When an object floats, it doesn’t always mean that it stays completely stable. Sometimes, objects fall over due to gravity pulling them into the earth. For example, if you drop a ball from the roof of a building, it falls straight down until it hits the ground. However, if you throw the same ball up high, it won’t hit the ceiling unless someone catches it before it does. Why is this? Gravity pulls everything toward Earth. When an object gets too close to the planet, it starts falling faster and faster. Eventually, it reaches escape velocity and becomes free-falling. Once it leaves Earth’s gravitational pull, it continues moving forward without slowing down. Objects that aren’t held together by friction start breaking apart once they reach escape velocity. They become smaller pieces of matter known as meteoroids. These small particles eventually burn up in the atmosphere. Some of these meteors enter the earth’s atmosphere and collide with each other. Others break apart further and continue traveling through space.

How do buoyancy and density apply to ships?

The ship displaces water when moving forward. As long as there are no leaks, the displacement of water equals the ship’s weight. When the ship moves backward, the water flows back into its original position. In other words, the flow of water around the hull creates pressure pushing upward on the bottom of the boat. This causes the bow to rise up until the net forces acting on the vessel cancel each other out. At this point, the ship is said to be neutrally buoyant.

The ship has reached neutral buoyancy.

Now let’s say that the ship sinks slightly below neutral buoyancy. Water now fills the void left behind by the sinking ship. Since the ship is heavier than the surrounding water, the downward force caused by the water pressing against the underside of the ship increases. This makes the ship move backwards again. But since the ship is lighter than the water, the upward force created by the water flowing past the top of the ship decreases. So the ship begins to move forwards again. Now imagine that the ship sinks so far that the ship is actually submerged under the surface of the water. There isn’t enough room for any additional water to fill the gap between the ship and the sea floor. Therefore, the ship remains stationary while the water level rises above the deck. This situation is known as positive buoyancy. A ship with positive buoyancy tends to stay upright.

In order to maintain stability during movement, a ship must have negative buoyancy or neutral buoyancy. Neutral buoyancy occurs when the ship is neither fully immersed nor fully exposed to the open seas. Ships with negative buoyancy tend to roll side-to-side instead of rolling fore-and-aft like boats with positive buoyancy.

Negative buoyancy happens when the ship is partially submerged beneath the waves. The ship may not be entirely covered by water, but the area underneath the ship is filled with water.

Using the right materials and designs

The design of the hull itself plays an important role in determining the stability of the ship. The shape of the hull determines what kind of forces are created by waves hitting the sides of the ship. If there is too much resistance at any point along the length of the hull then the wave energy cannot pass through and so creates more pressure against the side of the boat. This results in increased stress on the structure and ultimately leads to structural failure.

If you want your boat to remain stable regardless of how deep it goes underwater, make sure that the hull is designed properly. You can use different types of material depending on whether you’re going to sail across shallow waters or deeper ones. For example, if you plan to travel over relatively calm bodies of water such as lakes or rivers, you should choose a light wooden hull because they will float better and resist damage from rough weather conditions. However, if you intend to cross oceans where storms often occur, you’ll need something stronger. Steel is usually used for these kinds of vessels because it doesn’t corrode easily and won’t break apart even after being hit by large objects.

Does density of water affect the way a ship floats?

The answer is yes. The reason for this is that when you have more weight on top, it will push down harder than if there was less weight on top. This means that the center of mass moves to the bottom and thus the boat sinks.

A simple example would be two blocks stacked one atop another with an empty space in between them. If we put some weights into the middle then they’ll sink because their combined force pushes down hard enough to move the center of gravity below the surface of the water.

If you want to know how much difference there is, just measure the distance from the center of mass to the surface of the water before adding any extra weight and after. As far as why ships float at all, I’m not sure but I think it’s related to buoyancy. Buoyant objects tend to rise up out of the water while nonbuoyant ones fall back down. So by having lots of air inside your hull, you’re making yourself heavier so you don’t sink. It doesn’t matter whether or not the air is actually moving around though – it only matters that the volume of air inside the hull is greater than the volume outside.

Do cruise ships ever sink?

Cruise ship disasters are rare, but they do happen. In fact, there have been more than 100 reported incidents of passenger and crew deaths in the past century alone.

Can a cruise ship float upside down?

A floating vessel is not stable. It will tend to roll over and capsize if it loses its balance, or the wind changes direction suddenly. If you are on board when this happens, your best bet is to get out of the way as quickly as possible. The safest place to be in an emergency situation like that would be inside one of the lifeboats. You should also make sure all passengers know how to use them properly.

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 Harmony of the Seas at its debut in Southampton, England, in May 2016.

Courtesy of Royal Caribbean International

Harmony of the Seas at its debut in Southampton, England, in May 2016.

Why do cruise ships float?

From www.travelandleisure.com via Scoop.it :

Not everything that's heavy sinks right to the bottom of the ocean floor, and there's no greater proof than Royal Caribbean's Harmony of the Seas —which happens to be the biggest cruise ship on Earth.

The world's largest cruise ship boasts an ice-skating rink, a movie theater, 10-story dry slide, and weighs no less than 227,000 tons. And that's without any of its 6,780 passengers.

We know that giant ocean liners don't sink, even with their rock climbing walls and nightclubs. But have you ever wondered why?It's not magic or flying unicorns that keep the Harmony of the Seas plying the open waters. It's all about buoyancy: the power of a liquid to keep things afloat.

Colossal vessels stay above water by displacing an amount of water equal to their mass (the wide, U-shaped hull helps with this). As the ship moves forward and pushes water away, the water is ceaselessly trying to return to fill the space, with an energy that forces the ship upward.

I must admit, but I am love watching documentaries where the show how cruise ships are built and operated.  Many of these shows are on the Smithsonian channel.

I never really gave it much thought in the 25+ cruise I have been on how something so massive could stay afloat. Now I know.

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Ship Buoyancy and Stability: How Ships Float and Stay Upright

Container ship MSC Gülsün

Have you ever looked at a giant ship and wondered how such a heavy object made of steel floats in water? I know I have. As a matter of fact, I still marvel at the science behind giant cruise ships , with thousands of passengers, tons of supplies, and entire theme parks on board, not sinking to the bottom of the ocean! And how about those staggering 20,000 TEU container ships, carrying all the world’s TVs, smart phones, and t-shirts through wind and stormy weather? Well, let’s have a look.

Royal Caribbean's Oasis of the Seas 

So, how does it float?

For a ship to float, it must have lower density than water. At first sight, this might not make sense, as the steel that ships are made of is not exactly known for its low density. The thing is, the density of an object results from the relationship between its total volume and weight. If the total volume of a ship is large enough in relation to its weight, it has a low density. Simply put, because of all the air spaces inside, a ship has a lower average density than water, despite the heavy materials it is made of.

