interesting failure- dyneema rig terminal

[markxengineerin]

So that 'engineer' in your username, Lemme guess, civil?

Ahaha ouch --- yes, not my finest moment. I looked at a relatively huge diameter shiny steel threaded rod and though "no way will this fail". I did not analyze the right things, at the time was more worried about the aluminum cheeky tangs and the compression tube getting crushed by pre-load than the bolt itself. I'm used to working with bolts that are 5x higher yield.

As I'm finding out, a 5/8"-11 316 SS bolt can only take about 22 foot pounds of torque before it yields, and at that level only provides 3000is pounds of preload. This still sounds laughably low, but that's what I'm finding. I surely yielded it on assembly. With a 0.5 assumption for coefficient of friction, this isn't even close to preventing the cheeky tangs from slipping under the expected shroud tension. I don't know what it is exactly, but for analysis purposes am going to go with 5000 pounds each, unless someone has a better idea. Thats >5x the static load I set them to.

At least the failure makes sense so far. I'd still like to solve it without making new shroud lengths or cheeky tangs, but all the ideas presented so far are helpful for if there is no easier way forward. Now that I understand the bolt, next steps are to look at the cantilever situation, the compression tube/mast wall, and trying to prevent the bolt from seeing any bending.

[Benz]

Don't I have a memory of a thread about this very thing some time ago? Seems like I remember discussions about long lever arms and some about wall thicknesses....

[MicHughV]

You might be able to re-use the original bolt if you can't find a new one. Take it to a machine shop and have them lop off the bent end and cut a new thread . The bolt needs to pass thru' the mast, both new tangs, a washer each side, and enough thread for the nut plus a tad more.
Ideally, I'd get a new bolt as the old one might have been stressed.
If no machine shop is available, you'll have to cut the bent piece of, and drill a hole to accept a 1/8" s/s cotter pin. Allow at least 1/2" beyond the cotter pin location.
Cutting and drilling s/s requires special blades and drill bits and a bit of know how.

You will have to get new tangs made as well. I don't know your boat rig loads, but at a rough guess, something like 3/16" thick x 2" wide. Allow for about 1 1/2" meat above the bolt location. They need to be long enough so you can bend them to follow the shroud orientation. I'm guessing about 6". As above, allow at least 1 1/2" meat below the pin location for the shroud pin. You'll have a tough time bending these tangs yourself. A machine shop is you best bet here.
You could probably modify the existing black padeyes by sanding them or cutting them so they lay flush against the new tang.
Use these suggestions as a guide.

[markxengineerin]

Don't I have a memory of a thread about this very thing some time ago? Seems like I remember discussions about long lever arms and some about wall thicknesses....

https://www.cruisersforum.com/forums...ig-265066.html

Great evidence of analyzing the wrong thing, and me missing the fact that the stainless bolt has nowhere near the pre-load I assumed it would. You did mention the overhang, good call!

The original bolt (actually threaded rod w nut on each side) will not be re-used, as it's damaged and never could have worked from the start. Anything that involves changing the lengths of the shrouds is expensive, can't be done by me where I'm at, and nice to avoid if possible. I do agree with the suggestions in principle but trying for something that can be done in a random anchorage with minimum tools. I have a hack saw, drills and taps, normal hand tools.

[MicHughV]

Well, if you could find a new bolt, and some aluminum plate, thickness to match the existing plate/washer at the bolt location, you could fashion a plate that lay alongside the mast right underneath the bolt location. This would help to distribute the loads from those shroud padeyes to the mast...say something like 6"x"6, or 8"x8". You'd have to bend it to follow the shape of the mast. Cut a U-shaped slot in it, so you can slide it up around the existing thickened plate/washer. Once in place, the plate can be attached to the mast with some machine screws. Then I'd take some WEST system epoxy or any other epoxy you might have handy and fill the gap between the existing plate/washer and the new plate.

What you are trying to achieve is more bearing surface for those black padeyes.

