Mike and Doug,
My junk rig has an airfoil shape cut into the panels, with a draft of 1/3 the chord length. A junk rig with an airfoil shape is essentially the same as a big, modern square top mainsail. The only difference really is that the sail does get pushed around the mast on one tack....
But think about this. On the "favoured" tack with the mast to windward, the sail will set a great shape, but the mast will be exposed to the wind and creating drag. On the so called less favoured tack, the mast is hidden in the shadow of the sail, and creates very litle drag. So in actual fact, there is no noticable drop in performance due to the contact between the sail and mast. Sure it looks a little weird, but who really cares?
Also, the leading edge of a sail is very sensitive to turbulence, and a marconi rig with a mast right at the leading edge of the sail will hurt it's efficiency big time (unless the mast rotates), whereas a junk rig will not.
Regards,
James Brett
--- In harryproa@yahoogroups.com.au, Mike Crawford <mcrawf@...> wrote:
>
> Doug,
>
> From what I can see on Brett's NZ proa, he's got a standard
> single-skin junk rig that doesn't create an airfoil shape. If that's
> true, the sail would indeed be pushing against the mast on one shunt and
> pulling away on another.
>
> The swing-wing is a different design, though, with double-sided
> wishbones that create a sail with two skins.
>
> The way the wishbones work is that the leeward edge of the sail has a
> nice smooth curve to it, while the windward side gets the kink. This is
> less than ideal, but also still a better airfoil than a single skin
> sail. For each shunt, the leeward surface is smooth.
>
> To get a better idea of how it works, check out these photos:
>
> Nice leeward surface for generating lift:
>
> http://api.ning.com/files/OX6wi*XepeSFiOcN2A1LCruxG28UbW2T02hM1KC3mXMkg5J5D*9b2qh2350oH2Rv9oBLKC3AuWV9WVK3HiLPURXItsWuRpJb/178f2009042200PHAwithitsnewdoublewisbonesintandem.JPG
>
> Corresponding kinked windward surface:
>
> http://api.ning.com/files/8ZicrvcPCMAFxbwHBScmeROW*xJLvh1BnaXKVECpl9rETD3muuQn3pqV0nlR3LNyk6JoZyiD-TBUivh0Bmsl8yxjrNX*bsNh/178gUKJRA0002.JPG
>
> Interior (to see how things might push against the mast):
>
> http://api.ning.com/files/-1yR27opppagPAzdnm*JUW1PwlPrg*WP4CHraDW*Yptmb-pI6CheiXxsLX2itDcZSVU3AbJyOktv7NdXUO*0KzkHniwieOY0/176b200806010037.JPG
>
> - Mike
>
>
>
>
> Doug Haines wrote:
> >
> >
> > For the unbelievers, I might say that the thing looks like a pile of
> > crap - but somehow seems to go alright (James Brett's NZ proa on
> > youtube) and get's favourable comments.
> >
> > How can the sail have good curving when it has to go on one side of
> > the mast - which looks ok one one tack/shunt, but then it pushes into
> > the mast and must make a huge kink in sail shape when on your other
> > tack(shunt)?
> >
> > The battens would be pushed up hard against the mast (like sail
> > against spreaders on normal boat). The tack with the wind cxominjg
> > from ......need drawing.
> >
> > Wondering if we could infiltrate Sailing Anarchy, just so as to use
> > more photo uploading things
> >
> > DOug
> >
>
