When you look at a cruise ship, they are towering marvels of steel and glass rising out of the water. But then you start to consider – how does something reaching 40, 50 even 60 meters stay upright while it floats? Especially when being battered by high winds and heavy seas!
Cruise Ship Static Stability
Being bottom heavy by design!
The secret to keeping a ship upright and afloat is to balance the upward force of the water (buoyancy) with the downward force of gravity, and to make sure they stay balanced even as the ship leans over.
The 30-40 meters of the ship you see that forms the accommodation consists of cabins, lounges, restaurants and theatres. But when you think about it, it’s really just steel boxes filled with air – not really that heavy at all!
In contrast, every ship has thousands of tonnes of steel forming the structure and providing strength. They have huge tanks filled with fuel, water and ballast. They have enormous engines. To make sure it doesn’t become top-heavy, the vast majority of this weight is located very low in the ship, well below the waterline. This is also why cruise ships won’t have Olympic-size swimming pools on the top deck! Holding all that water up so high would make the entire ship less stable.
All this helps to make sure a ship stays vertical, but we know that sometimes they’re pushed to one side by wind or waves.
Cruise Ship Dynamic Stability
Bouncing back when we’re pushed!
The shape of the ships hull under the water is what determines how well it returns to a vertical position when it has been pushed over. If you consider a barrel in the water, and you push it to one side, you’ll see it rolling over and over. The buoyancy of the water presses evenly from all directions, so there’s nothing to stop it. Definitely not suitable for a ship!
However, cruise ships have tall, flat sides and a flat bottom, making a box shape underwater. As they lean over, this box shape means that more of the box is submerged on the lower side and less on the upper side. As a result, the buoyancy of the water pushes the ship back upright.
Even very large cruise ships can lean over 30 or 40 degrees and STILL come back safely!
This effect is great for a temporary force like a wave, where the ship is pushed to one side, but then released. The vessel can return to its original position with no fuss at all. But what happens when the force is constant, like the wind? Or something inside the ship, like water?
Cruise ships can move hundreds, if not thousands of tonnes of water around inside their tanks. We can manage fresh drinking water, wastewater and ballast water from the ocean. This means that we can both anticipate possible situations and react to what is happening around us, moving water from side to side or front to back to keep the ship stable. We even have designated ‘heeling tanks’ that can be used to quickly return the vessel upright if the wind picks up or if we turn quickly.
Even with all this at our disposal, there will still be some things we cannot overcome.
Cruise Ship Stability Challenges
Despite having so much water to play with, very strong winds can still cause the ship to lean over. On large ships, even 0.5 degrees list can be uncomfortable! In many cases, we can navigate the ship so the wind is hitting us from in front or behind rather than side on – this will have a fast effect, allowing us time to ballast the ship.
Another situation that can be particularly challenging is flooding. Whether from a broken pipe or a hole in the ship’s hull, this can result in thousands of tonnes of water pooling in the bottom. Modern cruise ships are designed to stay safe even with several of their watertight compartments wholly flooded, and they are designed to remain upright even with this flood water. In these situations, the actions of the crew in slowing or stopping the flooding and in navigating the ship can make a big difference, both for the better or for worse.
To keep everyone safe at sea, Officers are actively managing the stability of the vessel at all times, looking ahead to anticipate problems and making informed decisions when something unexpected comes up. Ship design is continually being refined, for stability, efficiency and comfort. As the cruise industry continues to learn and grow, ships are built to stricter standards particularly regarding their stability if they’ve been damaged.
So next time you look at a cruise ship towering above the dock, you’ll know how something floating in water can be so rock-solid!
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