Not exactly sure why you are tensioning that rod so much ??

Also not clear how much damage was done to the mast.

Anyway, just throwing out ideas for you too munch on.

[Celestialsailor]

Originally Posted by markxengineerin

As I'm finding out, a 5/8"-11 316 SS bolt can only take about 22 foot pounds of torque before it yields, and at that level only provides 3000is pounds of preload. This still sounds laughably low, but that's what I'm finding. I surely yielded it on assembly. With a 0.5 assumption for coefficient of friction, this isn't even close to preventing the cheeky tangs from slipping under the expected shroud tension. I don't know what it is exactly, but for analysis purposes am going to go with 5000 pounds each, unless someone has a better idea. Thats >5x the static load I set them to.

A 5/5"-11 ss bolt yields at 22ft/lbs.? I don't think so. Yield strength of the shaft itself is around 25,000lbs. Yield torque on the nut about 250ft/lbs.
To me, the obvious problem is L over D (length over diameter). In other words it is a lever. If it were mine, I would find some "surplus triangular plates like wire lowers use, use a large diameter spacer for the Dyneema to go around...keeping the force closer to the mast. As far as repair, I would put a 1/4" plate or so to cover and substantially overlap the dent, drill and tap it to the existing mast, slowly pulling it in with multi fasteners.

[markxengineerin]

My bolt analysis is highly simplified (just using a rod with no threads on it), but 3000 pounds applied axially on a 0.570" diameter rod shows localized yielding. Even if the stress is perfectly distributed, the cross sectional area is 0.26 inches^2, so at 3000 pounds that's a stress of 11,700psi. With a yield of less than 30ksi for 316 stainless, that's still < 9,000 pounds of pre-load possible. Where are you getting 25,000? I think that's for regular steel, not stainless.

Edit, here's a reference showing 96 foot pounds. That seems about right based on how I torqued it, which was about like I'd tighten a lug nut on a car wheel.
https://www.engineersedge.com/materi...teel_13353.htm
The point being, it's not even close to strong enough for this application.

As far as repair, I would put a 1/4" plate or so to cover and substantially overlap the dent, drill and tap it to the existing mast, slowly pulling it in with multi fasteners.

That's similar to what I'm working on now. It looks like the original design would have worked up to 5000 lb shroud load if I had used a Grade 8 threaded rod and torqued it to 250 foot pounds. I will post more when finished. Thanks for the input.

[markxengineerin]

What you are trying to achieve is more bearing surface for those black padeyes.

Not exactly sure why you are tensioning that rod so much ??

Also not clear how much damage was done to the mast.

Anyway, just throwing out ideas for you too munch on.

For the tension, I'm copying what the previous owner of the boat used, which was between 12 and 15% on 1/4" wire. I converted that to 1000 to 1200 pounds static tension. If that sounds wrong, please LMK, I'm new to this stuff. Dynamic load factor is unknown so I'm using 5. Also open for input on that one. The tension on the threaded rod needs to be enough to prevent sliding of the tangs on the mast. For 5000 pounds of shroud tension, that's about 20,000 pounds of pre-load. This requires a high torque. 250 foot pounds makes 27k pounds of preload, providing some margin.

I will do a more thorough mast evaluation hopefully tomorrow. The ideas are very helpful, keep em coming.

[Patrick of M]

Interesting bunch of suggestions/hypothesis above.
More grist: One of the downsides of dyneema rope is the elongation when cold. You set your tension at 30F which is cold, your mast was shorter, your rig was longer.
https://www.riggingdoctor.com/life-a...rmal-expansion

Your failure may be two-fold, a marginal design compounded with overtensioned rig. As my link indicates....tension rig on a warm day.

[Celestialsailor]

I wonder how other rigs with 3/8" to 1/2" stainless bolts survive...the horror! the horror!

[markxengineerin]

I wonder how other rigs with 3/8" to 1/2" stainless bolts survive...the horror! the horror!

