Webbing Layering Design Question

Hi everyone, this is my first post on the forums and I have a design question about webbing stitching. I’ve read as much material as I can about thread patterns, stitch count, differences in materials, etc., and I’ve researched how to maximize joint strength and seen various videos about MBS testing. However, I’m hoping someone might be able to help answer a question for which I cannot find an answer.

Is there any relationship between the strength of a joint and the number of webbing layers stitched together? I need to make a tether with one anchor and three legs of different lengths. I know there are multiple ways of doing this, but is one of the options below stronger than the other, and if so which is the better choice? If I’m missing something, please let me know!

To be clear, this is still in development, but I figured with all the experience and expertise on the forums it would be the best place to ask for help! Thanks very much to anyone who can advise.

For option A I would make the red webbing also form the eye making what is likely the highest loaded point, assuming your directions are also the direction of applied force, double thickness and the stitch more straightforward since you don't need to consider missing the red a possibility.

Edit to add: the better choice depends on what it hopes to achieve. Is being modular helpful? Then B if fewer items is preferred A. From an engineering standpoint changing the strength of the webbing is very easy and design for function should come first.

To maximise strength you need to think more about the metal attachment point webbing bent over a small radius will weaken it considerably, A is probably the stronger of the two but the design could be made to be quite a bit stronger. I'll roughly draw it up on my phone now.
Edit: I forgot how bad my phone is for drawing.

This design will have the outer webbing around a greater radius and the inner webbing will be loading the outer webbing rather than its low radius connection to the pin. 

I’m not a webbing guy but I did study some structural engineering in college.

When you look at a connection between two members joined by fasteners perpendicular to the direction of loading, the fasteners experience shear stress. A connection can be “single shear” or “double shear” with examples of both below:

The double shear connection, all other things being equal, is theoretically roughly twice as strong. The strength increase is a result of twice as much cross-sectional fastener area experiencing the same total force. Stress = force/area so more area equals less stress. If you continued to add layers you would theoretically increase the strength of the connection by an equal proportion with each layer. 

I expect that there are some interesting things going on with textiles, due to the flexibility of the materials, but the basic mechanics should be similar.

One of the sections from OPTION B could have just one of the red sections from OPTION A for another variation.

Thanks for the help everyone; appreciate it.

I've made a few revisions as per the advice, and I think the design is improved. While I understand there will always be shear loading to some degree in any application, the purpose of the tether is for backup/redundancy and the loading - should it ever be required - will be in a linear direction (so any shear loading should theoretically be negligible). And thanks for the note, but there is no requirement for it to be modular.

Here's the revised design. Does this look better?

Thanks very much again.

You're not really taking advantage of the wider radius of the blue webbing, that being said the weak points will be at th end of the tethers anyway so this seems like a pretty mute point. 

Assuming the stitching is sufficient, the weak point will be the same in both cases, the places where it is just a single strand of webbing.

Option b has extra links in the chain- more chances for failure.

Option a would be more fiddly if you most often need only two attachment point and have the third one dangling about.

Thanks for the replies and information about the strength-to-layers relationship; I appreciate it.

Russ Walling wrote: Sewing 4 layers is getting close to real industrial...

Why do you need 4 layers at all?

There is not any specific requirement for four layers, I'm just trying to keep the number of links (e.g. potential failure points as Ackley noted) to a minimum, and simplify the overall design as much as possible.

What sort of loads are these legs supposed to take?

The loads would not be very large. By my calculations (including a safety margin of course) they should never exceed 2kN. This tether is for backup/redundancy (I've explained the application below in more detail) and will likely never even come very close to 2kN. I do however wish to make something as safe as possible.

What is the diameter of the snap hooks?

The application requires a specific type of snap hook and of the options available, I will be using a snap hook with an eye which is 23mm wide (the largest available). I understand the webbing is 8.5% wider than the eye diameter, but I am limited by the options.

What is the end use, unless it is a secret?

The application is a safety tether for my autistic daughter for sailing. The tether needs a single snap hook which is constantly attached to her harness or life jacket, and then three legs (each with a snap hook), of which only one of the three is clipped on at any one time. The reason three legs of different lengths are required is due to the positions she likes to occupy around the boat. The boat has padeyes and jacklines installed at specific locations and the legs of the tether will correspond to these to ensure she can never fall overboard. For example, when she wishes to sit on the rail (edge), the shortest leg will be used to clip her in. When she's in the cockpit, the longest leg can be used as there is more distance between the center of the cockpit and the edge of the boat.

Due to the application, there is no specific fall factor calculation, nor is a belayer part of the equation. The concern is a making a tether strong enough to hold her safely in the unlikely event should she trip or slip over (but never with the risk of falling overboard).

The bespoke design is required as most sailing tethers are of a few select standard lengths, generally have inadequate hardware, and are too generic to be safely suited to the boat on which they're used.

I hope this makes sense and thanks everyone again for the help!

