Strop strength

zoidberg

zoidberg

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I'm making up some 'multi-part' strops using DM12 Dyneema, with a nominal breaking strain of 1000kg.

If I use 7 'loops' before a proper and full-on end-to-end splice, do I get 7000kg of nominal breaking strain.... or something else?
 
Provided the load is spread evenly across all seven loops you will end with selvagee strop which can, IIRC, be puddened with cloth to prevent any sharp edge cutting through a single loop and reducing the overall working strength.

If the nominal BS is 7000kg, a safe working load could be 1/5 to 1/3 of that figure.
 
Something else. "Breaking strain" is not the same thing as "breaking load" ("1000kg is a breaking load); and in this case the distinction is important, if the 7 lines are simply in parallel. If every one of your 7 strands had exactly the same extension under load (i.e. if it was a perfect join at each end, so that every strand took its equal share of the load, and extended by the same amount), then you might get something approaching 7x 1000kg. But that is unlikely. More likely is that one will be a bit shorter than its fellows, and end up taking most of the load before the others get much tension on them. so that will break at about 1000kg; maybe a bit more if the other strands are taking a bit of the load. The process will then repeat; ping, ping, ping... This is particularly likely with high-modulus fibres like dyneema. which have very little elastic extension for a given load.
BUT - you can alleviate this non-load-sharing effect by mutually twisting the strands, or plaiting them, so that friction between strands will help pass the load between them; this is of course what is done in the cores of kernmantle ropes.
(I'm a retired materials scientist. This would have been a good question to have set my students!)
 
sgr143 said:
Something else. "Breaking strain" is not the same thing as "breaking load" ("1000kg is a breaking load); and in this case the distinction is important, if the 7 lines are simply in parallel. If every one of your 7 strands had exactly the same extension under load (i.e. if it was a perfect join at each end, so that every strand took its equal share of the load, and extended by the same amount), then you might get something approaching 7x 1000kg. But that is unlikely. More likely is that one will be a bit shorter than its fellows, and end up taking most of the load before the others get much tension on them. so that will break at about 1000kg; maybe a bit more if the other strands are taking a bit of the load. The process will then repeat; ping, ping, ping... This is particularly likely with high-modulus fibres like dyneema. which have very little elastic extension for a given load.
BUT - you can alleviate this non-load-sharing effect by mutually twisting the strands, or plaiting them, so that friction between strands will help pass the load between them; this is of course what is done in the cores of kernmantle ropes.
(I'm a retired materials scientist. This would have been a good question to have set my students!)
Click to expand...

(If it is a loop there are only two pings. The loop and the cover.)

Exactly. You see this with lashings, generally breaking at 50-70% of the "calculated" strength. But loops are different, not tensioned from one end and better able to shift. This is why industrial lift slings are contained withing relatively loose covers, so they can float. They also used multiple loops, to reduce the risk of catastrophic failure if one loop fails. The answer is certainly somewhere in between.

I don't know the risk of failure in your application, but risk reduction is one of the nice features of most rope; limitied cutting is not fatal because they consist of many independent yarns. You would be giving up that redundancy.

I'm watching. Zoidberg, I'd suggest researching lifting slings. I've tested lashings, but never loops.
 
As Sarabande says it will also depend on how the load is applied, a 100mm circular pipe will produce an entirely different (and better) result to a thin piece of hard steel. Some lifting hooks can have rough edges - check - and add protection if necessary

The other factor is the speed at which tension is applied. If the tension is applied slowly the loops will have time to adjust - if its a snatch load the tension is likely to be applied progressively to each loop. For lifting the tension would be applied slowly.

X-Yachts used to sell their yachts with one of the keel bolts being used to lift the yacht (they may still continue the practice). They supplied a lifting strop which had a fitting 'in' it that fitted the bolt and the strop was lead through a hole in the cabin roof. The strop was made up of a single filament of Kevlar, this was before Dyneema and before textile became common place, wound in continuous loops, lots of single filament loops. The strop, or the loops, were contained loosely in a long continuous canvas sausage. Your idea is not original :). X-Yacht did not get the location of the bolt used for lifting quite right and a common practice was to have a man standing on the bow (made me shudder - but in HK life was then cheaper) or better a series of water containers to keep the deck horizontal.

I don't know what the lifting industry use as safety factors for textile (but it will be on their websites) but a 4:1 is common for chain and steel components made to European safety standards (EN818 ?).