The weight of water a ship displaces gives it buoyancy – a force acting against gravity on a body immersed in liquids or gases. This force is caused by the displacement, or pushing aside, of the surrounding medium (water) and corresponds to the weight of the deformed fluid. This relation between an object and the fluid it is immersed in is known as the  Archimedes’ Principle . It applies in the same way, whether we are dealing with a DIY wooden canoe or a giant container ship. A ship floats because the submersed part of it is lighter than the displaced water, and the total weight of the ship equals the total weight of the water it displaces.

Forces of gravity and buoyancy on a ship in water.  

However, it is not enough for a ship to simply float in a static state. It also needs to stay upright while sailing through the water in various conditions, and this is where it gets a lot more interesting.

How does it stay upright?

In shipbuilding and design, the term stability refers to the property of a floating body, such as a vessel, to maintain an upright floating position in still waters, or to right itself up again in rough waters. Several key factors determine the individual stability of a ship – the hull’s shape and size, as well as its mass and mass distribution, load weight and distribution, cargo behavior (ex. movement of bulk cargo in the holds), the dynamic behavior of the ship (ex. when changing course at high speed), and the free surface on board when cargo holds are only partially filled. External operating conditions that affect stability are wind, swell, drift, water density (saltwater/freshwater), and the risk of an additional load on the ship due to icing of the deck surfaces in cold weather.

Ice accretion from waves and spray may add load to the top of the ship and change its center of gravity, potentially making it unstable. 

The fundamental parameters of a ship’s stability are its center of gravity, center of buoyancy, metacenter, and metacentric height.

To picture the  center of gravity , imagine the entire down-acting weight of the ship, concentrated in one point. When the ship is loaded, the center of gravity retains its position within the ship, as long as all the masses in the ship remain in the same place. But if some cargo on board shifts, due to rough seas for example, its motion and new location may change the ship’s center of gravity.

To understand the  center of buoyancy , first think of the total upward force, caused by the hydrostatic pressure from the displaced water. Imagine this water trying to get back to its original location before it was displaced. The more water a ship pushes aside, the greater the buoyant force that the water exerts back upon the ship. The buoyant force may act on all parts of the immersed hull, but the side forces act in opposite directions and are of equal magnitude, therefore cancelling each other out and leaving only those forces acting upward on the hull. As there are too many points along the hull on which the upward forces act, mathematically we treat all of them as one single force acting at one single point. It is this point that we call the center of buoyancy. Technically speaking, this is the centroid of the displaced volume of water, i.e. the arithmetic mean position of all the points where buoyant forces act. Consider it as the one point, where the entire displaced water would remain in equilibrium. It is somewhat difficult to visualize this point in the case of water, as water does not stay in one place. So for simplicity, imagine instead trying to balance a piece of cutout cardboard on the tip of a pin. The point where the piece of cardboard would sit in perfect balance on the tip of the pin, is its centroid.

The  metacenter  is where the two lines of the original center of buoyancy and the newly shifted center of buoyancy intersect, as pictured in the image below. As the center of buoyancy moves laterally when the ship heels, the metacenter typically stays directly above it. The distance between the center of gravity and the metacenter, is known as the ship’s metacentric height.

Centre of gravity (G), centre of buoyancy (B), and metacentre (M) when ship is upright and when ship is heeled over to one side.

When the ship is in an upright position, the center of gravity and the center of buoyancy are aligned vertically above one another. If the ship heels to one side due to an external force, the center of gravity remains in its place relative to the ship, but it migrates to the side of the heel as a whole. The center of buoyancy migrates to the same side, into the center of the water currently displaced.

If the center of gravity and the center of buoyancy are no longer perpendicular to each other, and the center of gravity is below the initial metacenter of the ship, a so-called  righting lever  occurs. This righting lever creates a moment, known as the  righting moment , which returns the ship to its original position. If the ship is able to return to an upright position, it is said to be in  stable equilibrium .

Stable Equilibrium with righting lever (GZ)

In some cases, the vertical positions of the center of gravity and the metacenter overlap, and no righting lever is created during the heel. This means that the heeling moment does not generate a righting moment to bring back the ship to its original upright position. In this case, the ship is in a  neutral equilibrium .

Neutral Equilibrium ship

The danger in this situation comes if there is a cargo shift, and the heeling angle of the ship is increased due to the shift of the cargo’s weight. The ship then could quickly move from a state of neutral to a state of  unstable equilibrium .

Unstable Equilibrium ship

In an unstable equilibrium, the vertical position of the center of gravity is above that of the ships’ metacenter, creating a negative righting lever and thus, an even larger heel from the momentum. If the ship does not return to a stable equilibrium, and the heel increases, the ship will eventually capsize.

The capsized RoRo ship MV Cougar Ace after loosing stability during ballast water transfer.

I hope this article helped the concepts of buoyancy and stability “sink in”. See you next time!

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How the World's Largest Cruise Ship Floats

why do cruise ships float

The world's largest cruise ship is making its first transatlantic crossing from Finland to Fort Lauderdale, Fla., where it will make its U.S. debut. Though colossal, the ship relies on the same physical principles as its smaller brethren to stay afloat.

The massive ship, called the Oasis of the Seas and built by STX Finland for Royal Caribbean International, stands 20 stories high, is as long as four football fields, and can accommodate 5,400 guests at double occupancy.

The two typical measures of size are length and weight, which is measured as displacement, or the weight of water the ship must displace to stay afloat . "She is 1,180 feet long and displaces 100,000 tons," said Paul Miller of the Department of Naval Architecture & Ocean Engineering at the U.S. Naval Academy in Maryland, referring to the Oasis of the Seas .

For comparison, the RMS Titanic , which sank in 1912, was 883 feet long (269 meters) and weighed about 58,000 tons.

In terms of space available, the Oasis is nearly five times larger than the Titanic . Specifically, the Oasis can hold 225,282 gross registered tons, while the Titanic could hold 46,329 grt.

This measurement was derived long ago to describe a ship's space for a common cargo — wine. Since wine was shipped in "tuns" that each held 8 barrels or about 242 gallons, a ship that could carry 8,000 wine barrels was considered a 1,000-tun ship. "Tun" evolved into "ton" and then into "gross registered ton."

The Oasis is 50 percent larger than the runner-up, which is a group of Freedom-class ships (such as Freedom of the Seas ), according to Royal Caribbean International.

Staying above water

How does such a huge ship float?

"It's just like any other ship. It has to displace an equal amount of water to how much it weighs," said Matthew Collette, assistant professor of Naval Architecture and Marine Engineering at the University of Michigan. If it didn't, the ship would sink.

