Fish-bot Experiment

How does water move when a fish flaps its tail?

---

---

Fish-bot Experiment

 Let’s do an experiment using fishbots to determine if body shape matters. To visualize how body shape impacts a fish’s ability to move through water, we will focus on two differently shaped fish robots, one emulating a short-bodied swimmer, an angelfish, and one mimicking a long-bodied swimmer, a tuna. This activity can be done alone or in small teams.

• Fill the included tub roughly 2/3 with water. It should be deep enough for the fish to swim in and for dye to distribute without immediately spreading through the whole tank.
• As a demonstration, select one fish robot, it can be either of the two types. Place it in the water and observe how it swims. Describe where the forward propulsion force originates. How does it move?
• Next, each team/person will select a different DRY fish robot. Dye will not stick to wet fish! If you use the same fish, dry it off well.
• To determine the range of tail movement: 
  • Place your fish on a piece of paper.   
  • Measure the height furthest from the base and measure the width at the base. 
  • Gently push the tail to the leftmost and rightmost positions, marking each point with a dot on a piece of paper under the fish. Connect these two dots – this gives your tailbeat distance.
  • Record the sizes measured in the data table below.
  • When your fish is in the water, using the ruler you will record the width of the initial and final dye spread as it swims.

• Place a paper towel under the fishbot to keep your station clean. 
• Hold the fishbot over the towel and add 2-3 drops of food coloring along the top of the fish’s body. Because it is plastic, some of the dye may roll off, but as long as one or two drops stays on top it is sufficient. 
• Team members who are not launching the fish bot should be ready to measure the dye spread. If working alone, stretch a ruler across the tub, and be ready to record a video that you can look at later to check the spread.
• Record the width of the dye spread when you place the fish in the water, at 10 secs, 20 secs and 30 secs. 
• Once dye is added, the fish are ready to swim. Carefully place the robot into the water. Fishbots are water reactive and will begin swimming immediately, so they do not need to be turned on. The fish should be able to swim in a straight line for a few seconds, enough for data to be collected. If the fish tries to turn, gently and quickly redirect it.
• To repeat with a new fish, you may have to fill the tub with clean water so you can observe the dye patterns better.

Robot Type	Width of Dye Spread	Observations
Angel fish’ bot (short-bodied swimmer)	At 0 seconds (initial)   At 10 seconds:   At 20 seconds:   At 30 seconds:
Tuna bot  (yes, we know this is Nemo, a clown fish, we’re going for a long-bodied swimmer)	At 0 seconds (initial):   At 10 seconds:    At 20 seconds:   At 30 seconds:

Discuss the following with a team that had a different fish, or compare both fishbots on your own after completing the activity using each model:

• Which type of fish appeared to swim faster?
• Which fish swims straighter, which makes sharper turns? How may this be an advantage or disadvantage for survival?
• Compare and contrast how the dye spread varied for the different shaped fishbots. 
• Where did the propulsion come from?

Now, consider how the fish could be improved…

• What part of the body could be modified to increase thrust?
• If fins were modified to be longer, taller, wider, smoother how might this impact propulsion?

The body shape of fish is not the only anatomical trait that comes into play. Consider the fish’s fins as well…For the next activity, we will be testing out how different fin and tail shapes, sizes, and aspect ratios impact the way the fish swims, and how that affects the fluid surrounding them as well. 

---

Bonus: Next Steps

As a crafty next step, check out this exercise on fin modification.

---

Last updated: November 23, 2022.