If you check the DSM website they have an a article on dyneema chain made from sewn short loops of dyneema tape. The dyneema finds favour because it is light and does not damage the item being lifted. Most lifting slings I see, maybe exclusively, are tape. Apart from my X-Yacht example I have not seen, or noticed, multiple loops.

Synthetic Link Chains

I assume you are considering multiple loops - because you have an excess of same and don't have a length of beefy dyneema nor any tape.

The trouble with making something up and then covering it with a cover is - you cannot see what is happening inside - another advantage of tape.

Jonathan
 
zoidberg said:
I'm making up some 'multi-part' strops using DM12 Dyneema, with a nominal breaking strain of 1000kg.

If I use 7 'loops' before a proper and full-on end-to-end splice, do I get 7000kg of nominal breaking strain.... or something else?
Click to expand...

My interpretation is you you have need for a device, with a breaking strain of 7,000kg and you happened to have some spare 1,000kg dyneema - which you thought would do the trick. As Graham376 suggests there are other ways of achieving the same result (without using dyneema). Looking for a UTS of 7,000kg, on a yacht, is nothing to be sniffed at though the WLL might be down at between 2,700kg or 1,700kg (using Sarabande's safety factor) - but even these tension are not insignificant - but this is being put together by a gifted amateur, you (and how have you derived your UTS in the first place?)

I'd be twitchy as one of your crew, without further background. Your nearest chandler will have cordage that will be warranted to meet your needs, you can buy chain that will meet your needs, shackles you can get from Tecni (Crosby) and your local sailmaker can sew up a tape strop and as he will not be able to certificate it - he will make it comfortably over strength. If this is not good enough - your local lifting component supplier will make and warrant a strop for you.

I'm the first to sympathise with making your own kit - so as you have asked for advice (and we are now part responsible for your device) - tells us a bit more about it - and maybe we can help with more focus.

I'm also conscious you might not want to tell us your idea - it might be only half formed in your mind and you are searching for 'angles' - so excuse me prodding - and we can continue to throw out random ideas.

Jonathan
 
Thanks for the several responses. I'll need to dispel some 'ambiguities'.

The task is to make and fit a quite short 'continuous loop' strop between my forestay deck fitting and the lower end of my 7mm wire forestay, permitting a tubular bowsprit to be passed ( as per Selden style ). This strop will be mounted in what lifting sling makers call 'basket' configuration, with the pair of small 180-degree bends ( where a crane's hook might be passed ) being passed instead down around a pair of hi-load shackles, paralleled, which are secured to the foredeck fitting. This gives an advantageous D:d bend ratio.

The upper parts of the strop pass over the bowsprit and through a Wichard MXE10 saddle shackle, to which the forestay toggle attaches.

According to Marlow Ropes/Technical today, I should use a factor of 1.6 x nominal rope Break Load, which is valid for up to 5 'laps' made in a continuous spliced loop ( e.g. a 5-meter circumference continuous loop bent into 5 'laps' of 1-metre circumference )

I've found the 3mm D12 Dyneema cordage very difficult to work with, so have ordered some 4mm stuff. Five 'laps' of that should comfortably exceed the load capacity of the wire forestay. Similarly the 25mm nominal tubular antichafe sheathing proved 'too difficult', so I've ordered some 45mm stuff.

We discussed 'visco-elastic elongation' and 'creep' ( no, not a social media diss! ) and concluded that each is manageable by means of rigging screw - but with regular inspection and relaxation of the load whenever practicable.
 
I also found anti chafe sheathing, dyneema, difficult to instal. The trouble being if you pull it the braid tightens a locks up on the core. I found that if I milked the sheath and kept the braid open and loose (sorry but desperately difficult to describe) it could be installed with some patience. The killer is trying to do it quickly.

If you are installing a long cover it does help if you attach a mousing line (securely) to the core and thread it through the complete length of the cover and then as well as milking you can give the core some gentle encouragement to keep on the move.

Once you have the sheath on you then need to splice the cover into the core, but that's not so difficult as by this time you will value the use of patience.

This is actually not a task for early summer but one for winter sitting round a glowing fire. Possibly conducting the work out in the garden one balmy evening would also be suitable.

Thank you for removing the ambiguity (and I commend asking the manufacturers and their offering their wisdom)

Jonathan
 
I've seen video of these being made. The loops are made through the loose cover, the cover is bunched up, one strand pulled out and the splice made, and then everything milked back in and the cover sewn up.

