Lesson 4: Aerodynamics of the Oceanic Crab-Claw Sail: Bernoulli & Vector Forces
A sail does not drive a waka through the popular “equal transit time” story. Measure lift and drag, resolve the force vectors, and treat vortex lift as a hypothesis to test.
🎯 Ngā Whāinga Akoranga | Learning Intentions
Why the popular Bernoulli “equal transit time” account of lift is wrong, what pressure differential and flow turning actually describe, sail foil camber, force vectors (lift, drag, net driving force), and windward tacking.
Construct and test a miniature crab-claw sail vs triangular bermudan sail in a wind tunnel/fan stream, measure lift/drag ratio, and resolve force vector components.
🎥 Media Anchor & Pedagogical Scaffold
How Does A Wing Actually Work? — Veritasium
Video (Veritasium): A clear explanation of how wings and sails generate lift through pressure differentials and Bernoulli's principle, including force vector applications to sailing.
🧠 1. Before Viewing (Activate & Predict)
Some researchers have proposed leading-edge vortex lift as an explanation for crab-claw sail performance; others point to camber, angle of incidence, and drag. Treat the delta-wing account as a contested hypothesis, not a settled fact. In today’s model, which measurements could support or weaken it?
👁️ 2. During Viewing (Watch With a Job)
Watch the full 3-minute clip and collect evidence for these aerodynamic principles:
- The account being challenged: What is the popular Bernoulli explanation of lift, and which part of it does this video argue is wrong?
- Lift generation: Why does air speed and sail shape matter for creating lifting force?
- Drag forces: What is the relationship between lift and drag?
- Sailing applications: How do these principles apply to the design of sails and the ability to sail at different angles to the wind?
🗣️ 3. After Viewing & Kaiako Move (Process & Apply)
Kaiako Move: Use a smoke generator/streamer fan set up with paper wing models to demonstrate lift pressure differentials.
Immediate Task: Complete Section 4 of your Navigation Physics Logbook: Sail Lift Vector Resolution & Aero Force Diagrams.
⚡ Whakaoho | Do Now: A Sail Can Drive a Waka Towards the Wind
Two minutes: first write why that sounds impossible. Then have a genuine attempt at why it is not.
Very few people get this fully on the first try. Write your best attempt anyway — you will come back to it at the end of the lesson, after you have measured lift and drag with your own hands.
📖 Activity 1: Test Two Sails, Then Resolve the Vectors
Measure (13 min). Mount a crab-claw sail and a triangular bermudan sail in the fan stream at the same wind speed. Measure the force in the direction of travel at three sail angles each. Six readings, all recorded, including the ones that disappoint you.
Resolve (12 min). For your best angle, draw the force diagram: lift perpendicular to the airflow, drag along it, and the resultant of the two. Then resolve that resultant into the direction the waka actually travels. The driving force is only a component of the total — which is precisely why sailing towards the wind is possible.
📝 Activity 2: Navigation Physics Logbook & Problem Solving (20 mins)
Portfolio Section 4. Submit: (1) six force readings across two sails and three angles with the wind speed stated; (2) a scale vector diagram resolving lift and drag into a driving component; (3) your revised answer to the Do Now placed beside your first attempt, saying exactly what changed your mind.
🏫 Kaiako Planning & Pedagogy Notes
Year 10 Curriculum Alignment: Te Mātaiaho (2025) Science, Phase 4 — Motion and Forces; Matter Interactions and Energy. Integrates traditional Polynesian wayfinding (Mātauranga Waka) with Year 10 physics: mechanics, wave behaviour and optics. Year 10 sits at NZC Level 5, before NCEA.
“The action of forces on the movement of objects can be described using Newton’s Laws of Motion.”Te Mātaiaho (2025) · Science · Phase 4 (Years 9–10) · Motion and Forces (Knowledge)
“Predicting, observing, and measuring how resistance forces affect motion of objects (e.g. rockets, fish, swimmers, cars) in different environments (e.g. air, water, surfaces)”Te Mātaiaho (2025) · Science · Phase 4 (Years 9–10) · Motion and Forces (Practices)