Print Page | Close Window

Sail Twist

Printed From: Yachts and Yachting Online
Category: General
Forum Name: Beginner questions
Forum Discription: Advice for those who are new to sailing
URL: http://www.yachtsandyachting.com/forum/forum_posts.asp?TID=4787
Printed Date: 23 Apr 21 at 8:03am
Software Version: Web Wiz Forums 9.665y - http://www.webwizforums.com


Topic: Sail Twist
Posted By: ellistine
Subject: Sail Twist
Date Posted: 21 Oct 08 at 9:57am

Is an open or twisted leech a good thing or a bad thing?

I had always assumed it wasn't good but can it used for depowering? When we've been overpowered I usually put on lots of kicker and cunningham but that would straighten the leech when perhaps we're supposed to be letting the wind spill out?




Replies:
Posted By: JimC
Date Posted: 21 Oct 08 at 10:08am
This sort of thing is really very boat specific.

Lasers, for instance, are special cases and have quite different treatment for depowering than most other craft...


Posted By: ellistine
Date Posted: 21 Oct 08 at 10:22am

Laser Vago in this case.



-------------


Posted By: Chew my RS
Date Posted: 21 Oct 08 at 10:29am

In general you want an untwisted leach so that the air flows across the sail in a uniform way at all heights up the mast. In light winds some twist can be good, as the wind gradient means that the angle of the wind is different at different heights (due to friction with the rotating earth). Therefore twisting the sail to suit the wind direction maintains the uniform flow. You are right, twisting the sail in strong winds is one way of spilling excess power, but it is not generally the best way.  It is usually faster to flatten the sail as much as possible (lots of kicker, downhaul and outhaul), lift the daggerboard, hike like hell and ease the sails if needed.

In my experience it is better to have a flat, flogging sail than a full twisted sail (it is not really possible to have a flat twisted sail, 'cos the kicker must be eased to get twist, but easing the kicker straightens the mast and therefore powers up the sail). 

I've not sailed a Vago, but you have an untapered (and therefore stiff) mast I think.  This probably makes it hard to get enough mast bend to flatten the sail fully by pulling on the controls. 



-------------
http://www.sailns14.org - http://www.sailns14.org - The ultimate family raceboat now available in the UK


Posted By: ellistine
Date Posted: 21 Oct 08 at 10:43am

So if the mast is stiff then would twist be better suited it we're never going to get the sail particularly flat?



-------------


Posted By: JimC
Date Posted: 21 Oct 08 at 10:44am
> (it is not really possible to have a flat twisted
> sail, 'cos the kicker must be eased to get twist, but
> easing the kicker straightens the mast

Note for other folk, this is not really the case with modern full batten/large roach stable mylar rigs, where the downhaul is the key control for bending the mast. Different rigs really do march to quite different drummers.


Posted By: Chew my RS
Date Posted: 21 Oct 08 at 11:34am
Originally posted by ellistine

So if the mast is stiff then would twist be better suited it we're never going to get the sail particularly flat?

Maybe, but only as a last resort.  Its still probably better to get the sail as flat as possible and let the main out. 

And, of course, Jim's right. Rigs with full length battens and large roaches work differently. 



-------------
http://www.sailns14.org - http://www.sailns14.org - The ultimate family raceboat now available in the UK


Posted By: ellistine
Date Posted: 21 Oct 08 at 11:41am

Thanks all.

At Rutland at the weekend (very windy), while beating the wind was hitting the back of the sail so hard that it was popping the battens the wrong way.

Presumbaly in this case the sail was no where near flat enough?



-------------


Posted By: JimC
Date Posted: 21 Oct 08 at 12:02pm
Originally posted by ellistine

Thanks all.


At Rutland at the weekend (very windy), while beating the wind was hitting the back of the sail so hard that it was popping the battens the wrong way.


Presumbaly in this case the sail was no where near flat enough?



Sometimes that's inevitable if its windy enough, but the more you flatten the better off you'll be, especially at the top. Its a fairly conventional sort of UK rig with short battens lower down and long ones at the top. Tuning guides for other boats with similar rigs would be worth reading if there are no Vago ones available. I'm not the right person to advise on setting up such rigs.


Posted By: Jamesd
Date Posted: 21 Oct 08 at 2:57pm
Originally posted by ellistine

At Rutland at the weekend (very windy), while beating the wind was hitting the back of the sail so hard that it was popping the battens the wrong way.

Presumbaly in this case the sail was no where near flat enough?

