Surface Tension

With Bob Jackson

A brief respite in the pursuit of knowledge gives me a chance to dip into the old mailbag and answer some of our readers' questions. I see one here from Paul, in Minot, North Dakota . . . Paul writes:

"Bob, I've often wondered how those big cruise ships can float! Can you give me a clue?"

A clue? Paul, I can give you the answer! But to make things clearer, let's look at the question from the other direction: how do we keep those ships IN the water? And the answer is: we had to discover the magic of "surface tension."

Now, as you know, surface tension is a combination of forces that causes molecules of a liquid to resist separation. So, even though for centuries the desire was strong to develop the modern cruise ship, we had to wait for science to provide the answers that would allow us to build something that big and heavy, and KEEP it in the water.

You see, surface tension is constantly resisting the invasion of objects into the water, and once you do manage to submerge something, it pushes it right back out again. (Picture a balloon or a piece of wood popping back to the surface.) Keeping a cruise ship (or any ship, really) heavy enough to remain in the water is a real balancing act. Have you ever noticed that a ship never leaves the dock until all the passengers and their baggage are aboard? No way -- not until the delicate weight calculations are checked and re-checked to make sure that once it's released it isn't going to slowly rise up and get stuck under a bridge somewhere.

Have you wondered why the "head count" is so meticulous at the ports of call? Remember that a ship's passengers have been eating six times a day, depleting the stock of foodstuffs. Fortunately, the weight of all that food is retained by the passengers, so it isn't a problem, but ONLY IF THE SAME PASSENGERS STAY ABOARD FOR THE ENTIRE CRUISE! Again, that ship isn't going to untie from a dock or weigh the anchors that have been holding her down until the crew KNOWS she's within that narrow range of weight tolerance that insures she keeps her waterline where it belongs: in the water. On those occasions when someone does manage to get lost in, say, St. Thomas, the captain has to detail a party to go ashore, find an acceptable local and quickly dress and fatten them appropriately.

You can imagine the importance of knowing EXACTLY what everything aboard weighs at all times. For the crew, it's a constant worry and responsibility; tossing one too many crates of overripe carrots can land you in the hoosegow in nothing flat. In nautical parlance, a ship's jail is a "brig," and yes, there is a brig on a cruise ship, although it's out of sight to passengers.

So, Paul, thanks for asking! And remember, folks: if YOU have a question, it probably means that there are others out there with the same question, but they're reluctant to ask. Fire away!

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