basic aerodynamic theory - how a sail works. Dinghy sailing, Keelboat, Yacht.

1. BASIC AERODYNAMIC THEORY
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▸ 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.

3. HOW DOES THIS MAKE
THE SAILING BOAT MOVE?

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AIR ON THE LEEWARD SIDE HAS 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 AIR SPEED CREATES PRESSURE DROP ON THE
LEEWARD SIDE WHICH EFFECTIVELY ‘SUCKS’ THE SAIL TO LEEWARD

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PRESSURE:
Ambient
pressure
Ambient
pressure
LIFT
LOW
PRESSURE

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PRESSURE DIFFERENCES AROUND THE SAIL:
Ambient
pressure
Ambient
pressure
LOW
PRESSURE
LOW
PRESSURE

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FORCES ON THE SAIL:
TOTAL
FORCE
LIFT
DRAG
What is Lift?
▸ The force that pulls the
sail to leeward
▸ The combination of lift
over every part of the
sail’s surface is called the
centre of effort.

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WHAT CAUSES DRAG?
▸ Friction on the sail, hull and daggerboard/centreboard/keel
Which of the 4/ 5* Essentials will help us reduce drag?
▸ Centreboard/Daggerboard*
▸ Trim
▸ Balance
▸ Sail Setting
* = Dinghy Sailing

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SAIL SETTING / POINTS OF SAIL

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WHAT DOES THE CENTREBOARD / DAGGERBOARD / KEEL DO?
▸ The pressure of the wind on the sail alone causes the boat
to travel sideways instead of forward, therefore you need
the centreboard / daggerboard / keel to gain effective
forward motion.
▸ The aerodynamic and hydrodynamic forces combined
partially cancel each other out, resulting in the ability to
sail in almost any direction except straight into the wind.

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TRIM, BALANCE AND HEEL
▸ Pressure of the wind
on the sail/s + wind
on the hull = Heel

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BALANCE - HOW TO REDUCE HEEL ?
▸ Crew Weight
▸ Helm Weight*
▸ Ease out sails
▸ Reef Sails
▸ Remember a flat
boat, is a fast boat!
*Dinghy

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SAILING DOWNWIND
▸ In this instance, the sails are
merely set to trap the air as
it flows by, thus being
‘aerodynamically stalled’
▸ In stronger winds,
turbulence created behind
the stalled sails can lead to
aerodynamic instability.
This can create increased
risk of downwind rolling &
potentially an accidental
gybe.
TURBULENCE
WIND

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TRUE VS. APPARENT WIND
▸ 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
the apparent wind will be
forward of the true wind.

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ONLINE COURSES AVAILABLE
AT: MARINE-EDUCATION.CO.UK
Marine Education - RYA Training Centre
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