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Gui,

Thanks for an excellent set of instructions.

I have read through your post a couple of times already but I still need to reread it several times more to appreciate all its contents.

There are 2 comments I would like to make, always with due respect.

1. it would help to define the various types of stays on the mast for us non English readers.This to unsure that we correctly tighten the correct stay.

2. At one point you said you calculated the strength to be 15% of breaking strength. How do you arrive at this figure?

By the way you are spot on in your DISCLAIMER.

thanks again

xuraax
 

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Giu,

Sorry for getting your name wrong. This thread simply has to much info for me to appreciate all of it at one go.

I guess I was not clear enough when I asked for clarifications on the various terms. I have tried to upload a picture of the rig to clarify but for some reason Sailnet is not accepting the file.

So I will try with words.

The rig I am interested in has a twin spreader rig consisting of a wire A starting from the deck to the tip of the first spreader then to the tip of the second spreader and then on to the mast.

It also has a wire B starting from the same place as A going up at an angle to the mast at the lower spreader height.

There is a second wire C going up at an angle from the outer tip of the lower spreader to the mast at the height of the upper spreader.

I guess my question is: what are the names of wires A,B,C?

regards
 

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thanks a lot Marty.

Rereading Alex's mail several times I was deducing the same thing but being a pessimist it feels good to get a confirmation from others.
 

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From the web, Young's Modulus for 1x19 wire rope is 107.5 kN/mm^2.

Using the table of breaking loads for different diameters of AISI-316 wire rope as given in the book "Principles of Yacht Design" I got the following table for wire stretch for a 2000mm wire loaded to 5% of breaking load:

diameter(mm)____breaking strength(kN)___delta L(mm)

3_______________7.7__________________1.01
4______________13.8__________________1.02
5______________21.6__________________1.02
6______________30.o__________________0.99
7______________40.9__________________0.99
8______________53.5__________________0.99
10_____________69.1__________________0.82
11_____________83.5__________________0.82
12____________120.2__________________0.99
14____________160.1__________________0.97

This practically confirms what Alex is saying.

regards
 

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Right; I said the same thing. If the shroud length is constant; the amount of stretch required is the same. But you can't apply this universally to all shrouds because different boats have different lengths of rigging wire. If you plug in 4000 mm for your base length; the delta L will double to get the proper tension. Since that's true you can't use the 1mm/5% rule to get exact tension. If I tighten an intermediate 1/4" shroud using that rule the tension will likely be ~40-50% of breaking load because the shroud lenth is much shorter than the upper shroud; which goes from the masthead to the deck.
Keel,

As I understand it, if you mark off 2m of a shroud 6m long and then tension it to 10% of its breaking strength you should find that the marks that you made are now 2.002m apart.

If you mark off another 2m of a shroud that is 10m long and you tension this shroud to the same amount your 2 marks will again measure in at 2.002m apart.

this sound logical to me but hey...I am no expert, I could be wrong.

regards
 

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For type 316 Stainless; E = 28,000 kPSI
Keel,

When I convert your number to metric I come up with 193KN/mm^2 which is a lot higher than the one I quoted (107.5). When I looked up E again on the web the numbers I come up with now are closer to the ones you give so I guess it is more proper to use the constant you gave. The equation for Delta L is of course the same (Phewww!! thank goodness for that)

For 7/16 wire - S = 15% * (20,000#)/(Pi * (7/32")^2) = 15% * 133,040 PSI = 19,956 PSI
I note that American's frequently use the # sign. Can you explain what it means?

Use inches or mm for the shroud length; multiply by .000736 and get the length you need to stretch the cable.

For a shroud 55' long: dL = 660" * .000736 = 0.485"

For a shroud 20' long: dL = 240" * .000736 = 0.177"

It's -fairly- independent of wire diameter; but clearly dependent on length! You could use this for type 316; (but of course the standard disclaimer applies); and it does not take into account deflection of the rig or hull when you tighten the shrouds. Again; you should use an appropriate tension gauge to determine the actual rig tension.
I can understand the apparatus needed to measure an extension of a few mm over a length of 2 m.

What would you use to measure the 0.485" (say) in the 55' shroud to ensure that you don't tension it more than 15% of breaking strength?

regards
 

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I tuned my rig for twenty knots I figured that was good middle ground for us.
And did not exceed 15% on the lowers 20% on the caps.
How does one go about optimizing the rig for 20Knots?

The rig I would like to set is a twin spreader aft swept 7/8 rig with a set of lowers and intermediates. The mast is a keel stepped affair. The mast is held firmly at deck level but the base can be moved fore-aft.

Currently the boat is getting quickly overpowered in high winds, constantly broaching into the wind in the gusts. It looks like pulling hard on the backstay and cummingham has little effect in flattening the middle part of the sail.

My other problem is that I do not own the boat so, of course, before playing about with other people's stuff one must be extra careful on what one is doing.

From the excellent info found in this thread I conclude that, for this rig setup,
the rake will be determined by the forestay length while the prebent will then be primarily determined my moving the mast base fore-aft keeping the shrouds fairly tight. The uppers should then be set to the 15% loading mentioned earlier.

It would seem then, that to optimise the rig for 20knots, say, one has to play about with the lowers and the intermediates.

Currently reducing the tension on the lowers results in some significant bending of the rig forward when pulling the backstay but the bent is nowhere near the 2% of mast length originally mentioned by Alex. Also the leeward shroud now becomes fairly floppy when sailing at 20knots so I am not sure if we have created another problem with the mast bending to leeward.

By the way, getting a rigger with a loos guage did not help much as apparently all he did was set the tension in the upper shrouds in port and more importantly the broaching problems remained.

regards
 

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I think we need to know what type of boat you are sailing to determine if the problems you describe can be associated with the rig or not. Some issues with broaching or helm balance are more associated with the sailplan or a particular hull design and without knowing these things I can't say whether or not it is due to improper adjustment of your rig.
The boat is an Elan 37. The sails are Tape Drive racing sails.

Asking around on the WEB indicates that this boat does pretty well on the racing circuits even in high winds.

By the way I am not not necessarily saying that the rig is set up incorrectly. Merely that it is another paramater to check.

regards
 

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Thanks one and all for the many replies.

However this thread is not about whether this boat is a good racer or not. That may well be the final conclusion when all things are checked and confirmed to be correctly set.

The thread is about adjusting the rig, and recently it had shifted into how to optimise the rig for higher winds. That is surely an interesting argument.

regards
 
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