Spyro
Well-Known Member
I'm replacing my jackstays, sewn loops look good but are they as strong as a knot?
Webbing Jackstays, sewn loops or a knot?
- Thread starter Spyro
- Start date
sarabande
Well-Known Member
There are a number of sites dealing with the technical breaking strengths of rope and various knots.
Here's one table
from a climbing research group (bless them !)
http://www.caves.org/section/vertical/nh/50/knotrope.html
Stitching webbing has to be done carefully so that the needle does not cut the webbing fibres and reduce strength. The pattern of the stitching is also important, to ensure that the load is taken across all the stitching area and not just across a few lines.
Some specialised webbing has progressive failure of stitching built into the pattern, so that the webbing breaks gradually rather than suddenly.
Sewing threads must also be resistant to UV, and to degradation from contact with oils / diesel / acid etc. Specs for this can be found on sites relating to car safety belt construction.
I'll ferret around for some old research work.
I would not use a knot in a lifeline or safety harness webbing, unless for emergency purposes. As much as anything, a knot in tape or webbing will lock when loaded, and be very difficult to undo.
Here's one table
from a climbing research group (bless them !)
http://www.caves.org/section/vertical/nh/50/knotrope.html
Stitching webbing has to be done carefully so that the needle does not cut the webbing fibres and reduce strength. The pattern of the stitching is also important, to ensure that the load is taken across all the stitching area and not just across a few lines.
Some specialised webbing has progressive failure of stitching built into the pattern, so that the webbing breaks gradually rather than suddenly.
Sewing threads must also be resistant to UV, and to degradation from contact with oils / diesel / acid etc. Specs for this can be found on sites relating to car safety belt construction.
I'll ferret around for some old research work.
I would not use a knot in a lifeline or safety harness webbing, unless for emergency purposes. As much as anything, a knot in tape or webbing will lock when loaded, and be very difficult to undo.
FAITIRA
Well-Known Member
Re: Webbing Jackstays, sewn loops or a knot?
I prefer knots. Bill.
I prefer knots. Bill.
doug748
Well-Known Member
It's normally pretty thick webbing, do you intend to sew by hand?
sarabande
Well-Known Member
Re: Webbing Jackstays, sewn loops or a knot?
What's the preference based on please ? /forums/images/graemlins/confused.gif
What's the preference based on please ? /forums/images/graemlins/confused.gif
Boathook
Well-Known Member
Sewn loops would be best with a large overlap. Webbing used for lifting is always sewn and knots do weaken the rope/strap by up to 50%.
Ruffles
Well-Known Member
FWIW my jackstays were made up by a sailmaker in Weymouth. He charged less than it would have cost me to buy the tape.
bilbobaggins
N/A
Interesting - given the recent 'opinionated' thread regarding preferences for bowline vs double-figure-of-eight - that the Double F-o-8 shows up over 22% stronger than the bowline.....
....which is exactly what climbers and cavers have been aware of for decades, following Government-funded research from the National Engineering Laboratory in the 60s, and much more by the rope industry since.
I'm confident that some will still post on here, discounting all the published research and maintaining against all known truth that the bowline remains the best/strongest/prettiest knot in Christendom ( Oops! Not allowed to say - or think - that anymore! )....
Just how reactionary can yotties get? /forums/images/graemlins/grin.gif
....which is exactly what climbers and cavers have been aware of for decades, following Government-funded research from the National Engineering Laboratory in the 60s, and much more by the rope industry since.
I'm confident that some will still post on here, discounting all the published research and maintaining against all known truth that the bowline remains the best/strongest/prettiest knot in Christendom ( Oops! Not allowed to say - or think - that anymore! )....
Just how reactionary can yotties get? /forums/images/graemlins/grin.gif
AliM
Well-Known Member
My son, a climber, said he always used figure of eights because you could tell at a glance whether they were tied correctly by novices, whereas a bowline can deceive you. He didn't mention that it was stronger, but that evidence is pretty compelling.
I would not use any knot for jackstays because they are yet another thing to trip over, where a stitched jackstay lies flat. On the other hand, the stitching is likely to uv-degrade as fast or faster than the webbing itself, and the jackstay may chafe because the loop is fixed.
Are there any data on UV degradation of webbing (i.e. strength v. length of exposure)?
My vote is for stitched jackstays, regular inspection of stitching, taking them out of the sun when not actually sailing, and replacement as soon as any fading or degradation is seen.
Oh, and not falling in (and thus having to use them) in the first place!
I would not use any knot for jackstays because they are yet another thing to trip over, where a stitched jackstay lies flat. On the other hand, the stitching is likely to uv-degrade as fast or faster than the webbing itself, and the jackstay may chafe because the loop is fixed.
