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  Sail shape and Draft position (bosco)
Posted: 11:28:17 am on 12/21/2014 Modified: Never
 
 It has been thrilling to watch the Volvo Ocean racing unfolding day by day --- (Download the Volvo Ocean Race App. if you'd like to stay updated with positions, neat videos etc. etc.).    

One thing I have observed is that these boats all seem to carry the deepest part of their sails' drafts ( both main and genoas ) WELL  forward of the 50% of chord length point --Much further forward than is my practice. Is it their sails' construction/material --Their speed--or some other factor that causes them to do this -- apparently to advantage ??

Any ideas/comments?
Clare Jordan.   Aragorn


 
 
  Re: Sail shape and Draft position (davidww1)
Posted: 12:39:33 pm on 12/22/2014 Modified: Never
 
Until I was 16, I belonged to a club for kids like me and for adults who build flying model gliders and powered planes. We kids built gliders with a relatively thin wing section of low aspect ratio with the greatest depth of camber at about 45% of the chord (translation: short, wide wings whose thickest part was about 45% aft). Our gliders were relatively heavy and robust, and not terribly aerodynamic.

We had a few hotshots in the club who built FAI competition gliders - superlight, highly optimized craft that flew so efficiently that they had timed de-powering devices to prevent them from disappearing over the horizon. Their wings were very high aspect ratio, usually negative camber on the underside, with the maximum upper camber a bit farther forward than the "recreational" craft we kids flew.

I see our 27's (and most contemporary racer/cruisers) as roughly paralleling the recreational gliders: a moderate-drag, moderate-lift wing supporting a fuselage that is relatively high-drag (not easily driven), whereas the competition gliders parallel the Volvo boats and others in the rocket-ship class: higher-drag but much higher-lift sails (which is what I think you get draft-forward) on a very low-drag hull.

The Volvo boats sail very much closer to the speed of the wind, but I don't know enough aerodynamics to conjecture about the impact on desired sail shape.

And speaking of conjecture, this is all conjecture (a nice word for guessing) based on long-ago observation, so if anyone has any better info, I like Clare would be glad to hear it.


David Weatherston
Towser, Toronto
C&C 27 Mk IV