The 1.6 factor squares with testing I've done.
 
If the loops are anything close to circular, the tension in the line will be a lot more than the force applied radially.
 
No splice or knot / hitch retains full strength of the rope / material it is used on.

As a Seaman Officer .. we were always told that a good hitch / knot is rated 70% of the ropes SWL, a good splice 80% of ropes SWL ... (note that SWL is a fraction of the BS). We were taught to regard the works on such to always reduce the usable load.

Note that's GOOD quality work ...

Second - any turn round an object weakens the rope / item ...

Third - loops do not unless ends are seized properly give the number of loops increase - if the loops are not seized then the loops only add a part increase because the turn of the loop can still part at original rate. Again we were told doubling would give us about 50% increase subject to that turn round object.

The only way to actually multiply fully by number of lines - is to have each line individual and exactly set to equally carry load.

I'll leave you all to it now ...
 
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Refueler said:
No splice or knot / hitch retains full strength of the rope / material it is used on.

As a Seaman Officer .. we were always told that a good hitch / knot is rated 70% of the ropes SWL, a good splice 80% of ropes SWL ... (note that SWL is a fraction of the BS). We were taught to regard the works on such to always reduce the usable load.

Note that's GOOD quality work .........
Click to expand...

Yes, I thought those figures were optimistic. I like to assume I'm down to 50%, then I might have a bit in hand!
 
What"we were taught" is of varying quality.

  • Some manufacturers (Samson, who makes Amsteel, which I believe is what we are talking about) rate strength based on spliced dogbones. Yes, I have spoken with their tech people about this. Others wrap the rope on a drum (New England ropes). A dog bone is what matters in the real world.
  • Most knots are considerably weaker than 70% and it depends on the material. In Dyneema most are below 40%.
  • A good splice of Dyneema hollow braids is >90% based on many tests. A bad splice requires considerable disregard (it's an easy rope to splice well).
  • Round slings (what Zoidberg is proposing) are COMMON and well proven in the heavy lifting industry. Google that. This is NOT a new concept.
 
thinwater said:
What"we were taught" is of varying quality.
Click to expand...

How to introduce anchoring into a thread on strops :)

I suspect Thinwater's words may have raised an eyebrow or two.

Many of you came to sailing through family or learnt their skills from 'old salts' who were brought up with hemp, flax, Admiralty Pattern anchors and tar. Their skill and knowledge was centuries old and remained unchanged over those centuries and was passed from father to son or bosun to midshipman. What was true of linen, natural cordage, brass and bronze is not necessarily valid when applied to modern synthetic fibres, high tensile steels and modern sealants. What you were taught was probably true but may no longer be applicable. Skills remained unchanged for centuries - but suddenly in the last few decades many of those old skills have become, well - invalid (at least if you use modern materials). Just think of the fixation of the religion of catenary and scope :) and consider how many cadets are taught to use a sextant and calculate a position (when was the last time a ship's officer needed to use a sextant in anger? I may be out of date and sextants have been relegated to decoration - but it was a requirement until recently).

I have envied those who were introduced to sailing by their fathers and grandfathers, I learnt the hard way by my, many, own mistakes and video. I missed splicing hemp ropes, tying bizarre knots but have made as much mess with black Sikaflex as you must have done with tar.

But so as not to drift too far - what was true of traditional fibre ropes, even modern new fangled nylon is not necessarily applicable to dyneema in the same way that Dacron does not replicate Mylar.

What you were taught I am sure is correct - but we have moved on, specifically in this thread :)

What is sad is that much of. sailing knowledge has not moved on and we seem stuck in some sort of intermediate era trying to impose old skills on new product (or worse - ignoring the new and sticking with the old).

Apologies for the drift - I do become obsessed by the inability of the modern yachtsman's inability to accept modern developments. I'm also disappointed at the constant criticisms - we should be encouraging - we newbies have much to learn.

Take care, stay safe

Jonathan
 
Dear Neeves ...

Sadly - you assume that all modern materials improve or increase possibilities ... which unfortunately for you is not always true.
In the case of some super strength combos in cordage - some actually state you are advised not to splice as the material is too slick and does not 'bind' ........ I'll leave you to look that one up ...
As to your swipe at Anchors / Rodes and Catenary's - that certainly is way of the mark. Remind me not to anchor near you !!
 
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