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Activity: Ship Stability

NGSS Science and Engineering Practices:

NGSS Crosscutting Concepts:

Materials for parts A-B

  • Weighted Styrofoam for ship cross-sections (1 in thick Styrofoam board)
  • Test tank of water
  • Protractor
  • Waterproof writing utensil
  • 6 cm nail
  • Plumb line (string) with weight
  • Styrofoam (for plugs)
  • Knife or fine-tooth saw
  • Toothpicks
  • 8 ½  x 11-in sheet of blank paper
  • Pen
  • Fig.s 8.46–48
  • Scissors
  • Metal weights
  • Towels


Safety Note: Use caution when handling blades


A: Constructing a Styrofoam ship cross-section

  1. Use scissors to cut out the printed hull shapes in Fig.s 8.46–8.48 to create a set of templates.
  2. Trace the outline of each template on a piece of Styrofoam using the pen.
  3. Use the knife or fine-tooth saw to cut a Styrofoam ship cross-section.
  4. Lightly press a metal weight into the Styrofoam in the spot indicated by the black dot on the template. This weight will help determine the center of gravity.

<p><strong>Fig. 8.46.</strong> Template of a cross section of a round-bottomed hull that has a low center of gravity</p><br />
<p><strong>Fig. 8.47.</strong> Template of a cross section of a sailboat hull that has a low center of gravity</p><br />
<p><strong>Fig. 8.48.</strong> Template of a cross section of a flat-bottomed hull boat that has a low center of gravity</p><br />


B: Testing ship stability

  1. Obtain a Styrofoam cross-sectional model of a ship from part A.

<p><strong>Fig. 8.49.</strong> The Styrofoam model, when pushed to a 30° angle, lists to the side.</p>

  1. Tank-test the model for stability. NOTE: This test is for side-to-side stability only. If a model tends to fall forward or backward, support it gently with your hands.
    1. Place the cross-sectional model in the test tank in a vertical position.
    2. Test to determine whether it is stable. Push down on one side so that the model lists about 30˚(The nautical term list means to lean or tilt to one side.). Use a protractor to check what this angle looks like on your model. Release the model. See if it returns to an upright position. If it does, it is stable.
    3. Push the model down again on one side so that it lists approximately 30˚. Mark the 30˚ waterline across the face of the model (Fig. 8.49).
  2. Locate the center of gravity of the model. Do not use the test tank as you carry out the procedures below.
    1. Place a nail through the cross-sectional model at any position close to an edge. Hold the nail so that the model swings easily. Enlarge the hole if necessary (Fig. 8.50 A).
    2. Hang a plumb line from the nail as shown in Fig. 8.50 A.
    3. Draw a line on the model where the plumb line hangs. Repeat steps 3a and 3b, placing the nail near an edge and at least 5 cm away from its original position (Fig. 8.50 B).
    4. Locate the point where the lines intersect. This is the center of gravity (Fig. 8.50 B).

<p><strong>Fig. 8.50.</strong> The plumb line helps to locate the center of gravity by (<strong>A</strong>) hanging the model from the first nail position and (<strong>B</strong>) hanging the model from the second nail position.</p><br />


  1. Determine the center of buoyancy of the model at approximately a 30˚ angle in the following manner:
    1. Remove the weight(s) from your model and fill any holes with Styrofoam plugs.
    2. Cut each model in two along the 30˚ waterline (Fig. 8.51 A).
    3. Using the section of the hull that was submerged, find its center of gravity. Follow the steps in Procedure 3. Since this section has uniform density, you are locating its geometric center, which is also the center of buoyancy. Mark the center of buoyancy (Fig. 8.51 B).

<p><strong>Fig. 8.51.</strong> Find the center of buoyancy at 30˚ list by (<strong>A</strong>) cutting each model in two along the 30˚ waterline and (<strong>B</strong>) hanging the model from new nail positions.</p><br />


  1. Show the interaction between gravitational and buoyant forces on a sheet of paper.
    1. Reconstruct the cross-sectional model of the complete hull. Attach the two parts with toothpicks.
    2. On a sheet of paper, trace the outline of the cross-sectional hull positioned at a 30˚ list.
    3. Locate and label the waterline, the center of gravity, the center of buoyancy, and the nail holes on the paper.
    4. Draw the buoyant and gravitational force arrows pointing to the centers of buoyancy and gravity. Show the direction of force. Recall that these forces are always straight up and down.
    5. Determine whether the two forces acting at their respective centers restore the ship to an upright position or capsize it. Draw an arrow in the direction the ship should roll. Is the ship stable?
  2. Without placing your model in the water, predict the maximum list that it can have before it capsizes. Record your prediction and your reasoning.
  3. Re-attach the weight(s) to your hull model and test your prediction. Record how you tested your prediction.


Activity Questions: 
  1. Did your prediction in procedure B, step 6 match your observation in procedure B, step 7? Explain why you think your predictions and observations were similar or why you think they were different.
  2. Define the terms buoyant force and gravitational force.
  3. Define stability for a floating object. Use the terms buoyant force and gravitational force in your answer.
  4. Describe two characteristics of a ship that tend to make it stable.
  5. Explain how the plumb-line technique finds the center of gravity.
  6. A ship is considered tender if it slowly rolls back and forth to a stable position. It is considered stiff if it rights itself quickly, with little roll. What characteristics of the ship might determine whether it is tender or stiff?
  7. Using diagrams, explain each of the following terms:
    1. leverage stability
    2. weight stability
    3. center of gravity
    4. a ship righting itself

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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.