To displace this amount of water , and keep the ship stable without having a massive draft beneath the water, the designers created a wide hull.

"To keep it stable they had to make the ship very wide. It's 66 meters (217 feet) wide," Collette said. That means the Oasis can't fit through the Panama Canal, which is just 105 feet (32 meters).

About 30 feet (9 meters) of the ship sits beneath the water, which is a small percentage of the ship's overall height.

Dealing with the elements

The idea of a cruise generally means sunny skies, and such ships will change their ports of call to avoid large storms or hurricanes, Collette said. That's not to say rough seas are out of the question.

"It's hard to know for sure how this ship would ride without knowing the details of its design, but wide, shallow ships such as this tend to be 'snappy' in that they have so much stability they tend to snap back upright after a wave has passed, which can be uncomfortable to be on," Collette said. "The sheer size of this ship will also help it, as larger ships tend to be better rides in rough weather."

Another issue is the superstructure, or the part of the ship that extends above the main deck.

"We have learned how to design and build large ships successfully," said Miller, the engineer with the U.S. Naval Academy. "What is a bit challenging about this ship is the amount of superstructure. That can cause windage problems in hurricanes, but the basic idea of cruise ships is to run away from them."

While this ship will likely stay in warm tropical waters, what if it did stray and come upon a titanic iceberg? "The ship is bigger and generally able to take more damage than a smaller ship," Collette said. "However, as the ship is bigger, it has more kinetic energy to dissipate if it was involved in a collision, which may mean it would also suffer more damage."

And no matter the size of a water craft, rogue waves are always a concern. Rogue waves are rare but towering problems that can soar 100 feet and are known to sink large cargo ships. Scientists have only recently begun to figure out what's behind the once-mythical waves. A study in 2008 suggested that in rare circumstances, waves that would normally cancel each other out can combine to form tall monsters in quick fashion.

The Oasis is no low-riding cargo ship, however.

"If it was struck by one I would expect there to be some local damage at the point of impact — maybe some broken portholes or bent railings, but little else," Collette said. "All ships are designed to make the chance of large-scale structural collapse very remote."

Cozy quarters

And while the ship's behemoth size is garnering plenty of attention, the goal of such a cruise ship is to make it feel small. To do this, the Oasis is divided up into various themed neighborhoods, including parks, such as one themed after Central Park, boardwalks and a sports zone.

"It's an attempt to try to make it feel smaller, because you don't want the passengers to feel like they're just a commodity," Collette told LiveScience.

When you look under the hood, however, the giant cruise ship is pretty basic. "Most of the innovation of the ship is the size and the passenger amenities," Collette said. "In terms of the propelling machinery, the design of the ship, it's all very similar to the cruise ships that have come before. There's not a lot of new ground here."

Such a leviathan presents other issues, including figuring out an evacuation plan for so many passengers and being able to fit into ports.

Should we expect an even larger ship any time soon?

"Whether this ship is the limit I don't know," Collette said. "Because the economy has kind of turned south since they signed the contract for this ship, I don't think anyone is going to challenge it for the record in the next three to four years."

  • 10 Profound Innovations Ahead
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Jeanna Bryner

Jeanna served as editor-in-chief of Live Science. Previously, she was an assistant editor at Scholastic's Science World magazine. Jeanna has an English degree from Salisbury University, a master's degree in biogeochemistry and environmental sciences from the University of Maryland, and a graduate science journalism degree from New York University. She has worked as a biologist in Florida, where she monitored wetlands and did field surveys for endangered species. She also received an ocean sciences journalism fellowship from Woods Hole Oceanographic Institution.

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Building A Cruise Ship: The Float Out And Why Ships Do

Posted on March 2, 2015 March 2, 2015 Author Chris Owen

Anthem of the Seas, now under construction and the second of Royal Caribbean ‘s Quantum-class ships passed a major milestone recently as the ship completed the docking out process at Meyer Werft Shipyard in Germany .  This video shows the process in a unique and aesthetically pleasing way as lovely seafaring music plays in the background.  That’s for us to enjoy.  Let’s do that now, then read on.

Perhaps of more importance to those who have not sailed before, or those who have and wonder about the mechanics of cruise ships, are simple questions like “How does a cruise ship float?”, “If the ‘float out’ is a major milestone, what are other significant events?” and other related topics.  In the first of a series all about Building A Cruise Ship , we take a closer look at what goes into that process, from A to Z.

Since we have this new video at hand, let’s jump right into the process.

Photo- Royal Caribbean International

The Float Out Process Built in a dry dock facility, the initial stages of s cruise ship’s construction take place.  Put simply, in the case of 168-ton Anthem of the Seas and other large ships, giant blocks of the ship are made and welded together to form the whole ship.  Once that is done and before interior spaces are outfitted with what we see as passengers, that dry dock is flooded and the ship (hopefully) floats.   Which brings up the question: How do cruise ships float?

This is actually a question that comes up more often than might be imagined, most often from cruise travelers who once sailed a small boat with a sail on a lake.  That boat might have had a centerboard to help keep it stable and make maneuvering possible.  Cruise ships do not work that way.  They remain afloat thanks to two factors: buoyancy and displacement.

Why Cruise Ships Float Ships are designed to displace the amount of water equivalent to their own mass. At the same time, the ocean pushes up and keeps the ship afloat, or buoyant .   Basically, the pressure of the water pushing up on the hull of a cruise ship counters the force of the ship’s gravity down, allowing the ship to float on the water’s surface.

The science behind that is called  Archimedes’ Principle which concludes anything floats when it displaces more than its own weight of the liquid that it’s immersed in. But while critical and required to keep ships upright, buoyancy and displacement are just two factors that keep cruise ships floating.

Also Required To Keep Ships Floating To make buoyancy happen, a ship must be made of materials that are denser than water, such as extra-strength steel. Also, the ship must be designed in a way that allows them to displace their weight in water before being submerged. On cruise ships,  hulls are designed to push water out of the way which also helps them stay afloat.  Looking sort of like the letter “U,” water flows away from the ship, reduces drag and makes for a smoother rice.

Also a factor in keeping ships afloat, double and reinforced hulls.  Those, along with water-tight bulkheads that divide the ship into separate compartments, help keep the ship afloat should the hull be breached by icebergs, reefs, sandbars, and other large, sharp objects that can rip apart a ship’s outer layers.

Our Favorite Float Out In Recent Memory Speaking of floating out a new cruise ship, new Viking Star will debut in May 17 of this year.  In this video, Viking Star, the premier ocean ship from Viking Ocean Cruises makes her debut from dry dock to water on June 23, 2014, at the Fincantieri shipyard in Venice . Experience the Float Out of this state-of-the-art vessel with Viking chairman Torstein Hagen and ship godmother Trude Drevland.

