As with any other design challenge, students must begin by creating a drawing of their plan which includes labeling the materials they plan on using. There are many ways to make a self-propelled boat; paddle boat, air powered (balloon), and candle powered are the three most common student approaches I have seen.
Be aware - if you have balloons on the supply table, all students will choose that option. While there is nothing wrong with that, I prefer to have students display some variety in their approaches. Even if all students use balloons, they will need to find a way to slow down the flow of air so their boat makes it across the tank in a straight line. I do allow students to use research if they desire but I do not provide specific class time for that purpose. My experience with eighth graders is that they prefer to jump in and try things and refer to research when they get stuck.
The following image is an example of a group's preliminary sketch to show the type of detail I am looking for from students prior to allowing them to build.
For the next few class periods, students construct, test and improve their designs. I have managed the "improve" portion of this activity in many ways. My favorite is for the students to calculate the speed of their boat first with it holding no mass and then adding "slugs" of pennies (20 pennies duct taped together) to determine two things:
Students can use this information in their improvement redesign. This takes about 1 week to 10 days, so if time is a factor students can just try to increase speed without testing mass.
The following videos show students conducting some initial tests of their designs. The trays I use come from an old plant cart. Kiddie pools are fun to use and pose an additional challenge as students attempt to make their boats go straight across but they are a hassle to fill/empty.
To conclude the lesson, I like to have students reflect on the process. I have them choose 3 of the sentence stems to complete in their journals:
I like this type of reflection because it allows me some insight into what was happening inside the student's minds as they were working as well as give me some ideas on what I might need to add/edit for next year.