cruise ship double hull

What You Need to Know: Double Bottom vs Double Hull

What is a double bottom on a ship?

A double bottom is an additional layer of structure that lies beneath the ship’s main bottom. It creates an extra barrier between the ship’s cargo and the sea, providing added protection against potential damage or leaks.

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The double bottom is typically made up of two separate layers: the inner bottom and the outer bottom.

The inner bottom is the lowermost part of the ship’s hull , directly above the keel, while the outer bottom is located above the inner bottom.

These two layers are usually separated by a space called a ballast tank, which is used to adjust the ship’s weight and trim.

Why is it called a double bottom?

The term “double bottom” is used to describe the additional layer of structure beneath a ship’s main bottom.

The name “double bottom” comes from the fact that there are two distinct layers: the inner bottom and the outer bottom.

Advantages and Disadvantages of Double Bottom

When it comes to the double bottom feature on a ship, there are several advantages and disadvantages to consider.

Let’s take a closer look at them:

1. Enhanced Protection: The double bottom provides an extra layer of protection against potential damage or leaks.

By acting as a barrier between the cargo and the sea, it greatly reduces the risk of cargo leakage in the event of a collision or grounding.

2. Increased Stability: The ballast tank located between the inner and outer bottom helps adjust the ship’s weight and trim, ensuring that it maintains stability.

This is especially important in rough sea conditions where maintaining an even keel is crucial.

3. Safety: The double bottom enhances the overall safety of the vessel, crew, and the environment.

By preventing potential harm from spreading to the cargo holds, it protects both the cargo and the surrounding ecosystem.

4. Damage Limitation: In the event of a puncture or damage to the ship’s hull, the double bottom acts as a safeguard.

It helps contain the damage, preventing it from spreading to other areas and reducing the risk of a catastrophic situation.

5. Compliance with Regulations: Many regulations and industry standards require ships to have a double bottom.

By implementing this feature, shipowners can ensure compliance and avoid penalties or restrictions on their operations.

Disadvantages

1. Increased Cost: Constructing and maintaining a double bottom can be costly.

It requires additional materials , labor, and inspections, which can significantly increase the overall expenses of the ship.

2. Limited Space: The double bottom design may take up valuable space within the ship.

This can potentially limit the cargo capacity and affect the efficiency of the vessel.

3. Maintenance Challenges: Maintaining the double bottom can be more complex and time-consuming compared to a single bottom.

Regular inspections, cleaning, and repairs may be required to ensure its effectiveness.

4. Increased Weight: The double bottom adds weight to the ship, which can affect fuel efficiency and overall performance.

This is especially important for ships that need to optimize their fuel consumption.

5. Design Limitations: The double bottom design may not be suitable for all types of ships or operations.

Depending on the ship’s size, purpose, and cargo requirements, alternative safety features or designs may be more appropriate.

What is a double hull?

A double hull is a feature on a ship that consists of two separate hulls with a space between them, known as the double hull space.

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What is the benefit of a ship having a double hull?

The benefit of a ship having a double hull is two-fold: environmental protection and enhanced structural integrity.

Firstly, the double hull design greatly reduces the risk of environmental pollution.

The space between the two hulls acts as a secondary containment for cargo , fuel, or other substances, minimizing the potential for leaks or spills.

This is especially important when transporting hazardous materials or operating in sensitive ecosystems.

By preventing harm to marine life and ecosystems, the double hull plays a vital role in preserving the environment and maintaining sustainability in the shipping industry.

Secondly, a double hull provides enhanced structural integrity.

The dual-layer design offers extra strength and resistance to damage from collisions or groundings. This contributes to the overall safety of the vessel and helps maintain the integrity of the cargo holds.

Are double hull boats more stable?

The answer is yes!

The double hull design provides an extra layer of protection and stability for a ship. By having two separate hulls with a space in between, the ship gains added buoyancy and structural integrity.

This means that in rough seas or during turbulent weather conditions, the double hull helps the ship maintain its balance and reduces the likelihood of it rolling or tipping over.

Additionally, the dual-layered design of the double hull distributes the weight of the ship more evenly, which further enhances its stability. This is especially important for larger ships or those carrying heavy cargo loads.

Why do tankers have doulbe hulls?

Tankers have double hulls primarily for environmental and safety reasons.

