knuterikt
Well-Known Member
No knots in a dyneema lifeline...
For the rest show me your source of this insight.
And wire is how much more flexible?
From Stu
Thoughts on Dyneema for guard rails?
- Thread starter Heckler
- Start date
No knots in a dyneema lifeline...
For the rest show me your source of this insight.
knuterikt
Well-Known Member
Splicing uncovered dyneema is easy compared to other splices I know.
(Double braid polyester, three strand laid rope or twisted rope..)
The video I linked make it more complicated than necessary because of the thimble.
It's mostly about measuring correct and having a sharp scissor.
You can make a fid for this out of a piece of steel wire, like this..
knuterikt
Well-Known Member
Give better safety margin
I beg to disagree, I have been saved my the guardrail more than one time.
When moving on deck in heavy seas with boat heeling I never go upright.
I walk low, clipped on to the jackstay holding on to the boat.
If I loose my grip it's more like sliding across the deck than being thrown against or over.
I would rather stop at the guardrail than being towed by the tether...
Rossynant
Well-Known Member
As you prefer. This is personal preference.With 4 mm lifeline You have it may actually be right... 4mm... Christ...
I'm still alive (and still amazed that I am
) being saved by lines, "lifelines" included, numerous times. Would prefer not to hang on polypropylene for life, and when I climbed nobody else would. As said - it can serve for lifeline on boat. Just remember "breaking strength of fibre" has nothing to do with reality. It's not breaking of fibre or even rope we have to worry about - it's line. Has some end attachments, has to go through stanchions, has to withstand dynamic loads. Breaking strength is a laboratory parameter usefull to compare materials.
Dyneema rope will break under standard load from alpinist falling on it and is not used for climbing line - nylon is used instead, despite nylon fiber having 6 times lower (!) "breaking strength".
This is extreme example, please note. Just to show there is more to it. But guess what line my 'safety harness' has attached to...
To the point, since got into:
8mm steel rope has breaking strength about 4-5 tons; 8mm dyneema is advertised with strength 4-5 ton, sometimes even more depending on advertisement. So OK, similar breaking strength. Same elongation on breaking.
Safe load - 0,8 ton for steel; never found data for dyneema...
Other things to compare though: maximum possible strength of attachment point - 100% steel, 80% dyneema. To get 80% it's necessary to make splice, exceed bending ratio 5/1 - worthy to note. Polypropylene is destroyed by UV - no matter what advertisement says there will be loss in strength, in outer fibres (got dock lines of 'UHMPE" from company I trust - they were in black only, sensible idea I'd say). As any synthetic they chafe and have short life, strength loss is inevitable. Probably for replacement in a few years...
Came across advice to use dyneema of 2-3 times more breaking strength then steel it's replacing, to substitute for this loss - IMHO good advice. So indeed good idea to use 6 mm dyneema in place of 4 mm steel. (4 mm...
) Keeping all this in mind - nothing wrong with using dyneema instead.
But only reason to use it I see - apart from lower cost if that's possible - is weight. Density: 0,98 dyneema versus 7,8 of steel. Quite important to racers, personally I don't give a .... on 7 ton boat; let's be serious. I want the line to stay there for 20 years, put on and forget about with no worries
knuterikt
Well-Known Member
Strange comparison, you would never use wire for climbing for exactly the same reason....As you prefer. This is personal preference.With 4 mm lifeline You have it may actually be right... 4mm... Christ...
I'm still alive (and still amazed that I am
Breaking strength is a laboratory parameter usefull to compare materials.
Dyneema rope will break under standard load from alpinist falling on it and is not used for climbing line - nylon is used instead, despite nylon fiber having 6 times lower (!) "breaking strength".
This is extreme example, please note. Just to show there is more to it. But guess what line my 'safety harness' has attached to...
For climbing, rope with different properties are use depending on the use
-Rope with stretch when you are attaching to bolts while you climb - lead climbing (in case you have a fall where a bolt below have to take the load) http://en.wikipedia.org/wiki/Lead_climbing
-Stiff rope if you are using a top rope http://en.wikipedia.org/wiki/Top_roping
Load on attachment points is the same regardless..
Dyneema for dock lines ?
This is one application where some stretch in the rope is good.
(4 mm...
From http://www.sailing.org/tools/documents/OSR2012Complete24012012Colour-[11765].pdf
Keeping all this in mind - nothing wrong with using dyneema instead.
Ok so you have newer heard of wire or wire terminals failing - still is difficult to spot a weak point before it brakes...
