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DSYHS

Measuring tension in larger diameter shrouds

24 posts in this topic

Bump...

 

No one? How do you tune a rig on a 50ft+ cruiser without a mastjack? I imagine it would take similar tools?

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You could ask if the ROD RIGGING GAUGE withc will fit a 12mm wire by loose gives you repeateble number on dyform shrouds. Another possibilitie is to use : the kaver system smart eyes. You also have load pins that will fit std shroud toggles.

 

 

both the load pins and kaver system you can find at: http://www.tunedrigs.com

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Rigging tension gauges are designed to get you back to where you once were, in terms of repeat-ability of tension. Actually having them to tune your mast doesn't work. .

 

Especially when you consider that this is a (as far as I can see) no spreaders, no backstay, no lowers, wooden spar. You probably need enough tension to keep the leeward shroud from becoming VERY slack while going upwind. Anything more wont help you go any faster or hold the mast up better.

 

The 1905 Herreschoff NY30 I sail on is actually designed somewhat similar, except that the leeward shrouds fold forward so the gaff doesn't bend around them when going down wind. I know, I know the boat also does have runners, which really only hold the mast up going down wind (no backstay).

 

post-205-0-80858800-1383041879_thumb.jpg

 

Bam Miller

 

 

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Bam, thanks for your input. nice boat.

 

We use spars with 50-75cm prebend laminated into them. By tensioning the forestay, the mast is straightened and effectively pulled in its shrouds for sailing. This is the only way to get some forestay tension going on a spreaderless 18m rig with a 75cm shroudbase. (runners are not allowed)

To get a good base set-up, we want to have a certain tension (1200dN) in the forestay (design sag of the jibs) with equal tension in the shrouds. that's why we need some sort of gauge.

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Depending on where you are I think you could rent a Dillon or similar guy wire gauge for the day (they are expensive to buy, a couple of thousand dollars). Do the set up, then measure between some working surface (toggle or turnbuckle gaps) using a set of calipers. You can then use these working surface measurements for a repeatable base setup.

 

If you wanted to, you could create a scale and matrix, using the wire guy gauge and the working surface measurements, e.g. a lengthening of the forestay by 1 mm (1 mm expansion in the working surface measurement) results in a decrease of 1 kN in tension. This would take a few hours but would be worth it. We used to do this on a bunch of boats I sailed on that didn't have mast jacks, so that we could adjust rig tension in between races without bringing out the loos gauge.

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Bam, thanks for your input. nice boat.

 

We use spars with 50-75cm prebend laminated into them. By tensioning the forestay, the mast is straightened and effectively pulled in its shrouds for sailing. This is the only way to get some forestay tension going on a spreaderless 18m rig with a 75cm shroudbase. (runners are not allowed)

To get a good base set-up, we want to have a certain tension (1200dN) in the forestay (design sag of the jibs) with equal tension in the shrouds. that's why we need some sort of gauge.

Instead of trying to tune the mast for headstay sag using shroud tension forces, why not get a piece of small vectran or prestretched dyneema and pull that parallel, with lots of tension, to the headstay from the same attachment points; headstay sag can then be measured (ok, eyeballed) to suit the sails.

 

Also, isn't your shroud tension a dynamic changing number depending on wind pressure; wont that be affecting the headstay tension while you sail; the more wind, the more headstay sag - right? Are you adjusting the tension as you sail? How do you plan to take that into account?

 

While I am at it, Im not seeing 750mm of prebend, perhaps you meant 75mm. From what I can see, shrouds that all terminate at almost the same location, at or near the top of the spar. Can you explain how prebend is effected by the shrouds without having aft swept spreaders? Perhaps by the tension in the leech of the main, but without a backstay (which I dont see) changing the bend of the mast would be difficult, unless the mast was keel stepped, and you were bending the mast forward around a deck partner, something of which I havent seen before, except down wind in a star or an etchells.

 

Bam Miller

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Bam,

 

First the prebend.

 

I think i mixed up some terms here. While sailing the mast is straight (no prebend). At rest, the mast is curved backwards 75cm. This 'bend' is indeed no prebend, but effectively a huge spring to tension the forestay. This bend is laminated into the mast.

 

Let me rephrase what i'm looking for:

 

the sailmaker uses the forestay tension as a proxy for the forestay sag he designs the jibs for.

1200dN is what is generally used.

 

So for a base case, I am looking to have the mast perfectly centered, a couple degrees of rake, 1200 dN on the forestay and equal tension on both shrouds.

Then, i want to explore the relation between shroud tension an forestay tension.

Due to the "construction bend", the mast give about 800dN forestay tension while straightenend with slack shrouds.

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The only solution that I have would be to build your own tension gauge based upon the design RT-11 Rod tension gauge, only longer - probably 4 feet long. Then calibrate it against a load cell using one of your existing shrouds. It should be a fairly lineal progression up the scale as loads increase, so once you know the increment of movement for each 1000 lbs or whatever unit of measurement you like, you can extrapolate higher loads.

 

No one that I know of makes a larger version because of low demand.

 

Also, and this is probably ignorance on my part, but you are refering to "dN" or Deci Newtons, and when I convert 1200 dN, I get 88lbs. Can you clarify?

 

Bam Miller

 

post-205-0-43743000-1383262917_thumb.png

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If Bam found a torque moment, could one weld asocket to a split pipe or such and develope a scale to use a torque wrench to measure tension from a known set of shrouds?

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rig_tensiometer_165.jpg

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Use a shear pin load cell.

 

It's a pin that replaces the regular Clevis pin or bolt that attaches the headstays to the tang.

 

You can either fully install the readout, so you can see loads while sailing, or you can just plug them into the pin when setting the rig and otherwise leave them off the boat. In the later case a bunch if boats could share the readout's.

 

Lots of suppliers, but just to get you started . . . http://www.strainsert.com/product-categories/load-pins/

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Load pins are the right answer. Just be super careful about sizing, because a load pin has much lower shear strength than a normal pin with the same OD.

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Does anyone have an idea how to measure tension in 12 - 14mm 1*19 or dyform shrouds?

The larger loos gauges only go up to 10mm (http://loosnaples.com/tension-gauges/rod-rigging-gauge)

 

Seldon has a technique described in their rigging booklet, take a 2 meter yardstick and tape it at the top so lower end is just offset from turnbuckle upper edge. Then tension turnbuckle and measure change in gap. Right number gives you 20% of yield strength of wire regardless of size. Go to Seldon site.

Barefoot

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