The double hull design provides an extra layer of protection to prevent leaks and spills of cargo, particularly in the case of hazardous materials.

By having two separate hulls with a space in between, the risk of environmental pollution is significantly reduced.

In the U.S. by 2015, all tankers will be required to have double hulls. (based on www.ecologycenter.us)

Do modern ships have double hulls?

It should be noted that double hulls are not standard on all modern ships.

Smaller vessels like yachts and recreational boats are typically not equipped with this feature.

However, for larger commercial ships that transport significant amounts of hazardous materials, double hulls are considered an essential safety feature.

Regulations have been put in place to mandate their use in these cases to minimize the risk of environmental pollution and harm to marine life.

Double bottom vs Double hull

When comparing double bottom and double hull features on a ship, it’s important to understand their differences.

Here are some key distinctions to consider:

Double bottom refers to an additional layer beneath the ship’s main bottom, providing a barrier between the cargo and the sea.

Double hull, on the other hand, consists of two separate hulls with a space between them, acting as secondary containment for cargo or fuel.

Double bottom primarily aims to reduce the risk of cargo leakage in case of a collision or grounding. It acts as a safeguard against punctures and prevents damage from spreading to the cargo holds.

Double hull, on the other hand, focuses on environmental protection, minimizing the risk of leaks or spills that could harm marine life and ecosystems.

Many regulations require tankers to have double hulls, ensuring greater environmental protection.

However, not all ships need to comply with these regulations, and some may opt for a double bottom instead.

Constructing and maintaining a double bottom or double hull can be more expensive than a traditional single-bottom or single-hull design.

The additional materials, labor, and inspections required contribute to the increased cost.

Applicability

Double bottoms are more commonly found in older ships, while double hulls are more prevalent in modern vessels, especially tankers carrying hazardous materials.

The choice between the two depends on factors such as ship type, regulations, and specific needs.

Was the titanic double hulled?

The Titanic, one of the most infamous ships in history, was not double-hulled. It was built with a single-hull design.

This single-hull construction was common during the time the Titanic was built, and the concept of double hulls had not yet become widely implemented.

Do modern cruise ships have double hulls?

Modern cruise ships, like many other large commercial ships, typically do not have double hulls.

This is because cruise ships are not primarily designed to transport hazardous materials or carry a high risk of causing environmental damage.

Instead, their focus is on providing luxurious accommodations and entertainment for passengers.

While cruise ships prioritize passenger comfort and experience, they still have numerous safety features in place to ensure the well-being of passengers and crew.

These may include advanced navigation systems, state-of-the-art life-saving equipment, and rigorous safety protocols.

Cruise Ships, What Shape Are They Below the Water?

Have you ever seen a cruise ship you may be wondering what is below the waterline and how they actually manage to float?

A lot of a ship’s ability to float comes from the way that the ship is designed below the waterline.

Do Cruise Ships Have Flat Bottoms?

The bottom of a cruise ship is not perfectly flat, although it does look flat from a distance.

Under the water, the front of the ship is usually v-shaped or u-shaped, with the back of the ship having a square-like shape.

In this article, we will look at how the bottom of cruise ships are shaped – and why they are shaped like this .

We will also explore why the bottom of cruise ships are different to the bottom of Ocean Liners.

What Shapes Are the Bottoms of Cruise Ships?

If you look at a cruise ship from land, you may think that the bottom of the ship looks flat. Cruise ships often tower high into the sky and do resemble buildings in many cases.

If you were to take the ship out of the water. you’d see that the bottom isn’t perfectly flat.

I’ve been under a cruise ship. I visited the MSC Bellissima when she was being built and we were able to go under the cruise ship before the “Float-out Ceremony” – when the ship sails for the first time.

We were able to explore not only under the ship. but inside the ship too.

It was very surreal being in a ship that was mostly just steel. I’ve always understood the scale of cruise ships having cruised many times in the past, but being UNDER the ship was something different entirely!

Watch the video below to join me as we explore the ship:

How Does the Hull Shape Make a Difference to a Ship?

The hull is the main body of the ship. Hulls come in a variety of different shapes and can be built with different materials, depending on the purpose of the ship.  

There are a few different types of hulls, including:

Displacement Hulls

Ships with displacement hulls are designed to move through the water by pushing the water to the sides.