Are there any data on UV degradation of webbing (i.e. strength v. length of exposure)?
My vote is for stitched jackstays, regular inspection of stitching, taking them out of the sun when not actually sailing, and replacement as soon as any fading or degradation is seen.
Oh, and not falling in (and thus having to use them) in the first place!
davies
Well-Known Member
Climbers often tie tape using a "tape knot" - just a single overhand knot in the doubled end with the tape tweaked to make a smooth knot. I tie this in each end of the lazyjack, then stitch the end to the standing part so that the knot can't work loose.
Spyro
Well-Known Member
[ QUOTE ]
It's normally pretty thick webbing, do you intend to sew by hand?
[/ QUOTE ]
No.
It's normally pretty thick webbing, do you intend to sew by hand?
[/ QUOTE ]
No.
Norman_E
Well-Known Member
Mine were bought ready made with sewn loops. The stupid thing is that they would have been much better if stainless steel "D" rings had been put in before they were sewn.
FullCircle
Well-Known Member
In a previous life I designed aircraft seat belts and cargo nets.
The webbing stitching, pattern and thread used is crucial for acheiving maximum performance. Knots in webbing seriously degrade performance by creating point loads at the bends which increase as the knot takes up under load.
The load paths should all be designed to be in shear.
Do not use any thread with Kevlar in it, as Kevlar works best in tension, not in shear. It is particularly subject to embrittlement when it is folded into a loop and tautened, as in a sewing stitch.
If you are doing it yourself, go look at an aircraft cargo net for the stitch pattern size and the thread. It is approximate to your jackstays, as it is usually 25mm webbing.
The webbing stitching, pattern and thread used is crucial for acheiving maximum performance. Knots in webbing seriously degrade performance by creating point loads at the bends which increase as the knot takes up under load.
The load paths should all be designed to be in shear.
Do not use any thread with Kevlar in it, as Kevlar works best in tension, not in shear. It is particularly subject to embrittlement when it is folded into a loop and tautened, as in a sewing stitch.
If you are doing it yourself, go look at an aircraft cargo net for the stitch pattern size and the thread. It is approximate to your jackstays, as it is usually 25mm webbing.
Spyro
Well-Known Member
"I would not use a knot in a lifeline or safety harness webbing, unless for emergency purposes. As much as anything, a knot in tape or webbing will lock when loaded, and be very difficult to undo. "
--------------------
They will be attached to the toe rail by shackles so I would not need to undo them.
You couldn't undo a sewn loop /forums/images/graemlins/confused.gif
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They will be attached to the toe rail by shackles so I would not need to undo them.
You couldn't undo a sewn loop /forums/images/graemlins/confused.gif
FAITIRA
Well-Known Member
[ QUOTE ]
"I would not use a knot in a lifeline or safety harness webbing, unless for emergency purposes. As much as anything, a knot in tape or webbing will lock when loaded, and be very difficult to undo. "(quote)
The last thing I want is for it to 'undo', I,m sure the margin in the breaking strain is adequate.
How does one ascertain the condition of stitching for this purpose, genuinely interested?. Bill.
"I would not use a knot in a lifeline or safety harness webbing, unless for emergency purposes. As much as anything, a knot in tape or webbing will lock when loaded, and be very difficult to undo. "(quote)
The last thing I want is for it to 'undo', I,m sure the margin in the breaking strain is adequate.
How does one ascertain the condition of stitching for this purpose, genuinely interested?. Bill.
doug748
Well-Known Member
I think most domestic sewing machines would struggle. OK if you have the use of an industrial unit.
I have sewn lots of webbing of the seat belt type weight, using Dacron sailmaking thread, for things like lifelines and mast ladders - no problems so far, but I guess the real test is when you go over the side. Err on the side of caution with the overlap and I believe you will produce a breaking strain in excess of a commercial product.
I have sewn lots of webbing of the seat belt type weight, using Dacron sailmaking thread, for things like lifelines and mast ladders - no problems so far, but I guess the real test is when you go over the side. Err on the side of caution with the overlap and I believe you will produce a breaking strain in excess of a commercial product.
damo
Well-Known Member
This query comes up a lot, so FWIW here is my take on it again /forums/images/graemlins/smile.gif
A properly stitched loop (ie by a sling manufacturer) should offer about the same breaking strength as the webbing itself. All knots reduce the strength of tape or rope - up to about 50% for the weakest of the "standard" knots eg bowline, but only about 30% for the strongest eg the Figure-of-9 (a bit stronger than the Figure-of-8).
