Cruise Ships, What Shape Are They Below the Water?


Have you ever seen a cruise ship you may be wondering what is below the water 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 water.

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.

is the bottom of a cruise ship flat? bottom of MSC Bellisisma
Photo taken by me under the MSC Bellissima

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 prior to the float-out ceremony. where 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 needed 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 displaced too much water, it would sink but 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 traveling 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.

Why 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.

The sides of the ship as shown in the photo below, displace the water as the ship moves through the water.

MSC bellissima under a cruise ship displacement hull
Photo taken by me under the MSC Bellissima

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 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:

Protection

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 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 at. 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.

Do Cruise Ships and Ocean Liners They Have Different Bottoms?

When ocean liners were created, their primary purpose was transportation – how 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 liners that affect how the bottom of the ships are built. The chart below gives a basic overlook of the most important differences. 

ShapeWeightPosition in Water
Ocean LinersPointed bowHeavier, thicker, lower in the waterLower in Water (than Cruise Ships)
Cruise ShipsRounded bottomsLighter, sits on top of waterHigher in Water (than Ocean Liners)
How do the hulls of ocean liners compare to cruise ships?

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 hyrdodynamic 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 ocean liners. Queen Mary 2 features steel hull plating, ‘extra 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.

To learn more about the differences between cruise ships and ocean liners, check out this post:

To Conclude

Cruise ships don’t have perfectly flat bottoms, although from a distance it may look this way. The hull of a cruise ship is usually a V or U shape at the front and a flatter shape at the back.

Cruise ships have displacement hulls designed to move the water to the side as they move. They have this type of hull because it is stable, suitable for large ships and can be incredibly strong. Ocean liners and cruise ships have slightly different hulls with ocean liners sitting lower in the water than cruise ships.

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Emma Le Teace

Hey, I'm Emma, an award-winning cruise blogger, and YouTuber. I share cruise tips, tours, and videos on this website to help you master cruising on a budget. You can learn more here: About Me.

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