Ships 2: What Floats Your Boat?

What You Need


  • A bucket, pan, or tub of water. This container should be at least twice as deep and twice as long as the maximum size of the boat. If preferred, you can use one larger, common pool or tub for testing. You will need at least one set of a smaller container (e.g., plastic shoebox) within a larger one (plastic dishpan or 13 x 9 pan) to allow students to check the water displacement of their boats.
  • 2-4 cup liquid measure with milliliter markings
  • Aluminum foil (2-3 large sheets of 3-4 feet)
  • Craft or roll paper (3-foot-long sheet); waxed “butcher paper” works well
  • Wooden craft sticks and/or pipe cleaners (10-30 per group)
  • Craft glue and/or hot glue gun and glue sticks
  • Metal nuts, bolts, or screws, heavy plastic beads or buttons, dried beans, or other weights (10-30 pieces per group; it is preferable to use objects all of the same size and weight for consistency)
  • Waterproof markers or wax pencils for marking load lines
  • Cardboard shoeboxes for mounting and displaying ships
Ships 2: What Floats Your Boat? © 2011 Clipart.com


To design, build, and test the specifications (water displacement and load line) for a model boat. The lesson focuses especially on integrating design principles with inquiry-based experimental skills.


This lesson is the second in a three-part series on ships. The overall lesson series is designed to allow students to extend their understanding of floating, sinking, density, and buoyancy and apply it to the design and testing of boats.

This series of activities builds on previous simple explorations of floating and sinking (see the 3-5 Science NetLinks lesson Sink It for a related activity) and prepares students for more in-depth examinations of density and buoyancy. In addition, students should have some skill at designing and carrying out a simple experiment.

The activities in these lessons integrate historical and current information from several countries (United States, Great Britain, and Sweden) and utilize the true “world wide” nature of the Internet. They also point out the important relationship between engineering and scientific research. While the students learn that most ships are constructed very similarly—whether they are schooners or destroyers—they also learn that ships range widely in size and are built from very different materials, with very different tools, and to serve different purposes. The three lessons in this series are outlined below.

In Ships 1: Give Me a Tall Ship, student teams develop research and reporting skills as they gather information about a specific type of ship and report it to the class. The class, as a whole, compares and contrasts the different ships, noting similarities and differences among ships from different historical eras and ships built for different purposes.

Ships 2: What Floats Your Boat? teaches students about load lines and cargo. Using the information from the ship reports they developed in Ships 1, they note that different types of ships can carry different amounts of cargo. They learn that overloading ships has, historically, been a dangerous practice. Students often have the misconception that ships first fill with water and this causes them to sink as opposed to the reality that ships sink when they weigh more than the water they displace, whether that weight comes from water flowing into the ship or too much cargo loaded on the ship. This misconception is further challenged in the hands-on activity included in the lesson.

Students use what they have learned about ship design (along with some additional information on the Web about how ships are built) to design and build a small model boat, using limited materials. They then develop a procedure to determine the load line for their boat while it is in “calm” waters. Finally, they test their boat in “rough” waters to determine whether the load line is a practical one. Through these activities, students should learn that every design involves trade-offs and decisions. They may find that their initial design fails and they need to revise and rebuild. These hands-on activities not only allow students to explore the uses of technology and engineering, but they also hone experimental design skills and data collection and analysis skills.

In Ships 3: Grand Designs and Great Failures, students apply what they have learned to develop an explanation of why two real-life ships sank (the British Titanic and the Swedish Vasa). This application of knowledge to real-life situations demonstrates to students that even good designs can fail (the Titanic) and that the solution to one problem often leads to another (the Vasa). It also demonstrates how cultural and contextual conditions can affect both engineering plans and their implementation.

Planning Ahead

Note: The amount of materials needed by each group depends on the maximum allowed size of the boat; the materials listed below are for a boat that is 12 inches or less in length.


Point out some of the wooden and steel ships from the first lesson in this series Ships 1: Give Me a Tall Ship and ask students how the captains of these ships can be sure they will not sink, especially when they are loaded with thousands of pounds of cargo and/or people.

Remind students of the story they read about the sinking of the Titanic: I Survived the Titanic. Ask the class to consider whether the ship sinking can be described in terms of load lines.


Assign students to teams of 3-4 (or whatever works for your particular class size). Tell students that each team will be in charge of designing, building, and testing a boat.

Each boat should:

  • include the three major parts of all of the ships they studied in Ships 1 (a keel, ribs attached to the keel, and a hull);
  • be less than or equal to the maximum specified length (you specify the length);
  • be capable of carrying a load at least as heavy as the boat itself;
  • be “seaworthy” in both calm and rough waters; and
  • be constructed from the materials provided.

After the boats are built, challenge students to develop a procedure for determining the boats' load lines in fresh and salt water. In general, each group should float its boat in a tub or container of calm, room temperature, fresh water and add weight to the boat until it sinks; this weight represents an “overload” for the group's boat. Then students should repeat the procedure, adding weight only until they feel the boat has reached a safe load. Using a permanent marker, they should mark the fresh water load line. Then they can repeat the procedure using salt water (see below).