Cruise Vacation HQ

How Do Cruise Ships Float?

A single cruise ship carries thousands of people, and some of them weigh over 200,000 GT (gross tonnage). Having such a hefty weight and size to hold enough people for a small town, how do luxury liners stay afloat? Cruise ships can sail the waters thanks to their designs, which manipulate the physical principle of buoyancy.

Let’s go a little deeper and discover how objects like cruise ships manage to float. Aspects of their design, safety features, and other parts work with buoyancy to keep them upright even through the waves. 

Cruise ships float by exploiting the physics rule called Archimedes’ principle . In simple terms, the law states that objects must weigh as much as the water they displace to float. If they weigh more or are denser than the water they displace, the objects will sink.

Ships push on the ocean’s surface when they sail, but to float with Archimedes’ principle, they must displace quite a lot. A ship must have the right shape. Condensed items like balls and small weights sink because they do not push out much water. However, wide-pointed objects like a ship’s hull do it well.

When water displaces, it tries to reclaim the areas it’s pushed out of, from outside and below. When the force of the liquid pushing back equals the force of gravity caused by the ship’s weight in water, a cruise liner stays perfectly afloat.

Cruise ships have plenty of mass to balance. To certify the heavy ship stays buoyant architects employ many designs that help counter gravity and displace as much water as possible.

Cruise Ship Design Features Supporting Buoyancy

How do cruise ships displace enough water to float? The design of the hull holds the secret.

The most common modern cruise ships have U-shaped hulls. For water displacement to be successful, the hull must be denser than water, which is why engineers use extra-strength steel alloys. Steel is one of the only materials sturdy and light enough to supplant water without too much mass.F

Lastly, the upper portions of cruise ships need careful design, too. To float, the liner’s cabins and other facilities must be able to trap air. Since air is less dense than water, it becomes an additional aid for buoyancy. The upper portions of the cruise must also use lightweight materials to minimize the force of gravity pulling the boat downward.

Safety Features

Beyond the physics, cruise ships have a few safety features that aid in floating, particularly in emergencies. If an iceberg, rock, another boat, or large debris pierces the hull, there must be a contingency plan. That’s why most cruise ships adopt a double hull design where one floater rests inside the other, serving as a backup if the outer piece punctures.

Similarly, bulkheads keep a ship’s hull compartmentalized. Bulkheads are watertight, thick, sealed chambers that ship workers can open and close at will. If part of the hull punctures, sailors can close the flooding bulkheads to prevent water from filling up the rest of the cruise ship.

Why Don’t Cruise Ships Tip Over?

Even on wavy waters, the largest cruise ship won’t flip over. There are three reasons this happens: the ship’s center of gravity, its center of buoyancy, and its technology.

Cruise ships and other heavy boats have hulls with low centers of gravity. Much of the liner’s machinery, such as pumps and boilers, rests at the bottom of the vessel. The weight adds to the force of gravity pulling down on the cruise ship, bringing the center of gravity lower and increasing the boat’s stability.

Secondly, cruise ship hulls have varying centers of buoyancy by design. Each comes with a centerline, usually resting flat underwater. If the wind or other forces push on the cruise and it tilts, the hull’s design shifts the boat’s center of buoyancy back beyond the centerline. Therefore, the cruise hull’s design brings it back to the upright position when it begins to tilt one way or the other.

Lastly, technologies on cruise ships regulate their balance. Ballast tanks and anti-heeling systems are pump-based regulators found near the ship’s hull. When the boat is swaying or tilting, operators use them to pump seawater from one side of the hull to the other. The force of releasing that water helps to balance the boat when it leans or sways.

Most modern cruise ships also have objects called stabilizing fins attached to the hull. They help the boat maintain a forward direction and a steady center of gravity.

A combination of natural physics and human engineering allows us to experience giant cruising vessels. The science of the modern cruise ship’s U-shaped hull balances the force of gravity above and the push of the waters below. Now that you know how cruise ships float, you can marvel at the feat of engineering while you relax as a passenger on the deck.

why do cruise ships float

Bermuda Cruise Port Guide

Planning your cruise to Bermuda and trying to figure out what to expect at the port? There are multiple ports in Bermuda for cruise ships to dock: King’s Wharf and Heritage Wharf are both located in the Royal Naval Dockyard on Ireland Island North at the western tip of the Great Sound (the busiest cruise …

why do cruise ships float

How Much Is a Disney Cruise?

The happiest place on Earth brings smiles to the seas with all the amenities offered on a Disney Cruise vacation. Between the pools, spa, shows, entertainment, and dining options, a Disney Cruise is a vacation with something for everyone. If you’re considering a Disney Cruise vacation, you’re likely wondering, “how much is a Disney Cruise?” …

why do cruise ships float

How Much Does an Antarctica Cruise Cost?

For many people, seeing Antarctica is a dream come true and the trip of a lifetime. The best way to experience it is by taking a cruise. Flying requires expensive charter flights with expert pilots, but cruises let you make multiple stops. Generally, a cruise to Antarctica typically costs at least $10,000 per person. Still, …

why do cruise ships float

About Fred Dean

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How Do Cruise Ships Float With So Much Weight?

Have you ever looked at the size of a cruise ship, especially the modern-day mega-ships in particular, and wondered how they float?  

This is even before knowing how much some of these vessels weigh, with no less than 5 of the largest Royal Caribbean cruise ships coming in with a gross tonnage of over 200,000 and the largest Wonder of the Seas, a staggering 236,857 gross tonnes.  

It’s a wonder how they stay afloat.

How Do Cruise Ships Float

That’s even before all the guests’ weight and luggage is put onboard.    

Not to mention the weight of hundreds of crew members who live onboard the cruise ship.

In this post, I aim to explain as clearly and simply as possible how a cruise ship floats no matter how large they are built and then go into a little more depth, illustrating each of the key factors more clearly.

How Does a Cruise Ships Float?

Although made of steel, much of a cruise ship hull is hollow, less dense than the amount of water it displaces.  The ship’s weight is spread over a large volume, including a wide hull and overall surface area, which spread over a wider area of water and offers more of an upward force, keeping it afloat.  This is known as a buoyant force or buoyancy.

Let’s look at each aspect in more detail.

Whether an object floats or sinks comes down to the laws of physics, in this instance, its buoyancy.

So what is buoyancy?

Buoyancy is the ability of an object to float on a fluid. 

To expand further.

Buoyancy is an upward force exerted by fluid, in this case, water, that opposes the weight of an object partially or wholly immersed, ie, a cruise ship.

