Moment of Science: Buoyancy

“Be brief, be buoyant, and be brilliant.” -Brander Matthews
We're discovering how a 220,000-ton ship can safely float, in this week's "Moment of Science". ⬆🚢⬆
Published: Nov. 1, 2022 at 11:26 AM EDT|Updated: Nov. 1, 2022 at 11:37 AM EDT
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Welcome aboard the Harmony of the Seas, one of the largest cruise ships in the world! She’s nearly 1200 feet long, and weighs as much as 17,000 African elephants, which kind of makes you wonder: How in the heck does it even manage to float? This week’s episode is all about buoyancy.

* In basic terms, buoyancy is the ability of an object to float in a fluid -- such as water or air. (For ease of explanation, we’re starting with water.) There are two key factors here: how dense the object is, and how much water it displaces. If the object is dense or heavy enough, the force of gravity overcomes that “upthrust” and the object sinks. If that upthrust is stronger than gravity, it’s buoyant -- and if the density is equal to the displaced water, it’s still buoyant, but it just doesn’t go anywhere.

* Legend has it that Archimedes discovered this principle in his bathtub (246 BC), when he climbed in and noticed the water spilling over the sides as he displaced it. He then ran through the streets naked, shouting “Eureka!” or “I have found it!” He used that knowledge to prove to a king that his crown wasn’t entirely worth its weight in gold -- which is probably why he didn’t get arrested for indecent exposure.

* That principle isn’t exclusive to boats: blimps and zeppelins are buoyant enough to float in air, and even party balloons are a simpler example. The thing is, air is compressible... and water is not, which also helps a massive ship like the Harmony stay afloat. The ocean pressure pushes upward against the hull with an equal or greater force of the ship’s mass -- displacing about 120,000 tons of seawater in the process.

* Floating is one thing... stability is another. Maybe the Titanic isn’t the best example of floating ships, but it WAS the longest ship in the world over a century ago (882.5 ft) -- that honor now goes to the Harmony (1188 ft), which is 35% longer -- but nearly 250% wider. Engineers found over the years that a wider, rounder, deeper hull allows for a lot more weight with much less rocking back and forth. The center of gravity cannot change, but the center of buoyancy does shift, rocking it back upright. On our cruise, we tilted no more than 2 degrees to either side -- good news for anyone prone to seasickness.

Whatever floats your boat, make sure you stay topside until next week, when we take a dive into the deepest corners of the deep blue sea.

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