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Sailing Aerodynamic theory

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Sailing Aerodynamic theory

  1. 1. Another tutorial bought to you by www.instructorresources.co.uk
  2. 2. Basic Aerodynamic Theory Q: How a sail works?
  3. 3. When your sail is flapping like a flag, the wind passes down each side of it without being diverted from its course. When you shape your sail, the wind has to split at the sail’s front edge and flow down each side of the curved ‘wing’ shape.
  4. 4. How does this make the dinghy move?
  5. 5. Air on theleeward sidehas to travel a longer distance in the same time as the air passing the inner surface; therefore it moves over the outer surface faster. This difference in airspeed creates a pressure drop on the leeward side which effectively ‘sucks’ the sail to leeward.
  6. 6. Pressure Ambient pressure Ambient pressure Low LIFT pressure
  7. 7. Pressure differences around the sail:- Ambient pressure Low Low pressure pressure Ambient pressure
  8. 8. Forces on the sail What is lift? •The force that pulls the sail to leeward. • The Lift combination of lift over every Total force point of the sail’s surface is called Drag the ‘centre of effort’.
  9. 9. What causes drag? • Friction on the sail, hull &centreboard Which of the 5 E’s will help us reduce drag? •Centreboard, Trim, Balance & Sail setting
  10. 10. Sail setting – Points of sail
  11. 11. What does the centreboard do? • The pressure of the wind on the sails alone causes the dinghy to travel sideways instead of forward, therefore you need a centreboard to gain effective forward motion. • The aerodynamic & hydrodynamic forces combined partially cancel each other out resulting in the ability to sail in almost any direction except straight into the wind.
  12. 12. Trim, Balance & Heel Pressure of wind on the sail/s + wind on the hull = Heel
  13. 13. Balance - How to stop heel? •Helm & crew weight •Ease out sails •Reef sails A flat boat is a fast boat!
  14. 14. Sailing downwind • In this instance, the sails are merely set to trap the air as it flows by, thus being Turbulence ‘aerodynamically stalled’. • In stronger winds, turbulence created behind stalled sails can lead to aerodynamic instability, which can create an increased risk of downwind rolling of the dinghy.
  15. 15. Further info: • The air flowing at the sail surface is not the ‘true wind’. Sailing into the wind causes the apparent wind to be greater than the true wind and the direction of apparent wind will be forward of the true wind.
  16. 16. For further tutorials & resources please visit: www.instructorresources.co.uk

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