In simple terms, if a 100,000 gross tonne cruise ship can displace an equal weight in water, in this case, 100,000 gross tonnes of water, it will be buoyed up by force created, and the pressure from the seawater will force the cruise ship to stay afloat.

Various factors come into play to ensure that a cruise displaces an equivalent amount of water to its own weight.

Archimedes Principle

Buoyancy is an ancient law of physics known as the Archimedes’ principle , which was discovered around 246 BC and named after the Greek Mathematician and inventor who discovered it.

The Archimedes Principle states that an object immersed or partially immersed in a fluid will float when the weight of the water it displaces is equal to the object’s weight.    

It’s the water that is displaced, pushed aside by the object, that is trying to push its way back to its original position that creates the upward force that meets the downward force of the object, that when equal, meets a floating point.  

How buoyant an object is depends on two main factors:

  • Displacement – the amount of water the object displaces

Various other factors also come into play which we have broken down and expanded on below to give you a clearer understanding of each one’s contribution to keeping a cruise ship afloat. 

A stone is a dense object. 

It’s a solid compact object all the way through .  

Therefore no matter how large or small it is, from a large rock to a tiny pebble, because of their density, they will all sink.

This is because of its density, the stone weighs more than an equal volume of water.  Therefore the stone sinks.

Compare this to a stick or even the whole trunk of a tree. Because wood density is less than water, the stick or tree trunk will float.  The weight of the wood to an equal amount of water is less.

Even though cruise ships weigh thousands of tonnes, because they are not dense objects, they will float if they displace enough water equal to their weight, they will float.

Even though cruise ships can weigh thousands of tonnes, many hollow areas are filled with air .  It’s not like the ship is a solid mass of steel.

These hollow air-filled spaces reduce the average density of the cruise ship’s volume.  

If the average density of the cruise ship is ever more than the weight of the water, it displaces, it will sink.  

This was the case with the Titanic , which hit an iceberg damaging the ship and allowing water to enter, increasing the ship’s density. Once the ship’s density is more than the density of water displaced by the ship, the ship sinks .  

However, the cruise ships’ designers still need to ensure they not only float but that they float upright and don’t tip over , even in rough seas.

Two Forces Meeting

In understanding buoyancy further, it is worth explaining the role of forces.

When a cruise ship is launched into the water , two forces come into play and act on it to keep it afloat.  

Firstly you have the weight of the cruise ship .  

Once it’s in the water, the weight of the cruise ship meets what is known as the “ upthrust ” or “buoyant force”. This is the upward force of the fluid , in this case, the seawater.

If there were no upthrust, nothing would float.

When the two forces meet, it’s here that an object will float or sink.

When the weight of an object is more than the upthrust , it will sink.   If the weight is less, it will float.

This is known as the Archimedes Principle, as outlined above. It’s the action of an upward force on an immersed object (cruise ship) in fluid (the sea).  The force is equal to the weight of the displaced fluid by the object.

Buoyant Force

Weight and Volume

Although cruise ships can weigh thousands of tonnes, this weight is spread over a vast volume . 

To see how this is relevant, let’s look at an example.

If you took an apple that weighs 100g and a nickel coin that weighs 5g.  

Even though the apple weighs 20 times more than the nickel if you place them into a container of water, which one will float?  

The apple will float, and the nickel will sink to the bottom.

This is because the apple’s weight is spread over a much larger volume in relation to its weight.  Its density is less than the nickel coin.

This is similar to a cruise ship.  

The ship’s enormous weight is spread over an equally colossal volume (and it’s not solid like a nickel).

Surface Area

Buoyancy is the upward force pushing up on objects that come into contact with it.  

The larger the object’s surface area, in this case, the hull of a large cruise ship, the more weight it can support.

The large surface area of the cruise ship means there’s a very wide area of water to push up against the surface and, therefore, more force to keep the cruise ship afloat.

Displacement

A cruise ship floats because although it is made of dense steel its shape and design enable it to enclose large volumes of air, the overall cruise ship’s density is less than the water it displaces .  Therefore allowing it to float.

By displacement, we mean that some of the water must move for the cruise ship to sit in the water.  The water and the ship cannot occupy the same space.  The cruise ship pushes water to the side. This is displacement.   

To understand this concept, fill a glass of water right to the top.  If you place an object into the water, some water will be displaced and fall over the sides of the glass.  The same thing happens in the sea, the water is pushed to either side of the ship, it’s displaced.

A cruise ship, although made of steel can be lighter in weight than the seawater it displaces because it is not solid steel.

The hull of a cruise ship is hollow , therefore primarily full of air and much lighter than the equivalent amount of water it displaces.  

If the hull were damaged and water was to get in and fill the space occupied by the air, the density of the hull and the cruise ship would increase in relation to the water it was displacing and the ship would likely tip over, and eventually, the cruise ship would sink .

Shape of Hull

The larger the cruise ships have become, the more water they need to displace for them to be able to float.

Therefore cruise ships are designed with wide U-shaped hulls, which displace more significant amounts of water which is moved out of the way on either side of the ship, thus creating a greater force that keeps the ship afloat.  

Wide U Shaped Hull of Cruise Ship

The more water that is displaced, the more buoyancy force is created and the more weight it can support as the water tries to return to its original location .   Remember, the ship has to displace an equal weight in water to the weight of the ship itself to float. 

The hulls of a cruise ship are often referred to as displacement hulls because they are designed to displace (move) as much water as possible to the sides.  Their rounded edges help to dissipate the drag enabling smoother sailing.

You can see this more clearly when you look at the bottom of a cruise ship .

This is in contrast to many ships with V-shaped hulls that almost bounce on the waves compared to U-shaped cruise ships that move in a much flatter stable motion.

However, the disadvantage is that they move slower than their V-shaped counterparts.

Weight Distribution

Weight distribution is vital in ensuring the cruise ship has a low center of gravity.

The lower part of the ship is much heavier than the higher decks.  

The lower decks are weighed down with:

  • Machinery in engine rooms
  • Freshwater water tanks
  • Food Storage

There can be as much as 2000 tonnes of fuel and 3000 tonnes of freshwater on some of the largest cruise ships.

The upper decks are much lighter in comparison , there’s much more open space that is full of air.  

There are some heavy areas on the cruise ship, such as the swimming pools, which, when full, can weigh over 200 to 250 tonnes, but if, for any reason, the center of gravity needs to be adjusted, they can be emptied.

How Deep Does The Water Have To Be For A Cruise Ship To Float?

The water depth needed for a cruise ship to float has to be at least a couple of feet or a meter deeper than the cruise ship’s draft , which could be 25ft to 30ft (7.6m to 9.1m ) for a mega cruise ship.

