Why Don’t Cruise Ships Tip Over?

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Around 35 million people are expected to sail on a cruise ship in 2024. And cruise ships keep getting bigger and bigger.

You can’t talk about big cruise ships without mentioning Royal Caribbean’s new giant Icon of the Seas.

Right now she is the largest cruise ship in the world. Who knows how long she will hold that title but for now this mega ship has 20 decks. 7 swimming pools, 6 water slides and can carry up to 7,000 passengers.

Surely there has to be a point when cruise ships simply can’t get any bigger?

Icon of the Seas – Royal Caribbean

It was seeing the Icon of the Seas that prompted me to research the question why do cruise ships not tip over?

I know that they don’t, but I wanted to understand the science behind it!

Several factors combine to mean that it is almost impossible for a cruise ship to tip over:

• Stabilization Systems
• Low Center of Gravity
• Ballast Tanks
• Weather and Hazard Detection Systems
• Highly Trained Crew

Understanding Ship Stability

Cruise ships are marvels of modern engineering designed to provide stability and safety for all the millions of passengers who sail each year.

Cruise ships are one of the safest forms of travel.

Incidents are very rare.

When we think of cruise ships capsizing we naturally think about the Costa Concordia which ran aground and tipped over with 32 people losing their lives.

Photo: Paolodefalco75, CC BY-SA 3.0

This tragedy was completely avoidable and found to be the result of the negligent and criminal action of the Captain, Francesco Schettino, who was subsequently jailed for 16 years.

The tragic Costa Concordia sinking should never have happened.

Cruise ships possess a wide range of features that prevent them from tipping over even under harsh sea conditions.

The fundamental design of a cruise ship’s hull, its wide base, and its lower center of gravity contribute significantly to its stability.

These elements ensure that the cruise ship remains upright and can safely navigate through various sea conditions, from calm waters to high waves.

Cruise ships are also subject to stringent safety regulations and undergo rigorous testing scenarios. These tests, including the ‘inclining test,’ evaluate how the ship behaves when tilted, ensuring it can recover from significant angles of heel.

Further contributing to a cruise ship’s stability are the sophisticated ballast systems, which are integral to maintaining balance.

These ballast systems can pump water into tanks within the ship’s hull to counteract any unwanted tilt or list.

Additionally, the proportion and distribution of weight throughout the ship are carefully calculated to ensure a well-balanced structure. This is one reason why all the heavy equipment is housed towards the bottom of the ship.

The Science of Ship Stability

The stability of a cruise ship depends on its center of gravity and the relationship between buoyancy and water displacement.

Center of Gravity

The center of gravity is the point where the weight of the cruise ship is evenly distributed and it is crucial for maintaining stability.

A low center of gravity in a cruise ship helps to ensure that the ship remains upright.

This is achieved through the distribution of heavy machinery and fuel tanks near the bottom of the hull, creating a lower center of gravity.

Buoyancy and Displacement

Buoyancy is the force that allows a cruise ship to float on the water.

Remember at school learning about Archimedes’ principle? This states that the buoyant force on an object is equal to the weight of the fluid displaced by the object.

For a cruise ship, strategic hull design ensures that the displaced water exerts an upward buoyant force greater than the weight of the ship, therefore keeping it afloat.

Displacement refers to the amount of water a cruise ship displaces when it’s afloat.

The design of the ship incorporates wide beams and multiple watertight compartments, enabling it to displace a larger volume of water and therefore enhancing stability.

Engineering and Design Features

Cruise ships incorporate advanced engineering and design features to ensure stability and prevent tipping over. These features are precisely calculated to handle various sea conditions and maintain the ship’s equilibrium.

Ballast Systems

Ballast Systems serve a critical role in maintaining a cruise ship’s stability. These systems take in seawater into tanks, known as ballast tanks, which are strategically located throughout the ship’s hull. The process involves:

• Adjusting Weight Distribution: By filling or emptying these tanks, the ship’s center of gravity can be lowered or shifted to counterbalance the effects of wind and waves.
• Stabilization During Voyages: During transit, ballast water is adjusted to ensure optimal stability based on the current load and sea conditions.

Hull Design

The Hull Design of a cruise ship is a central aspect of its stability:

• Wide Beam and Deep Draft: Cruise ships boast a wide beam (the widest part of the ship) and a deep draft (the portion of the ship below the waterline), providing a substantial base to prevent capsizing.
• Hydrodynamic Efficiency: Modern cruise ships are designed with a hull form that ensures both stability and hydrodynamic efficiency, allowing them to move smoothly through the water with minimal resistance.

What is Draft?

Ship draft refers to the vertical distance between the waterline and the bottom of the hull (keel) of a vessel. It is the depth of water that a ship displaces when it is floating.

The draft of a ship is crucial for maintaining the vessel’s stability.

If a ship is loaded too heavily, the draft will increase, and the vessel may become unstable, increasing the risk of tipping over or capsizing.

Properly managing the draft ensures that the ship’s center of gravity is low and that it has enough freeboard, which is the distance from the waterline to the deck.

Cruise ships are designed to meet the most stringent safety standards, and the highly trained crews that operate them are always monitoring and checking that safety standards are met.

The draft on Icon of the Seas is 30′.

To compare this to smaller ships, Disney Wonder has a draft of 25.3′ and Disney Wish has a draft of 27.2.

Most modern cruise ships have a draft of between 25 to 30′.

Weather and Routing Strategies

Modern cruise ships are equipped with state-of-the-art weather and navigation systems.

Ships’ officers consider weather forecasts and sea conditions to chart a course that will minimize the impact of the elements on the vessel.

• Advanced Monitoring Systems: Cruise ships employ sophisticated technology to monitor weather patterns, enhancing their ability to adjust routes in real-time.
• Route Planning: Captains employ historical data along with real-time information to navigate away from adverse conditions, prioritizing passenger comfort and safety.

This is why, even in adverse weather conditions cruise ships are safe. Having sailed during hurricane conditions several times I have no hesitation in booking a cruise during hurricane season.

Related: Should You Book a Cruise in Hurricane Season?