Tension measurement for synthetic fibre rigging

There you go. There is an engineering spec and a simple way to measure. Since you are going to fine tune under sail, putting 15% tension in using a rule and calipers as a starting point is pretty easy.

Isn't this the information that you were trying to get for DDux over a year ago? You had to do your own tests didn't you?

Did you find that DDux has linear stretch over the 5-20% MBL range?

I think you can make the DDux work, you are able and willing to do research that few others are. My impression is that the maker of the rope and the retailers could not or would not answer your basic questions. If this is not correct and there is proper data available now I'll be more than happy to retract some of my statements about the rope.

 

At least over my test range up to 40% MBL the elongation is linear. Not only that but the elongation is directly related to the weight of the rope.

I think what happened is that the recreational market sort of snuck up on Hampidjan. Their primary business is supplying the commercial fishing industry. They had all the data but in a format that a trawler skipper wants to see. Even worse, it was for European skippers who had some idea how to use Newtons/tex. It took some time to get it published in a form that we could use but even now it takes some time to convert it to a format that I as a structural engineer am used to working with.

Taking the standard engineering results from my testing and working backwards correlates within 1 or 2% with the charts on John's page.

I do see a strong need for an accurate means of measuring tension in Dux however. Probably more so than wire. The elongation rate is so low that one turn of a turn buckle has a very large effect on tension.

 

This weight/elongation relationship is similar to SS wire then?

Turnbuckle thread pitch and length of the stay are related. Once the total length of the stay is known. The turnbuckle becomes a dead accurate tensioning device.

 

What I mean is that both elongation and ultimate strength are directly related to the weight of the rope. In fiber the weight per given length is used rather than cross sectional area because regardless of how tightly spun there is always some void. Unless you know the diameter of each fiber and have the patience to count a few million of them it is impossible to know the exact cross sectional area.

A given piece of Dynex Dux that weighs X per foot will have twice the MBL and half the stretch of one that weighs 1/2X per foot. I believe this is because, at least in the diameters up to 20mm, the angle of the fiber to the load doesn't change and the opposing braid eliminates any tendency to rotate. In contrast as wire rope increases in size the geometry of the wind changes slightly so the rate of elongation goes down as diameter goes up.

The last thing I have to work on to complete all the required data in an engineering format is the formula for creep. I have developed an exponential equation that describes the creep curve in my testing of 9mm. I applied it to 11mm and compared it to the curves on John's site. It works well up to about 10% MBL then goes crazy so I am doing something wrong.

Hampidjan gives the constants for the Baily-Norton creep formula but I can't figure out how to apply it to the finished rope.

 

You can not measure stretch with the turnbuckle. I guess the problem is that DUX streches very little at the creep limit. According to this: http://www.colligomarine.com/Colligo-Synthetic-Systems/Dynex-Dux.htm
DUX stretches only about 0,05% at the creep limit. This is so small, that it can not be measured with a ruler.

I would think, that measuring the tension with a decent accuracy should be quite easy by measuring the force needed to deflect the rope to a defined angle.

 

If I read that chart correctly 11mm DD has .000468" stretch per inch per 1000 pounds of load?

40 foot shroud = 480 inches = 0.22464" stretch per 1000 pound load?
11mm DD is rated at 40480 pounds, 10% is 4048 pounds?
So round numbers at 10% load that 40 foot shroud will stretch 4 times .22464"? or 0.89856"?

I'm pretty sure I can measure .9 inch.

A rigging screw with 18 TPI moves .1111" per turn (double the thread pitch), so It would seem I have a 8 full turn range between 0 and 10% load?

Am I reading the chart wrong?

Take that 40 foot shroud in SS 1x19 using the 1mm over 2000mm = 5% rule of thumb. 10% load stretches the wire .48" ... about 50% of DD?

 

Why on earth are you making the calculations in mixed units instead of SI?

It seems that there is something badly wrong in the table. It shows a larger strecth for 1000 lbs than 1000 kg while 1000 lbs = 454 kg. Actually the 1000 kg stretch = 0.454 * 1000 lbs stretch while it should be divided by 0.454.

Looks like the 1000 lbs value is more reasonable and strecth is very close to 1x19 at target tension (10%MBL for DUX 20%MBL for SS).

When you tension the rope using a turnbuckle there is much more happening than just streching of that rope. Other ropes stretch as well, the hull changes shape, mast compresses and changes chape etc. Thus you can not calculate tension from the turnbuckle. You need to fix the ruler to the rope.