If I were you I would skip the sewing and get a rabbit runner and tie it like this. Or if you want some adjustability, this might be an option though I don't know if the metal parts might have trouble in a marine environment.

Russ Walling wrote: In my estimation, you are way over thinking this.  The load potential is so small, that almost any configuration will work.  What I would do is probably make the main loop have two legs, and insert one piece of webbing to make the third leg.  
Top loop does not really need to be doubled, but if you did, do a Metolius style loop to make the webbing doubled at the connection point, yet it does not extend into an increasing layers problem.
I’ll try to add a diagram, but on this device it might be impossible!
edit:  in fact, something like a standard or adjustable daisy chain gets you near infinite adjustment in the range is is totally bomber for the potential loads.  Have you looked into this??  Even a length of perlon with a RopeMan would sort you out good style.

The ISO test for sailing lanyards is actually a drop test involving a 100kg weight, in use they tend to be clipped directly to metal bits on the boat! And those are with min BL of 2040kgs (4500lbs). Most lanyards nowadays also have "pop" windows to show they have been impacted, normally in the 2-400kg range. Child lanyards (depending on age) are nornally hitched to the harness, first so they can't fiddle with the attatchment and second so they don't get smashed in the face by the karabiner. Three legs on a lanyard is going to be a nightmare, two is bad enough!  

Personally I'd check what the USCG  and the insurance required first.

Maybe use cord instead of webbing, with knots, spliced, or sewn eyes.  Also instead of a snap on each end, use a single carabiner, so hardware not being used isn’t flying around.

What's it for? I would guess an adjustable tether but I suppose it could also be a multi-attachment tether though I doubt the latter because getting the length of the legs correct would be nearly impossible. If an adjustable, why wouldn't you use something like a PAS? Having those extra attachments hanging around, possibly with carabiners or snap hooks in them seems annoying but that's based on my assumed use.

Bill Czajkowski wrote: What's it for? I would guess an adjustable tether but I suppose it could also be a multi-attachment tether though I doubt the latter because getting the length of the legs correct would be nearly impossible. If an adjustable, why wouldn't you use something like a PAS? Having those extra attachments hanging around, possibly with carabiners or snap hooks in them seems annoying but that's based on my assumed use.

I suppose actually reading the thread is too much to expect.......

The loads in this application might surprise you. I have hit the end of my tether once, after sliding down the deck of a boat heeled around 30 degrees, and it was pretty jarring; I don’t want to think about what it would feel like to actually fall on a nylon webbing tether attached to a solid piece of deck hardware. There’s just no shock absorption in a system of PFD-harness/tether/padeye.
 
I second Jim’s recommendation to girth hitch it to the harness rather than using a carabiner. 

Thanks everyone; I really appreciate all the ideas.

Russ Walling wrote: In my estimation, you are way over thinking this.  The load potential is so small, that almost any configuration will work.  What I would do is probably make the main loop have two legs, and insert one piece of webbing to make the third leg.  
Top loop does not really need to be doubled, but if you did, do a Metolius style loop to make the webbing doubled at the connection point, yet it does not extend into an increasing layers problem.
I’ll try to add a diagram, but on this device it might be impossible!
edit:  in fact, something like a standard or adjustable daisy chain gets you near infinite adjustment in the range is is totally bomber for the potential loads.  Have you looked into this??  Even a length of perlon with a RopeMan would sort you out good style. 

Thanks for the suggestions. I hadn’t considered a length of perlon with a RopeMan, but I don’t think it would be suitable (obviously the application is "bulky" now matter what, but minimizing that is important).

A main loop with a third inserted in between sounds like a good idea – I’ve redrawn the design below which looks much improved compared to the four layers of my second design. Does this look better?

Alex R wrote: If I were you I would skip the sewing and get a rabbit runner and tie it like this. Or if you want some adjustability, this might be an option though I don't know if the metal parts might have trouble in a marine environment.  

This is a good suggestion, though there is no specific need for adjustability; the different lengths of the three legs essentially function as the adjustability. And thanks for the note about the environmental considerations - I’ve already contacted all the manufacturers of the parts I plan to use to ask about issues relating to marine conditions and they were really helpful. Also, while knots are certainly secure, the design requires a need for a release at the anchor, so a snap hook is required at all termination points.

Jim Titt wrote: The ISO test for sailing lanyards is actually a drop test involving a 100kg weight, in use they tend to be clipped directly to metal bits on the boat! And those are with min BL of 2040kgs (4500lbs). Most lanyards nowadays also have "pop" windows to show they have been impacted, normally in the 2-400kg range. Child lanyards (depending on age) are nornally hitched to the harness, first so they can't fiddle with the attatchment and second so they don't get smashed in the face by the karabiner. Three legs on a lanyard is going to be a nightmare, two is bad enough!  

While I generally always advocate two-leg tethers to ensure a constant connection to the boat, I totally understand where you’re coming from and agree completely that three legs is certainly more fussy than two! In this case however, a three-leg design is essential to ensure the length of the leg which is in use is never longer than the distance to the edge of the deck.

