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Jib Height and pointing #153060
08/20/08 10:52 AM
08/20/08 10:52 AM
Joined: Aug 2006
Posts: 297
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rexdenton Offline OP
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rexdenton  Offline OP
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R

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I recently cured a 'manufacturing issue' on the forestay of my F18 to modify the jib height to accomodate better purchase on the jib tack. Interestingly, I also noticed, all other aspects being more or less equal, that my boat began pointing a bit higher, and seemed to have more drive. I assume that the drive aspect is related to a more efficient overlap, but I am somewhat stumped about the pointing improvement. (I'll take it though!)

Can someone give me so quick insight into how what happened to make the overall boat improvement be fundamentally dependant on the tallness of the 'little guy' up front, and whether or not any experience tells you where to position the head of the jib for maximal performance? Thanks in advance.

R


Nacra F18 #856
-- Have You Seen This? --
Re: Jib Height and pointing [Re: rexdenton] #153061
08/20/08 11:17 AM
08/20/08 11:17 AM
Joined: Jun 2001
Posts: 3,293
Long Beach, California
John Williams Offline
Carpal Tunnel
John Williams  Offline
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Long Beach, California
How much higher up the forestay are we talking?

When you made the change, did you find that you were getting more tension in your luff? If you have changed the entry, you have changed the flow across the sail - more luff tension in the right condtions means more efficiency which will pull your apparent around, letting you sail a bit higher. An easily-adjustable jib luff-tensioner is important, in my opinion.


John Williams

- The harder you practice, the luckier you get -
Gary Player, pro golfer

After watching Lionel Messi play, I realize I need to sail harder.
Re: Jib Height and pointing [Re: John Williams] #153062
08/20/08 12:47 PM
08/20/08 12:47 PM
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rexdenton Offline OP
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About 6-7 inches higher up the forestay. Previously, the tack was more or less right at the bridles. It had tension, but not enough, and it sagged a little. Shorter bridles from the mfr resulted in some ability to adjust, but the tack was right at the grommet, and still was not very adjustable. I moved the head up with a modification. Now it has all the tension I need.

Entry must have have been changed as a result of the jib slot determined as a function of the angle of the forestay and ever closer to the mast and farther back as the jib goes up the forestay. My question is what determines the ideal geometry of this slot, and how do you determine this setting? Tell tales? Closness to the spreaders? Dunno.
Thanks,

R

Last edited by rexdenton; 08/20/08 12:51 PM.
Re: Jib Height and pointing [Re: rexdenton] #153063
08/20/08 02:05 PM
08/20/08 02:05 PM
Joined: Jun 2001
Posts: 3,293
Long Beach, California
John Williams Offline
Carpal Tunnel
John Williams  Offline
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Long Beach, California
Yeesh. I'll hope that others with better backgrounds weigh in on this. My understanding is that you want that jib tack as low as you can get it, not higher. I would bet that your observed increase in pointing ability is entirely due to you finally getting a firm luff. Tack that sucker down and get a firm luff on it. Your slot should be controlled by a limiter or track on the front beam in combination with mast rotation. A good starting point is to have the leech of the jib about two inches off the spreader with your rotator pointing back at the front of the leeward daggerboard.


John Williams

- The harder you practice, the luckier you get -
Gary Player, pro golfer

After watching Lionel Messi play, I realize I need to sail harder.
Re: Jib Height and pointing [Re: John Williams] #153064
08/20/08 08:14 PM
08/20/08 08:14 PM

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Shackle your jib as low as possible and then control luff tension at the top. Most Tornados are now runing a 4:1 Halyard. The loose end runs down through the luff of the jib and then back to cleat on the beam with a 2:1 purchase. This will give you 8:1 luff tension which is more than we all used on our mains 10 years ago. Set the halyard so you can remove the majority once it is up.

Re: Jib Height and pointing [Re: ] #153065
08/21/08 07:16 AM
08/21/08 07:16 AM
Joined: May 2003
Posts: 4,451
West coast of Norway
Rolf_Nilsen Offline
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West coast of Norway
Some jibs need to go higher than others, depending on the rig geometry, the jib track and the cut of the jib. At least that is my experience.

Re: Jib Height and pointing [Re: Rolf_Nilsen] #153066
08/21/08 03:31 PM
08/21/08 03:31 PM
Joined: Feb 2008
Posts: 242
Brisveagas
Aido Offline
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Brisveagas
raising the tack of the jib will make your sheeting angle steeper. That will definetly give you more height.


Aido
Viper 288
Re: Jib Height and pointing [Re: rexdenton] #153067
08/21/08 03:42 PM
08/21/08 03:42 PM
Joined: Nov 2005
Posts: 1,203
uk
TEAMVMG Offline
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Pointing issues are related to leech control [on both jib and main]
By raising the jib, the sheet is pulling down the leech, giving it more tension, squeezing the slot, making you point higher.
this quite often leads to chocking of the slot which has a negative effect on speed. Methinks that the front monkey was not sheeting jib hard enough before or your jib leech is shagged![Maybe more than a couple of seasons old?]


