Dyneema life lines?

[mel80]

This kind of discussion comes up a lot in climbing forums. To summarise: Dyneema is exceptionally strong, but has such low stretch that despite its strength, it will tend to fail under a lower dynamic load. It also has a lower melting point and can fail easily if loaded over an abrasive surface. Thus, it is great for when you can carefully control the peak load (or it is under a static load), but in less predictable situations it will (at best) put a much higher stress on the rest of the system (and your body).

I can't see any real advantage for a lifeline.

 

[estarzinger]

The melting point for dyneema is around 130C, and the racing community (who use dyneema life and jacklines quite a bit) has never see a 'melting' failure.

My own personal experience with dyneema life lines is that they last longer than wire, because the wire will strand where it work fatigues coming out of the stanchion holes (and I replace it when even one filament strands) while the dyneema will not wear at all if the stanchion holes are carefully smoothed.

Regarding jacklines and tethers . . . . my personal sense is that the elasticity should be (primarily) in the tether and that in my testing 8.5mm dynamic climbing makes terrific tethers. The jacklines should not be able to stretch very far (in absolute distance) as their purpose is to keep you on board and a long stretch will allow you top go over while still clipped on. I personally use 'hybred' jacklines that have dyneema cord inside tubular webbing where the webbing is several cm longer than the dyneema to allow several cm of initial elasticity/shock absorption but to then exactly limit the total stretch.

I do think we need to be careful with the climbing analogies/engineering, because the data suggests that 'falls' are very very different in climbing vs in sailing. They are much more pure free falls with higher dynamic loading in climbing and more like slides with less dynamic loading in sailing.

 

[prv]

I personally use 'hybred' jacklines that have dyneema cord inside tubular webbing where the webbing is several cm longer than the dyneema to allow several cm of initial elasticity/shock absorption but to then exactly limit the total stretch.

Are you sure you don't have the long/short backwards there?

Pete

 

[mel80]

I do think we need to be careful with the climbing analogies/engineering, because the data suggests that 'falls' are very very different in climbing vs in sailing. They are much more pure free falls with higher dynamic loading in climbing and more like slides with less dynamic loading in sailing.

That is probably fair, but I can imagine situations in sailing where dynamic loads might be quite high (particularly with a longish tether). Even if it didn't fail, you would be much more likely to hurt yourself. In any case, I wouldn't use dyneema unless there was a shock absorber somewhere in the system. The hybrid system that you describe sounds like a neat compromise.

 

[estarzinger]

Are you sure you don't have the long/short backwards there?

Pete

yes, thanks, I wrote too quickly - as you say . . . . should be longer dyneema/shorter webbing

 

[prv]

That is probably fair, but I can imagine situations in sailing where dynamic loads might be quite high (particularly with a longish tether). Even if it didn't fail, you would be much more likely to hurt yourself. In any case, I wouldn't use dyneema unless there was a shock absorber somewhere in the system. The hybrid system that you describe sounds like a neat compromise.

Frankly, if you fall far enough that the elasticity of your tether is an issue, on the average boat you're probably going to have hurt yourself already by hitting and scraping against the deck and various gear. I'll take a few bruises over drowning any day, and not worry too much whether they came from harness straps or a sheet car.

For climbers the issue is different, as they must expect some pretty large falls on their ropes as a matter of course.

Pete

 

[mel80]

Frankly, if you fall far enough that the elasticity of your tether is an issue, on the average boat you're probably going to have hurt yourself already by hitting and scraping against the deck and various gear. I'll take a few bruises over drowning any day, and not worry too much whether they came from harness straps or a sheet car.

For climbers the issue is different, as they must expect some pretty large falls on their ropes as a matter of course.

Pete

Technically, the length of the fall isn't relevant. The peak force is proportional to the ratio of fall length to rope/ tether length. e.g. you can easily snap a 30cm dyneema sling with a 40-50cm fall. But, you are quite correct that the two situations are not strictly comparable.

 

[estarzinger]

Technically, the length of the fall isn't relevant. The peak force is proportional to the ratio of fall length to rope/ tether length.

Just to be pedantic . . . technically it's a function of the fall factor (as you say) AND the acceleration.

My sense of the sailing data is that our "fall's" are 99.9% of the time way under 1g. The actual measured data I have seen shows peak loads less than 3x body weight in most sailing 'falls' using low stretch dyneema jacklines and dacron webbing tethers (which does not give the system much shock absorbing capability).

In french single handed racing we have some experience with systems with almost zero elasticity (dyneema jackline combined with dyneema tethers). In climbing falls such systems would inevitably both break themselves, and break the attached human body. But we have zero instances of either in this sailing sample and there have been numerous MOB's and knockdowns. We also have quite a bit of prior experience with wire jacklines (with various tether types) that also had minimal stretch with essentially zero human breakage and most of the gear breakages I am aware of were from obviously improperly speced/assembled systems (eg did not meet the ISAF OSR standard).

That said, I am a big fan of and think it is clear we should be building elasticity/shock absorbing into the system as practical, and as I mentioned above, I currently think the 'ideal' system is 8.5mm dynamic line tethers with 'hybrid' jacklines.

