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Old 12-12-2015, 14:28   #1
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Leech confustion...

Consider the following:

Tightening the outhaul flattens the lower part of the sail and opens the leech.

Tightening the vang closes the leech.

Seems to be contradictory since both have the effect of straightening the leech and decreasing draft depth.

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Old 12-12-2015, 14:35   #2
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Re: Leech confustion...

I would say yes but, the vang is pulling down on the aft end of the boom, placing more pull on the leach which would be like tightening the leach control line too much. As opposed to the outhaul which is removing draft by pulling the clew out to the aft end of the boom.
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Old 12-12-2015, 16:52   #3
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Re: Leech confustion...

Tightening the outhaul doesn't straighten the leech. It straightens the foot.
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Old 12-12-2015, 23:36   #4
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Re: Leech confustion...

It works a bit like the genoa sheet and its fairlead. Pull the fairlead sternward and you shift the pull onto the foot. This allows the leach to twist as it has reduced load on it. That's like puling on the outhaul.
Put the fairlead forward and you shift the load onto the leech and this also softens the foot. This is like pulling down the vang to increase the load onto the leech without increasing the force on the foot.
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Old 13-12-2015, 02:38   #5
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Re: Leech confustion...

Quote:
Originally Posted by dlymn View Post
It works a bit like the genoa sheet and its fairlead. Pull the fairlead sternward and you shift the pull onto the foot. This allows the leach to twist as it has reduced load on it. That's like puling on the outhaul.
Put the fairlead forward and you shift the load onto the leech and this also softens the foot. This is like pulling down the vang to increase the load onto the leech without increasing the force on the foot.
Good explanation.

The obvious advantage of the vang/outhaul over the fairlead is that you can adjust both edges independently.
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Old 13-12-2015, 03:25   #6
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Re: Leech confustion...

Wow looks like I have much to learn, all of that sounds like gibberish to me.
It's like listening to a friend of mine talk about his Linux computers, I know just enough to use one but there are lots of words that are alien to my ears.

Where's that book of sailing terms?
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Old 13-12-2015, 06:03   #7
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Re: Leech confustion...

Quote:
Originally Posted by dlymn View Post
It works a bit like the genoa sheet and its fairlead. Pull the fairlead sternward and you shift the pull onto the foot. This allows the leach to twist as it has reduced load on it. That's like puling on the outhaul.
Put the fairlead forward and you shift the load onto the leech and this also softens the foot. This is like pulling down the vang to increase the load onto the leech without increasing the force on the foot.
Thanks for that explanation. I wasn't looking at it from the standpoint of shifting the load on the leech. I was thinking about it schematically, ie tightening the outhaul moves the clew further from the mast which takes slack out of the leech, makes the leech more of a straight line from the head of the sail to the clew, decreases draft depth and "opens" the leech. Adding vang tension would then take even more slack out of the leech, etc but is said to "close" the leech.

Makes more sense to simply say "opening" is allowing more twist and "closing" is allowing less twist.

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Old 13-12-2015, 06:12   #8
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Re: Leech confustion...

Quote:
Originally Posted by dlymn View Post
It works a bit like the genoa sheet and its fairlead. Pull the fairlead sternward and you shift the pull onto the foot. This allows the leach to twist as it has reduced load on it. That's like puling on the outhaul.
Put the fairlead forward and you shift the load onto the leech and this also softens the foot. This is like pulling down the vang to increase the load onto the leech without increasing the force on the foot.
I don't think it is quite like that. When you move the fairlead forward on a jib, you can pull down on the leach more with the sheet. If you just moved the fairlead forward while keeping the exact same amount of sheet distance from fairlead to clew, the leach would twist off, not tighten up. The opposite as you pull the fairlead back.

With a fixed boom, pulling out the outhaul does tighten the leech because the distance from the masthead to the boom increases. Loosening the outhaul has the opposite effect - shortening the leech distance to the masthead and letting the leech twist off.

If one used a vang or the mainsheet/traveller to compensate, then it would act like a jib in that the length would still increase and tighten the leech as you pulled the outhaul back, but that could be taken into account by other controls.

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Old 13-12-2015, 07:39   #9
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Re: Leech confustion...

Quote:
Originally Posted by Salty Mike View Post

Where's that book of sailing terms?
I think there is a glossary of terms (or a link) somewhere on the forum but I don't remember how to access it.

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Old 13-12-2015, 14:14   #10
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Re: Leech confustion...