If a cruise ship is coming into a port with mixed water levels, a maritime pilot or harbor pilot, as otherwise known, will be sent out to guide the cruise ship through the deepest water levels to dock.   

The maritime pilots will be experts in the depths and conditions of the dock’s waters, helping the cruise ship to avoid any higher surfaces of rock or mud, etc.

Video – Why Do Big Ships Float?

For an overall explanation of all aspects that keep a cruise ship aflot, check out this video.  It has some good visual examples to illustrate some of the key points I’ve highlighted above.

Frequently Asked Questions

Is it easier for a ship to float in seawater.

If you have ever noticed your body is more buoyant in seawater, you may have wondered if it’s the same for cruise ships.

Seawater is denser than freshwater, increasing the buoyancy of any objects floating on this.

Freshwater has a density of 1000kg/m^3, whereas seawater has a density of around 1030kg/m^3, although this can vary.  

An extreme example would be the Dead Sea which has a density of 1240kg/m^3, where your whole body will float without trying. 

What Is The Largest Cruise Ship Afloat?

The Royal Caribbean Wonder of the Seas is the largest cruise ship afloat, with a gross tonnage of 236,857 and 362.04 m 1,187.8 ft in length.

Royal Caribbean Harmony of the Seas is the longest at 362.12 m or 1,188.1 ft and slightly lower gross tonnage of 226,963, making it the 3rd largest by that metric.  

How Do Cruise Ships Not Sink?

While a cruise ship’s weight equals the water it displaces, it will not sink.

Once the weight of the ship is more than the weight of the water it displaces, it will start to sink, but as it does so, it will displace more water. 

If the cruise ship can maintain airtight spaces on the cruise ship via the ballast tanks and utilize inbuilt bulkheads, it can prevent sinking if it can keep its weight at any point equal to the amount of water it is displacing.   The more airspaces and less dense the cruise ship can be made, the less chance it has of sinking.

As you can see, there are several design factors that come into play in allowing even the largest of cruise ships. Fundamentally it comes down to physics and creating a ship that is equal in weight to the water it displaces.

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Reader's Digest

The 10 Biggest Cruise Ships in the World

Where to float on the biggest boats.

E very vessel that's once held the title of "biggest cruise ship" has one thing in common: It is a destination in and of itself. Of course, these enormous passenger cruise ships also transport guests to thrilling ports, exotic islands and remote locations too. It's almost as if you could live on a cruise ship!

These massive ships each have so much going on: theater and music on a multitude of stages, water slides, roller coasters and go-kart tracks, zip lines, ice skating rinks, laser tag arenas—you name it. There are also hidden cruise features waiting to be discovered and more restaurants than you will ever need to feed yourself. They are, simply put, awe-inspiring creations. Have you ever thought about how cruise ships float?

One of the most important cruise tips for vacationing on one of these big boats is to accept that you won't be able to do it all on your first visit. Thankfully, with many itineraries to choose from, you can sail on the largest cruise ship again and again, learn cruise ship code words and have new experiences every single time you're on board one of these massive ships.

What is largest cruise ship in the world?

The current reigning champion is Royal Caribbean's Wonder of the Seas. But next year, there will be a brand-new biggest cruise ship, the Royal Caribbean Icon of the Seas . Planned with 20 decks, 2,805 staterooms, seven pools and nine hot tubs, the future largest ship in the world is currently being built in Finland. Here are the details about the biggest and grandest ships to sail on.

Wonder of the Seas

Cruise line: Royal Caribbean

Length: 1,188 feet

Width: 215 feet

Guest capacity: 7,084

A true wonder, the current biggest ship in the world delights guests every week with a feast of activities, shows, meals and memories. Royal Caribbean's Wonder of the Seas , an Oasis-class ship that is a city at sea, has 16 guest decks, 2,204 crew members from every corner of the planet and more than 10 bars and 20 dining destinations—nine of which are complimentary. Curiously, even though the Icon of the Seas will eventually take the mantle as the largest cruise ship, Wonder will still have more staterooms than its new big sister, a whopping 2,867 of them.

It also features a carousel on a real boardwalk, a dry slide that twists down 10 decks, surfing simulators, a laser tag arena, an ice skating rink and a zip line that spans the interior width of the ship. It also has interior rooms with balconies that look out over the restaurant and a tree-lined neighborhood that looks like Central Park. Wonder of the Seas will soon lose its crown as the biggest cruise ship in the world, but it'll never lose its ability to provide unique vacations thanks to cheap cruises for every kind of traveler.

World Europa

Cruise line: MSC

Length: 1,093 feet

Width: 154 feet

Guest capacity: 6,762

MSC is not only a privately held shipping company, the largest in the world, they are also the maker of some of the largest cruise ships in the world. The World Europa , which took to the sea for the first time in 2022, is currently escorting passengers all over the Mediterranean, using one of the cleanest marine fuels to do so.

The World Europa features some of the best rooms on a cruise ship, thanks to having the most cabins with a balcony. It also has the world's longest dry slide at sea, taking adventurous cruisers down 12 decks in a shiny, twisting stainless steel tube, and bumper cars in a SportsPlex arena that also hosts rollerblading—not at the same time, however. People looking for something a little less dramatic will enjoy six pools, some surrounded by tropical greenery, 14 whirlpools and MSC's first-ever department store at sea. Finally, you'll never go hungry on this ship, given its 10 restaurants, including four main dining rooms and three buffets.

Cruise line: P&O Cruises

Length: 1,130 feet

Width: 138 feet

Guest capacity: 6,685

Whether you want the sunshine of the Caribbean or the culture found in and around Mediterranean ports, Arvia is P&O's largest cruise ship and one of the biggest cruise ships in the world, with 1,800 crew members, a three-screen cinema complex, a wide range of staterooms to choose from and four pools, including an infinity pool with a swim-up bar. There are also nearly two dozen whirlpool spas, a sauna, steam room and a hydrotherapy pool.

This means that whether you're taking a singles cruise or have children in tow—the Arvia is a family-friendly ship with age-specific kids clubs on board—serious rest and relaxation are definitely on the agenda. Then, when you're feeling spry and ready for some adventure during your vacation at sea, you can try an escape room or ropes course, play in the sports arena or strut your stuff in a dance class.

Costa Toscana

Cruise line: Costa Cruises

Length: 1,105 feet

Width: 137 feet

Guest capacity: 6,600

A sister to the Smeralda , the Costa Toscana became the fleet's largest cruise ship when it took to the water in March 2022. The ship uses LNG propulsion engines to make sea travel more sustainable, both while cruising and in ports of call around the world. An international crew of 1,678 works to deliver dream vacations for up to 6,600 passengers who will love to discover all there is to do, see and experience on board.