 

Sorry ... I grew up with Imperial so I "think" in those units. Riggers working in NA use Imperial sized rigging except for the wire supplied with Seldon/Furlex units.

That table is very confusing. That the table was created by the retailer of the rope so it is the only data to work with? Why not relate stretch to rated strength? If you conclude that the rope has the same stretch at target tension and the target is tension is 50%, then the rope stretches 50% less for the same load?

If we agree that the tension transfers from the unloaded side to the loaded side, then at the design load with 50% pretension, the leeward shroud is slack and the windward shroud carries 100% of the load. The mast moves some distance to leeward under those conditions correct? That distance should be 50% for the stiffer rope correct?

Cycle the rig between 50 and 100% of design load at a given frequency. One rig moves twice as far as the other in the same time. The means the accelerations are double for the stiffer rig correct? This doubles the cyclic load on the fittings does it not?

Now combine a stiff rigging material with a slack rig ... what happens to the loadings on the fittings? The soft rig treats its fittings more gently than the stiff rig. Has anyone taken this into consideration when they replace rigging with a rope has has only 50% of the shock absorption of the material it replaces? I spent about an hour rolling about in swell with very little wind during a race yesterday. As the boat rolled though 20 degrees I thought about what would happen if the rig was slack. Instead of a smooth transfer of load from one side to the other in the rig there would be a violent shock with each roll. Even with proper tension the fittings are subject to accelerations that place large shear loads on them. Fittings like tangs and clevis pins (if properly designed) will show zero deformation after 100's of thousands of load cycles, double the acceleration loads and how soon will you see elongated holes in tangs and clevis pins that show signs of shear stress?

Many boats built in the 1970's had 3/8" pins on 1/4" wire ... I used to keep a handful of damaged pins and a few tangs with oval holes to show my customers when I suggested that they use 1/2" pins and replace chainplates and tangs when they replaced rigging ... Has anyone done a cyclic loading test on fittings designed for SS wire when subjected to stiff synthetic rope instead? After a year rolling about on the mooring with a slack DDux rig, how much damage will there be?

Yes, you fix the ruler to the rope or wire so you measure the elongation of the rigging. Fixing a 2 meter measure to the rigging and measuring the change is a simple method that riggers have used for years.

If the same data was available for rope, the same method would work. I made up a spreadsheet using the standard cross sectional area of 1x19 to verfiy the 1mm per 2000mm stretch rule and satisfied myself that for the range of wire sizes I used to work with 1/8" to 1/2" diameter (3mm - 13mm) that the 1mm per 2000mm was a useful tool.

 

No, that was not the conclusion. The conclusion was, that the strecth is about the same.

Steel would be dimensioned to be tensioned to ~20%MBL and would then stretch about 0.2%.

If DUX would be dimensioned to ~10%MBL due to creep and would then strecth about the same 0.2%. Thus DUX would has twice the breaking load, but equal stretch at about the same diameter.

Maybe at 10%MBL DUX still has too much creep. Thus you may have to go even to 5%MBL. Then you would have about 50% less stretch and about 40% bigger diameter.

All the above was based on equal absolute tension in the rig.

 

The data suggests that 20% is well into the creep zone.

If I understand correctly:
If Dux is working between 10% and 20% of MBL it will have about the same stretch as a Steel rig working between 20% and 40% of MBL at the same tensions?

That if the Dux must be limited to 10% MBL to eliminate creep that it will have 50% of the stretch of a Steel rig?

Do you agree that 50% of the stretch doubles the cyclic loads?

If you were rigging with DUX what sizes are direct replacements for Steel?

 

I was referring all the time to static loads. I don't think creep is a problem during dynamic loads, which typically are very short compared to static ones.

A typical pretension is 20%MBL for steel. What happens at dynamic situations depends on many factors. The leeward shroud is never slack in my boat. In some other boats it is slack already at 15 degrees heel.

For DUX you can not use 20%MBL pretension, since it would creep like crazy. Even 10%MBL seems to creep about 0.25%/year, thus you would need to adjust the tension several times a year, if you like to keep the pretension constant. You can not use turnbuckles since they would run out of adjustment in 1-2 years.

At 5%MBL creep is less than 0.05%/year, thus pretension would be almost constant for a year and turnbuckles would be OK. Thus I would choose that.

