Sails: Floppy Leech Gone?

[swchandler]

Actually mac, Kevin Kan's photo depicts his sail under dynamic load very well.

Brimar, interesting comments. However, maybe you could pictorially highlight "winglets" in sails. Is it something to do with the features (the cupped areas) in the leech between the battens and the battens ends themselves above the boom?

[BRIMAR]

None of our sails have winglets Per-Se but twist has the same effect as winglets do on a airplane.
Although Bruce did experiment with an actual rotating device at the top of a sail but for whatever reason it didn't work.
I have laid awake many times trying to conceptualize a winglet for a fin and people have done it but they are so fragile that it's impractical.
Something that would spin around or flop over on each new tack!
Truth is that with controlled fin twist (just like a loose leech sails) fins are pretty damn hydrodynamic.

[hammerton]

There are two things going on that we should separate:
1. Loose leech - as in the floppyness an lack of tension
2. Twist - the change of angle of attack going up the sail

It used to be to get twist you had to have loose leech. Modern sails are designed with the twist built in and don't need to be forced to do it via excessive downhaul tension. When you look at the angles rather than the looseness you can see there is still a fair amount of twist.

Twist is important and can be considered as both static and dynamic. By that I mean the static twist is the twist designed into a sail that will be exhibited when the sail is loaded normally. The dynamic twist is that induced by external forces such as gusts and sailing over rough water which causes movement in the system. Bear in mind everything is a compromise, but there is a desired degree of twist depending on the angle of the apparent wind that the sail should be seeking. There is also a desired range of dynamic movement. Too much movement and you are loosing drive and the sail can feel quite unsettled and actually upset the board. Too little dynamic and the sailor has to work harder and has trouble maintaining everything in balance. Overall more twist is more forgiving most of the time than too little.

So I think we as designers are getting better at identifying what we want a sail to do and actually making it do it. Over twisted sails are actually a bit of a crutch that can compensate for stability issues. Now we have more stable sails we don't need such forgiving levels of twist.

Now in the case of GPS vs GP2 we see that the GP2 has more twist than the GPS because it is more forgiving, but it is at the expense of power and continuous drive. The GPS requires more of the sailor but is eeking out every last bit of performance.

The KS3 as an example has low static twist, but with the big roachy head it has quite a lot of dynamic twist. This gives it very good low end drive but keeps it feeling light and reactive. This only works because the sail is very stable.

Now having said all this if you look at any of those sails rigged on the beach you will see almost no tension down the entire leech. BUT as soon as the sail has wind in it and the proper mast deflection the WHOLE of the leech tightens up and much off the looseness is no longer apparent. Generally we keep a little in reserve not to limit the dynamic twist too much so there will still be little bit of looseness right at the top, but nothing like you would have seen 10 years ago.

[BRIMAR]

Got it!
Can you confirm my observation/ theory about the cut away clew and the batten tips aft of the outhaul.
It seems to me that with heavy back hand pressure the batten tips would pressure the battens to flatten out the sail effectively depowering the sail and making it faster and more controllable in one motion

Thanks,
Fastfhucker

[hammerton]

Yes, the cutaway clew effectively moves the tension inboard which helps to lock the draught forward and flattens the exit which reduces drag.

I should also mention that the boards are also driving this trend. Wave boards have gotten a bit bigger and generally people are using smaller sails. Lots of low end power and drive has therefore become more important. It is a similar situation in racing where generally boards are wider and fins are longer than 10 years ago. It may not have the same absolute top speed but it gets you around the course faster 100% of the time. Races are almost never won on top speed, it is usually in the corners and the lulls where positions change. But as is demonstrated in Luderitz modern race sails are not lacking for top end!

[konajoe]

Does every sailing location have 'know-it-alls' who tell folks how to rig sails without having any experience or knowledge of that sail? This discussion is highlighting that there are differences in sail design philosophies, even from year to year on the same sail model. With the availability of smart phones, the best advice someone can give is 'look up your rigging guide'.

I avoid manufacturers who don't have detailed tuning guides online, since I'm not a mind reader.

[Jorionw96]

I have north hero and idol 4 batten sails. After much tinkering I find that you want the leech pretty tight compared to older 5 batten sails. They have amazing low end grunt but do not have as much top end range as my old 5 batten sails. Instead of staying with to big of a sail downhauled to the max I just rig the next size down. When I tried rigging sails with floppy leech they performed terribly. Also they need more outhaul than my old sails to compensate for the reduced downhaul. This is just what I have found works best with my sails. I also notice that all the local freestyle experts have super tight leeches.

[Arrgh]

[airwave]

This conversation indicates to me that.......

We need inflatable battens and luffs.
Oh, yea, and a pump.
And a boom that collapses for travel and storage.
Oh, and a furled foot to adjust for the wind condition.

And most importantly....A clue.
Because we don't have a clue.

[koogzah]

Not so "Intelligent Design"?