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Why don't cruise ships tip over?

Why don't cruise ships tip over?

If you have seen today's mega cruise ships, you might have wondered why don't cruise ships fall over?

With all that height above the water and not much below the water line, what's keeping it upright.

Looking at a cruise ship, there is a large amount of the ship above the water, and a small amount below the water. So what forces are actually acting on the ship?

Oasis Class ship diagram

The weight of the ship is pulling it down in the water, which is balanced by the buoyancy, which is pushing it up. And if the buoyancy is greater than the weight, the ship would continue to move upwards.

But to work out why the ship doesn't tip over, we need to think about where exactly these forces are acting. Those points are the center of gravity for the weight, and the center of buoyancy for the buoyancy.

Forces on ship

Center of Gravity

If everything in the ship weighed exactly the same center of gravity would be right in the middle.

But the engines, machinery, fuel stores, and those sort of things weighs an awful lot more than the cabins and passenger spaces, stuff like theaters, which are mainly just air.

This has the effect of dragging the center of gravity downwards. So we know the center of gravity will be towards the bottom half of the ship.

Center of buoyancy

For the center of buoyancy, we are interested in the stuff under the water and to find the center of buoyancy, all we need is the center of the water plane area.

This is just the middle of the part of the ship that's under the water.

The buoyancy of an object on the water depends on its density. If the object is denser than water, then it will sink. If, however, it is less dense then water, it will float.

What keeps a cruise ship from sinking is an opposing upward force or pressure from the water. This upward force weighs the same as the water that was displaced, keeping it from sinking completely. 

How a ship floats

How it stays upright

Since we can calculate the center of gravity compared to the center of buoyancy, this is a matter of managing weights.

But what happens when an external force, such as wind, comes along? Since the wind will likely push the ship on one side, why doesn't it topple over?

Essentially, a cruise ship stays upright because they keep all of the heaviest equipment below deck.  This has the effect of keeping a low center of gravity.

Celebrity Apex

In addition, the shape of a cruise ship's hull is wide and rounded, which helps it move through the ocean smoothly and with minimal drag.

Round edges also increase the ship stability, preventing the vessel from swaying and rocking and passengers- from feeling seasick.

In addition, ships have something crucial for the ship’s balance, ballast tanks. They contain water that can be pumped from one side of the ship or the other. In case of an emergency or rough seas, it helps to keep the ship balanced, counter the waves and reduce rocking. A large cruise ship usually has several ballast tanks.

So the combined effect of a ship's buoyancy, low center of gravity, and ballast keep the ship from tipping over.