Even worse when you just "forget about with no worries"
Rossynant
Well-Known Member
Still have to test my 40 year old rigging wires, but nothing wrong on visual inspection of terminals. No swages, nor 1x19, granted.
Used wire for climbing - not for safety lines which must be elastic. But for, how can I say - descending into caves? any elasticity would be a pain, moreover - proverbial pain in the ass
Wire will not cut on rocks.
Yes, partially dyneema (rather something similar) for dock lines. Bit stiff, but strong, and floating. Quite nice. When we lost 28 mm we had before... Well, wanted strong; diameter is bit restricted by cleat size.
Polish safety rules specify - "lifelines of steel not less than 4 mm, stanchions spaced no more than 2 m" Never imagined a lifeline less than 4 mm, quite frankly never seen lifeline of only 4 mm. "in cockpit area lifeline may be replaced with tape of breaking strength not less than wire line". No mention of synthetic lifelines, though.
Anyway if you know about climbing, and line tested for "fall factor" of laughable 2 feet loaded along the line - and what breaking strength should it withstand, moreover only once or twice - what more can I say
Used wire for climbing - not for safety lines which must be elastic. But for, how can I say - descending into caves? any elasticity would be a pain, moreover - proverbial pain in the ass
Wire will not cut on rocks.
Yes, partially dyneema (rather something similar) for dock lines. Bit stiff, but strong, and floating. Quite nice. When we lost 28 mm we had before... Well, wanted strong; diameter is bit restricted by cleat size.
Polish safety rules specify - "lifelines of steel not less than 4 mm, stanchions spaced no more than 2 m" Never imagined a lifeline less than 4 mm, quite frankly never seen lifeline of only 4 mm. "in cockpit area lifeline may be replaced with tape of breaking strength not less than wire line". No mention of synthetic lifelines, though.
Anyway if you know about climbing, and line tested for "fall factor" of laughable 2 feet loaded along the line - and what breaking strength should it withstand, moreover only once or twice - what more can I say
Woodlouse
Well-Known Member
As far as I'm aware you use stretchy rope for climbing because if you take a fall with a rope that won't stretch then there's a chance you will break when the line stops you. Since dyneema is happily used for sheets on board very powerful boats, I don't think the risk of it breaking from shock loads is something to be concerned about as long as it's been specced correctly.
Dyneema's main advantage over wire is weight, however it can also be stronger with less stretch.
Wire is an advantage if chafe is going to be a big issue. But if it is then no matter what I was using I'd try and solve that issue.
Dyneema's main advantage over wire is weight, however it can also be stronger with less stretch.
Wire is an advantage if chafe is going to be a big issue. But if it is then no matter what I was using I'd try and solve that issue.
billcowan
Well-Known Member
There is very nice stuff called 'Parafil' which is a very strong folded plastic core covered with soft white UV stable PVC.
They use it for safety rails on the lifeboats. and I have it on my boat.
Quite cheap, but the terminals are quite dear, so I just bent it round SS thimbles and whipped it to itself (
covered the whipping with self amalgamating tape to protect from UV). then shackled it to the pulpit and used lashings at the pushpit ends,
If I need to remove it I just cut and redo the whipping.
They use it for safety rails on the lifeboats. and I have it on my boat.
Quite cheap, but the terminals are quite dear, so I just bent it round SS thimbles and whipped it to itself (
covered the whipping with self amalgamating tape to protect from UV). then shackled it to the pulpit and used lashings at the pushpit ends,
If I need to remove it I just cut and redo the whipping.
Rossynant
Well-Known Member
Maybe to clarify - it's not gradual chafe with time, nor breaking because "breaking strength" is exceeded, that I'd worry.
It's the sudden chafe, from shock loading - friction that will appear when somebody actually falls on the line - line is moved then and may, so to say, suffer injury to the fibres where there are splices, bends or just where passing through holes in stanchions. For this some safety margin better be provided. I'd go for some 2 times breaking strength of steel wire (or more if fits) in this application, so to have extra for this possible injury to line, also for aging, accidental surface chafe or cut etc. We are talking here of very small diameter line, so surface fibres of it are quite big proportion of the whole. Since line is lighter weight anyway there are no arguments against using bigger size.
But original OP's question was
So having been accustomed to 5mm steel as normal, and dyneema strength parameters about equal - would prefer bigger diameter dyneema, for this safety allowance. Should be possible to fit some 7 mm without modifications.
Another thing to clarify - stretch, like of elastic nylon, is not necessary in guardrail naturally; whole guardrail system, stanchions and pulpits included act as elastic system, giving a bit or bending when loaded. Some 3% dyneema has is enough.