If you were to think about a ship’s hull, this is probably what you think of by default.

When you put a ship into water, it has to displace water in order to be able to float. The amount of water it needs to displace for the ship to be able to float is carefully calculated when designing the ship.

The weight of the water that is displaced weighs the same as the ship according to Archimedes Principle.

If a ship’s hull displaced too much water, it would sink. Too little and the ship would sit too high on the water.

Planing Hulls

Ships with planing hulls are designed to glide across the top of the water.

An example of this would be a speedboat, where the front of the ship comes above the waterline.

Ships with Planing Hulls usually are designed to go faster than ships with Displacement Hulls.

Semi-Displacement Hulls

A ship with a semi-displacement hull, acts like a displacement hull when travelling at slow speeds, pushing water to the sides of the ship.

When the ship gains speed, it is able to act like a Planing hull.

What type of Hull Does a Cruise Ship Have?

Cruise ships have displacement hulls.

Engineers use lightweight but sturdy materials to help achieve the buoyancy needed to keep the ship afloat.

You can see in the photo below that the sides of the ship are built to displace the water as the ship is sailing.

Why Are Cruise Ship Hulls The Shape That They Are?

Elimination of drag.

The rounded edges of the hull help to dissipate the drag or force of the water acting against the ship.

The shape allows the ship to easily travel through the water without too much resistance.

Fluidity and Stability

Ships with Displacement Hulls provide a steadier sailing, than ships with Planing Hulls.

Cruise ships are designed with the onboard experience in mind and being able to keep the ship as stable and steady as possible is very important.

Cruise ships also have stabilisers to help keep the ship as stable as possible for the guests. On the majority of cruise ship sailings, you will not experience any movement of the ship.

To learn more about how much of the movement on a cruise can be felt, check out this post below:

You May Feel Movement on a Cruise Ship – Here’s When and Why

Most cruise ships are built with a double hull (a hull within a hull) to help prevent damage.

Cruise ships are designed to last for decades and to be able to sail day after day without stopping to take any breaks.

The round bottom shape also helps with further protection from reefs, icebergs, and other damage to the ship.

Are There Any Downside to Displacement Hulls?

The drawback of a Displacement Hull is that the ships can’t go as fast as they might with a different hull shape.

That said, cruise ships still sail at around 20 knots, which is almost 25 miles per hour.

The Ocean Liner Queen Mary 2  has a maximum speed of just over 30 knots and a cruising speed of 26 knots. This is faster than a cruise ship.

The speed that a ship can go at is determined by a speed-length-ratio meaning that the length of the ship determines how fast it can realistically travel. The calculation is worked out as below:

Maximum speed (knots) = 1.34 x the square root of the waterline length (in feet) – source.

How Do Cruise Ships Compare to Ocean Liners?

Most people aren’t aware of the difference between cruise ships and Ocean Liners.

They may appear similar but there are significant differences in purpose and design. The design differences affect the hull in multiple ways.

Find out more about the differences in the article below:

Cruise Ships vs Ocean Liners – Measurable Differences You Need to Know

Do Cruise Ships and Ocean Liners Have Different Bottoms?

When Ocean Liners were created, their primary purpose was transportation – to get from point A to point B. Arguably, the most well-known Ocean Liner in history is the Titanic.

There are many important differences in cruise ships and Ocean Liner design.

This affects how the bottom of the ships are built. The chart below gives a basic overlook of the most important differences. 

Ocean liners tend to sit quite low in the water and have a pointed bow. They are designed this way to be able to deal with bad weather and to minimize the feeling of movement onboard.

Cruise ships often sit high on the water and can often be strange shapes that aren’t hydrodynamic at all.

Cruise ships aren’t designed to deal with particularly bad weather, although modern ships do have powerful stabilizers.

Most cruise ships will try to avoid stormy weather and may slow down or change their route to do so, Ocean Liners are less likely to have to do this.

Ocean Liners generally have a stronger hull than Cruise Ships. Queen Mary 2 features steel hull plating – ” Thick for rigidity during Atlantic crossings .”

There is only one modernly designed ocean liner currently still in service, and that is the Queen Mary 2 (QM2), which is owned by Cunard.

It was completed in 2003. Cruise ships are still being built in large numbers.

I was lucky enough to sail on the last Ocean Liner, The Queen Mary 2. It was a very different experience to the normal cruises I take.

