interesting failure- dyneema rig terminal

[double u]

there was a mastcompany, that hung the shrouds internally in the mast: shroud entering through a slot & some bar passing through swage, IIRC.
Mabe you could adapt the system? Is there a spreader-bar traversing the mast? Maybe the "sheaves"/Thimbles could be fed through the slot & hung on the spreader-bar? Would be much neater too

[MicHughV]

You know off course, when you use threaded rod, you have to take off the depth of thread from the actual rod diameter, which can make a big difference in the x-sectional area of the rod, which in turn makes a big difference in the strength of the rod.

Better to find a solid rod and thread only the protruding ends where the nut attaches.

If you were to make a finite analysis of the difference between a threaded rod and a solid rod of the same diameter, it would surprise you as to the difference that little bit of extra diameter makes.

[dkenny64]

interesting problem.
I'm wondering what happened to the other side?
since the rig is dyneema is it that hard to resplice? the separate

the 2 connection points on the mast


-dkenny64

[markxengineerin]

The black parts are anodized aluminum, not poly, wasn't clear. Off center holes would have been smart, but I chose not to at the time for ease of manufacturing (lathe part without any offset fixture required).

many boats have much higher forward lower tension than aft lower so be selective on which one goes inside in your arrangement if you can’t combine.

Yes good idea. I also use running backs which I believe lowers the force on the aft lowers, but have yet to develop a good intuition for how much to tension them. I want to learn some more about rigging loads, haven't found a good practical example of my style rig at different points of sail and wind/sea, so might set up a simulation at a later date.

Yes, Solidworks Simulation. For some reason it gets a bad reputation but I have benchmarked it extensively against more expensive tools and it's the same math for 99% of what I need to do.

Corners will be rounded at the end, easier to put them in once all other geometry is finalized.

Internally hung shrouds- yes would be cleaner but more difficult to implement, and the cantilever issue would remain unless I make new hangers or support the outboard end. Supporting the outboard end would be elegant with an internal design, I like the idea and will think some more on how it might be done w the tools I have.

Threaded rod vs. non threaded- I agree, but hopefully the 5/8" fully torqued to 250 foot pounds makes it a moot point by being "strong enough". Threaded rod is $14 from McMaster and available next day vs. waiting for a custom piece, which would probably need to be heat treated to get the same strength, add more time for that.

The other side looks failed too, but only slightly. I was mostly on starboard tack w/ the recent west wind we've had. I am guessing this gave way when reaching with 3rd reef + staysail then staysail only when that got overpowered for the direction I wanted to go. Apparent wind was 40-45 knots for a whole night and I wasn't out there watching. I still don't have good intuition for how the loads are generated in the rig, but that's my guess.

I thought of a way to justify the design as "good enough", without having to guess at a load. The original designer spec'd 1/4" stainless wire for these shrouds. Hayn catalog says breaking strength for that is 6900 pounds and that 40% of that should not be exceeded (safe working load). Does anyone know where that 40% number came from? Either way, we are in the same region as the 5,000 pound guess, so if I can make it survive 6900, I will feel very safe.

Re splicing dyneema, the splicing is easy. The first one took me 3 hours, and by the end less than 30 minutes each. But after splicing, have to re-tension, and there is length change as the splice re-sets. I didn't record that value, but I believe it was significant. It's also tricky to put the splice in the right position so that the final length is as desired. The method that worked for me was to pre-stretch each piece to 4000 pounds for 30 minutes, remove tension, wait another 30 minutes (over which time it shrunk by roughly 1/4"), put the marks on for splice locations, do the splicing, then repeat the 4000 pound load, remove load, and check length some time later (I used 30 minutes or longer when convenient). This procedure got me within .25" of target on all 7 pieces, and turnbuckles right where I wanted them after tensioning on boat. Others may do it differently, but this worked for me, and I'm sufficiently spooked about the difficulty to not want to attempt re-splicing and length alterations, especially without having my come along and load cell to help.

[markxengineerin]

This is shameful, I went back and looked at the notes I had for tension measurements, adjustments, and photos. The time of failure is obvious and should have been noticed. My only defense is that I was sleep deprived, cold, and worrying about 20 other things on the shakedown cruise (chafe, alternator not charging correctly, missing pin lock in headstay, bilge switch not working, multiple deck water leaks, etc). My optimistic/lazy mind attributed the complete loosening of the lower shrouds to a shift in the hanger position, which in my mind must have been sufficiently stopped, because the rig tensioned up again. Failure to not think about this more deeply, and even worse not to connect all the dots until now. I even have a photo from Nov 21 which I think shows the problem (hard to make out, but bad looking angle on starboard side).

[thinwater]

Blaming this on software is amusing to me, since my engineering training predates widespread use of computers. We had them, but would not have bothered for something so simple. This would have been an exam problem, with only a pencil, calculator, and pad of paper allowed. I'm pretty sure the Brooklyn and Golden Gate bridges were designed using a pencil. No finite element analysis required. There is old math that does this. Or you could figure out the math using the calculus and geometry you learned in high school. You know you could.



Now I would use software ... and then I would double check it using back-of-the-envelope stuff, just in case I entered something wrong. Always use two calculation methods that share as little as possible.



But that is off the point. The design was not robust or mechanically efficient.