It probably wasnt flat enough. but i know in some classes if you over tighten the kicker the batterns will invert. so perhaps it needed to be eased a few inches.

also when dropping the mainsheet the jib should be going down as well to stop the front of the main backing. it keeps the slot open as well. Its far faster in those conditions in all venues (well all the ones ive visited) to keep the bow down ease jib and main together and power on through the gusts and watch the guys that pinch and rag their sails go backwards. only problen is that it is seriously physically demanding.

btw this is just something that ive found in the RS classes, it does vary as Jim has said between boats, and sailors



Posted By: ellistine
Date Posted: 21 Oct 08 at 3:05pm

I like the idea of the crew working the jib sheet. Why should I have to do all the hard work

I'll give that a go too.



-------------


Posted By: G.R.F.
Date Posted: 21 Oct 08 at 3:45pm
Sails should twist to work efficiently.

Now before y'all start citing this class and that class, all boats do
something else, they heel and therefore spill wind off the head.

Whereas in that more sail efficient world that I have always existed and I
note will come to pass in the future with foiling moths, the rig is canted
over to weather, so the sail must work automatically and efficiently to
ensure continued power and no stalling action so twisting off at the head
or what became known as 'floppy leech' worked out to be very very much
faster than sails that didn't twist and remained tight leeched and more
stable.

The reason is exactly the same as having a tuned exhaust on a race car,
get the fuel in extract the power and get it out, so it is with air, use it and
exhaust it before it turns into something else.

I've also been told down the years by everybody that knew anything about
dinghies, that 'twist' and the way twist occurs is very important, so if you
can adjust your rig so the lower half remains powerful with a tightish
leech, then the further up the leech the more it 'twists' off in the puffs,
the better it is.

But equally I've also found that the way the jib reacts and the gap that
brings a venturi effect of speeding the airflow over the main, without
backing the leading edge of the main is also of paramount importance to
an efficient and fast rig, that bit I found out for myself, the hard way and
the jib itself is very inefficient in that it has a propensity for stalling the
flow rather than twisting off at the head, which in turn adds to the
turbulence over the main, another reason not to have it sheeted too hard
in a breeze.


Posted By: ellistine
Date Posted: 21 Oct 08 at 3:54pm

We've always sailed with the jib hard on when going up wind pretty much regardless of wind strength.

It sounds like it's more sensible to let it off a little to depower and reduce the main backing when it too is let off a little?

All these new things to try. I'll have to right myself a list to take out with us.



-------------


Posted By: Chew my RS
Date Posted: 21 Oct 08 at 4:09pm

On some classes (RS400 for example) the boat is EXTREMELY sensitive to jib sheet tension.  Always let the jib out if you let the main out. 

BTW GRF, the jib slows down the air flow over the main, not speed it up.



-------------
http://www.sailns14.org - http://www.sailns14.org - The ultimate family raceboat now available in the UK


Posted By: tgruitt
Date Posted: 21 Oct 08 at 4:39pm
Originally posted by Chew my RS

BTW GRF, the jib slows down the air flow over the main, not speed it up.



Are you sure? I was led to believe it was the venturi effect?


-------------
Needs to sail more...


Posted By: G.R.F.
Date Posted: 21 Oct 08 at 4:56pm
The tendency of the jib is very similar to the lee bow effect you get from
another boat ahead and to leeward of you.

Too tight and it will 'head' the main which backs it. Even I know that
much, but still made that mistake early on because at the end of the day
the crew operates it and when it's windy we were more intent on
surviving, we're past all that now.

But it is a fact that the slot between the jib and the main, and this intel
goes back to Scorpion times, actually venturis the airflow as well as
slightly heading the main sail hence why you do all your pointing with the
jib leading edge and it those tel tales you watch.

Balanced properly a two sail dinghy rig is really acting as one unit as far
as propulsion is concerned, but if you get the balance wrong you are
effectively creating an air brake, which done wrong produces all those
parasitic drag things I crack on about.

We most of us these days with these modern boats are over canvassed
more often than not, so getting the depower right is very important and I
wish I could sort the magic rig formula, the controls open to us are so
over complicatedly useless compared with what i"m used to.

Bang the Gnav on, it will tighten the leech, but it also now bends the
mast forward and flattens the sail unless you tighten the lowers which
then prevents the cunningham from bending the mast to twist the top off
because the jib tension will bend the top of the mast ahead of the bottom
unless you then tighten the upper shrouds which then rakes the mast
back and it doesn't point as well... it goes on.