The ship has 1,550 staterooms ranging from interior cabins to elaborate suites, a baker's dozen pools and whirlpools, and a total of 20 restaurants, bars and lounges. Costa's new flagship vessel is a smart city at sea, offering subtly high-tech sailings that provide fun for the whole family, thanks to Peppa Pig character parties, an aqua park with water slides and a mind-bending skywalk 213 feet above the ocean! So book a vacation on one of the largest cruise ships in the world, then pack your cruise ducks and your appetite for learning how to cook at Toscana 's Food LAB cooking school, among other adventures at sea.

Celebration

Cruise line: Carnival

Guest capacity: 6,631

As cruise lines jockey for position at the top of the biggest cruise ship in the world leaderboard, the original family-fun ships are also getting larger. A total of 1,735 international crew, famously considered the best in the business, will ensure that every family's cruise on the Celebration not only maximizes the fun but also the memories, and all at some of the most affordable prices in the cruise industry.

This ship has BOLT, the first roller coaster at sea, Dr. Seuss children's themed cruises with character appearances, and incredible water parks with multiple slides. Delicious food is made fast-and-fresh at Guy's Burger Joint, Shaq's Big Chicken and the Seafood Shack, as well as tasty street-food carts located near the pools. Plus, with all the extra space on the largest cruise ship in their fleet, Carnival's Playlist Productions has more stages to dazzle you and your kids with famously fantastic live music and musical theater.

Cruise line: Norwegian

Length: 1,094 feet

Width: 136 feet

Guest capacity: 3,998

With 1,700 crew members and more than 2,000 staterooms, including suites in The Haven—Norwegian's luxurious, private and tranquil ship-within-a-ship concept high atop the ship, which comes with 24-hour butler service and its own lavish sundeck—the Encore ranks among the largest cruise ships in the world. Curiously, this vessel from 2019 is still the largest in the Norwegian fleet! Both Prima  and the forthcoming Viva were built after Encore but are considerably smaller in size, bucking the cruise industry trend of "bigger is better."

Encore shines as a superior and super big ship, the fourth in the Breakaway Plus class, and features a multi-deck go-kart track, the Cavern Club music venue, fashioned after the famous Liverpool room where The Beatles became big, a virtual-reality gaming room, an outdoor laser-tag arena and some of the best food at sea. No doubt, this will become one of every passenger's favorite cruises.

Symphony of the Seas

Width: 216 feet

Guest capacity: 6,680

None of the many all-inclusive cruises take place on the largest cruise ships, but you may be forgiven for thinking you're enjoying an all-inclusive experience on Symphony of the Seas . The ship's many spectacular activities include a zip line that soars above the boardwalk many decks below, an old-fashioned carousel, Broadway-style theater shows, surfing simulators and the 10-story Abyss dry slide. There's also delicious food all over the ship that is 100% complimentary. And while you may fly on the zip line over and over again, you'll probably still wonder, Why do cruise ships have wings?

After its maiden voyage in April 2018, Symphony was the biggest cruise ship in the world. But because of its ingenious neighborhood concept, guests rarely feel overcrowded. Seven distinct areas carve up the ship into unique spaces to play, eat, stroll, sleep and enjoy live entertainment, and the traffic flows brilliantly to make passengers feel as though they're on a more intimate ship and not cruising with upward of 6,680 other people in more than 2,700 staterooms, some of which are interior cabins with balconies that boast views of the Central Park neighborhood's greenery and eateries.

Cruise line: AIDA

Guest capacity: 6,654

While American travelers looking to cruise the Caribbean may not encounter her, the AIDAnova is not only the largest cruise ship in the AIDA fleet, she's also a revolutionary lady. The ship's Four Elements adventure deck, with its three water slides and climbing garden under the dome of a retractable glass roof, drops jaws on every sailing. The ship's more than 2,600 staterooms with 20 different types, ranging from a glorious two-deck penthouse to budget-friendly interior cabins, allow guests to sleep well and arrive in port well rested, and the two-deck spa offering 80 different treatments provides even more healing and luxury.

But AIDAnova is also impressive because it made history as the world's first cruise ship to be powered in port and at sea by liquefied natural gas (LNG), the world's cleanest burning fossil fuel. It's important that big ships can be more sustainable, but did you ever wonder why there are big balls on cruise ships?

Sun Princess

Cruise line: Princess

Length: 1,133 feet

Guest capacity: 4,300

The Love Boat is getting much, much bigger! Yes, this is the namesake ship from the beloved '70s TV show. With 2,150 cabins, the Sun Princess is close to carrying 4,300 passengers to Europe for its inaugural season later this year. And then all around the world, people will be asking, "Why do cruise ships have to stop in Canada?"

The third ship in the history of the fleet to be graced with the iconic name, the S un Princess is poised to be a fan favorite. There are more than 29 bars, lounges and restaurants on board, with the Sea View Terrace and Bar ensconced in glass at the top and front of the ship, book-ended by the Wake View Terrace at the back, which has a stunning infinity pool hanging off the edge. Drawing inspiration from the tiered terraces of Santorini, Greece, this evocative area will have an indoor/outdoor pool to enjoy supreme relaxation during the day. But once the sun says goodbye, the pool becomes a stage and the dome above transforms into an entertainment venue with eye-popping lighting effects.

Length: 1,085 feet

Width: 141 feet

Guest capacity: 6,334

Come June 2023, one of the newest and biggest cruise ships at sea will be carrying eager pasengers, and more than 1,700 dedicated crew members, to world-class destinations in Northern Europe and the Mediterranean. While on board the Euribia , cruisers will enjoy a massive amount of personal space (100-plus square feet per person) at any one of five unique pools, including one of the most intricate waterparks at sea, covering more than 10,000 square feet.

These watery areas of rest, relaxation and revelry will comfortably accommodate more than 1,000 guests at a time. Indoors, a 360-foot-long Mediterranean-style promenade is lined with myriad shops, along with some of the 20 bars and 10 restaurants found on the ship. It also features a domed LED screen that magically displays the sky and other animations throughout the day and night. MSC continues to be a favorite cruise line of environmentalist travelers looking to ease their mind and minimize their footprint, as the Euribia has an advanced onboard wastewater treatment system that doesn't allow any water to leave the ship without being treated and cleaned.

The post The 10 Biggest Cruise Ships in the World appeared first on Reader's Digest.

High Angle View Of Cruise Ship On Sea

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  2. Why Do Cruise Ships Float When They're Actually Really Massive?

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  4. How Do Cruise Ships Float?

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COMMENTS

  1. How Do Cruise Ships Float?

    In the simplest terms, ships float because their weight is less than the weight of the water they displace. If you've ever carried a bucket of water, you know that water is incredibly dense ...

