Answer Geek: Why Do Boats Float?
<br> -- Q U E S T I O N: I live on the Great Lakes and it has always astonished me how these gigantic freighters can stay afloat. Why is it that huge seafaring ships can stay afloat even though they may weigh hundreds of tons, but I can’t stay afloat in the bathtub?
— Shane
A N S W E R: Why is it that you sink while a gigantic seafaring ship made mostly of metal and carrying tons of heavy freight floats? Quite forward of you, Shane. Compared to the ship, you are very dense.
But, please, don’t take offense. We’re not talking about your mental acuity here. Density, in this case, is a function of weight divided by volume.
Every substance has density, including water. If the density of a given object is greater than the density of the water around it, it sinks. If it is less, it floats. Simple as that. That supertanker may be made of material that is denser than water, and it may outweigh you by a few thousand tons, but much of the area enclosed by the ship’s metal hull is just air. And the average density of the total combination of metal, air, and cargo is less than the density of the water it floats in.
‘Eureka’
There is a law of physics that covers this. I’ll give you a hint. It has something to do with a bathtub. And a Greek mathematician.
If you didn’t come up with the answer, your high school science teacher is very, very disappointed. Remember Archimedes’ Principle? According to the story, Archimedes lowered himself into the bathtub one day, watched the water spill over the edge, and realized that the amount of water he displaced was related to how much he weighed and how much space he occupied. Rumor has it he jumped out of the tub and ran out into the street naked shouting “eureka,” which is Greek for “I found it.”
Once all the excitement died down, Archimedes sat down and came up with a basic definition for why things float: “An object immersed in a fluid is buoyed up by a force equal to the weight of the fluid it displaces.” That force is sometimes called upthrust. What that means out on the high seas is that a boat that weighs, for example, 1,000 tons will sink until it displaces 1,000 tons of water. At that point, the upthrust from the water will equal the downward pressure of the ship. If it has been designed correctly, that will happen before the vessel is completely submerged.
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