Author Topic: GNAV & Cunningham Rigging  (Read 799 times)

Norman Vallette

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GNAV & Cunningham Rigging
« on: September 18, 2017, 12:08:30 PM »
Does anyone have pictures of how the cunningham is supposed to be rigged?

Also, our GNAV is only adjustable after putting an extension in the loop that goes around the mast... any tips (and pictures) on fine tuning these systems is greatly appreciated.

Peter Beardsley

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Re: GNAV & Cunningham Rigging
« Reply #1 on: September 18, 2017, 04:13:19 PM »
This will take a few posts since the forums limit the size of photos that can be posted. 

Cunningham: start here: http://forum.viper640.org/index.php/topic,1639
This system also describes (although does not have a photo of) my older system, which I've added to subsequent boats, with some minor modifications.  For whatever reason, 269 did not have an eyestrap on the mast-end of the boom, so instead, the line that loops through the tack grommet now goes to a soft shackle that is attached through the clam cleat on the back of the mast (i.e., the stock cunningham clam cleat that we don't like using since it would involve getting off the rail to adjust the cunningham).

We end up with a 4:1 cunningham - 2:1 through the grommet, and 2:1 from a bullet block that has a line that dead ends on each chainplate U-bolt. 

The cunningham class rule is 6.6 - be very conscious of the last sentence, which does not allow you to mount the cunningham cleats more than 20 cm aft of the back of the mast (i.e., you can't lead it back to the driver, and the cleats end up being forward of the shrouds at best).  We prefer leading them to the rail instead of the mast.  Sometimes you end up with too much slack on one side, and sometimes that slack can find its way into the jib sheet ratchet block at inopportune times, but not often and so far never with tragic results. 

Finally, note that you need to add two bullet blocks for the redirect to the rail.  If you don't want to drill more holes than needed, you can tie micro bullets into the eyestrap for the vang redirect on the floor using Opti sail-tie line.  See the prior post linked up top for additional installation tips. 
« Last Edit: September 18, 2017, 04:28:19 PM by Peter Beardsley »
Larchmont YC Fleet
Viper 269 "Great Scott!"
Formerly Viper 222 "Ghost Panda" and Viper 161 "Vicious Panda"

Peter Beardsley

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Re: GNAV & Cunningham Rigging
« Reply #2 on: September 18, 2017, 04:26:58 PM »
Vang: operative rule is Section 6.4.  There is a bit more flexibility w/r/t vang rigging than cunningham.  For instance, I have seen people move the vang cleats from their builder supplied location, claiming they are protected by the first sentence of Section 6.4(c):  "The boom gnav system does not have to be identical to that supplied by a class approved builder."  Personally, I think this is a bad argument, or if I wanted to give those people the benefit of the doubt, I'd argue that our rules are not written well in Section 6.4 since 6.4(b) says "The boom vang may be mounted with the cam cleat at either the top or the bottom of the vang. The vang shall not otherwise be changed from original configuration."  Presumably those who rely on 6.4(c) will say that 6.4(b) deals with rope vangs that run from the boom to the bottom of the mast, and that 6.4(c) deals with GNAVs that go from the boom to a point above the gooseneck on the mast.  ANYHOW...

Since we're talking about GNAVs since I thought the old rope vangs were outlawed by late 2011 or so and arguably should be written out of our rules, the stock setup usually needs to be modified to make the GNAV work better upwind. 

We have a 16:1 vang, which is the most you're allowed under the rules.  We have the stock 4:1 purchase on the boom - photo attached showing our standard GNAV track -- we loosen the screws on the stock track to slide it aft 1-2" or so from the factory location since no matter how hard you pull on the GNAV, you can't get the car to the front of the track, but we did want the ability to ease the GNAV enough in light air so as not to have more leech tension than necessary, esp. downwind.  Note also in the attached photo that we have a scale on both sides of the boom so that we can see where the car is located to allow for consistent tune. 

Finally, for the real eagle-eyed Viperers, note in that photo that we've filed down the starboard side of the red track stopper so that the block for the GNAV purchase slides over the stopper more easily.  Sometimes we would have a situation where the block would get jammed into the track end stopper and prevent us from pulling on more GNAV -- it doesn't do that anymore after we spent some time with the file. 

Next, we have a 4:1 purchase down low.  If you look at the photo in the previous post entitled "mast area.jpg", you'll see the double carbo bullet block that we added.  This is now attached via soft shackle on our boat to allow for easy removal when we're trailering or stowing the boom so that the vang doesn't foul the instrument brackets.  When we had a 4:1 with a cascade, we found that the vang bottomed out too easily in breeze before we were done adding leech tension. 

Next, if you look at the photo in the prior post titled "cunningham floor", you'll see that we added a triple carbo bullet block to one eyestrap, and take my word for it though I don't think it's visible in the photos, there's a double carbo bullet block on the other eyestrap.  Vipers generation Mark III and newer (after hull 102 or so) have two eyestraps immediately aft of the mast step standard.  The stock setup is just a single double, so to rig this, you need to buy another double for the part immediately below the gooseneck, and a new triple for the unused eyestrap on the floor.  This allows you to rig both the cunningham and vang properly and be able to redirect the cunningham to the rails. 

A 16:1 vang is pretty powerful.  Make sure you ease a fair amount in breeze going around the weather mark if you don't want to damage your gooseneck long-term.  I've never broken it so far but there's definitely a lot of load on it on our boat in 15+ kts. 
« Last Edit: September 22, 2017, 08:30:33 AM by Peter Beardsley »
Larchmont YC Fleet
Viper 269 "Great Scott!"
Formerly Viper 222 "Ghost Panda" and Viper 161 "Vicious Panda"

Norman Vallette

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Re: GNAV & Cunningham Rigging
« Reply #3 on: September 18, 2017, 04:38:16 PM »
Thanks Peter!

Peter Beardsley

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Re: GNAV & Cunningham Rigging
« Reply #4 on: September 19, 2017, 10:31:53 AM »
The margins are cut off in the in-line photo posts - if you click on the actual .jpg file, you can see the entire photo and all of the text. 
Larchmont YC Fleet
Viper 269 "Great Scott!"
Formerly Viper 222 "Ghost Panda" and Viper 161 "Vicious Panda"

Dan Tucker

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Re: GNAV & Cunningham Rigging
« Reply #5 on: September 21, 2017, 10:07:43 AM »
For whatever reason, 269 did not have an eyestrap on the mast-end of the boom
The reason is that the eyestrap on the top of the front of the boom was only intended for storage of the above-boom GNAV purchase while the boom was off the mast. People were breaking boom ends and such by running the GNAV purchase off that eye, rather than to the dog collar around the mast. So we eliminated that possibility for the sake of durability. Soft shackles for quick disconnect is an excellent idea!

While the rules allow for 16:1 vang purchase, and it makes adjustment easy, please be aware that it's a LOT of power. If you're not paying attention, it's possible to over-vang and break inboard boom ends, or bend the gooseneck fitting on the mast. Watch the sail for the desired trim, please don't just wail on the vang with 16:1 :)  Most who will put on 16:1 understand this, but beware of the overenthusiastic new crew!
Race it like you stole it.