interesting failure- dyneema rig terminal

[markxengineerin]

I believe my failure (bad design aside) was ultimately caused by insufficient pre-load on the threaded rod. This allowed the hangers to slide down when enough force was applied, thus putting the bolt into a bad bending situation. The compression tube was also too small a diameter and yielded with the bolt it's probably a chicken vs. egg situation about which happened first, but it doesn't matter for my situation as long as I correct both.

What size are your cut threads, and what torque did you tighten to? Also would need to know the geometry of the parts and the shroud diameter (by design) to give a more meaningful answer. You don't have the overhang issue I do, but still slightly concerning and worth a check IMO.

[Sailer_Med]

I think this all misses the underlying problem. You have a fastener in single shear and you are loading it, probably in a cyclical manner (masts do move with the load). If you want to eliminate this as a failure rethink the design a little but convert the faster to double shear design. You'll never have to think about it again.

[MicHughV]

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.

by itself....yes, dyneema is fine, but it is easily abraded or cut, which is the point I was trying to make.
yes, stainless rigs have been known to come down, but taken as a percentage of the 1,000's of boats out there with stainless wire, it would be a minute fraction. The wire is usually not the problem, rather it is the fitting on the end.

At the end of the day, it's each to his own, but in this particular instance, the existing connection of the dyneema to the mast needs to be re-thought. I'm still of the opinion, that some kind of tang would provide a solution.

[markxengineerin]

Not only loading it, but loading it out of plane in a way that reduces pre-load. I am down a rabbit hole of how to properly analyze this in a realistic way that doesn't involve bonding everything together as I did for the initial checks. The math is intense to do by hand, above my head https://www.researchgate.net/publica..._Bolted_Joints
I'm working on an idea that moves the hangers inboard, pilots them into the compression post, and might possibly allow double shear. This + easy to fabricate would meet all my goals and satisfy (some of) the reviewers here.

[MicHughV]

We all wait with bated breath to see what you will come up with

[seandepagnier]

Originally Posted by markxengineerin

I believe my failure (bad design aside) was ultimately caused by insufficient pre-load on the threaded rod.

You mean the bolt needed more tension? I am not sure this makes sense.

What does seem obvious is how far out the outer shrouds connected on the bolt giving them a lot of leverage. To make this work, a larger diameter rod, or perhaps a stronger alloy of some kind could work (inconel??) Or putting both loops on a single thimble to get them closer, or separate bolts, or what I did is have a tang on the outside as well to support the bolt on both sides of the attachment of the shroud, but I could not find any information on anyone else doing this...

What size are your cut threads, and what torque did you tighten to? Also would need to know the geometry of the parts and the shroud diameter (by design) to give a more meaningful answer.

The original design used 1/4" stainless wire. My bolt is 1/2" stainless.. I simply cut standard threads. I dont think the bolt is meant to be in significant tension at least in my case since it is also supported on the outside. I tightened it snug but not more than I could torque with a 6 inch wrench. Take standard tangs for example.. you could use a pin without any tension on it correct? The purpose of making it tight is it doesn't slide around, and the shrouds attach as close to the mast as possible.

Originally Posted by MicHughV

The wire is usually not the problem, rather it is the fitting on the end.

Agree completely.. but then at some point you have to face this issue with expensive fittings. I saw a boat where the guy bent 1/19 stainless around thimbles and I asked if you were really meant to do that and he said it was fine... Ive seen stainless wire break one strand at a time too.

I'm still of the opinion, that some kind of tang would provide a solution.

The issue with tangs are, now how do you connect the tang to the thimble? You can use a shackle but now you have the issue of the extra weight and cost of both a tang and shackle. I dont really trust the cheaper stainless shackles although they are probably fine, but you are now back to trusting stainless in the rig unless you use titanium shackle and tang or something.

[Sailer_Med]

An idea, half baked, yet maybe useful to consider. Doesn't change the loading, however, it does change the location at which the load is applied significantly.

[neilpride]

Originally Posted by markxengineerin

Not only loading it, but loading it out of plane in a way that reduces pre-load. I am down a rabbit hole of how to properly analyze this in a realistic way that doesn't involve bonding everything together as I did for the initial checks. The math is intense to do by hand, above my head https://www.researchgate.net/publica..._Bolted_Joints
I'm working on an idea that moves the hangers inboard, pilots them into the compression post, and might possibly allow double shear. This + easy to fabricate would meet all my goals and satisfy (some of) the reviewers here.

Take a break from numbers or ideas, your main problem its the dent in the mast, and in a really bad place, below spreaders, measure the depth of the dent and guide your efforts there, if you dont make the mast strong again in the dent area your mast have a dark future.
My 2 cents, sleeve or replace...

[MicHughV]

If you go back to post # 4, you'll see the tang arrangement....the single cheeky tang they call it.
But instead of only one dyneema padeye, use two, one on either side of the tang, and bend the tang out so that it matches the shroud alignment.

