This is the final activity of three dealing with density and displacement. There are several common materials and techniques between the three activities, and each is able to be a stand-alone activity. This activity requires the greatest amount of materials and set-up, and it assumes that students are familiar with how to use graduated cylinders and scales. They will also need to be instructed in setting up a cup full of water and a bowl to measure displacement and volume.
For this activity, you should have on hand, two rolls of “heavy duty” aluminum foil per class. You will also need up to 600 to 700 milligrams (about 1.5 pounds) of metal washers or other small weights per boat to be tested at the same time. This is the most the boats will hold, so you can do with less, but you can expect to use at least 10 pounds at one time.
The number of boats you can test is limited by the size of you water trays. When choosing a tray, keep in mind that as the boats fill, they lower in the water. If the water is not deep enough, some boats will rest on the bottom of the tray before they are full. I suggest having a deeper bucket available to test these boats if needed.
Students begin this activity making a boat of the same design. If they finish in time, then they are free to come up with their own design for a second boat using a separate piece of aluminum foil. Both boats will be tested in the same manner. The first boat is designed with right angles making it suited to the calculations in the activity.
Before testing their boats to see how much weight they can hold without sinking, students use two methods for calculating the theoretical maximum weight each of their boats could support. The first is to measure their boat’s dimensions and calculate the volume geometrically. The second method is to fill the boat with water on the inside and measure the volume of the water contained. Ask students if they can come up with a third way to calculate the maximum displacement of their boats.
There are many interesting extensions of this activity. Ideal boats for this challenge are more like barges. You could add additional design constraints to this activity that had to do with hydrodynamics. You could allow students additional materials. You could have students investigate real world design parameters for ocean vessels. How much of a boat’s maximum capacity can be used safely? You could also investigate the stability of different shapes and arrangements of weight within the boats. Have students research primary and secondary stability. You could agree to test your boats with a level of waviness in the water to see how that affects their results.