This is only a general procedure. Allow students to develop their detailed procedures (e.g., where the load will be distributed on the boat) and present it to you for approval.

Note: If preferred, you can use the method for procedure development outlined in the 3-5 Science NetLinks lesson, Sink It. Remember that the load line should be at a level carrying as much cargo as possible, but safe for varying weather and water conditions.

After you have approved the procedure, students can determine the load line for their boats in both fresh water and in a tub of salt water (3.5% salt, that is, 3.5 grams of salt per 100 ml of water). Students should find that the boats can carry additional weight in salt water without sinking due to the greater density of salt water and, therefore, the greater weight displacement of the boats in salt water. It is important to allow teams enough time so that, if their initial designs are not successful in floating and carrying a load, they have time to adjust their design and repair their boats.

Next, students can determine their boats' water displacement by using the same procedure as Archimedes. Using the set up described in the Planning Ahead, set the dishpan on a flat surface and set the plastic shoebox in the middle of the dishpan. Completely fill the shoebox to the very rim with fresh water, making sure not to spill water over into the dishpan. Students should then float their unloaded boat on the water in the shoebox and begin loading it carefully and evenly with weights (nuts, bolts, etc.) until they reach the load line. Water will spill over the side into the dishpan. When their load line is reached, they should carefully lift out the boat, then the shoebox. Pour the water in the dishpan into the liquid measuring cup. This is the amount of water the boat displaces when fully loaded (that is, at the load line).

Remember that water displacement is measured in the weight of water displaced rather than the volume of water displaced. In this case,

1 milliliter of fresh water = 1 gram of water

Therefore, you can measure your water displacement in grams. If students are curious about how the water displacement of their boats compares to the ships they researched, use the following approximate conversions:

Unit Grams
U.S. Ton (2000 pounds) 1,100,000
Metric Ton 1,000,000

For example, if the ship they researched displaces 7,000 tons of water, this is equivalent to

7,000 U.S. tons x 1,100,000 grams/ton = 7,700,000,000 grams!

Finally, each team can test its boat in “rough” water. Teams should load their boats with a full cargo (at the load line) and test them in a tank where another team is creating waves with plastic lids or plates. The water should be rough enough to create a turbulent “ocean” but not so rough as to destroy the boats. Ask students what modifications they could make to their boats to make them more “seaworthy” in rough weather.

Distribute the Ship Specifications student sheet. Each team should complete this, including naming their boat. Using an upside down shoebox, teams should mount their boat on top, and glue or tape the boat's specifications on the side. If you prefer, students can be encouraged to decorate the mounting box for their boat. Each team should describe to the class how they built their boat, how they determined its load line, what load it will carry in fresh and salt water, and what the water displacement is.

To complete the lesson, ask each group to write a song about their ship. This can be in a contemporary format or can follow the format of more traditional seafaring songs such as "A Cheer for Plimsoll" (found on the Load Lines page of the National Maritime Museum website), or "The Edmund Fitzgerald."

The song should include at least two verses and a chorus and tell about at least one event in the life of their ship; have students record their songs on the Ship Specifications student sheet.


Assess student understanding with a class discussion, using the following questions as guidelines. If preferred, you can assign these questions to each group to answer in writing:

  • Which boats seemed to hold the most cargo? Were they the biggest boats? Were they shaped differently than the other boats?
  • When you added weight (cargo) to your boats, did you put it in one end or along the whole length of the boat? Why?
  • What did you find about loading your boat to travel in salt water versus fresh water? Why do you think that’s true?
  • How did your boat fare on rough seas?
  • How would you improve your boat design if you were planning to build another one?


Complete the third and final lesson in the Ships series: Ships 3: Grand Designs and Great Failures.

Allow groups to try warm versus cold water to test the load lines on the boats. Most ships are marked with load lines for both salt and fresh water and for warm and cold water loads. Since cold water is denser than warm water, the boats should carry more weight in cold water than warm water. Obviously, you can extend this activity by allowing students to add ballast, sails or other propulsion methods, a rudder, and stabilizers to their boats. They could also consider adding one or two decks. Other ship types can be modeled such as oil tankers and aircraft carriers. The very different designs of these ships point out the importance of, for example, a deep, heavy keel to keep the deck of an aircraft carrier stable and the wide and long shape of a tanker to distribute the weight of the cargo.

Additional resources about sinking, floating, and boats can be found in the following resources:

  • The Way Things Work, by David Macaulay, Houghton Mifflin; ISMB 0-395-42857-2; 1988. This book has a good section on boats and submarines.
  • NOVA Online’s Buoyancy Brainteasers has activities and problems to solve on floating and sinking.
  • The Mariners’ Museum website has good overviews of the history of sailing, going as far back as 3200 B.C. Although the website contains few pictures, there are good, easy-to-read narratives.

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