I don't really understand the need for the bolt to be tensioned so much in a tang application, as the load on the tang bolt will not be in tension, but in shear, two entirely different things.

A tang such as depicted above is also a good idea..but again. I'd bend the lower portion of the tang out to replicate the shroud alignment.

The original installation, would have had the bolt in both tension and shear.

I'll stick with my original opinion. A tang arrangement, however it is concocted, will offer a better solution than trying to replicate the original. ie, get the load as close to the mast as possible.

I'd like to see how the compression tube was installed inside the mast and if it is able to be removed for inspection.

As others have pointed out, and I agree, the bolt should be a solid bolt, not a threaded rod. There are different grades of bolt material, which affects their tensile strength rating, which in turn will affect the shear strength.

In this particular boat, there is not much room above the bolt location for additional screws, so the bolt will be the sole piece of equipment to provide the necessary strength to support the shrouds.

[MicHughV]

Yep, the ding in the mast is an issue. Wish we had more info on that.

Ideally, the mast should be dropped for a better looksee and determination for a path forward.

[markxengineerin]

The buckled portion of the mast is getting cut out entirely, that's why I haven't focused heavily on it. The mast is still straight as best as I can see, and the failure looks local. It was raining and gusting to 40 today, so I didn't get up there to do a layout on damage and the available space for new screws. The position of the spreaders being so close up top and the curvature of the mast starting at some point are both challenging to work around.

This problem is kicking my butt, design wise. Nesting into the mast and making the bolt double shear did not work out. I also gave up on trying to prove to myself that the massively cantilevered bolt would work. It might, but proving it is beyond the scope of what I want to take on. Here is the latest, which still has a few specific problems noted in the PDF. Advice on those is much appreciated.

[markxengineerin]

About tension in a bolt, in single shear, I think it's a good thing to have enough to keep the bolt from seeing any bending.
https://www.steelconn.uk/single-post...ternal-tension
If you look at the image in this link, if there is enough pre-load, (most of?) the shear forces if the plies were pulled to the left/right will be reacted by friction at what they call the interface/faying surface. The bolt will see no (or very little?) shear/bending.
If there is no pre-load or insufficient preload, the bolt sees shear (if the axial length is small/ minimal moment present like in that picture) or shear + bending (my case where there is a long cantilever). Resolving internal bolt forces under this loading scenario by math or FEA is complicated, I looked into it and gave up, but I think I understand the principle of what's happening. There may be an ideal torque value for each single shear scenario, where you don't over-tension to the point where bolt material is close to yielding before the moment is applied, but enough to achieve tension to support the moment.

Double shear is a different situation. If the tangs are strong enough in your design, seandepagnier, there is no unreacted moment at all, and probably no concern. Shear is less concerning to me than the moment. In my new side by side design, I have reduced the moment by a lot, which should be much better for the bolt. I'll also use 5x stronger bolts and the loads are cut by half because there are now two. That's no longer the weak link, I think it's now more about the mast and mast interface of the stuff i'm bolting in.

[markxengineerin]

If you go back to post # 4, you'll see the tang arrangement....the single cheeky tang they call it.
But instead of only one dyneema padeye, use two, one on either side of the tang, and bend the tang out so that it matches the shroud alignment.

This sounds like it would twist the tang, if I understand correctly, because the forces of the shrouds are not in the same plane. To be fair, my original had the same problem. I think Sailer Med's arrangement, bent out, would be ideal. The original wire shrouds for this boat attached exactly that way. I wanted to eliminate the additional tang, set me down the bad path. Now there's no way to get back there without new shroud lengths, due to the spreader location preventing me from going "up" at all.

[markxengineerin]

Someone earlier mentioned G/flex epoxy. Maybe G/flex is a solution for both my perceived problems. The interface I'm worried about, the 1/4" plate to mast, has about 6 inches^2 surface area as currently modeled, above the centerline of the 5/8 bolts. If G-flex has 2500 psi of "adhesion", this is a big help. They didn't advertise aluminum on aluminum, but that's roughly what the other materials show. https://www.westsystem.com/specialty...adhesion-data/

[MicHughV]

Nah, I don't see how this could twist the tang. This is pretty much how most lower shroud tangs are.
As posted elsewhere, I would make the tang thicker, as it will be supported only by the single bolt.
Forget about attaching those padeyes (what do you call those things anyway ?) next to the mast as shown in your sketch above. That connection should be the tang, and only the tang. Only one bolt required, forget about two holes. Every hole you drill eats at the mast wall area.
Once the shrouds are attached and tightened, that tang will find it's own neutral "alignment", another reason to not overtighten that bolt. In fact, attach the tang so it is free to rotate, ie, tight, but not too tight.

I know you are looking for a solution that saves you having to adjust the dyneema shroud lenght, but it's time to bite the bullet here....