Amerigo Vespucci at the ship parade in the NYC harbor

What makes a heavy ship float?

Buoyancy illustration - gravity mass of object vs. density of the fluid

The reason a boat or ship carrying a heavy load can float while the same load would sink if thrown overboard and why a person can float when stretched out flat but sink when curled up into a ball is due to the scientific principle of buoyancy or floatation. This principle is based on the amount of water pushing against an object. When a person stretches out flat, more water pushes against them because their body is spread out more, which creates more upward force and causes them to float. Conversely, when they curl up into a ball, less water is pushing against them, which reduces the upward force and makes them sink. Want to test this for yourself? Try this experiment:

Take a piece of clay and split it into 2 identically sized pieces. Take one of the pieces and roll it into a ball. Take the other piece and fashion it into a flat boat shaped object.
Now place both pieces into a sink full of water. Which one floats, and which one sinks? Both? Neither?

So you see, if the total area of the object that makes contact with the water is large enough, the object floats. The object must make room for its volume by pushing aside or displacing an equivalent (or equal) volume of liquid. The object is exerting a downward force on the water and the water is therefore exerting an upward force on the object. The solid body floats when it has displaced enough water to equal its original weight.

This principle is called buoyancy. Buoyancy is the loss in weight an object seems to undergo when placed in a liquid, as compared to its weight in air. Archimedes’ principle states that an object fully or partly immersed in a liquid is buoyed upward by a force equal to the weight of the liquid displaced by that object. From this principle, he concluded that a floating object displaces an amount of liquid equal to its own weight.

Principles of flotation science experiment supplies

Supplies: Clay

Image Credits

In-Article Image Credits

Amerigo Vespucci at the ship parade in the NYC harbor via Wikimedia Commons by Harley D. Nygren - NOAA with usage type - Public Domain. July 1976
Archimedes principle via Wikimedia Commons with usage type - Creative Commons License. June 25, 2018
Buoyancy illustration - gravity mass of object vs. density of the fluid via Wikimedia Commons with usage type - GNU Free. May 11, 2008