We've got a new main now, a good metre and a half smaller, but with very
little twist at the head, I've not used it in anger yet, but it'll be ages
before I work out what to do with it and when, life's a lot simpler for a
windsurfer with just two controls, downhaul and outhaul.


Posted By: ellistine
Date Posted: 21 Oct 08 at 5:18pm

Originally posted by G.R.F.


Bang the Gnav on, it will tighten the leech, but it also now bends the
mast forward and flattens the sail unless you tighten the lowers which
then prevents the cunningham from bending the mast to twist the top off
because the jib tension will bend the top of the mast ahead of the bottom
unless you then tighten the upper shrouds which then rakes the mast
back and it doesn't point as well... it goes on.

Could you break that down a little? Our shrouds and lowers are still on factory settings. The lowers are if anything quite loose. What effect does the lowers tension have on sail shape?  I was as interested as you with the explanations of the adjustments made on some of the boats in the Endeavor Trophy. Somebody mentioned they moved the spreaders foreward and out. I can't work out in my head what that would do.

It's all interesting stuff. Keep it coming!



-------------


Posted By: AlexM
Date Posted: 21 Oct 08 at 5:33pm

Sail twist (wind 25-30knts)

 



-------------


Posted By: ellistine
Date Posted: 21 Oct 08 at 5:41pm
Originally posted by AlexM

Sail twist (wind 25-30knts)

So was that by accident or intentional?



-------------


Posted By: AlexM
Date Posted: 21 Oct 08 at 5:46pm

intentional. (i hope)

It was very windy and some big waves, so the rig tension was let off making the jib slot open up.   we had max kicker and loads of cunningham opening up the top of the main sail but notice the main was still brought to the middle.  Made us go very fast that day



-------------


Posted By: ellistine
Date Posted: 21 Oct 08 at 5:53pm

I think this is where it gets confusing. I would've expected max kicker to tension the leech and therefore stop the sail twisting.

I suppose the cunningham would flatten the sail at the top allowing it to twist.



-------------


Posted By: Chew my RS
Date Posted: 21 Oct 08 at 5:58pm
Originally posted by tgruitt

Originally posted by Chew my RS

BTW GRF, the jib slows down the air flow over the main, not speed it up.



Are you sure? I was led to believe it was the venturi effect?

Apologies for taking this of topic, but yes, I'm absolutely certain.  Take a look at these for a thorough explanation of the process:

http://www.arvelgentry.com/magaz/Another_Look_at_Slot_Effect.pdf - http://www.arvelgentry.com/magaz/Another_Look_at_Slot_Effect .pdf

http://www.arvelgentry.com/techs/A%20Review%20of%20Modern%20Sail%20Theory.pdf - http://www.arvelgentry.com/techs/A%20Review%20of%20Modern%20 Sail%20Theory.pdf



-------------
http://www.sailns14.org - http://www.sailns14.org - The ultimate family raceboat now available in the UK


Posted By: ellistine
Date Posted: 21 Oct 08 at 6:18pm
Originally posted by Chew my RS

Originally posted by tgruitt

Originally posted by Chew my RS

BTW GRF, the jib slows down the air flow over the main, not speed it up.



Are you sure? I was led to believe it was the venturi effect?

Apologies for taking this of topic, but yes, I'm absolutely certain.  Take a look at these for a thorough explanation of the process:

http://www.arvelgentry.com/magaz/Another_Look_at_Slot_Effect.pdf - http://www.arvelgentry.com/magaz/Another_Look_at_Slot_Effect .pdf

http://www.arvelgentry.com/techs/A%20Review%20of%20Modern%20Sail%20Theory.pdf - http://www.arvelgentry.com/techs/A%20Review%20of%20Modern%20 Sail%20Theory.pdf

I might come back to them in a year or two's time



-------------


Posted By: G.R.F.
Date Posted: 21 Oct 08 at 9:34pm
Originally posted by Chew my RS

Originally posted by tgruitt

Originally posted by Chew my RS


BTW GRF, the jib slows down the air flow over the main, not speed it
up.


Are you sure? I was led to believe it was the venturi effect?


