quote:
Originally posted by howley
Steve,
quote:
If twinskin foils in low air speeds are so efficeient why are they not used in racing yachts and windsurfer sails?
I just thought I'd let you know that many yacht and windsurf sails are twinskin. Most of the new Formula Windsurfing sails are about 30% twinskin. C class Catamarans are now nearly all using twinskin sails, and moths are moving that way too. The reason they are not in general use is they are expensive and complicated. They do have andvantages though, eg. they are super efficient.
See
http://www.moth.asn.au/news/wa_2001_nov.html
www.team-invictus.co.uk/Team-Invictus/Invictus2/invictus-2.htmhttp://
Luke
G'day Luke
The Yellow pages cat that holds the speed record used a twin skin sail ( I think?), and the earlier version Jacobs Ladder used twin skin Flexifoil power kites stacked together.
The Moth sail looks like it becomes a single skin effectively when sheeted in hard, and looks extremely impractical for everyday use eh?
The Invictus cat team link is a further development of the yellow pages concept, and it looks like that for outright speed records this is the direction a lot of sailors will head for.
The point I picked up on with dachopper was that he was using false or assumptive logic to claim that bridled parafoil kites were inherently more efficient than single skin LEI designs, because of the ratio of flat to projected area being smaller for that type of kite, which is simply not true.
I know from many years trying to as fast as possible with kites on a variety of surfaces that parafoil kites are efficient, especially at higher wind speeds, but so are single skin kites. Big parafoils, and all big kites used for light wind, have practical limits forced upon them by the availability of current technology. Turning these beasts has always been a major issue, and must be factored into any efficiency claim,
Like I have said above we do not want just outright speed as kitesurfers (thats the overwhelming majority of us). Nor do we want abolute minimum light wind performance, its all about 10 to 30 knots or so, isn't it?
At the end of the day, we are using kites, not sails or high speed aircraft wings, even though there are some similarities between all three.
At the very extremes of each application of wind and flight, we can find people obsessed with records and speed, but even they have to finely balance drag, with lift, there is no such thing as a 100% efficient wing, is there?
Most efficient, well lets leave that for the record stalkers. The tips of LEI and parafoil sleds are needed, and not solely drag inducing and therefore cannot be judged as 100% parasitic to the efficiency of the kite, which is another point that I don't think dachopper is prepared to accept just yet.
The ratio of flat area to projected area is not an indicator of efficiency, when taken on its own. Cabrinha are very cleverly wording their marketing and are not stating that either, from what I have read. All the claims I have read are in relation to a "classic arch style kite" or something to that effect. The inclusion of the word classic has me thinking they are talking about an old and virtually obsolete design of LEI, also invented by Bruno.
I think the depower/wind range claims about this kite design are the most interesting thing I want to test. Depower and wind range still do not make the design the most efficient though.
Cya and
Goodwinds
Steve McCormack
