How a Cruise Ship Floats—In Terms Even a Toddler Could Understand

There are many complicated ways a physicist could explain how a 100,000-tonne cruise ship can stay afloat in the water, but a four-pound brick will sink to the ocean floor. They could use math (a lot of it), they could draw a free-body diagram (the one with the arrows that you didn't like in high school), or they could quote complex laws of physics (basically, the quickest way to send your eyes to the furthest reaches of your head).

But if they’re Nicole Sharp—a self-defined "elder millennial" with a PhD in aerospace engineering and founder of the FYFD (formerly known as Fuck Yeah Fluid Dynamics) blog (though calling it just a blog would be reductive, considering it's a successful multi-platform effort)—they could instead do so by telling you about how their infant son got frustrated during bath time.

"I remember the very first time we put this turtle toy in the bathtub with him, and it drove him crazy," Sharp tells me. "As soon as he saw it floating in the water, you could see his little brain decide, 'No, this is wrong. This cannot be.' He was determined that things sink in water because he sank. He saw himself, and he was like, 'I don't float in the water. Why would this thing float in the water?'"

"This is exactly the same principle as a cruise ship," Sharp explains. "Only none of us is big enough to push the cruise ship under the water and force it to stay there."

All of which is to say, you don't really need to learn about buoyancy and Archimedes' Principle to understand this idea (though it doesn't hurt to know that, technically, these are the formal concepts involved). It can be much simpler than that: What does the turtle toy have in common with a cruise ship? A suitable shape.

"Think about it: If I throw a bar of steel into the water, it's going to sink," says Sharp. "But if I take that steel and I shape it into something that's thinner and that takes up a large volume, all of a sudden I have a canoe, and the canoe floats."

So shape—and therefore, volume—have a direct effect on whether or not an object can float, regardless of how big it is.

Then there's the matter of an object's mass, or how much it weighs. Specifically, how much water the mass of an object pushes aside. That's ultimately what does or doesn't allow it to float. If the object is able to push aside enough water, it will float; if not, it will sink.

"In order for something to float, there must be an upward force that's balancing the weight of the thing that's floating," explains Sharp. "When an object pushes water aside, the mass of whatever amount of water it has moved provides the force that counters its weight."

To visualize this, Sharp suggests picturing a rubber duckie. "The duck is lightweight, which is great. It doesn't have to move very much water [to float]," she says. "It's also shaped in such a way that very little of it has to fit into the water before it's pushed enough of it aside to counter its weight at this buoyant force."

A brick sinks for the same exact reason. It's too heavy for its small shape, so it can't move the necessary amount of water to receive a force strong enough to float.

As for cruise ships, Sharp explains, "What shipbuilders are doing is designing these boats in such a way that even though they're super heavy, they push aside enough water that they're able to counter their weight." Yet, like a brick, if a cruise ship kept the same weight and shape but was transformed to assume the size of a pea, "it would sink straight down to the bottom." That's because there wouldn't be enough water displacement and water-based force to keep such a pea-liner afloat.

Interestingly, though, Sharp says you don't need to be on a cruise for fluid dynamics to relate to your life. A rainy day will do the trick, too. Or as she tells me, "I was walking to a restaurant to pick up lunch one day. It was raining, and there were puddles with these big bubbles on them. I was like, 'How are these giant bubbles showing up in the puddles on the sidewalk? Did somebody pour dish soap everywhere? That doesn't make sense.'

'The answer is that it hadn't rained in a while, and there was a bunch of dust and oil and things on the road. What was happening was, it had rained enough that the puddles were fairly deep, and when new raindrops came down, they would create a crater and trap air that would turn into bubbles. And the reason the bubbles were sticking around for as long as they were is because the water wasn't pure."

Whether aboard an enormous cruise ship or trying not to get your shoes soaked after an afternoon storm, that is fluid dynamics for you.

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