1X19 stretch

Hayden Watson · Jan 15, 2018

smokey73 said:
I've started another thread on a related note about shroud tension - particularly the effect of cold weather. Please visit and give me your experiences, especially those of you in colder climates. My shrouds seem to have "un-tensioned" in the cold weather.

Your shrouds will be a bit looser in very cold weather than they are in warm weather because the coefficient of thermal expansion is about 30% - 40% greater for aluminum than it is for stainless steel. That said the change is not very significant and will only result in reducing the preload on the static rig. The final ultimate tension on the rig is only dependent on the properties of the rig/boat. Ideally you want your stays to be tensioned so that the leeward stays just start to come slack (start to dance) when at your normal max angle when fully powered up. On my boat that is at 22º. The rig tension or lack there of will NOT CHANGE THE MAX TENSION IN THE RIG. The rig tension will be greatest in the longest cables and decrease as they get shorter. You should do final adjustment under sail fully powered up and adjust as needed to keep the mast in-column, ie not bending to either side. And FWIW, yes I am a licensed Structural Engineer.

graphic reposted from https://loosnaples.com/how-tos/tension-gauges

SG · Jan 15, 2018

Nice post, Hayden. The concept of prestressing is a very important part in understanding what you are doing when tuning a rig.

Of course...The backstay is usually longer than the forestay. Whether the keel is deck or keel stepped has an effect (with possible different temp swings). My J's back stay is over 15% longer then the distance from the masthead to the coach roof. The forestay (with our mast relatively far forward) is closer to the same distance to the deck. Rod rigging vs. Cable Rope have different coefficients of expansion. Etc. :^)))

P.S. - The comment about adjusting stay tensions on a cable supported bridge at night is an interesting one. I'm not sure that statement is correct.

The analysis and "setting" of cable dimensions is pretty complicated. You have wire rope, bridge decks made of sometimes concrete, sometimes steel. You have expansion joints that "sort-of" slip. The top of the deck is different than the bottom (and sometimes middle of the deck "structure"). The loads imposed are different in construction, than under imaginary "static" load, and then there are the dynamic forces imposed by traffic and wind of varying velocities and loads.
So many variables! Adjusting a bridge support system might not be best done at night, I would suspect. Not all cable supported structures are created "equal".

Hayden Watson · Jan 15, 2018

SG said:
Nice post, Hayden. The concept of prestressing is a very important part in understanding what you are doing when tuning a rig.
...
P.S. - The comment about adjusting stay tensions on a cable supported bridge at night is an interesting one. I'm not sure that statement is correct.

The analysis and "setting" of cable dimensions is pretty complicated. You have wire rope, bridge decks made of sometimes concrete, sometimes steel. You have expansion joints that "sort-of" slip. The top of the deck is different than the bottom (and sometimes middle of the deck "structure"). The loads imposed are different in construction, than under imaginary "static" load, and then there are the dynamic forces imposed by traffic and wind of varying velocities and loads.
So many variables! Adjusting a bridge support system might not be best done at night, I would suspect. Not all cable supported structures are created "equal".

OFF TOPIC ALERT!
As a structural engineer I can tell you that the expansion joints in bridges is not a "'sort-of' slip" joint. The bearings are Teflon on mirror polished stainless steel to provide the minimum bearing frictions possible.

Expansion joints can be designed with up to 48" of movement. The actual expansion setting of one of the most critical aspects of most bridge projects and are completely weather dependent. The temperature much say within a specified narrow range of change typically not more than 10º in 12 hours. The joint then needs to be set to a specified width based on the casting temperature. On one bridge that provided expansion joint inspection, the contractor waited for 1½ months for the conditions to be met.

Hayden Watson · Jan 15, 2018

capta said:
I don't remember ever seeing anybody work on the GG bridge at night. If there were adjustments being done, I'm pretty certain they were done during the day.

Golden Gate bridge is a suspension bridge which is not the same thing as a cable stayed bride. In either regard the expansion joint is done during construction prior to the bridge being opened and they usually work 2 - 3 shifts.
FWIW, they like to do maintenance during the day, especially during rush hours just to mess up the traffic or retaliate against political enemies.

All U Get · Jan 15, 2018

So have you found a method to tune the rigging? I would like to check mine but couldn’t find a Loos for 1/2 inch cable. My mast has a rake from the second spreader to the masthead.

All U Get

NotCook · Jan 16, 2018

capta said:
No gauge that big available anywhere in the Caribbean, as far as I know.

That’s been our experience, too, Capta. We have the same size wire. Following the comments here with interest...

NotCook · Jan 16, 2018

capta said:
I'll be fine tuning next time I can get 20+ knots of wind without any seas

LOL good luck with that!

Hayden Watson · Jan 16, 2018

NotCook said:
That’s been our experience, too, Capta. We have the same size wire. Following the comments here with interest...

You can calculate the tension in wire based on the amount of stretch. The basic equation for elongation is δt = WL/AE with δt= elongation (inches), W = tension (pounds), L=length (inches), A=area of wire (0.153sq-in for ½" wire), E= young's modulus (15,590,000psi for 1x19 grade 316 SS).
The minimum breaking strength of ½"ø 316 is 26,678 lbs so at 20% you have a max working tension of 5,330 lbs.
The max elongation would be = 5330 x L / (0.153 x 15,590,000) = 0.00223 x L.
For a 65' wire (per original post) δt = 0.00223 x 65' x 12"/' = 1.74".
so what does all of this mean? Slack your uppers and / or fore & aft stays till they just touch the pins. Then measure the gap between the ends of the studs in the turnbuckles. from that point, you can reduce that gap to 1.75" and will have 20% UBS.
5% = 7/16"
10% = 7/8"
15% = 1 3/8"
http://www.ronstantensilearch.com/w...RPA112_Structural_Cable_Catalogue_72dpi_0.pdf

smokey73 · Jan 17, 2018

Hayden Watson said:
You can calculate the tension in wire based on the amount of stretch. The basic equation for elongation is δt = WL/AE with δt= elongation (inches), W = tension (pounds), L=length (inches), A=area of wire (0.153sq-in for ½" wire), E= young's modulus (15,590,000psi for 1x19 grade 316 SS).

http://www.ronstantensilearch.com/w...RPA112_Structural_Cable_Catalogue_72dpi_0.pdf

Interesting engineering based approach. With a little math manipulation I can use this to calculate how much the pre-stress in the shrouds would be expected to change by the difference in thermal expansion of the aluminum mast and the stainless steel shrouds. As most have said I don't think it is really much of a big deal but its an interesting engineering exercise just the same. I'll need the length of the shrouds and their angle with the mast for the trig portion to be really accurate but I like the engineering approach. Thanks Hayden, brings back some memories of engineering school.

By the way, I was back to the boat yesterday and the tension is not as low as I had "imagined" It is a little lower than when I set it at 90 degrees F or so but not as loose as I thought.

Will Gilmore · Jan 17, 2018

Does an aluminum or carbon fiber mast move more with temperature change than steel stays?
What does that do to an in-mast furling system?

-Will (Dragonfly)

SG · Jan 17, 2018

Yes, the expansion coefficient is greater (i.e., there is more difference resulting from temperature) for the same length of steel. A deck stepped mast is going to be 8' shorter (or so) than a deck stepped mast. That might affect calculations.

The backstay is usually longer than the forestay -- so you will have some differences between the forces changed (greater or lesser) on mast from those two shrouds. The side stays presumably are the same; but, also impose different resulting release of forces.

Again: Usually, the combined affect will be to reduce the "pretensioning" of the spar -- so the stays often don't become "loose".

1X19 stretch

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