Don't Rock the Boat

3.6 based on 18 ratings

Updated on Feb 08, 2012

Grade Level: 3rd - 6th; Type: Physical Science


To investigate how a bottom keel helps keep a boat from rocking back and forth.

Research Questions

  • Is there a way to keep a ship more steady out in the ocean?
  • What problems can the rocking motion of a ship cause?

Ocean waves can be choppy and ships can be tossed back and forth making people seasick. Engineers devised a keel or bottom fin to help keep a ship from rocking back and forth.


  • 2 empty 2-liter plastic bottles (one will need a lid)
  • Sharp knife (for adult use only)
  • 20 marbles
  • rubber cement
  • marker
  • tape
  • large tub of water, bathtub, or pool
  • stopwatch
  • paper
  • pencil

Experimental Procedure

  1. Gather the necessary materials.
  2. Have an adult cut a rectangular hole in one of the plastic bottles. The hole should be about 6 inches from top of the bottle to bottom and 2 inches wide. When tipped on its side, the hole will be at the top of the boat. Leave the lid on the bottle.
  3. Use rubber cement to adhere the marbles to the bottom of the boat making sure they are centered so that they don’t tip the boat one way or the other.
  4. With the marker, draw a line lengthwise dividing the top of the boat and the bottom of the boat.
  5. Place your boat in the water and make sure that it floats. Tilt the boat to the line you drew with the marker. Then release. Set the timer. Count the number of times the boat rocks back and forth until it stops. Stop the timer. Record the number and the time in seconds. Repeat the procedure two more times. Then figure out the average by adding
  6. the data and dividing by 3 (the number of trials).
  7. Make a keel using the other bottle. Have an adult cut the top and bottom off the other bottle. Then cut the remaining piece in half lengthwise. These two pieces will be used to make two bilge keels.
  8. Fold each keel in half lengthwise and attach each to one side of the boat using tape.
  9. The keels should extend out from the side at a 90 degree angle. You may need to trim the keels to fit your boat. Be sure that the keels are even on both sides of the boat.
  10. Repeat step 5.
  11. Analyze the data and draw a conclusion.

Terms/Concepts: keel: a structural part of the bottom of a boat used to keep the boat from rocking back and forth buoyancy: the force that causes something to float; Ocean waves can cause a boat to rock back and forth. Bottom keels help to steady a boat.


“How Cruise Ship Stabilizers Work” by Barb Nefer at “What Keeps a Ship Afloat?” by Ernest Capraro at

Nancy Rogers Bosse has been involved in education for over forty years - first as a student, then as a teacher, and currently as a curriculum developer. For the last fifteen years she has combined a career in freelance curriculum development with parenthood - another important facet of education and probably the most challenging. Nancy lives in Henderson, Nevada with husband and their three teenagers.

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