Apologies for taking this of topic, but yes, I'm absolutely certain.  Take
a look at these for a thorough explanation of the process:


http://www.arvelgentry.com/magaz/Another_Look_at_Slot_Effect
.pdf
- http://www.arvelgentry.com/magaz/Another_Look_at_Slot_Effect
.pdf


http://www.arvelgentry.com/techs/A%20Review%20of%20Modern
%20Sail%20Theory.pdf
-
http://www.arvelgentry.com/techs/A%20Review%20of%20Modern%20
Sail%20Theory.pdf



Lot of interesting stuff there and probably relevant at the time, but things
have long moved on since then, and, the jibs and mains I have been
exposed to react differently. Now, this could get very very technical, a lot
of what that theory debunking the original venturi theory I believe was
probably correct, but it doesn't explain, nor take into account the fact
that inorder to function, the main HASto have a faster airflow over
the leeside in order to function. And it's my theory it doesn't slow the
airflow over the main, it deflects it and the balance is drawn through the
gap which evens out the pressure, but obviously speeds and 'heads' the
main (Drawing needed here).

Back then they were still tied up in Bernouilli, fretting about boundary
layer separation, AND they were stuck with traditional cloths that by
their very nature had tight trailing edges that induced massive tip
vortices, and the speeds and apparent wind effect was a much much less
part of the equation.

I accept the point you and they are making, and agree my view was
couched in the original theory debunked in that article, but it has always
been a black art, with theory countering theory.

I'd agree with the jib sheeted in, there would be no increased airflow
between the two foils, particularly with full jibs with overlap (Our current
jib is a self tacking affair so the gap is bigger)but, ease the sheet increase
the gap and that airflow will increase and so will the efficiency of the
main.

I'm minded to set up a bit of a test with those hand held anemometers, it
would be an interesting experiment.

We know, from sailing tandem windsurf boards, that the aft sail
experiences huge pressure drops if the lead sail is sheeted in, both sails
have to be balanced exactly so there is an airflow between them before
the sailors can for example hook into their harnesses.

So it is only when the airflow between the sails is restored that they work
anything like their maximum, but it is also true that neither sail feels or
works like it would if sailed singly, so the whole becomes less than the
sum of its parts. Which kind of makes both theories true.


Posted By: PobodysNerfect
Date Posted: 21 Oct 08 at 11:20pm
Basic Theory:
Twist = power (unless you have a huge amount and all the wind spills out of the top of the sail, called scandalising the main)
Flat = speed

In light/medium winds, you want more power, and hence a deeper, more twisted sail. This is especially true when sailing in choppy waves.

In fully powered up conditions, you want more speed (flattish sail), and in heavy winds you want less power (flat sail).

It was also mentioned earlier in the thread that cracking off a bit and easing sails gives you a lot more speed and stability than pointing high. You'll end up not losing much to leeward because you'll be making a lot less leeway, and you'll go a LOT faster. Also raising your centreboard a fraction can help (especially if you rake the mast back in heavy winds), as can dropping some rig tension in very windy conditions.

In a perfect world, you'd always have a perfect amount of power. This never actually happens, so you have to be constantly asking yourself one question: am I searching for power, or am I trying to lose power? Once you have answered this question you can begin to take action by adjusting sail shapes etc.


Posted By: redback
Date Posted: 22 Oct 08 at 8:43am

I'll try and keep it simple.  Ignoring drifting conditions you sail in 2 basic wind conditions - you are either under-powered or over-powered. 

Under-powered requires a rig set-up that produces maximum force and if it produces a lot of heeling force that is of little consequence because you can always hike a bit harder.  You therefore want full sails sheeted as close to the centre line as they will go.  Too close and they will stall especially at the top so some twist is good and you control that with mainsheet tension.  No kicker or downhaul will be required with a conventional rig.

Over-powered requires a rig that minimises heeling force (you still want as much forward force as possible).  Flat sails produce less power and less drag so apply kicker to bend the mast but this hooks the leach so apply downhaul which opens the leach.  Open the slot too by moving the jib fairleads out or back.

You could of course lose power by setting the sail up badly by not appling any kicker or downhaul but you then have a sail which flogs.  A flogging sail produces a lot of drag and is "snatchy" (either on or off) which is very difficult to control.

So in strong winds you want flat sails and if you apply a lot of kicker and downhaul you'll see the top batten go almost straight meaning that the top of the sail is producing hardly any power (and hence hardly any heeling force).  And because the sail moves in and out like a barn door it is easy to modulate hence you can easily increase and decrease the heeling forces that fluctuate so much in the turbulent air of strong winds.  Incidentally a bit of twist in inevitable in strong conditions.



Posted By: NickA
Date Posted: 23 Oct 08 at 11:09pm

Sheeted as close to the centre line as possible?  Are you sure?  Won't that hook the lower part of the sail?  Surely that can't be fast.

