How a Boat Floats

Yesterday I was sorting through a pile of old books someone was getting rid of, and I came across a children’s book called The Way Things Work.  I used to read books like this all the time when I was a kid.  (I thought I would grow up to invent something useful.)  Unfortunately, a lot of the basic principles I learned back then have been long forgotten until yesterday.  I flipped through the pages and happened to come across some drawings of boats.  It asked the age old question: How does a boat float?  When I was in third grade, we did a unit called sink or float.  I have a vivid memory of making shapes out of blue playdough and dropping them into a cup of water.. And that's about all I remember.

So how does a log float, but a steel needle sink?  And how does a steel needle sink, and a steel ship float?  Well, let’s start with density.  An object’s density must be less dense than the density of water in order to float.  And how do you figure out the density before you risk sending your ship to the bottom of the sea?  Find the mass of the ship and the total volume, then divide:

Density = Mass/Volume

Ok, so we got that.  We probably remember that from some point in our lives.  Let’s think about the structure of a steel ship.  It is full of air, unlike a steel needle (hence why a ship is less dense than water).  And, it is long and wide (a hot dog laying down) rather than tall and skinny (a hot dog standing up).  That is because the ship relies on water displacement to float.

Good ol' Archimedes was the first to record the definition of floating.  He said, "An object in a fluid experiences an upward force equal to the weight of the fluid displaced by the object."

So if we fill our 60 pound boat to its maximum capacity of 450 pounds, it will sink unless it displaces 510 pounds of water.  Thankfully, we know Uncle John has taken care of our water displacing fears.

Check out the picture below.  In the third picture, the mammouth displaces more water by adding sides to his raft.  You go woolly mammouth.





From David Macaulay's The Way Things Work