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Old 07-29-2013
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Re: Twist

Quote:
Originally Posted by ctlow View Post
Thanks, Jeff. I've got a few dilemmas in there. Let's say "fairly flat seas but with a bit more wind than 'moderate' for the boat". You're suggesting twist in the foresail but not the main. Did I understand that correctly? The books say that usually both sails will have similar twist.
Ideally, yes you want to match the twist of each sail and keep the slot consistent, but when you are over-powered you might want to break that rule.
Quote:
I'm not sure what "blading out" is. But strangely - and I've come across this in several places and confirmed it with some high-level racing sailors - easing the mainsheet, letting the boom rise and freeing the leech (opening, even though the tack and head are on a shorter line to each other) adds twist but flattens the main. This of course means raising the traveller to restore angle of attack with the boom near centre, whereas you say lower it. I can't quite visualize why that flattens, although I can see how it adds twist. But so be it. It also means having a very tight backstay.

That was one of my original points about "twist": one can flatten and twist at the same time (they say!).
"blading out" is just racer-speak for flattening your sails as much as you can. That means cranking on the outhaul, the backstay and probably the cunningham. Once the sheet is set in upwind trim, increasing backstay tension will flatten the sail and increase twist because the curve of the bent mast stretches the material between the mast and the leech. At the same time the mast tip comes aft, reducing the distance between the clew and the head, reducing leech tension and therefore increasing twist. That is why you will find that after you make a backstay adjustment, you will need to make a sheet adjustment as well.

Quote:
Stronger winds need flatter sails (all other things being equal), and twist. Flatter sails allow more of the sail to be closely aligned with the wind, giving more propulsion and less heel. At least, that's what the books say and how I conceptualize it.
Twisting the sails off in stronger winds is somewhat of a last resort when you have used all of the other de-powering tools and you are still over powered. High performance fractional rigs are "self-twisting" to a certain extent; when a gust hits the unsupported mast tip will allow the mast to bend and temporarily twist the main off at the top.) Flat sails actually give you less propulsion (and less drag) than fuller sails, but flat is good when you have more wind power than you need. Keep in mind that in this context when I say "full sails" I am not referring to baggy old cruising sails, but good sails that haven't been "bladed"!

Quote:
Then there's draft position - we haven't got into that yet.
Draft position is fairly straight forward. Cunningham and halyard tension are draft controls. Typically you want to start with the draft at about 50%. That gives you a nice smooth rounded entry at the luff. Increasing luff tension pulls the draft further forward which will open up the leech. You will find that as you pull on backstay and bend the mast you will have to pull on cunningham to maintain draft position where it should be, hence, the "blading out" procedure will likely include grabbing a big handful of cunningham too!
Older sails usually need quite a bit of cunningham because as sails age, the draft moves aft. If the draft is too far aft, you end up with a very flat entry on the sail which is very hard to steer to, and at the same time the leech is excessively rounded.

It is difficult to get used to visualizing draft position, which is why many racing sails have "draft stripes" on them as a visual aid. Sighting up the sail from under the boom helps too.
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