Friends asked this week’s question.
QUESTION: How can a 100,000-ton ship float but a 5-ton truck will sink?
ANSWER: Very lightweight things, like a cork or piece of Styrofoam, will float. Heavy things, like a brick, will sink. But that is not a very useful rule. The weight of an object is less important than how much water the object has to move out of the way.
Whether or not something sinks or floats depends on the overall density of the object compared to water. Density is how much it weighs per volume, rather than how much it weighs overall.
Those ancient Greeks were among the first to figure out how this sinking vs. floating works. They found that if you put something in a container of water, it takes up space that water previously occupied. We can name the water pushed out of the way as displaced water.
Sink or float? Compare the weight of the object with the weight of the water displaced. If the object weighs more that the weight of the water displaced, down it goes. If the object weighs less than the weight of the water displaced, it floats.
Archimedes Principle states that an object experiences an upward force (buoyant force) equal to the weight of the fluid displaced.
A ship is shaped in such a way that the weight of the boat is less than the weight of the water pushed out of the way. A good portion of the interior is air. The overall density of the ship, a combination of steel and air, is less than the density of water.
All ships, up to about the middle of the 1800s, were made of wood. It is not possible, the conventional wisdom said at the time, to make a boat out of steel. Wood floats, steel sinks. The French and British built ironclad gunboats in the late 1850s. During the American Civil War, the Confederates built the Merrimac (USS Virginia) and the Union North had the Monitor for an epic battle in 1862. The outcome was inconclusive.
Warship sizes are generally measured in displacement. The large aircraft carriers in our Navy fleet, such as the USS George H. W. Bush, the USS Abraham Lincoln, and the USS Harry Truman are in the ballpark of 100,000 tons.
The cruise industry goes by Gross Registered Tonnage. It is found by dividing the volume of the space enclosed by its hull (measured in cubic feet) by 100. With emphasis on luxury and comfort, the sizes of cruise ships have gotten bigger through the years.
Some of the largest cruise ships active in the world today, such as Royal Caribbean Quantum and Oasis class ships are over 225,000 gross tons. They are about 1200 feet long and 154 feet wide (beam) and accommodate over 5,400 guests on 16 passenger decks.
Most rocks sink, but there are exceptions. Pumice is a light and porous volcanic rock that forms when gas-rich lava solidifies rapidly.
Most woods float, but there are exceptions. Some rosewoods and black ironwood are denser than water. They sink when placed in water. Ebony is a dense black hardwood that sinks. Some of these dense woods are used in making musical instruments. Lignum vitae is one of the densest woods known. The trees are indigenous to the Caribbean and the northern coast of South America. The belaying pins and deadeyes, used for rigging sails, aboard the USS Constitution (Old Ironsides) and many other sailing ships were made from lignum vitae.
The three Scheckel boys had epic sea battles on the Seneca farm in the late 1940s and into the 1950s. Our ocean was the cow tank, and our ships were large hollowed-out cucumbers. Favorite battle tactic was ramming your brother’s boat.
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Larry Scheckel is a retired Tomah High School physics teacher.