By not being cantilevered this badly. For example, the turnbuckles are 1/2 UNF and have 14,000 pound breaking strength, load perfectly axial, no issues. It's just a bad design here.

Your failure may be two-fold, a marginal design compounded with overtensioned rig. As my link indicates....tension rig on a warm day.

Could be, I haven't gotten that far yet, but will before I proceed. Without numbers I don't know how to respond, and nobody has provided any that I know of. I have a tension meter and will check tension vs. temp over time once this thing is back together, but unless there is a significant temp change in a short amount of time, it might be hard to differentiate from creep. It can also be calculated, which I will try, but the hull movement probably comes into the picture too, and every boat will be different. I will assume the hull is rigid and doesn't move with temp for the first check.

Colligo's recommendation is to tension the rig on the coldest day, not hottest, so...

Here is what I have so far for a fix- all stuff I can do in the field:


More analysis I used to get here in PDF. There's an error in there i haven't fixed on the last bolt analysis slide- shroud loads were missing by accident, actual stress is 943ksi obviously way too high. It is essential that the assembly does not come loose and the bolt never acts as a pin. The fix needs more check but looking hopeful so far.

[fxykty]

I have no dog in this fight and am not an engineer, but do have two comments. The first is regarding your proposed solution and based on what knowledgeable others have posted. The second is simply personal opinion.
1) Use a single hanger for both shrouds - stacking the spliced eyes is not a problem as there is no movement, or you could make your hangers wider to accommodate side by side. The original problem was caused by the cantilever caused by two hangers - why continue with that design?
2) Please do not refer to your hangers as “Cheeky Tang”. That is a registered trademark of Colligo Marine.

[markxengineerin]

Yes, for the reasons described, I don't want to re-make the shrouds and the hangers. Doing so would cost several thousand dollars vs. several hundred for the proposed plan, and there are lots of ways to make something more expensive and stronger. I just want to be strong enough to not worry about it.

Sorry, I did not realize cheeky tang was trademarked and didn't know what else to call them. I was pretty clear about the Colligo design being superior/not having this issue, and have nothing but respect for that business. John has helped me plenty on other aspects despite my wanting to DIY the project- 5 star review for Colligo Marine.

[RaymondR]

You may have got away with it with the big washer things machined from aluminium with the holes as far off-centre towards the top as possible. I'd be inclined to straighten the bolts, build the spools from alloy and have another go with the existing arrangement.

The poly spools compressed enough to allow what should have been largely shear loading on the bolt to become a bending moment. Relatively incompressible alloy washers with off centre holes should prevent this.

[ABJ87]

Originally Posted by markxengineerin

By not being cantilevered this badly. For example, the turnbuckles are 1/2 UNF and have 14,000 pound breaking strength, load perfectly axial, no issues. It's just a bad design here.


Could be, I haven't gotten that far yet, but will before I proceed. Without numbers I don't know how to respond, and nobody has provided any that I know of. I have a tension meter and will check tension vs. temp over time once this thing is back together, but unless there is a significant temp change in a short amount of time, it might be hard to differentiate from creep. It can also be calculated, which I will try, but the hull movement probably comes into the picture too, and every boat will be different. I will assume the hull is rigid and doesn't move with temp for the first check.

Colligo's recommendation is to tension the rig on the coldest day, not hottest, so...

Here is what I have so far for a fix- all stuff I can do in the field:


More analysis I used to get here in PDF. There's an error in there i haven't fixed on the last bolt analysis slide- shroud loads were missing by accident, actual stress is 943ksi obviously way too high. It is essential that the assembly does not come loose and the bolt never acts as a pin. The fix needs more check but looking hopeful so far.

Can you get both dyneema shrouds onto a single tang? Would go a long way to reducing the cantilever. Also 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.


Solidworks simulation? Please, please backup your FEA with some hand calculations. Is a nice tool but is only as good as the setup.

Think about rounded corners on the repair plates an inserts.