I've always thought I should use the kicker to bring the leech close to parallel with the boat centre line and leave the boom away from centre to maintain that shape a the bottom of the sail  - and if it's really windy, let off the lowers so that the kicker also bends the mast and flattens the sail.

Anyway - with the Vago specifically, the mast despite being straight section, is really quite bendy; it manages gust response by having a fat headed main which causes the top of the mast to bend off more than the rest of it in gusts - ie the mast bend characteristic is determined by the sail shape rather than by the mast section, so you can use lots of kicker in confidence. 

Quite the contrary was my old laser 2 "fun" that had a very stiff straight section mast (more like a scaffold pole). All applying kicker did was pull the leech in without flattneing the sail, which made the sail more powerful not less. Not good.

So on a Vago in lots of wind, let off the lowers, apply lots of kicker and as Redback says, apply some cunningham too to open the leech a bit more. You could also reduce the batten tension a bit before putting the sail up. 

The jib should be in tight and you can head up wind to de-power, a loose jib will flog and pull you sideways without driving you forwards.

... and AlexM :  I'm intrigued by your letting rig tension off in a strong wind.  How does that work?    I've been piling ON rig tension in a blow to stop the jib hogging and induce some mast pre-bend.  Then making the slot wider by raking the mast back, which also has the advantage of twisting off the jib leech to dump a bit more power. (irrelevant on a vago as it can't handle rig tension anyway!)



-------------
3604 ...lapse of reason
Javelin 558


Posted By: redback
Date Posted: 23 Oct 08 at 11:40pm

Well you can't quite get the sail on the centre line.  On a 4000 we use a strop to get the boom as near as we can - which proably puts the lower leach more or less parallel to the boat axis.  You have to remember that a fast boat like a 4000 can be beating at 8 or 9 Knots in a 10 Knot breeze so that'll mean the apparent wind is only about 25 degrees off the bow - so the sheeting has to be narrow compared to say a Laser which is not going to get above 5 Knots.

It very important not to hook the leach at the top which is why the kicker isn't used and its controlled by the mainsheet tension which can be continually hardened or eased acording to pressure.  If the top does hook the sail stalls and the rudder suddenly goes dead.

I might just mention something which at first seems counter intuative.  On faster boats the increased speed which come from increased pressure is worth taking advantage of.  So unlike slower boats which luff when over powered and thus gain hieght.  The faster boats drive off in the gusts and just go faster.  We actually have to ease the jib a bit.

But I was trying to keep it simple.



Posted By: NickA
Date Posted: 24 Oct 08 at 2:36pm

Understood. 

Unfortunately it just ISN'T simple is it?

The next time "moderate" conditions occur I shall go out and try beating up and down the lake with no kicker on. It will be good for my arm strength if nothing else!



-------------
3604 ...lapse of reason
Javelin 558


Posted By: G.R.F.
Date Posted: 24 Oct 08 at 3:41pm
I can tell you, from very bitter experience beating with no kicker in strong
wind is a very unpleasant experience especially when you have to do it for
the best part of 10 mile or so. Everything redback said is true.


Posted By: Merlinboy
Date Posted: 24 Oct 08 at 4:24pm
When our Gnav exploded in the 14 i had to sail it with no kicker it killed my gloves in no time and took all the skin off my fingures.  I had to Duck tape them up for the rest of Abersoch dinghy week.  I also could barelyh lift my arms after a further hour on the water!

-------------


Posted By: redback
Date Posted: 24 Oct 08 at 7:28pm

OK a bit more detail about the kicker.  I'm talking about under powered conditions.  The kicker goes slack when you apply maximum mainsheet tension but there has to be something to stop the sail twisting right off when you bear away at the windward mark.  So you need enough kicker to stop excessive twist on the reach but not so much that you're going to bend the mast and/or hook the leach when beating.  So my judgement is you should see all the weight go off it when you sheet in and then you can play the leach tension when you're going upwind.  The idea is to keep the boat powered up to the maximum that you can so keep bringing the leach up tighter until you see the top tell tale stall then ease and start the cycle all over again.  That way you're keeping the sail at its most powerful. 

As soon as you are over powered its a different ball game, then you start using the kicker to de-power the rig and keep the boat from heeling and incidentally save your arms from having to take all the leach tension through the mainsheet.  Without kicker in those conditions the mainsheet will be unbearably heavy in a boat like a 4000 and the sail will rag causing a lot of drag.




Print Page | Close Window

Bulletin Board Software by Web Wiz Forums® version 9.665y - http://www.webwizforums.com
Copyright ©2001-2010 Web Wiz - http://www.webwizguide.com