I consider myself to have a decent set of sea legs -- I’ve never been hit with a bout of seasickness, I can handle a kayak or paddleboard on my own and I’m a pro at sunbathing on the front of a motorboat. I can understand the physics of a boat to a degree, enough that I know not to lean too far to one side and stay low when coming up on an intense swell of waves. I also know that too many people in a small canoe is a recipe for disaster -- namely, sinking.
So, pray tell: How does a 200,000-ton cruise ship with 5,000-plus passengers sail the open seas with no problem?
I already have a love-hate relationship with cruises. I went on a Disney cruise with my family years ago, complete with a private island in the Bahamas and a ship with magic around every corner (the walls in one of the restaurants on board gradually changed from black and white to technicolor during your meal); it was perfect for my preteen wonderment.
Now, though, I’m a little skeptical of boarding a cruise ship; between being out in international waters and being forced to sit with strangers every night for dinner, I think I’d just rather book a flight to the Caribbean these days.
Then again, if Oprah were on a ship handing out free tequila shots, I’d probably show up.
But that’s besides the point. Ships are 20 stories high with an overwhelming amount of stuff stacked on top. You’re telling me they stay afloat with no wizardry involved?
Turns out yes, and the theory of displacement is to thank. Displacement accounts for how much water an object must move out of the way in order to float.
Cruise ships are constructed in a way that displaces an equal amount of water as the mass of the ship. As the ship pushes forward, moving water out of the way, the water is continuously attempting to return to fill the space and pushing upward to do so, hence floating the ship.
Rather than thinking about how your cruise ship stays afloat in terms of weight, we have to consider the effects of buoyancy (a liquid’s power to keep things floating) and density of the ocean and how it relates to the displacement of water with the ship; the boat presses down, and the water responds by driving upwards, so it creates a buoyant little balance.
The structure of the ship’s hull, or main body of the vessel, has a lot to do with that: The typical U-shaped hull is rounded, wide and deep, so the weight of the cruise liner gets evenly distributed throughout it.
So, no matter how many bars and tennis courts are built on top of the ship, its main structure will always have lots of open space and be less dense than the water it’s riding on. Engineers have had lots of practice to get this equation right.
Sounds fake, but OK.
So, go nuts -- drink that fourth brunch margarita and rest assured that your boat isn’t going anywhere but to the next island. Where you’ll have four more margaritas.