# Tension measurement for synthetic fibre rigging

Discussion in 'Sailboats' started by patrik111, Nov 23, 2009.

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### GashmoreJunior Member

In my experience significant number of boat owners pay no attention to the shroud tension at all. This stuff is definitely not for them.

It amazes me some of the people who buy big sailboats. I was talking to a salesman at the Annapolis show and remarked at the size of a galley and that there was no place for the cook to brace. He said the reason is that they sell to the wife, not the husband. He just wants a boat and will buy whatever she gives the OK on. He is definitely not a candidate for synthetic rigging.

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### JoakimSenior Member

No the unit of force is N. Here on earth at sea level 1 kg = 9.81 N.

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### GashmoreJunior Member

The density of dry air at sea level and 27C is ~1.176 kg/m^3 or 11.53 N/M^3 but because the kg is a unit of mass and the Newton of force the standard expression is in kg/M^3. You get the same result using either but the result will be in the units you started out with.

In a more practical way, consider that the total cross sectional area of the rigging is a bit over a square meter. The Cd of a flat plate perpendicular to the wind is 1.17. Think about holding a square meter of plywood up to a 20 knot wind. The force will be considerably more than a 11 pounds.

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### JoakimSenior Member

Density can only be mass/volume thus kg/m^3. When you use F=0.5*Cd*rho*A*V^2 formula you just put SI units in and get SI units out.

F = 0.5 * 1.0 * 1.176 kg/m^3 * 1.24 m^2 * (10.3 m/s)^2 = 77.35 kgm/s^2 = 77.35 N (by definition [N] = [kgm/s^2], remember F=m*a => [N]=[kg]*[m/s^2]).

Dividing that with gravity acceleration of 9.81 m/s^2 you get that 7.89 kg mass has the same gravity force. And then divide with 0.454 kg/lbs and get 17.4 lbs.

It so much easier with pure SI units....

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### JoakimSenior Member

Haven't tried that, but no problems holding 6.5 m^2 sail of my windsurfer at 20 kn apparent. Even a 60 kg (132 lbs) surfer could do that. If the force was much more than her weight, the sail would make her fly.

I can also bicycle at 20 kn speed in calm for an hour. The surface area is somewhere between 0.5 and 1 m^2. I'm sure I can't output more than 200 W for a longer time. 200 W at 10.3 m/s => P = F * V => F = P/V = 200 W / 10.3 m/s = 20 Ws/m = 20 N. ([W] = [Nm/s]) 20 N = 4.5 lbs. That 20 N includes rolling resistance as well.

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### GashmoreJunior Member

Well, you are right. In canceling units I forgot we squared the m/s. All SI units but needing a different conversion. (I went to Tech in the slide rule, abacus and imperial era.)

So windage is a minor issue if it is an issue at all. Definitely out weighed by the improvement in righting moment.

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### jfrantaJunior Member

The Data has been on the website for over 2 years now in one form or another.

As for the incessant name calling, snake oil salesman, marketing BS, amateur. I do all the big shows, Annapolis, Chicago, Miami, Oakland, la Rochelle, Southampton, would be glad to buy you a beer and see how you respond to a person when he is in front of you. Let me know and I will get you on my schedule.

John Franta, Colligo Marine.

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### jfrantaJunior Member

No your statement is not correct. Gashmore has confirmed our stretch and creep data that has been posted on the website for over 2 years now. You need to retract your statements as any professional would do. FYI: If you can do it without any name calling it would add greatly to your credibility.

John Franta, Colligo Marine.

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### RHoughRetro Dude

As noted.

The data is available, not in a format that is generally used, but it is available. Gashmore asked for it in a standard engineering format so he could make comparisons and ended up with:

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

and

"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."

Joakim says:
"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."

So is some data available? Yes
Is it correct and usable by an engineer? No, Yes, Maybe?

Do I owe you a retraction? Has the data you provide changed in the time it had taken for Gashmore to run his own tests only to conclude that there is something wrong with either the formula or his application of it?

