Issues with synthetic rigging?

[steveR]

If the mast were composite............would there be any issues along these lines???

[echidna]

So let's look at some actual data for coefficients of linear expansion:

Aluminum 6061: 24E-6 per deg C
Stainless steel 316: 16E-6 per deg C
Dyneema: -12E-6 per deg C

I'm not sure how accurately the value I found for Dyneema applies to Dynex Dux, but I'll assume it does.

So, if for the sake of simplicity, we assume a shroud of equal length to an aluminum mast (as would be a good approximation for a deck-stepped, masthead rig), we have the following differential coefficients of linear expansion between mast and (cap) shrouds:

SS 316 rigging: 8E-6 per deg C
Dyneema rigging: 36E-6 per deg C

So the effect of differential expansion is 4.5 times greater for Dyneema in this case.

To get an idea of what this means with Dyneema/Aluminum, consider a variation in temperature from 0 to 30 deg C. This gives a relative length change of 30*36E-6 or 0.0011. Assuming all the elasticity is in the Dyneema (i.e., neglecting elasticity of the hull and mast), this corresponds to a change in load in the shroud of order 6% of breaking strength, based on the Colligo data. This value is probably greater than the pre-tension acceptable in terms of creep.

For comparison, with a SS 316 shroud, the change in load would be of order 2.5% of breaking strength, which should be compared with a typical pretension of say 15% of breaking strength for SS.

I worked these numbers quickly, so I’m open to correction on the math, but assuming it’s correct, the difference between Dyneema and SS is quite significant in this regard. Obviously I’ve made a number of simplifications in my model, but I think the general results will be reasonably valid.

[Brion Toss]

Hi,
And thanks for the numbers. I can see that this might be an issue, but would like to throw in some qualifiers. First, the range, in rational measure, is from freezing to 86 degrees. Not likely to happen in a day, or in a given season. Next, the 15% tension you give for stainless is typical for long wires, like uppers and jibstays. Next, it seems to all depend on what kind of pretension the Dux would be set at. If these numbers are accurate, one could have shrouds at about 6% when slackest, and 12% when tightest, which would keep it under the worst creep levels. In my climate, this varying tension works out for stainless, as the winds tend to be highest when temperatures drop, so this could actually be a feature.
Anyway, looking forward to further documentation and analysis. And to the adaptations we come up with in response.
Fair leads,
Brion Toss

[echidna]

Hi Brion,

Well, I would say that my 0-30 Centigrade temperature range is quite reasonable, even conservative, for a boat left in the water year round (or hauled for winter rig-in) in New Jersey (where I used to sail), having on the one hand wilted in summer heat in the high 90's (Fahrenheit), and used my former boat as an ice breaker in the destination harbor (and near frozen to death) on a December trip to Mamaroneck, NY.

Regarding loads and creep, I just went to the Colligo site to refresh my memory on their creep data, and could not find the relevant information. It seems to have been removed - Google cache still has it though.

Assuming the old data was correct, a load 12% of breaking strength would cause creep of over 3" per year in 50ft at 22 deg C. I'm not sure that is acceptable.

The question now is, was that old Colligo data valid, and what happened to it? Insofar as creep is probably the dominant consideration in sizing Dyneema for stays, it's important to have good information on it.

I'd also like to raise another point related to temperature effects, and that is the temperature dependence of creep. Creep is typically strongly dependent on temperature, and the equation on the old Colligo graph would imply that the creep rate for a given load in Dyneema doubles with every 4 deg C temperature increase. So in discussing creep, it is surely essential to indicate the expected ambient temperature. Temperatures in the tropics could easily be 5-10 deg C higher than the 22 deg C reference point used in the old Colligo data. Probably a topic for another thread, really.

[Brion Toss]

Hi again,
No, no, the range is quite reasonable, just not so likely to happen in one day, for most of us. In the boats I've seen and rigged, day-to-day temperature changes don't seem all that significant. As for the creep data, John Franta from Colligo will be checking in here shortly.
Fair leads,
Brion Toss

[jfranta]

Originally Posted by echidna

Hi Brion,

Well, I would say that my 0-30 Centigrade temperature range is quite reasonable, even conservative, for a boat left in the water year round (or hauled for winter rig-in) in New Jersey (where I used to sail), having on the one hand wilted in summer heat in the high 90's (Fahrenheit), and used my former boat as an ice breaker in the destination harbor (and near frozen to death) on a December trip to Mamaroneck, NY.

Regarding loads and creep, I just went to the Colligo site to refresh my memory on their creep data, and could not find the relevant information. It seems to have been removed - Google cache still has it though.