  2. How do cruise ships float?

    A cruise ship displaces an amount of water equivalent to its own mass. The pressure of the sea pushes up against the vessel's hull to counter the downward force of the ship's mass. Unlike air, water cannot be compressed, so the combined forces create buoyancy. As the cruise ship moves forward, water is pushed away and that water tries its ...

  3. Why Do Cruise Ships Float When They're Actually Really Massive?

    Here's the short answer: Giant cruise ships can stay afloat because they can displace an amount of water that is equivalent to their mass. While the vessel is propelled forward, it forces the water away, but the water attempts to return, ultimately creating a force that pushes the ship upward. Let's take a closer look.

  4. How Do Cruise Ships Float?

    It's all about buoyancy: the power of a liquid to keep things afloat. Colossal vessels stay above water by displacing an amount of water equal to their mass (the wide, U-shaped hull helps with ...

  5. How Do Cruise Ships Float And Not Tip Over?

    Cruise ships are designed not to be unstable or to tip over (capsize). The combination of a wide hull, (known as "the water plane area") low centre of gravity, fuel, and ballast tanks filled with water keeps a cruise ship from tipping over in rough seas. A cruise ship's centre of gravity can be moved up or down by adjusting weights within ...

  6. How do Cruise Ships Float? The Science Behind Cruising

    Put simply, cruise ships float because their mass is low in relation to the volume of water they're sailing on. This creates an effect called buoyancy, which keeps the ship upright and afloat. Cruise ships must be able to withstand the weight of the ship, the crew, luggage, supplies and passengers. This mass can total anywhere upwards of ...

  7. How Do Cruise Ships Float And Not Tip Over? (Explained)

    7- Ballast Tanks: Help Cruise Ships Float And Stay Upright. Ballast Tanks are ingeniously used to help cruise float and stay upright. The ship can alter its center of gravity by adjusting the amount of water in these tanks. This creates an upward force that counteracts the downward force of the ship's weight and effectively keeps the ship stable.

  8. Why do Ships Float?

    When a ship sinks, it is because water enters the ship. This forces out the air, making the average density of the ship greater than that of the water. One of the most famous disasters is the sinking of the RMS Titanic. The ship struck an iceberg off the south coast of Newfoundland in April of 1912.

  9. How Do Cruise Ships Float?

    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 ...

  10. How Cruise Ships Float, More from Engineers

    Why a tree trunk will float but a brick will sink despite the tree trunk weighing more. Where is the engine located, and how are cruise ships powered? Engineers: Cruise ship engines are located near the bottom of the ship as they are heavy, and they are usually towards the aft of the ship. Cruise ships use Diesel-Electric propulsion systems.

  11. How Do Cruise Ships Float? Science Behind Ship Buoyancy

    The answer lies in the principle of buoyancy, which is the ability of an object to float in a fluid due to the upward force exerted by the fluid. Cruise ships are designed to displace a large amount of water, which creates a buoyant force that supports their weight. The shape of the ship's hull is also critical in ensuring that it stays afloat.

  12. How Does A Cruise Ship Float?

    A buoyant ship does exactly what a name suggests; it floats due to air trapped within the structure of the ship itself. These ships use ballast tanks filled with seawater and/or sand to keep themselves afloat. When these ballast tanks fill up with water, the ship sinks until the tank empties again. If you've ever seen a boat bobbing around in ...

  13. Why do cruise ships float?

    It's all about buoyancy: the power of a liquid to keep things afloat. Colossal vessels stay above water by displacing an amount of water equal to their mass (the wide, U-shaped hull helps with this). As the ship moves forward and pushes water away, the water is ceaselessly trying to return to fill the space, with an energy that forces the ship ...

  14. How Do Massive Cruise Ships Float?

    In this episode of Explainers, we explore the science behind how gigantic cruise ships stay afloat.Subscribe to Travel + Leisure! - http://www.youtube.com/us...

  15. Ship Buoyancy and Stability: How Ships Float and Stay Upright

    A ship floats because the submersed part of it is lighter than the displaced water, and the total weight of the ship equals the total weight of the water it displaces. Forces of gravity and buoyancy on a ship in water. However, it is not enough for a ship to simply float in a static state. It also needs to stay upright while sailing through the ...

  16. How the World's Largest Cruise Ship Floats

    The world's largest cruise ship departed from Turku, Finland, on Oct. 30 en route to Fort Lauderdale, Fla., where it will make its U.S. debut. (Image credit: Oasis of the Seas.)

  17. Building A Cruise Ship: The Float Out And Why Ships Do

    The Float Out Process. Built in a dry dock facility, the initial stages of s cruise ship's construction take place. Put simply, in the case of 168-ton Anthem of the Seas and other large ships, giant blocks of the ship are made and welded together to form the whole ship. Once that is done and before interior spaces are outfitted with what we ...

  18. How Do Cruise Ships Float?

    Cruise Ship Design Features Supporting Buoyancy. How do cruise ships displace enough water to float? The design of the hull holds the secret. The most common modern cruise ships have U-shaped hulls. For water displacement to be successful, the hull must be denser than water, which is why engineers use extra-strength steel alloys.

  19. How Do Cruise Ships Float With So Much Weight?

    Buoyancy is the ability of an object to float on a fluid. To expand further. Buoyancy is an upward force exerted by fluid, in this case, water, that opposes the weight of an object partially or wholly immersed, ie, a cruise ship. In simple terms, if a 100,000 gross tonne cruise ship can displace an equal weight in water, in this case, 100,000 ...

  20. How Does a Ship Float? A Scientific Explanation of Buoyancy

    Three basic concepts explain how and why a ship can float: Principle of Buoyancy: According to the principle of buoyancy, an object immersed in a liquid will face an upward force. When the upward force is greater than gravity (downward force), the object floats. The upward force exerted by the liquid is the buoyant force.

  21. Why Do Ships Float?

    Why Do Ships Float? This infographic demonstrates in an easy to follow manner why ships (no matter how big they are) float. This is a great resource for teaching children. If you have enjoyed reading this infographic, please share with your friends.

  22. Why do big ships float? [Buoyancy and flotation explained]

    Join our Exclusive Community over on Patreon: https://www.patreon.com/CasualNavigationDo you look at enormous ships out at sea and wonder how it is possible ...

  23. Why Do Ships Float?

    Ever wonder why something as heavy as a cruise ship could float above the water? Learn about displacement with Jessi and The Giant Squid, Squidstravaganza!--...

  24. The 10 Biggest Cruise Ships in the World

    Where to float on the biggest boats. E very vessel that's once held the title of "biggest cruise ship" has one thing in common: It is a destination in and of itself. Of course, these enormous ...