Have you provided the test to failure results of the rigging assembly's you sell as I asked for over a year ago? The ones that will show that your splicing method and end fittings do not seriously lower the MBL of the rope?

What is your response to this from Gashmore in post #31?
"In my experience significant number of boat owners pay no attention to the shroud tension at all. This stuff is definitely not for them."

How does that comment fit with your statements about 100's of cruising boats using DIY rigging?

Gashmore concludes that his rig will need adjustment every 3 months. How many SS rigs require that?

What does buying space at a boat show have to do with anything? Are you saying that boat shows screen renters to make sure that the claims they make are valid? I've done too many shows myself to believe that.

I have said time and time again that I would love to be convinced. So far I have your claims as the retailer and at least two people that seems to know something about rigging questioning the formulas you provide. The engineer that started asking questions, still has questions and has concluded that constant adjustment over the life of Dux rigging is acceptable.

When you compare two products they must be comparable. For a replacement of SS wire in rigging, that must include the ability to hold a tune. At a minimum customers should be advised of the increased maintenance requirement.

We can sell you Dux rigging. The benefits are: X,Y, and Z compared to your SS rigging. However, to take advantage of these benefits, you must do A,B, and C more often.

The reduced weight aloft comes at a price. Increased cost of the rigging itself if you consider the fittings and professional splicing and installation. Increased maintenance if you have to pay to keep it in tune.

What to you think when you hear claims that offer something for nothing? Less stretch, less weight and NO cost?

From what can be learned here:
Same stretch as SS but will creep and require frequent adjustment for reduced weight benefit.
Less stretch than SS at no creep sizes but higher cyclic loading and less weight reduction.

Does anyone get that from you? You claim direct replacement for SS with Dux, until your replacement recommendations result in rigs that are as low maintenance as SS (no frequent re-tuning needed), they are not really direct replacements are they?

Synthetic rigging has a future. Until you come clean and make people aware of the trade-offs, I'll keep holding your feet to the fire.

Fair enough?

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### GashmoreJunior Member

Randy, this conversation definitely illustrates the validity of my great grandfather's dying words "'Tis the friction of minds that causes the spark of truth."

What I believe is the foundation of this problem is that Dynex Dux is a product originally intended for one market and unexpectedly adopted by another. Hampidjan serves the commercial fishing and heavy marine industries. They talk the language of hawsers and tow lines. It has taken them some time to learn the language of recreational sailing and even longer to understand the market. In fact, after talking to a couple of their engineers I am not sure they understand it fully even now. That said, there is a lot more detailed data available today than when I started this project two years ago and my testing indicates that the data is valid.

Today there are a number of vendors from Europe to Australia selling Dynex Dux for standing rigging and of them all John has made the most effort to discover and publicize the available data.

Are there compromises to be made? Certainly. A boat is one big compromise. From the material the hull is built from to the size of holding tank. Discovering and understanding the effects of these compromises is my primary goal in this effort and I have to say that your role as devil's advocate has been a great help in the process.

I believe I have enough information now to answer the questions presented here and on other forums and over the next few days will organize it all in a report that lays out the choices. I will put it on my web site and post a link here. Hopefully I can sell a simpler version to one of the sailing rags. After all, I have to buy a couple thousand bucks worth of rope soon.

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### RHoughRetro Dude

If anyone follows your lead and goes in with eyes open as to the compromises and makes an intelligent (based on knowledge rather than sales claims) decision, then this forum has done it's job.

Your willingness to share information is a great example to others. IMO we have all learned something. I have decided to wait and see before I decide.

Until tension measurement in Dux is known and repeatable, you have to be very careful not to pretension too much. What I'm hearing is that 10% is a target to limit design load to 20% to limit creep. If you miss 20% on SS wire, there is no harm done, SS is elastic way past 40% MBL.

I'm hearing that static preload in Dux is more critical than in SS wire and there is no tool or procedure to measure the tension accurately.

