Basic Aerodynamic Sailing Theory Tutorial

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2. Basic Aerodynamic Theory
Q: How a sail works?

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. How does this make the
dinghy move?

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. Pressure
Ambient
pressure Ambient
pressure
Low
LIFT
pressure

7. Pressure differences around
the sail:-
Ambient
pressure
Low
Low pressure
pressure
Ambient
pressure

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. 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. Sail setting – Points of sail

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. Trim, Balance & Heel
Pressure of
wind on
the sail/s
+ wind on
the hull
= Heel

13. Balance - How to stop heel?
•Helm & crew weight
•Ease out sails
•Reef sails
A flat boat is a fast boat!

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. 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.

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