Rigging tension

Moderator: GreenLake

Rigging tension

Postby Shagbark » Tue Dec 20, 2016 11:31 pm

I've read various postings about setting the rig tension so that a certain 'note' is achieved when twanged. I would like to go off of something a little more concrete. The Doyle tuning guide recommends 50 pounds on the forestay when using a loos gauge while Dieball recommends 250 pounds on the forestay. Quite a big difference between the two. Has anyone on this site used a loos guage to set their rigging, and if so, what tension have you had success with?
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Re: Rigging tension

Postby talbot » Wed Dec 21, 2016 6:43 pm

250lbs? Yikes! The original Johnson stay adjuster for 3/32" wire only had a breaking strength of 1000lbs. That would make the stay tension 25% of the breaking strength, even before any loading or shock. Most people have upgraded to 1/8" wire and bigger hardware, but 250 still seems like a lot. I would look at some other sources. Try North Sails.

In past discussions of rig tension on this site, some prominent racers have argued that there is no one perfect setting. Most people start with the mast tip about 6" aft of its base, measured by the main halyard hanging slack. All the wires should then be adjusted so there is no wiggle in the rig. The idea is not just tuning of the mast shape, but minimizing the shock loading during tacks and gybes. Don't try to get all the tension out with the turnbuckles; you can freeze up the screw threads. I would get the mast angle where you want it using the forestay, then fasten the main halyard to the end of the boom and tension the mast with the main sheet. With tension off the shrouds, you can tighten them by hand a few extra turns. When you releast the sheet, the wires should be tight. You can sight up the mast to see if you are still straight, and readjust as needed. (I guess you can also do it in reverse, setting the shrouds first. But you don't have as convenient and powerful a tensioning system in front of the mast. You would have to really crank on the jib halyard.)
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Re: Rigging tension

Postby GreenLake » Wed Dec 21, 2016 7:12 pm

The 250 lbs sound about right for 1/8" shrouds. The forestay would be less, by geometry.

In a discussion on mast steps I recently found mention a "rule of thumb" that the compressive force on the mast step roughly equals displacement.

Assuming the shrouds as nearly vertical (and dominating contributors to the downward pull on the mast) 250 lbs each gives 500 lbs, or about 5/6th of the value predicted by the rule of thumb.

When I upgraded to the new shrouds, I measured the tension (I have a mast jack where the shrouds are fixed length and the number of turns predicts the tension). I recorded the number of turns I needed and lo, I can "twang" out the same note as before, when I set up the rig by feel.
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Re: Rigging tension

Postby talbot » Wed Dec 21, 2016 7:58 pm

The original question had to do with the forestay, not the shrouds.
If the shrouds have 250lbs each, would the forestay have the same amont of tension?
Is it possible both figures are correct? About 250 for each shround, and 50 for the forestay?
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Re: Rigging tension

Postby GreenLake » Wed Dec 21, 2016 9:16 pm

The shrouds lead barely aft, while the forestay leads pretty far forward (about 6 feet for 15 in height, something like that). So, yes, the tension in the forestay would have to be much less to match the stays. 250 simply can't be the right number, but I'm not going to try to do the math to see whether 50 is in the ballpark....
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Re: Rigging tension

Postby Shagbark » Fri Dec 23, 2016 12:45 am

The Dieball literature states 250 lbs on the forestay, 350 lbs on the shrouds.
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Re: Rigging tension

Postby GreenLake » Fri Dec 23, 2016 1:31 am

OK, if the shrouds are at 350 each, 250 becomes conceivable. I don't think I went that high on my shrouds - but we don't get winds in the upper ranges that often.
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Re: Rigging tension

Postby Shagbark » Fri Dec 23, 2016 3:08 am

350 just seems high. North Sails lists 150 lbs on the shrouds, which would put roughly 100 on the stay.
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Re: Rigging tension

Postby GreenLake » Fri Dec 23, 2016 1:21 pm

Those numbers seem like the low end of the range. My jib has a luff wire, and (in principle) I could add significant tension with that as well. And variable under way. But to take full advantage of that, I would need a 2:1 purchase on the halyard.
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Re: Rigging tension

Postby talbot » Fri Dec 23, 2016 3:04 pm

Here is the full text from North. Their procedure attends to the geometry of the rig. The wire tension is a result of (not cause of) the rig tuning:
MAKE RAKE AND SHROUD TENSION
To measure the aft rake of your mast,
hoist a tape measure on your main halyard
and hold it tight at the intersection of the
transom and rear deck. This measurement,
without your jib up and your rig set
“snug” (no play in any shroud or forestay),
should be 24 11” to 25’ 1”. Then hoist
your jib and pull the rig forward through
either a magic box, a block and tackle,
or using the forestay (as a bow and
arrow effect) until you achieve a 25’1”
to 25’2” measurement at the transom.
With this rig tension (approximately
150 lbs on the shrouds) the leeward
shroud should not go slack until the wind
velocity is approximately 10 mph.
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Re: Rigging tension

Postby Shagbark » Sat Dec 24, 2016 9:04 pm

The North rigging guide sounds pretty sound. I'll still purchase a loos guage and test if afterward, but the measuring of distances sounds like a good the way to go. By the way, just opened my Christmas gift from my wife - cuff links of a fractional rig day sailer. Pretty sweet.
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Re: Rigging tension

Postby K.C. Walker » Wed Jan 04, 2017 5:57 pm

I believe the rig tension numbers from North were developed with Dave Keran and the Dieball numbers were developed with Mike Gillum… Both national champions and fierce competitors but also best friends. And, Skip Dieball now works for North! The boats they use are definitely reinforced to withstand those rig tensions and they do not leave the boats rigged at that tension except for racing. The high rig tensions adds more flexibility to the mast for more sail control and gust responsiveness.…

A loose rig is more likely to have a high shock load which is more damaging by far than a lower constant load.

I admit to not having a gauge. I set my rig tension with a 5:1 tackle system on the fore stay. I really have to yank on it to get it connected. I know that Dave used his trailer winch to get enough tension to connect his fore stay. Both using the jib halyard.
KC Walker, DS 1 #7002
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Re: Rigging tension

Postby Shagbark » Thu Jan 05, 2017 9:02 pm

I never thought about using the trailer winch! After all the struggling of trying to get the forestay connected, embarrassing that I never thought of something so simple and readily available.
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Re: Rigging tension

Postby tmittner » Tue Jun 27, 2017 5:45 pm

I've read mast prebend should be take into consideration your mainsail's built-in luff curve and you should ask your sailmaker how much mast prebend is right for your sail. I believe rigging tension is the mast prebend adjuster on a stock DS II with floating spreaders. (No fore aft mast butt adjustment) Something to consider for DS II 'ers.
Tom
DSII #8483 Old Fox
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