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1.2.1 Why Ships Float

Teacher Background

Salt content differs from sea to sea, and ships often sail from one body of water to another. This is very important since ships are only stable with the right combination of cargo or ballast and ocean conditions, including salinity. Ships can become unstable in stormy waters if their center of gravity is too high in relation to their center of buoyancy. Ships can swamp and sink if they ride too low in the water. Meredith Olson, who contributed several Activities to this Guide, including this one, lives in the Pacific Northwest, and tells of several heavily-laden trawlers being lost at sea when they sailed from salty waters-where their cargo weight was safe-to waters diluted in salt content by fresh water melting from glaciers! Ice, salt, and why ships float: it's a matter of life and death.


Students will conduct experiments demonstrating the effects of varying rates of salinity on how high out of the water a simple "ship" will float.

Materials (for each group of 3/4 students)

  • 600-ml. beaker or wide mouth glass jar
  • lightweight 8 in. plastic test tube
  • sand (for ballast)
  • wood splint (popsicle stick marked in 1 cm intervals) which fits in test tube
  • metric ruler
  • Activity 1.2.1 Student Worksheet, "Why Ships Float"
  • Blackline Master #11, "Facts About the R/V Polar Duke, and its Equipment"


Ask the class to name large bodies of both salt and fresh water. In which would floating be easier? Why? Water in the Dead Sea is about 27 percent salt. If you floated vertically, your head and shoulders would stick out above the surface. You could say your body's "water line" passed under your arm-pits. If you stretched your arms out, you'd lose balance because you'd be top heavy. Your "center of balance" would not coincide with your "center of buoyancy". And you wouldn't swim very well because you couldn't stay under water sufficiently to propel yourself.



  1. Review experimental procedure on Activity 1.2.1 Student Worksheet. As students complete their investigations, they should add their data to class record sheet on chalkboard.
  2. Debrief. Analyze class data set. Gather conclusions based on the data.


Discuss the following real-world applications:

What would happen if a ship was loaded to the fresh water line in a salty sea port and then took its cargo to a fresh water port?

Winter seas in the North Atlantic are very rough so a ship must be less heavily loaded. What is a safe load level for a trip to Antarctica? The Polar Duke sails from South America to Antarctica. There is ice and fresh water at the surface around Antarctica. What will happen to the Polar Duke's water line as it moves along its journey?

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Last Update: 1/18/97
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