 

[Woodlouse]

This kind of discussion comes up a lot in climbing forums. To summarise: Dyneema is exceptionally strong, but has such low stretch that despite its strength, it will tend to fail under a lower dynamic load. It also has a lower melting point and can fail easily if loaded over an abrasive surface. Thus, it is great for when you can carefully control the peak load (or it is under a static load), but in less predictable situations it will (at best) put a much higher stress on the rest of the system (and your body).

I can't see any real advantage for a lifeline.

I don't understand why climbers would ever consider using dyneema. The last thing you want when falling off a cliff is a rope that will bring you to an immediate stop, however I'd expect fittings to fail before a reasonably specced length of dyneema. If the line can't handle the dynamic loads then the line isn't big enough and I think that applies to climbing just as much as it does sailing, however I would never consider dyneema for the former for all the reasons you have given.


Anyway, back to lifelines, if you are worried about weight and don't want to go down the dyneema route then I did go on board a boat last year that had guard wires made of titanium rod.

 

[prv]

I don't understand why climbers would ever consider using dyneema. The last thing you want when falling off a cliff is a rope that will bring you to an immediate stop

Certainly you don't want dyneema as your main rope in that situation, but I imagine advanced climbers have all sorts of other rigging that they might use it for.

Pete

 

[mel80]

I don't understand why climbers would ever consider using dyneema. The last thing you want when falling off a cliff is a rope that will bring you to an immediate stop....

Bit off topic but.... Dyneema is used in climbing for slings (i.e. a static element in the system like anchors or carabiners). Ropes (made of nylon) are the shock absorbing element is the system. The problems occur when people use dyneema slings as tethers to attach themselves directly to anchors. In these cases, any slack in the sling can be enough to cause serious injury or sling failure. Having a line that can handle the dynamic load is all well and good, but not much use if the mass at the end can't! DMM made a nice video showing the problem: http://dmmclimbing.com/knowledge/how-to-break-nylon-dyneema-slings/

As others have said though; perhaps not that relevant to sailing.

 

[Roberto]

Could someone (Evan?) explain this:
http://www.sailing.org/tools/documents/OSR2015AmendmentSheet12012015v2-[18222].pdf

The Amendment says that starting 2016 high modulus textile guardrail/lifelines will not be allowed any more on higher category races 0,1,2,3

What are the reasons ?

regards

 

[PuffTheMagicDragon]

I don't know if this answers your question. I assume that this was because, unlike stainless steel, Dyneema is prone to catastrophic failure (without any warning). According to the ISAF site:

"Following several accidents in 2014 where crew fell in the water when Dyneema lifelines failed it was decided to require stainless steel lifelines for monohulls racing in offshore special regulation race categories 0 to 3. Dyneema lifelines remain permitted for category 4 monohulls and multihulls. This amendment will come into effect on 1 January 2015."

 

[J Neeves]

Interesting

Soft shackles, made from Dyneema, for use in as many applications as 'normal' shackles (plus some more) are all the rage. One wonders why the Dyneema lifelines failed catastrophically - that has not been anticipated nor noted in other applications.

I'm not doubting the failure of the lifelines but wonder how many, other, failures might be waiting to happen.

Jonathan

 

[PuffTheMagicDragon]

One reason could be that soft shackles do not 'rub' when they are under tension. Another could well be that since, by their very nature, they are handled each time that they are used any warning signs are more likely to be noticed. No documentation, this is just my personal opinion.

 

[J Neeves]

Soft shackles are being recommended (by some knowledgable people) as a replacement for chain hooks to attach a snubber, or snubbers (in case of a bridle) to an anchor chain. Failure is hardly life threatening, but certainly inconvenient - though as you suggest warning signs would be, very, obvious. But I have seen no account of soft shackles, used in a snubber application (or any application), failing 'early' but equally I would have hoped that those responsible would have checked lifelines, though one would think the crew (sitting adjacent to the lifelines) would have noticed wear themselves.

We did try a soft shackle as a chain hook replacement but found it less than convenient - possibly 8mm chain is a bit small to work with.

But you make mention of catastrophic failure - which implies, unforeseen? though possibly you meant expensive and life threatening. Are you able to elaborate on 'catastrophic'?

I confess to be puzzled - in the absence of more information.

Jonathan

 

[PuffTheMagicDragon]

Catastrophic.

Well. with wire you get broken strands. These are so evident that we commonly refer to them as 'meat hooks'. Usually, although not always, these happen where the wire passes though a stanchion and they make themselves known even though you aren't looking for them. The strength of the wire is not drastically reduced if one or two strands have broken and the wire is normally replaced well before there is a risk of failure; if for nothing else to remove the actual risk of painful gashes.

With Dyneema there is no equivalent warning. Fibres in the outer sheath do chafe through, again usually in the region of the stanchions, but they are not so evident unless through close inspection. I would say VERY close inspection because here we have plaited strands forming the sheath and each strand has scores of fibres that are thinner than a hair. There is also the factor of degradation through continued exposure to UV radiation; not very 'visible'.

The point that I am trying to make is that damage to wire is more easily seen and rectified in good time while damage to Dyneema progresses silently and, in practical terms, invisibly until it suddenly snaps. -> "Catastrophic".