Paul

teamvmg.weebly.com
Re: Jib Height and pointing [Re: TEAMVMG] #153068
08/21/08 04:26 PM
08/21/08 04:26 PM
Joined: Aug 2006
Posts: 297
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rexdenton Offline OP
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rexdenton  Offline OP
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Quote
Pointing issues are related to leech control [on both jib and main]
By raising the jib, the sheet is pulling down the leech, giving it more tension, squeezing the slot, making you point higher.
this quite often leads to chocking of the slot which has a negative effect on speed. Methinks that the front monkey was not sheeting jib hard enough before or your jib leech is shagged![Maybe more than a couple of seasons old?]

Thanks everyone! Lots of good thoughts here. I think there may be something in just about all of the posts that are valid, but I do believe is may have been a combination of a sagging luff, because it was not tight enough, and the angle of the sheet on the leech, because the jib was too far forward.

Both of these suggestions make the most sense to me, in that if the luff was loose and the jib a little too far forward, there would not be a tight luff (bad laminar airflow) and not the appropriate/maximal leverage on the clew from the sheet (because the angle of the clew is l ess than ~90 degrees at the attachement point of the self-tacker. Both of those things probably led to some inefficiency of the jib. Now, to figure out the best height! Will start with getting it as low as possible to the deck, with out adversely moving the clew forward, such that maximal force can be applied to the clew from the sheet. And, BTW, it is a new sail (new boat), so the leach had better be alright!.

Thanks again


Nacra F18 #856
Re: Jib Height and pointing [Re: rexdenton] #153069
08/21/08 07:19 PM
08/21/08 07:19 PM

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Try setting the jib all the way down and then moving the pick up point up the clue board. I'm assuming you know how to work out the right sheeting angle using the upper and lower wind indicators. If you find you're not getting enough leach tension slowly move the jib up until you reach the right point. Thinking about it I'd set the tack height so that your jib is on the second top clew hole. This will leave you a little scope for on the water adjustement.

Re: Jib Height and pointing [Re: rexdenton] #153070
08/22/08 08:42 PM
08/22/08 08:42 PM
Joined: Jun 2002
Posts: 806
Toronto, Ontario
pitchpoledave Offline
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Toronto, Ontario
Rex how did you do this move? On the Infusion there isn't an easy way to move the jib up. The halyard S hook, hooks onto a ring which is connected via eyelet and then swaged to the forestay. Maybe if you bypass the factory ring?

Can you post pics of your jib setup?

In the stock setup the tack will be 4-5 inches from the bottom of the front chainplate (where the plate sticks out)

Re: Jib Height and pointing [Re: pitchpoledave] #153071
08/22/08 10:08 PM
08/22/08 10:08 PM
Joined: Aug 2006
Posts: 297
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rexdenton Offline OP
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rexdenton  Offline OP
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Quote
Rex how did you do this move? On the Infusion there isn't an easy way to move the jib up. The halyard S hook, hooks onto a ring which is connected via eyelet and then swaged to the forestay. Maybe if you bypass the factory ring?

Can you post pics of your jib setup?

In the stock setup the tack will be 4-5 inches from the bottom of the front chainplate (where the plate sticks out)


Yes, I bypassed the lower ring assembly. (Actually, it was carefully removed with a hacksaw and prying.) Pics later when I get a chance to ip it over.

In short, I bought a furling jib option on the boat with two pigtails. So I simply scuttled the lower pig, and bought a rectangular shackle and used it, insted of the ring, through the upper eye, to 'hook up' the jib. The halyard went to a block I installed at the hounds' eye of the forestay. A little electrical tape here and there to prevent the spin fouling in the shackle and it works fine. Its a little high at the moment, but I will add a length of line at the head of the jib and attach that line to the hook for the appropriate height adj. The lower pigtail was either too long or too low to gain any purchase on the tack. Went up and hooked first try! <img src="http://www.catsailor.com/forums/images/graemlins/shocked.gif" alt="" />


Nacra F18 #856
Re: Jib Height and pointing [Re: rexdenton] #153072
09/17/08 08:29 PM
09/17/08 08:29 PM
Joined: Jul 2006
Posts: 1,147
Bay of Islands, NZ
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warbird Offline
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Quote
I recently cured a 'manufacturing issue' on the forestay of my F18 to modify the jib height to accomodate better purchase on the jib tack. Interestingly, I also noticed, all other aspects being more or less equal, that my boat began pointing a bit higher, and seemed to have more drive. I assume that the drive aspect is related to a more efficient overlap, but I am somewhat stumped about the pointing improvement. (I'll take it though!)

Can someone give me so quick insight into how what happened to make the overall boat improvement be fundamentally dependant on the tallness of the 'little guy' up front, and whether or not any experience tells you where to position the head of the jib for maximal performance? Thanks in advance.

R


On my old Hydra the Jib went best lower down but with more tension.
The tension was what I found most important and I wanted the jib lower for better center of effort and more flow across the bottom of main..


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