To address the issue of the two loose/unused legs, I plan to include a simple D-ring at the harness / life jacket anchor to which they can be attached when not in use. This means they will be stowed safely and not present a significant hazard when not in use.

Equally, because of my daughter’s autism, managing her tether will not be her responsibility, but mine or my wife’s. Therefore my design has to allow for my daughter’s reduced understanding of the nature and use of safety tethers.

Brocky wrote: Maybe use cord instead of webbing, with knots, spliced, or sewn eyes.  Also instead of a snap on each end, use a single carabiner, so hardware not being used isn’t flying around. 

Julian H wrote: If you use rope you can splice it. 

Thanks for the ideas, but rope is not the preferred option for the design. First, using webbing as safety lines helps differentiate it from rope used for mechanical operations; and second, rope tends to role more easily underfoot when on deck which means it presents more of a general hazard. For my design and application therefore, webbing is the better option. I appreciate the idea though, thanks.

Also, thanks to those who provided suggestions and examples of sewing. Once I have finalized the design, I plan to have it constructed by a sailmakers with an industrial machine. Not only will this be better than anything I could do myself, but also provides more flexibility in terms of what sewing configurations are possible (e.g. number of layers, stitch counts, and patterns, etc).

Dylan B. wrote: I don’t know anything about anything on this thread, but why not use this design, but make it out of only one piece of webbing? Where the blue in this diagram ends on the “bottom,” just continue on and connect to where the red ends on the bottom. 

Thanks for the suggestion, and it would certainly be solid, but I think that would generate too much bulk for each leg. Also, extra sewing would be required just to keep the layers together, which I’m thinking might negatively affect the flexibility of the webbing (e.g. reversing internal/external layers when it changes position) and make it too stiff to function comfortably.

Jared Chrysostom wrote: The loads in this application might surprise you. I have hit the end of my tether once, after sliding down the deck of a boat heeled around 30 degrees, and it was pretty jarring; I don’t want to think about what it would feel like to actually fall on a nylon webbing tether attached to a solid piece of deck hardware. There’s just no shock absorption in a system of PFD-harness/tether/padeye. 

Thanks very much for bringing this up - this is a great point and something which I have considered extensively! I plan to address this in the subsequent stages of the design which will involve thinking about the role of (center located) jacklines to provide shock absorption (and possibly thinking about inserting shock cord inside the tubular webbing – not for storage purposes as per standard elasticated sailing tethers, but basic shock absorption). The purpose of the design is more about “holding” my daughter (possibly stopping her after a slip or trip) but way more often just to prevent her enthusiasm overcoming her safety (I hope that makes sense!).

Thanks again everyone for any more comments; I really appreciate the help and replies.

G C wrote:
The application is a safety tether for my autistic daughter for sailing. The tether needs a single snap hook which is constantly attached to her harness or life jacket, and then three legs (each with a snap hook), of which only one of the three is clipped on at any one time. The reason three legs of different lengths are required is due to the positions she likes to occupy around the boat. The boat has padeyes and jacklines installed at specific locations and the legs of the tether will correspond to these to ensure she can never fall overboard. For example, when she wishes to sit on the rail (edge), the shortest leg will be used to clip her in. When she's in the cockpit, the longest leg can be used as there is more distance between the center of the cockpit and the edge of the boat.

Sorry, I missed your application previously. 

In case you're not familiar with the PAS and rabbit runner and would be happy with a commercial solution: (https://www.metoliusclimbing.com/slings.html).

Thanks very much for bringing this up - this is a great point and something which I have considered extensively! I plan to address this in the subsequent stages of the design which will involve thinking about the role of (center located) jacklines to provide shock absorption (and possibly thinking about inserting shock cord inside the tubular webbing – not for storage purposes as per standard elasticated sailing tethers, but basic shock absorption). The purpose of the design is more about “holding” my daughter (possibly stopping her after a slip or trip) but way more often just to prevent her enthusiasm overcoming her safety (I hope that makes sense!).

A jackline will definitely add some shock absorption to the system, but will also deflect a ton - probably feet, depending on the length of the jackline. You could probably mitigate that by running the jackline through a few intermediate padeyes but keeping the webbing continuous. 

If only one lanyard at a time is to be attached, they should be sewn together or formed from 1 strand to keep loose strands from tangling etc

Dynamic cord with a scaffolding knot at each end and at the mid points would hold the snap hooks in place, not tangle as easily, and reduce the force of a fall.

https://www.bluewaterropes.com/product/dynamic-prusik-cord/

Lanyard from dynamic prusik cord. Scafolding knot at each end. choice of scaffolding, overhand on a bight or other at mid points. Carabiners used cause thats what I have.
Could be larger chunk of dynamic rope, say 8 mil half rope. And what sailor leaves rope lying around to fall over, especially when it is supposed to be attached to his daughter.