All of the above is based on the charts and tables provided by Colligo Marine. I have no experience on synthetic standing rigging and I'm not considering to buy one.

I don't think that lack of stretch is a problem to rigging. Too much pretension might be.

 

RHough's calculation is correct but it also illustrates what I was saying about turnbuckle adjustment being very sensitive. In that 40' shroud if 8 turns creates an extension equalling 4,000 lb, one turn adds 500 pounds to the tension. When you are trying to add 200 pounds it gets pretty exacting.

BTW, The first rule for synthetics is to design for creep rather than load. If I designed for load my cap shroud would be 9mm Dynex or 3/8" wire but creep would be unacceptable so 11mm is the correct choice. I have run several simulations of creep based on the cap shroud of my boat projected over a year of relatively heavy use in the Caribbean.

Assumptions:
Shroud size = 11mm
Shroud length = 58'
Design load at RM30 = 4,150 lb (10% MBL)
Pretension = 2,000 lb (5% of MBL)
Average angle of heel 25 degrees.
Passage making hours (52 days @ 24 hours) = 1248
Day Sailing hours (5 hours on alternate non passage days) = 640
Total = 1888 hours (22% of the year and slightly higher than the SCCA survey results)

Results;
Creep in port = .16"
Creep under sail in windward shroud = .30"
Creep under sail reduced in leeward shroud = -.04"
Total creep = .42"

This does mean that the rigging will require some degree of attention. As the shroud creeps pretension will decrease so to maintain the correct pretension a 3/4" turnbuckle with a 16 tpi thread will need to be adjusted by an average of a little less than one turn every 3 months. A 3/4" turn buckle has an adjustment range of about 3" or 48 turns so at 4 turns a year that is about 12 years before it bottoms out. By that time I will be pushing 80 and probably driving a trawler.

For those worried about the added windage I also ran the wind load calculations for the shrouds on my boat. At 27C, standard sea level pressure and 20 knots apparent at 30 degrees, the 11mm Dynex added 59 pounds of drag. That results in 51 pounds (cos(30)*59) opposite the driving force over 3/8" wire and 29 pounds (sin(30)*59) in the heeling direction. However, that has to be balanced against a weight saving aloft of about 90 pounds.

51 pounds is a very small percentage of the total driving force at 20 knots apparent so I am not terribly concerned about that but the question remains as to the relationship between the heeling force of the additional wind load and the increase in righting moment from the less weight aloft.

 

There is something wrong here. 59 lbs would be a very high drag. I get something like 50 N = 11 lbs for total drag of standing rigging (assuming 4*18 m of rope and Cd=1). The difference to 9 mm would be about 10 N = 2 lbs.

I have no idea about your boat, but just based on the length of the shroud I guess it has a drag of 1000-2000 N (200-400 lbs) on beat.

 

Density was in kg/m^3 so the result is in kg. I converted to pounds.

Also should have stated that this was the wind load on 370' (including the back stay) of 11mm Dux vs 3/8" wire and used a Cd of 1 for wire and 1.2 for Dux to avoid any argument about the roughness of rope over wire. That probably overstates the difference by close to three times but I was trying to be very conservative. Double check my calculation:

I used (Cd*A*p*V^2)/2
Cd = 1 to 1.2
Area 3/8" = 9.5mm * 112.78m = 1.07m^2 and 11mm dux * 112.78m = 1.24m^2
Density (p) @ 27C = 1.176 kg/m^3
Velocity = 10.3m/s
For 3/8" drag is 66.7kg
For 11mm Dux at Cd of 1.2 drag is 92.6kg
At a Cd of 1 drag is 77.2kg
So the diffference is somewhere between 10 and 26 kg or 22 to 57 pounds.

 

Just for grins what is the next size up from 11mm? 13mm?

What does that do for your creep numbers, weight, windage?

Set the Pretension @ 10% and see what % you have under sailing loads. My concern (not for you) is that my experience has been that sailors don't check/adjust rigs more than once a year (if that) unless they are racers. The rig in 3/8" SS would get tuned once, checked once after a week or two then can be forgotten for years. I think you are talking about a level of care that I just have not seen on cruising boats. Okay for you, you are a geek (meant as a compliment), but for 95% of the boats I see, the tape on the turnbuckles as old as the rigging. Well designed and tuned rigs are set it and forget it ... if anything changes and a once taught shroud is slack, there IS a problem. If the habit is to wind it up a turn every 3 months, there is no reason to go up the mast and look to see why something changed.