Thousands and thousands of falls have been taken by climbers over the decades onto tape knots, and failures have been known to occur only when there is prior damage, or there is a sharp edge or crushing. My jackstays have knotted tape loops, and I am perfectly happy with that system.
50mm polyester webbing with a static breaking strain of say 25kn, will fail at about half that in tests with a tape knot, but still has way more than the desired safety factor of 5kn.
"The maximum loads felt by a faller are suspected to be at most 15 kN ( 3372 lbs ). This is the load at which other parts of the system start breaking, such as the rope at the tie in knot, the top karabiner through the protection, and the faller's body being broken by the forces of de-acceleration. Thankfully, very few falls reach forces anywhere near this high, but we can view it as an upper limit"(Black Diamond - harness manufacturer)
In simple terms what this means is that the maximum possible shock is only a little more than the point at which a tape knot in a webbing jackstay is likely to fail, and that shock load is way more than a sailor can experience, unless it is a mast fall from above the anchor point.
So, the (informed /forums/images/graemlins/blush.gif) opinion of this opiniated poster is that:
a) A professionally stitched tape loop is the strongest, but most expensive
b) A tape knot, properly tied, with the end stitched down to prevent slippage, is plenty strong enough
c) ANY method of securing is only as strong as the weakst point, and environmental damage and abrasion will weaken the strongest method - including tape stitching.
A properly stitched loop (ie by a sling manufacturer) should offer about the same breaking strength as the webbing itself. All knots reduce the strength of tape or rope - up to about 50% for the weakest of the "standard" knots eg bowline, but only about 30% for the strongest eg the Figure-of-9 (a bit stronger than the Figure-of-8).
Thousands and thousands of falls have been taken by climbers over the decades onto tape knots, and failures have been known to occur only when there is prior damage, or there is a sharp edge or crushing. My jackstays have knotted tape loops, and I am perfectly happy with that system.
50mm polyester webbing with a static breaking strain of say 25kn, will fail at about half that in tests with a tape knot, but still has way more than the desired safety factor of 5kn.
"The maximum loads felt by a faller are suspected to be at most 15 kN ( 3372 lbs ). This is the load at which other parts of the system start breaking, such as the rope at the tie in knot, the top karabiner through the protection, and the faller's body being broken by the forces of de-acceleration. Thankfully, very few falls reach forces anywhere near this high, but we can view it as an upper limit"(Black Diamond - harness manufacturer)
In simple terms what this means is that the maximum possible shock is only a little more than the point at which a tape knot in a webbing jackstay is likely to fail, and that shock load is way more than a sailor can experience, unless it is a mast fall from above the anchor point.
So, the (informed /forums/images/graemlins/blush.gif) opinion of this opiniated poster is that:
a) A professionally stitched tape loop is the strongest, but most expensive
b) A tape knot, properly tied, with the end stitched down to prevent slippage, is plenty strong enough
c) ANY method of securing is only as strong as the weakst point, and environmental damage and abrasion will weaken the strongest method - including tape stitching.
Spyro
Well-Known Member
Thanks for info
Difficult to ascertain the load when a person falls in. I suspect if a person did go over the side on my boat whilst clipped to a jackstay they would hit the water first before the safety strop became tight and pulled on the Jackstay. Thus forces applied would more likely to be caused by a person being dragged through the water. This being dependent on boat speed and the drag caused by the size of the person and whatever they were wearing.
It would be interesting to know if these forces have ever been tested.
Ian
Difficult to ascertain the load when a person falls in. I suspect if a person did go over the side on my boat whilst clipped to a jackstay they would hit the water first before the safety strop became tight and pulled on the Jackstay. Thus forces applied would more likely to be caused by a person being dragged through the water. This being dependent on boat speed and the drag caused by the size of the person and whatever they were wearing.
It would be interesting to know if these forces have ever been tested.
Ian
Ruffles
Well-Known Member
[ QUOTE ]
The maximum loads felt by a faller are suspected to be at most 15 kN
[/ QUOTE ]
Unfortunately, with the jackstay stretched tight, the load on the webbing is likely to be an order of magnitude more than that experienced by the faller.
Not much you can do about it. Except try not to fall overboard!
BTW the heavyweight polyester tape is actually 3 ton. About to order some.
The maximum loads felt by a faller are suspected to be at most 15 kN
[/ QUOTE ]
Unfortunately, with the jackstay stretched tight, the load on the webbing is likely to be an order of magnitude more than that experienced by the faller.
Not much you can do about it. Except try not to fall overboard!
BTW the heavyweight polyester tape is actually 3 ton. About to order some.
dratsea
Well-Known Member
Valid question but only if your lanyard is knotted! If lanyard is sewn, no problem as long as jackstay is same calibre tape.
John
John