Soft Fish-Tail Propulsor Performance Study

ISEF Category: Robotics and Intelligent Machines

Subcategory: Biomechanics  ·  Difficulty: Intermediate  ·  Setup: School Lab  ·  Time: 1 to 2 Months

The Hook

Fish swim with a secret trick. They do not just flap, they tune their tails to make the water push back harder. You can build a soft propulsor that copies that idea and see which motion gives the most thrust for each watt of power.

What Is It?

This project studies a soft robot tail that moves through air pressure and valve timing instead of motors and gears. The goal is to see how changing the tail’s beating rate affects thrust, which is the forward push, and power use, which is how much energy the system needs. In simple terms, you are asking which motion gives the best speed for the least energy.

A useful idea here is the Strouhal number. That is a dimensionless number, which means it has no units. Engineers use it to compare swimming and flying patterns across different sizes. For fish, a certain range often lines up with efficient motion. Your project asks whether your soft tail performs best when it moves in that same range.

You can also watch the water flow with food dye. The dye acts like a trail marker, so you can see vortices, which are spinning swirls in the water. Think of them like tiny water fingerprints left behind by the tail. If the swirl pattern changes with frequency, you can connect the shape of the wake to measured thrust and efficiency.

Why This Is a Good Topic

This is a strong science fair topic because you can change one variable, measure a real output, and compare your results with biology. The project connects robotics, fluid motion, and animal biomechanics, so it has a real-world link to efficient underwater drones and soft robots. You can also collect original data with school-level equipment, then analyze how frequency, wake shape, and power use relate to each other.

Research Questions

• How does tail beating frequency affect thrust in a soft pneumatic fish-tail propulsor?
• What is the effect of valve timing on thrust per watt for the propulsor?
• Does matching the Strouhal number range of real fish improve propulsion efficiency?
• To what extent does tail stiffness change the wake pattern and forward force?
• Which frequency range produces the largest net thrust for a fixed input power?
• How does the dye-tracked vortex pattern change as the propulsor speed changes?

Basic Materials

• Soft silicone sheet or casting silicone for the tail body.
• Hobby air pump.
• Solenoid valves rated for the pump setup.
• Flexible air tubing.
• Fish tank or clear water test bin.
• Food coloring or water-safe dye.
• Phone camera with slow-motion mode.
• Meter stick or ruler for scaling video.
• Digital kitchen scale or force sensor setup for thrust measurement.
• Power meter or multimeter for input power tracking.
• Clamp stand or frame to hold the propulsor in place.
• Waterproof tape and connectors.
• Notebook or spreadsheet for data logging.

Advanced Materials

• Soft elastomer casting kit for repeatable tail fabrication.
• Pressure regulator and inline pressure sensor.
• Data acquisition system for synchronized valve timing and force data.
• Load cell with amplifier for thrust measurement.
• High-speed camera or calibrated smartphone video rig.
• Particle image tracking software or ImageJ for wake analysis.
• Flow tank access or recirculating water channel.
• Power analyzer for electrical input measurements.
• 3D-printed mounting hardware for alignment and repeatability.
• Optional dye injection system for cleaner wake visualization.

Software & Tools

• ImageJ: Measures tail motion, wake spacing, and frame-by-frame displacement from video.
• Tracker: Tracks markers in phone video and estimates motion frequency and amplitude.
• Python: Organizes data, calculates efficiency metrics, and makes graphs.
• Google Sheets: Logs trial results and compares thrust, frequency, and power across runs.
• GeoGebra: Helps fit trends and inspect relationships between variables.

Experiment Steps

1. Define the main variable you will change, such as tail frequency, valve timing, or tail stiffness.
2. Decide how you will measure thrust, power, and wake shape so each trial has the same metrics.
3. Build a fair control setup that keeps tail size, water depth, and mounting position constant.
4. Plan a way to estimate the Strouhal number from your measured frequency, tail amplitude, and flow speed.
5. Choose a comparison method that links dye patterns to efficiency, not just to visual appeal.
6. Design your data table and graph types before testing so you can spot the best operating range quickly.

Common Pitfalls

• Measuring thrust while the propulsor mount wiggles, which mixes body motion with water force.
• Changing both frequency and tail amplitude at the same time, which makes the source of any result unclear.
• Using uneven dye release, which hides the wake pattern and makes vortex spacing hard to compare.
• Ignoring power draw from the pump and valves, which can make an inefficient setting look good.
• Comparing trials with different water conditions or tank positions, which adds noise that can swamp the trend.

What Makes This Competitive

A competitive version of this project goes beyond a simple frequency sweep. You can test a clear efficiency metric, compare your tail against a baseline design, and connect the wake pattern to measured thrust with statistics. Strong projects also explain why one operating range works better, then test whether that pattern matches the Strouhal window seen in real fish.

Project Variations

• Test how tail material stiffness changes thrust per watt at the same valve schedule.
• Compare a single-tail design with a split-tail design to see which wake pattern gives better efficiency.
• Swap fish tank dye tracking for load-cell-only analysis and focus on the link between wake frequency and force output.

Learn More

• NOAA Fisheries: Search for fish swimming efficiency, locomotion, and biomechanics background.
• NASA Glenn Research Center: Search for basic fluid dynamics and vortex explanations.
• PubMed: Search review articles on fish locomotion, fin mechanics, and soft robotic propulsion.
• MIT OpenCourseWare: Look for introductory fluid mechanics and robotics materials.
• Journal of Experimental Biology: Search for articles on fish swimming kinematics and Strouhal number.
• USGS Water Science School: Use it for clear visuals on flow, drag, and water movement basics.