All this makes Dux an experimental rigging product IMO. I don't feel it is ready for the prime time retail market. I'm willing to think that it could be, but it is not there yet. The attitude of the people selling it as DIY kits is not presenting a good case. So I call them out on it.

So thank you for the kind words and for sharing your test findings. Your attitude is professional and much appreciated.

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### JoakimSenior Member

Well this depends a lot on how you look at it and what is your goal.

If your goal is to have a well known pretension for the whole season, you are much better of with steel (or really over dimensioned DUX). E.g. if you are measuring your settings from turnbuckles like I do, creep is not a nice feature.

If you use too much pretension with steel, you are more likely going to have fatigue issues and eventually something will break. Going over 25%MBL pretension is not a good idea.

Due to creep DUX will in a way pretension automatically. If you put 20%MBL pretension, it will creep quite fast until it reaches a low creep pretension. Then you could measure the turnbuckles and set up the same pretension for the next season. Eventually you would reach a constant pretension, but it may not be the pretension that is optimum for your rig and sails.

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### RHoughRetro Dude

That is a novel idea! Tension the rig, see if it creeps. Let it creep to reduce the tension. Once it is stable, see if it has enough pre-tension, if not go up a size and repeat the process!

I agree about not going over 25%MBL on SS rigging. Point was that if if you are close to 20% a small error of 5% does not damage the rig in most cases. 5% in SS wire is about 10% of the elastic range (IIRC the deformation load is about 50-55%?). For Dux, the no-creep range is 0-15%? so 5% is 33% of the target range? 10% of the working (no-creep) range is 1.5% for Dux, thus you need more accurate measuring tools?

Way back in this thread I suggested using a halyard to heel the boat to 20-25 degrees to get a ball park pretension. Centre mast, add some pretension, heel the boat and see what goes slack and adjust from there. The actual numbers are no so important if the rig is sized properly, just make sure nothing goes slack before 20-25deg of heel. A SS rig should hold this tune until the service life of the wire is reached. When it no longer holds tune, the wire is due for replacement, it has reached the point where the elastic range is lower than the normal sailing loads.

I have run into situations where a new wire will not hold its tune. The culprit has been a rig that was marginal with 302SS wire and a replacement with 316SS has made the normal sailing loads to high for the wire size. Production boats fro the 1970's are apt to see this when a NA owner wants to re-rig with "Tropical" 316 wire. I try to do the basic math on rig to make sure that a change to 316 will not reduce the safety factor too much if the owner requests 316.

I think what I find hard to accept is the concept of creep being manageable and acceptable. I equate Rig not holding tune = Problem I can fix. I just cannot see a customer that will choose Dux after hearing, "Your new rigging might creep and require re-tuning depending on how you sail the boat. This is normal. The rig tension must be checked every 3 months or so until we know how often you rig will need to be re-tensioned." I'm a rigger and I would not put up with that, I want my rig to be stable, even if it means using larger diameter Dux ... I'd have a better feeling about the sizing suggestions if they were "no creep" replacements. Then the only thing to check is the safety margins of fittings to handle the increased loads. The approach seems to be to size Dux just big enough to keep creep in the "managable" range ... for me that is zero.

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### JoakimSenior Member

According to this the yield limit for steel rope is 70-85% MBL: http://www.fatzer.com/contento/Link...glisch_Drahtseiltechnik.pdf&tabid=241&mid=734

But I think fatigue is much more important limit than yield for steel in this case.

I don't think it is good to compare %-values here. For steel using only 10%MBL pretension is too little and 30%MBL is too much. Thus missing the target pretension by 50% leads to problems. I don't think many would accidently use 30%MBL since that would not be easy to achieve on most boats and many owners don't dare to use even 20%MBL.

For DUX your target may be 5%MBL. 10%MBL would be 100% error in pretension and an extremely tight rig equal to 40%MBL for steel. How many turnbuckles could handle that without ruining threads?

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