Assuming the old data was correct, a load 12% of breaking strength would cause creep of over 3" per year in 50ft at 22 deg C. I'm not sure that is acceptable.

The question now is, was that old Colligo data valid, and what happened to it? Insofar as creep is probably the dominant consideration in sizing Dyneema for stays, it's important to have good information on it.

I'd also like to raise another point related to temperature effects, and that is the temperature dependence of creep. Creep is typically strongly dependent on temperature, and the equation on the old Colligo graph would imply that the creep rate for a given load in Dyneema doubles with every 4 deg C temperature increase. So in discussing creep, it is surely essential to indicate the expected ambient temperature. Temperatures in the tropics could easily be 5-10 deg C higher than the 22 deg C reference point used in the old Colligo data. Probably a topic for another thread, really.

Due to many issues with understanding the creep data (or misunderstanding it), we are keeping it interrnal now. We can assist with sizing Colligo Dux Rigging with anyone who asks. For the time this is how we size the line in order of priority:

1. Stretch
2. Creep
3. Breaking strength.

We almost always have the line at much less than 10% of strength effiiciency, mostly based on stretch. Creep is figured at pretension and 0.1" per year is a maximum target. (@22C). This seems to be a good conservative number as we have many boats now rigged with normal travel turnbuckles that have not gotten into creep issues. One 65' monohull in the Carribean. For offshore boats we factor in long term dynamic loads as well. Keep in mind that creep happens over time, so average temperatures are what you need to look at, or time at temp if you have access to that kind of data.

A couple of comments on pretension: Since steel has such a low creep rate, sailors for years have been over tensioning with no short term consequences. This makes it easy to over tension at the dock and cover for any load conditions you may encounter. Some long term issues may result, stainless stress crack corrosion, Creep in the fiberglass at the chainplates, etc., but who cares about the long term stuff we are Americans!

If you oversize slightly you will be really surprised how little tension you need with Colligo Dux Rigging. I just installed/tensioned the diamonds on a F27 yesterday. We used 9 mm dux to replace the 1/4 inch wire, oversized for creep. The 1x19 1/4" wire was previously tensioned at around 2,000 lbs to get the required mast bend. With the 9 mm Colligo Dux we tensioned to around 400 lbs to get the same amount of mast bend! Yes, we have loos gauge calibrations for it now. For a 50 foot length of 9 mm UHMWPE line at 1200 lbs pretension you will get about 0.1 inches of creep per year. So this is 400 lbs and only about 20 feet long, so creep will be much less @ 22 degrees C and can tolerate much more temperature before you get into creep issues.

I don't have the time now to go thru the thermal expansion math again but suffice it to say that pretension seems to take care of it. You should expect to adjust some over long periods of environmental cycles. Fall to winter, etc will need some adjustment if pretension is critical. I can tell you I met a customer at the Oakland show this month that has a Catalina 36 (with turnbuckles) and Colligo Dux standing rigging in the Bay area, (pretty big daily temp cycles) and he was literally jumping up and down with joy about the performance of his boat. The pretension seemes to mask the linear expansion/contraction of his rigging and it still stays in the performance window for stretch.

We did have a boat (50 foot ketch) that sailed from Argentina to Greenland that used lashings initially but after getting into the cold north needed to change to turnbuckles. Lashings are somewhat limited in their ability to create pre-tension.

So bottom line as I always say, this is a new material, it is not steel and should not be sized like steel. Stretch and creep are its limiting parameters. Do not, and I repeat do not size it for breaking strength. We also like to think of it more as plastic than rope as rope carries with it certain paradigms that do not apply here either. Did I say do not size it for breaking strength???

Hope this helps. Would like to say more but no time. Please stay tuned to our website for Newsletters and our Facebook page for more and more examples of this rigging. The proof is in the pudding.

John Franta, Colligo Marine

[echidna]

Hi John,

Thanks for your response. I would like to return to discussion of thermal expansion issues, which was one of the original topics of this thread. I will start a new thread to address the creep question.

As a starting point, there is the question of the coefficient of thermal expansion for Dynex Dux.

Do you believe the value I quoted above of -12E-6 per deg C (n.b., that's a negative value), which I found quoted in various places for Dyneema fiber, to be correct for Dynex Dux line? If not, what is the appropriate value?

[brian eiland]

Lets see, I spent about 45 minutes yesterday composing a posting with links for this discussion, only to loose the whole thing when I selected 'preview'. So before I go thru that again, I want to run a test.