[markxengineerin]

Where did anyone blame software? And thank you, all agree it was not robust or mechanically efficient.

[MicHughV]

That photo on post # 35 sez it all for me.
It would be interesting to see that threaded rod once you've removed it from the mast.

At the end of the day, it is simply not a good setup and repeating it, even with some modifications, will likely result in the same failure or some other kind of failure.

Though you're reluctant to switch gears, I think it's time for you to accept that a different way of doing this is the way to go. Re-doing the dyneema shroud lenght, from the sounds of it, is not that hard or difficult to do.

You can always revert to "old school" s/s wire and give up this dyneema experiment. Not exactly sure why you opted for this in the first place. Those dyneema shrouds look quite thick to my eye, increasing wind resistance.

Say what you want, abrasion or a sharp cut will spell the end of the dyneema shroud whereas s/s can live to see another day.....'jes sayin'.....

The cost of another failure could be steep, ie, you could lose the mast...most likely at an inappropriate time......

As above....I'm an old school structural engineer, and while computer models are great and all that, input a wrong number and it will spit out a wrong answer...

[markxengineerin]

Dyneema experiment- because I think it's interesting, and tend to be on the R&D and risk taking side of engineering. There are risks and unknowns with Dyneema (what happens in a lightning strike, for example, compared to steel rigs, which seem to survive with extremely good probability?), but I have developed a personal dislike of "stainless" steel. I would love to have a rig that never needs to be replaced, but to my knowledge this does not exist. Dyneema is irrational reaction to frustration of the situation, and at the time I originally planned it, it was much much cheaper than wire, since I could make the fittings myself. Since then, dyneema has become more expensive, and if I had a do over, would probably not end up here.

[MicHughV]

Well, it's your boat, buddy, and you are free to do with it what you want.

Cost of replacing your entire rig should also enter your experiment.
Mast support is kinda like a chain....the weakest link can break an entire chain.
One broken shroud or stay can bring the stick down...

Stainless steel has a proven track record, whether you like it or not, and with modern-day end fittings, simple to assembly/repair yourself with only some basic tools.

[Iron E]

Originally Posted by markxengineerin

Dyneema experiment- because I think it's interesting...

Damn right it is. We wouldn't be living our best life if we all stuck to the "This is how it's always been done" philosophy.

I love what people are doing with Dyneema.
I definitely look for new ways and I definitely find this interesting af

I just wanted to enter the thread to say thanks for sharing this.

[markxengineerin]

Yes, understood, trying to make it a calculated risk but it's a risk nonetheless.

Does anyone have info on young's modulus of Dynice Dux vs. temperature? I have the following info

I have found the stretch rate of 11mm Dynex Dux to be .001205mm/mm/1000kg (.00055in/in/1000lb)

From a forum post in 2008 https://www.boatdesign.net/threads/s...perties.24097/

The chart on Colligo's website suggests 0.00099 in/in/1000lb based on my visual graph interpolation. That's a pretty big difference.

Influence of temperature on this is something I doubt we'll find, but if it's like this

The dux would get less stiff as it gets warmer. This is a helpful effect, since the aluminum mast is getting longer, and dux shorter as temp goes up. Having the dux more flexible will lower the forces some as the center distance increases and tries to increase force.

Temperature change can be ruled out as a root cause, as tension was set and failure happened between maybe 25 and 35 degrees F, but still good to understand this better.

I just wanted to enter the thread to say thanks for sharing this.

Thanks!

[Celestialsailor]

One thing that stands out that I do not see you have addressed is the all thread. Basically the 5/8-11 has a minor diameter of 1/2". This is why professional applications are always a solid shaft bolt with only the thread extending out of the mast for the most part. Also the threads act as a stress point for a bend to take shape. After reading your responses, I will take back my suggestion of a DIY fix and say the rig should be taken down and repaired by a reputable rigging expert who's blue collar approach will result in a rig done correctly.

[seandepagnier]

I find this failure very interesting.

On my own boat, I have 2 bolts, one for forward lowers, the other for rear lowers.

Both have compression pipes as well. There is compression of solid aluminum also filling the space in the stainless thimble the dyneema goes around. It was difficult to find bolts of the correct length but also not threaded for exact amount, so I cut my own threads from stainless rod and put nuts on both ends. I think this gives even lower strength on the threads (vs rolled threads), but correct me if I'm wrong, but I think the bolt itself bending is the weak point?

On the outside of where the dyneema is attached, I have a 3/16 thick stainless steel tangs on both sides connecting the two bolts. I thought this would help compensate for the attachment being slightly further from the mast than a traditional tang? I could not find much info on this though and I made it up, so curious what you guys think.

So far no failures but I have not really pushed the boat above 12 knots of boat speed.

[smj]

Originally Posted by MicHughV

Well, it's your boat, buddy, and you are free to do with it what you want.

Cost of replacing your entire rig should also enter your experiment.
Mast support is kinda like a chain....the weakest link can break an entire chain.
One broken shroud or stay can bring the stick down...

Stainless steel has a proven track record, whether you like it or not, and with modern-day end fittings, simple to assembly/repair yourself with only some basic tools.

I don’t know of any rig failures due to dyneema, but do know quite a few due to stainless. I’m not saying there hasn’t been or won’t be a dyneema rig failure, but so far it’s record has been pretty good.