Hobie Cat Forums

Can someone explain WHY a sail is trimmed with less outhaul and downhaul in light breezes? Please explain the aerodynamics behind it rather than "...because it makes you go faster!".

Thanks,

Mark

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1989 Hobie 18 Magnum "Rhapsody"
2007 Hobie Tiger

its about lift and attached air flow. in lighter air greater lift is needed so you set a fuller shaped sail to encourage a greater differential in pressure an either side of the sail. there is a limit to fullness where you then disrupt airflow and the air does not stay 'attached' going around the sail, ruining lift.

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Chris
'88 H18SE Arís

The three main attributes of a sail in this context are draft (how much "belly" is in the sail), the position of maximum draft and vertical sail twist.

Tightening the downhaul / outhaul flattens the sail - they reduce draft and "open up" the top of the sail - increasing twist and spilling power from the top of the sail.

They have opposing actions with regards to the position of max draft - the downhaul pulls it forward and the outhaul pulls it aft. In general, you want the draft further forward in light air and further back in heavy air.

A flat sail has reduced power (you don't need it all) and the flow remains attached in higher winds, reducing drag.

A fuller (deeper draft) sail produces more power in lighter air. The lower wind velocity flow can stay attached to the greater curve, also reducing drag.

Think of the difference between the wing sections of a Cessna 172 (thick & round for slow speed) vs. a supersonic jet (almost knife-like). Same principle, just exaggerated.

Sailing is based on Bernoulli's Principle. One of the most fascinating aspects is that a sailboat can exceed the wind-speed by almost 50%.

http://ffden-2.phys.uaf.edu/211_fall200 ... basic.html

Hobie-specific information can be found in the "Hobie University" booklet: http://www.hobieclass.com/site/hobie/ih ... HobieU.pdf