Sailing Aerodynamic theory

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Basic Aerodynamic Theory Q: How a sail works?

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 yoursail, the wind has to split at the sail’s front edge and flow down each side of the curved ‘wing’ shape.

How does this make the dinghy move?

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.

PressureAmbientpressure Ambient pressure Low LIFT pressure

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

Forces on the sail What is lift?•The force that pullsthe sail to leeward.• The Liftcombination oflift over every Total forcepoint of the sail’ssurface is called Dragthe ‘centre ofeffort’.

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

Sail setting – Points of sail

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 forcescombined partially cancel each other outresulting in the ability to sail in almost anydirection except straight into the wind.

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

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

Sailing downwind• In this instance, the sailsare merely set to trap theair as it flows by, thus being Turbulence‘aerodynamically stalled’.• In strongerwinds, turbulence createdbehind stalled sails can leadto aerodynamicinstability, which can createan increased risk ofdownwind rolling of thedinghy.

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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Sailing Aerodynamic theory

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