Jib: luff wire - trim question

Moderator: GreenLake

Jib: luff wire - trim question

Postby GreenLake » Thu Jun 19, 2014 11:46 pm

I have a Jotz jib that has a wire inside the luff. The halyard pulls on the wire and the sail cloth is fixed to the wire with a lashing.

I was sailing last night with the setting sun showing the shadow of the luff wire. I noticed it was not under tension because I could see it making a series of S shapes inside the "hem" of the sail at the luff. When I pulled tight the halyard (single purchase) I could make the curves almost disappear. The luff did have less sag, but I began to wonder:

With a setup like that, what am I aiming for with the adjustments? And does this setup ask for a purchase on the jib halyard?

My mast has a mast-jack, so that there's a single value for the overall rig tension. That seemed about right for the conditions, but I'm open to suggestions how I could verify that. With the luff wire, I should be able to selectively increase the effective forestay tension, giving me back some relative adjustment between forestay and shroud tension.

I've gotten some head wagging-style advice from the local set of dinghy sailors, a somewhat eclectic group with relatively wide-ranging experience in terms of different boats and rigging, but not with the DS or the mast-jack system. So I'm curious what the group here will come up with.
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Re: Jib: luff wire - trim question

Postby swiftsail » Fri Jun 20, 2014 1:17 pm

Hey GreenLake,
The top guys are running about 180 lbs on the forestay and in light to moderate conditions putting just enough tension on the jib halyard to leave a bit of scallops in the leading edge of the jib. I think I'm running about 6:1 on the jib tension system. It's over kill but makes it easy to adjust.

Steve
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Re: Jib: luff wire - trim question

Postby GreenLake » Sat Jun 21, 2014 12:27 am

In the thread on tension gauges I describe the Mast Jack that I use and how it relates to rig tension.

With a regular sail (no luff wire) the halyard would simply adjust cloth tension. If the tension is high, it would offload the head stay, and the sail would sag more, because the cloth, while carrying tension, will stretch more than the wire.

With a luff wire in the sail, I can in principle create tension that is higher than the forestay was set at, without losing the low-stretch characteristics of having a wire holding the tension.

Because I don't have a separately adjustable forestay, to get 180 lbs, I would have to pull that hard on the halyard. The cloth tension would have to be adjusted by re-doing the lashings that hold the sail cloth top and bottom to the luff wire eyes and therefore control the relative tension. (Which is what Jotz told me, but not in language I was prepared to fully understand way back then :) ). However, I would be hard pressed to see scallops - when the luff wire supports the sail, that is with enough halyard tension, the hanks are redundant and the sail is supported by the pocket the wire runs in (like a hem).

With something like a 6:1 purchase, pulling 30lbs would get me 180lbs of tension, so that sounds doable. Except that I would have no way to separately dial the cloth and wire tension. Will have to think about what that means - for competitive purposes, setting up for moderate winds (to light) would cover the majority of our events.
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Re: Jib: luff wire - trim question

Postby swiftsail » Sat Jun 21, 2014 12:12 pm

What I think you should do is, set the length of you shrouds and forestay to such lengths that when you adjust the mast jack to get 180 lbs on the forestay, you also get the proper prebend in the mast for the main that you are using, then you can use the jib halyard just for luff tension.

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Re: Jib: luff wire - trim question

Postby GreenLake » Sun Jun 22, 2014 5:00 am

"set the length" implies "get new ones of proper length" as none of the wires is adjustable in this setup.

In the separate thread on tension gauges, there was a suggestion how to estimate tension without a gauge. Next time I have the mast up, I'll see what tension level my usual setup results in. I'll also observe whether there's detectable mast bend. I'll report back what I find out.
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Re: Jib: luff wire - trim question

Postby K.C. Walker » Mon Jun 23, 2014 10:53 pm

Because your luff wire is quite loose, I think your jib is acting like one without. So your halyard tension is basically your luff tension, up to the point that the wire inside the luff hem it's straight.

Some boats, like the 420, rig loose and then tension the setup with the luff wire, at I believe 2 to 1. I don't think they have adjustable luff tension, though.

If you wanted to tension your rig with the luff wire, and you wanted to have adjustable luff tension, you would need to rig a Cunningham at the bottom to pull on the luff.
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Re: Jib: luff wire - trim question

Postby GreenLake » Tue Jun 24, 2014 2:05 am

Right, K.C.. The implication when I talked to Hank Jotz (before I had enough experience to really process his input) was that adjusting the lashings that hold luff to luff wire eyes was an acceptable compromise. I realize that this would be like setting a ratio, not an independent value, but it seems plausible that high luff tension and high rig tension more or less often go together than not.

I understand Swiftsail's post that in his experience people go for fixed rig tension and then adjust only luff tension. If that is because of the inability (inconvenience) of changing rig tension under way, or whether it confers other inherent benefits, I can't make out from his post.

Now, each sailmaker may build in some expectations into his sail, and Jotz would not have added the luff wire, if it wasn't part of his design... (musing).
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Re: Jib: luff wire - trim question

Postby GreenLake » Tue Feb 16, 2016 6:07 pm

GreenLake wrote:"set the length" implies "get new ones of proper length" as none of the wires is adjustable in this setup.

In the separate thread on tension gauges, there was a suggestion how to estimate tension without a gauge. Next time I have the mast up, I'll see what tension level my usual setup results in. I'll also observe whether there's detectable mast bend. I'll report back what I find out.


I since replaced my stays, and upgraded to the 1/8" ones from D&R. Those come with adjusters. The overall tension would still be set from the mast jack, but the relative length of forestay/shrouds can be adjusted with this system. Time to look a the advice from this thread again and play with those adjusters. (So far, I've only used them to reproduce the same lengths as the existing stays. That succeeded well enough - the boat handles the same - but perhaps due to initial stretch, the mast-jack now requires a few additional turn to get to the same tension I had been using).
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Re: Jib: luff wire - trim question

Postby TIM WEBB » Wed Feb 17, 2016 10:07 pm

Had a somewhat similar initial experience when I replaced the stays on TRW, but with differences (DS2 with hinged mast, no mastjack). Went from fixed length shrouds and a turnbuckle on the forestay to turnbuckles on the shrouds and a lever on the forestay. Knew I wanted a bit more aft rake to the mast, so it was a pretty straightforward process to set up the rig. I pinned the forestay to the top hole on the lever and closed it. Tightened the shroud turnbuckles until the rig was snug, opened the lever, then tightened them more bit by bit until I couldn't close the lever. Then I backed them off until I could close the lever. Haven't measured rig tension or the distance from the masthead to the transom, but the rake "just kinda looks about right", and the boat definitely handles better than before, so I'm leaving it at that.
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