interesting failure- dyneema rig terminal

[MicHughV]

Glad to see you're going to pull the mast......

[wyb2]

Why are you keeping the attachment point so close to the spreaders? I would think the mast would be plenty strong enough to carry the load a few inches.

If you move your attachment points down, say 2”, then you can cut around the old boss (see pic) and have more space for fasteners. Bonus, you would have more space to add an external hanger to take the cantilever off the the bolt, like others have suggested (2nd picture).

Kinda surprised you are getting suggestions, and going with, steel plate. How often do you see non-stainless steel on the exterior of a fiberglass boat? It’s an aluminum mast, go with aluminum. People say cyclic loading, but again, it’s an aluminum mast. Heck, your spreader attachments are welded on, worst of the worst as far as fatigue goes. The point is to make it strong enough that fatigue is not longer much of a concern. You can google fatigue charts, for 6061 t6 I’m seeing on the order of 100 million cycles to failure at 10ksi. That’s a lot of sailing. This isn’t an engine component that sees thousands of cycles per minute.

[MicHughV]

The OP doesn't want to have to re-do the length of his lower dyneema shrouds...simple as that...

[Dave_S]

Sorry, I haven't read all the posts, this may have been mentioned.

My money is on a bracket with a suitable bend and form such that the loads from the shrouds are inline with the strongest usable point of the mounting structure (the point where mast face and bolt head meet).

The bracket,

The top bend of the bracket is parallel to the mast and through bolted to a mirror image designed bracket on the other side of the mast with a compression collar between the brackets.

The bracket extends down far enough to clear a thick hardened washer, under the bolt head. After that point, it then bends away from the mast, at the same angle, inline with the direction the shrouds are running and this bend extends far enough so that you can fit an attachment point for the shrouds on the inner and outer side of the bracket with clearance.

The bracket should then branch into either an inverted T or a Y (whichever is best to clear the shrouds) and dog leg back to the mast where it is reattached to the mast to give the bracket stability and greater resistance to fatigue.

You will need to match the top bolt size and grade to suit the shear loads not tensile loads. If the angles of your bracket are accurate the lower fixings will not carry much load. The bolts for the attachment points through the bracket will need to exceed the loads in shear.

You will also need to support the mast where it has deformed.

[lo2jones]

I agree with Kenbo and NeilPride: that looks serious. Rigging is an area that, in my opinion, requires a professional, and replacing the mast looks like a real possibility, especially in that location.

[seacap]

I have had a Dyneema rig for 15 years now. Have basically the same setup on the lowers as you, with one large through bolt. I don’t have an actual picture of my Dyneema, but I do have a pic of the previous setup I had that I just modified with a new slightly wider plate to accommodate the Dyneema end fittings.

So you will have to have some custom Plates made, but most machine shops can do that. What I had made is wider than in the pic so they could fit side by side.

[admiralslater]

Originally Posted by neilpride

To much lever arm? both lowers should be as close posible to the mast wall , i see in the pic from colligo how both lines run to just a single cheeky tang?

My thoughts as well

[Pleiades]

I suggest not stacking the sheeves against the mast to reduce moment. Attach an angled tang close to the mast and attach the sheeves either side of the tang. Note that this will result in a twisting moment at the tang so make it out of 1/4” 316 stainless.

[kryg]

I fully rerigged my mast with Dyneema as well. The problem I note is the stays are out of line to the mast wall. Because your Tang fittings act as a lever as your stays are too far away from connecting to the mast, kind of like a crowbar effect if you like pulling down as a lever. Check Also the bolt it maybe you need to change it to a higher grade of stainless steel or titanium.

I have attached 2 photos of my old tang note the design and note how my stays are in line with the tangs note also it is impossible for the rigging to bend the through bolt as there is no leverage to do so. Yours seems to be a very badly designed tang connection system.

Have no idea what you can do about repairing the mast, hard to assess. Who should pay for the repair is the tang fitting supplier. Good luck with it.



Kryg, Skoiern IV

Originally Posted by markxengineerin

I discovered this a couple days ago, sailing off the coast of NJ. It happened sometime between Maine and here. This was the first sailing after a refit which included the synthetic rig. I am working on a detailed analysis and fix strategy, but wanted to get input from others here before I spoil the group creativity by sharing. Thanks!

[Cheechako]

Originally Posted by markxengineerin

I discovered this a couple days ago, sailing off the coast of NJ. It happened sometime between Maine and here. This was the first sailing after a refit which included the synthetic rig. I am working on a detailed analysis and fix strategy, but wanted to get input from others here before I spoil the group creativity by sharing. Thanks!

I'm just noting that your welds are cracked in case you dont know.

[rgleason]

I like kyrg and wyb2 suggestions, but you might be able to use a robust double or single SS tang from your current attachment point, with the deadeyes bolted to each side at the end of the tang.

[Djarraluda]

Originally Posted by 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.

Calculator? now you are showing your young age. We only had slide rule or log tables!

[burgoynebc]

The design looks horrible. The attachment points need to be as flush as possible to the mast so a tandom attachment would be better to eliminate the "stickout". IE horizonal or vertical 2 points.

You may be able to get the mast "dent" out by using the existing through hole and internal tube. Make a "puller" to surround the mast with two threaded rods and two plates shaped to the mast profile. Make sure that a replacement through bolt is pushed into the tube while pushing the old bent one out. Use the replacement rod in compression to push the "dent out from the inside. The dent is on the neutral axis of the mast hollow structure so it is subject to the horizontal loading of the spreader only, which is not high. The "pad eye" both sides of the through hole is not sufficient in thickness nor size, so re enforce this with new external plates curved to fit tight on the mast - best to surround the spreaders if possible - simply attach the plates with small screws as far from the area as possible and maybe epoxy them in place to distribute the side load even better. The dent should be removed as much as possible however, so as to eliminate compression buckling of the mast.
Richard
SV Zolanda
Nanaimo

[markxengineerin]

Here's the final final, and lots of documentation in the PDF. Since the last version, I changed bolt positions, a few radii got bigger, added backing plates to get better thread engagement, and changed from 5/8-11 threaded rod to 5/8-18 bolt. The last thing was unnecessary, but it won't hurt either.

I checked every load case I could think of vs. a baseline and can't find any problems or even near-problems. Let me know if I'm missing something.

Reading this got me re-educated on bolted joint design:
https://www.instarengineering.com/pd...ok-Dec2022.pdf
really great resource. Even using all the calculations in there, this one had to be checked with FEA- too complex a situation for the assumptions required to analyze by hand.

Repairs are scheduled to be done by Jan 1, a little slower than I was hoping, but, holidays, and I will go "home" to make the parts.

[Celestialsailor]

Originally Posted by markxengineerin

Here's the final final, and lots of documentation in the PDF. Since the last version, I changed bolt positions, a few radii got bigger, added backing plates to get better thread engagement, and changed from 5/8-11 threaded rod to 5/8-18 bolt. The last thing was unnecessary, but it won't hurt either.

Wasn't necessary? Maybe all your computer generated models say so but in practical life situations a person would never use all-thread for such an application.