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I don't think it is quite like that. When you move the fairlead forward on a jib, you can pull down on the leach more with the sheet. If you just moved the fairlead forward while keeping the exact same amount of sheet distance from fairlead to clew, the leach would twist off, not tighten up. The opposite as you pull the fairlead back.



With a fixed boom, pulling out the outhaul does tighten the leech because the distance from the masthead to the boom increases. Loosening the outhaul has the opposite effect - shortening the leech distance to the masthead and letting the leech twist off.

If one used a vang or the mainsheet/traveller to compensate, then it would act like a jib in that the length would still increase and tighten the leech as you pulled the outhaul back, but that could be taken into account by other controls.

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I've highlighted a the critical point. If you adjusted the fairlead position, you wouldn't leave the sheet unadjusted - if you did, then everything would be looser. Dlym was correct in describing how the relative forces on the foot and leech change as you move the fairlead.

The tightening of the leech is minor compared to the tightening of the foot and as with adjusting the sheet after moving the jib fairleads, you will generally adjust the vang or mainsheet after moving the outhaul to compensate for the small change in leech tension
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Old 13-12-2015, 15:16   #11
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Re: Leech confustion...

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Originally Posted by StuM View Post
I've highlighted a the critical point. If you adjusted the fairlead position, you wouldn't leave the sheet unadjusted - if you did, then everything would be looser. Dlym was correct in describing how the relative forces on the foot and leech change as you move the fairlead.

The tightening of the leech is minor compared to the tightening of the foot and as with adjusting the sheet after moving the jib fairleads, you will generally adjust the vang or mainsheet after moving the outhaul to compensate for the small change in leech tension
Oh, I didn't mean to disagree how things work for the jib. I only meant to point out that this cannot be translated to the main - unless you qualify it by saying that after the outhaul is tightened the leech will be put back into position using other controls.

Because the leech definitely does tighten when the outhaul is moved back (this is just simple geometry of a triangle). Our boat has a fixed vang with a single point mainsheet which means a fixed boom height at all times (not the best for sail control, but a compromise). When I tighten the outhaul, the leech definitely tightens.

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Old 13-12-2015, 15:19   #12
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Re: Leech confustion...

The important part of this discussion from a VERY PRACTICAL STANDPOINT is to keep confusion to a minimal. So when one thinks about adjusting leech tension (twist) you must think about using main sheet tension and forget about your vang unless the boom extends beyond the traveler. Then you would set your vang with enough tension to keep the boom from riding up when it extends beyond the length of the traveler and therefore increasing more twist in your mainsail.
Also keep the function of the outhaul in your mind to control depth of foot and do not start playing around with thoughts of leech tension as related to your outhall unless you are on a very long sail and are bored to death.
In windy top end conditions you can flatten the main by not only tightening outhaul but also tightening the vang which will flatten the sail and sometimes allow you in sail in conditions you would otherwise have to reef. Of course I am speaking in the context of racing otherwise you should always reef sooner than later!
In conclusion both vang and outhall can be set up for the conditions you are initially sailing in and will need very little adjustment whereas your mainsheet is much more dynamic.
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Old 13-12-2015, 16:28   #13
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Re: Leech confustion...

I can see that the boom might change the characteristics of the mainsail. I think this happens because the out haul is so short or the main is sliding in a slot for some sails. When you have the fair lead set back on the headsail, there's a longer length of sheet between the clew and the fairlead and this allows the foot of the headsail to lift and give twist at the top of the sail.
The vang stops the mainsail from lifting if it's already tight.
However, the mainsail is not a planar triangle and pulling horizontally on the clew might not be translated into extension of the foot and leech. The out haul force might pull the foot and somewhere diagonally through the sail and this might lead to a looser leach
Mainsails are odd creatures,, Why do we tighten the luff with a Cunningham and not use the halyard?
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Old 13-12-2015, 16:45   #14
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Re: Leech confustion...

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Why do we tighten the luff with a Cunningham and not use the halyard?
In our case, we use the cunningham when we want a situational change and don't want to luff the sail to tighten the halyard more. And it also gives a bit more fine control.

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Old 13-12-2015, 16:53   #15
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Re: Leech confustion...

Also you always want max height.
Pulling the main higher may cause a "little problem" when you pull the head of the main through the sheave.
And as the previous post stated much easier to simply pull the cunningham.
On a well laid out race boat that should be accomplished without having to leave the rail.
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