As for stretch and side load - someone was wandering what I mean - I'm lost with proper 'scientific' terminology, but in simple words.
This is probably called bridle suspension. Example is horizontal line on which a weight is hanged. Line will deflect a bit downwards, forming shallow "V". Forces acting along the line are bigger in this situation than weight suspended. Just a case of vectors of forces being analyzed, basic physics, which most people realize, but not everyone realize how big the difference in load is, and it may be huge if this bridle V is shallow. Speaking from memory, but in case line is very taut and stiff, a V may have deflection of just 1 degree (that is angle of V at 179 degrees) - in this case load in line will be about 30 times more than weight (side load). The principle was used to shift heavy objects, as purchase power is so great, but a bit forgotten today.
Now in case of a man falling for some distance onto the lifeline, his body may give a force of about a ton, this falls on line which is then loaded 30 times more... such situation never will happen in practice, as no line would hold - it breaks.
With bigger stretch in line V angle will be greater and tension in line much less, but still in practical situation can be some 5-10 times more than side load, depending on line and it's attachment. So some elasticity in the system should be incorporated, and guardrails are rarely going in straight line.
LadyInBed
Well-Known Member
I''ve been thinking about using bungee cord!
It will stop people trying to move my boat by pulling and pushing on it
It will stop people trying to move my boat by pulling and pushing on it
Woodlouse
Well-Known Member
What is actually more likely here is that the stanchions will bend or break whilst the wires, or dyneema would hold.
Rossynant
Well-Known Member
Exactly - stanchions are the elastic point there, so to say; line is there to catch the unlucky. Stanchions just keep it in right place. So it's all about the line - should hold
josephmoore
Well-Known Member
Apologies for resurrecting an old thread, but I came across it researching suitable options for guardwires.
You are quite correct that polypropylene is susceptible to UV damage, however Dyneema (or Spectra, whichever brand name you prefer) is not polypropylene; it is Ultra-high-molecular-weight polyethylene - http://en.wikipedia.org/wiki/Ultra-high-molecular-weight_polyethylene. A very different plastic.
greenalien
Well-Known Member
Don't think I'd trust any plastic to be unaffected by UV in the long term. Question is, exactly how long?
grumpy_o_g
Well-Known Member
What are the regulations for stanchions? I don't think I've ever seen stanchions that I felt would my full weight against on any less that 55' regardless of how strong the guardrail/lifeline is.
josephmoore
Well-Known Member
Quite. I don't know - some googling will probably yield a datasheet or two - reputedly better than polypropylene though. Just wanted to make the correction that they are in fact two different plastics with different properties.
josephmoore
Well-Known Member
Stanchions and guardrails are basically sacrificial items. They're intended to bend or flex to absorb the shock of someone falling hard against the wires, kind of like the motorway crash barriers are designed to break away and contain a truck rather than being an immovable wall.
Buck Turgidson
Well-Known Member
I believe the ISO standard says something like:
The configuration shall withstand a load of 1787 N in
any direction without failure to the extent that it can
no longer perform its purpose.
Not exactly sacrificial!
charles_reed
Well-Known Member
I've used Dyneema/Spectra forguard rails for the last 15 years.
Pros - less damage to your body/limbs when you get thrown against it - less likelihood of people trying to hand-off from your boat - stronger then similar diameter coated SS - lighter and, if you cost accurately (include the swaged end-fittings), cheaper than plastic-covered SS wire of the same diameter.
Cons - stretches - 8 years before chafe demands replacement - knots weaken it, so you need to de-rate its load.
Look also at Vectran - as strong or stronger and less stretch - sometimes sold at a discount.
josephmoore
Well-Known Member
Sounds like a big number, but:
F = 1/2 m v2 / s
Where s is the slow down distance of the impact (citation here: http://www.engineeringtoolbox.com/impact-force-d_1780.html)
So... rough and ready, but for an 80kg man (at least, he'll be wearing clothes, you'd hope!) falling across a foredeck (say an vertical actual height of 1m?) with a guardrail deflection of what, 20cm? Far enough to bend a stanchion but not snap it off or stop it working?
1/2 * 80 * 4.43^2 / 0.2 = 3924.98N
Now that number will obviously be spread across the tension in the guardwires and several stanchions, but it seems to me like if someone had a pretty big fall against one (certainly on your average size yacht) the chances are they'd crumple and absorb some of the impact. Certainly if someone fell hard against my stanchion I'd expect it to stay attached and the guardwires not to snap, but I'd be replacing it when I got home.