Find out all about that sailing here:

Before You Go

Find out more about Ocean Liners – including what happens to some when they are taken out of service – below:

Ocean Liners, They Still Exist: Here’s Everything You Need to Know

I stayed in the cheapest cabin when I sailed on the Queen Mary 2. Find out whether I enjoyed the experience here:

I Cruised in The Cheapest Cabin on the World’s LAST Ocean Liner – Queen Mary 2

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• Ask a Cruise Question

Double Hulls

By Bounders , January 19, 2014 in Ask a Cruise Question

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Does anyone know what, if any, cruise lines use double hulls all the way to the water line in their ships?

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I thought they only had double bottoms, not full double hulls.

CruiserBruce

It is my understanding double hulls is more about preventing contents from spilling, for example, in an oil tanker incident, rather than keeping water out. Water tight compartments are for keeping water from spreading throughout the ship.

That being said, I know double hulling was promoted by a certain TV show, to prevent a Costa Concordia type incident. Of course TV is always right, with their perfect 20/20 hindsight. Just like the recent report that a female falling from a Carnival ship was caused by lack of newest technology motion detection cameras mounted on the side of the ship. No cruise ship has them currently, but because someone has developed them, Carnival is at fault for not installing them, regardless of any research indicating their effectiveness or value. Not the female who was admittedly drunk at the time, and was rescued by the Carnival ship.

Not a clue. Why do you ask?

If any do, it would be the newest ships. I have not just now gone back, but have looked in the past at the company web pages for new ships by Princess and NCL and do not recall any mention of double hulls. Also, my memory agrees with a previous poster that it is used to contain leaked fluid cargo. No matter how drunk some cruise goers get, they are NOT yet fluid cargo. Hence, I doubt any do and doubt it would help.

Hi Bounders.

Welcome to cc, and congrats on a very weird first post.

Ifaik, no cruise ships have double hulls, but there are many more knowledgable people on here who may say differently.

Does not having a double hull concern you?

MCC retired

Double hulls ? Unusual concern for sure.

The Titanic had a double hull .

Double hulls ? Unusual concern for sure. The Titanic had a double hull .

Correction : Double Bottom .

phish tales

double hull ?????????????????????????

double bottom ?????????????????????

well now we have something else to worry about.:confused::confused::confused:

I believe that you are correct. If a hole was struck in a double hull then the void space would fill with water and while it might not fill the rest of the ship it could fill enough to sink the ship dependant on the sea conditions and the size of the void. At a minimum it would cause the ship to sink lower in the sea. Think about it, it is the same type of principle that a submarine works on. The submarine takes on water, it sinks, it shoots the water out, it surfaces. If a ship had a large void take on water with no way to discharge it, it would be a huge problem.

If the OP is worried about something like the Costa Concordia happening, I would worry about a million more things than that. Remember the people who "drive" the ship are trained professionals who know where every single piece of land underneath the water is, and certainly do not want to hit anything. The issue in that ship was the idiot captain ordered the crew to do something that was plainly wrong.

Furthermore, the ships that travel around are taking the same exact route over and over again, so they will now deviate from the previous routes anyways.

So, bottom line, I would not worry about the ship hitting an obstruction in the water.

I am not "worried" about ships not having double hulls. I am curious.

If the OP is worried about something like the Costa Concordia happening, I would worry about a million more things than that. Remember the people who "drive" the ship are trained professionals who know where every single piece of land underneath the water is, and certainly do not want to hit anything. The issue in that ship was the idiot captain ordered the crew to do something that was plainly wrong.   Furthermore, the ships that travel around are taking the same exact route over and over again, so they will now deviate from the previous routes anyways.   So, bottom line, I would not worry about the ship hitting an obstruction in the water.

So, "wrong" things and route deviations do happen even with the highly trained professionals.

I don't think it would be practical on a cruise ship. It's used in oil tankers, but the disadvantages outweight the advantages for a cruise ship.

I wanted to find the technical difference between Double Hull and Double Bottom so went to Wikipedia for Double Hull and found this line:

"Double hulls or double bottoms have been required in all passenger ships for decades as part of the Safety Of Life At Sea or SOLAS Convention." It gives a footnote link which goes to nothing more.

Not correct as others have said, but would not be the first time Wiki was wrong.

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