Impact of knots on rope strength


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neilm
neilm
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There is some good data on the extent to which various knots weaken ropes at
http://caves.org/section/vertical/nh/50/knotrope-hold.html



Dick
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neilm - 5/15/2019
There is some good data on the extent to which various knots weaken ropes at
http://caves.org/section/vertical/nh/50/knotrope-hold.html 
To summarize, double and single sheet bends give only about 50% of the rope strength, and then only if backed up by half hitches to resist pulling out.
The fisherman's knot gives 75% strength, and is safer with half hitch backups

For loops, a bowline is about 65% of a splice, and double figure eight loop about 75%

Splices have the full strength of the rope, if well done.

On a related issue, an article on the impact of water on strength of climbing ropes shows that wet nylon has only about 30% of the strength of dry nylon when subject to abrupt shock loads, such as a falling rock climber.  I suspect the situation is not quite so bad in the less abrupt shock loads seen in anchor rodes.  Data on tests would be very interesting.
Hi Neil,
It is nice to have sources for this data and to be reminded of the limitations our ropes have when knots are tied in them.
Your admonition of splices needing to be well done is, I believe, very well taken when it comes to braid. I taught myself how to splice braid years ago and then decided that if I did not do them regularly, they would not be done well enough to distribute strain equally. And there is so little call for splices that there would be no regular practice. I then hired riggers to do the splices who I knew did them regularly.
I have figured a 50% hit on rope strength generally for most knots used on a boat: this allows for a certain safety margin. On cruising boats, we generally have a fair amount of wiggle room when it comes to rope strength as we like lines big enough to be comfortable in the hand.
For me, bowlines, rolling hitches and buntline hitches are the knots I use that see the most strain. I tend not to use knots unless I use them with some regularity, so I do not use the fisherman’s knot nor sheet bends of any sort, although I see them as good knots. Do you use them regularly? If so, for what application?
As to the strength loss reported for wet nylon, my experience with nylon makes me somewhat skeptical of this. Shock loading always stresses rope: was the break by any chance at the knot or where the line was going over a bend? Luckily, sailing does not subject lines too often to shock loading, tethers being one exception where this needs to be taken into consideration. My nylon line that is often wet for much of its length and gets loaded hard is my snubber, which is fairly light as these things go (7/16 inch) and has weathered squalls and storms and aggressive backing up to set the anchor.
Interesting discussion: thanks for initiating.
My best, Dick Stevenson, s/v Alchemy

neilm
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Dick - 5/15/2019
neilm - 5/15/2019
There is some good data on the extent to which various knots weaken ropes at
http://caves.org/section/vertical/nh/50/knotrope-hold.html 
To summarize, double and single sheet bends give only about 50% of the rope strength, and then only if backed up by half hitches to resist pulling out.
The fisherman's knot gives 75% strength, and is safer with half hitch backups

For loops, a bowline is about 65% of a splice, and double figure eight loop about 75%

Splices have the full strength of the rope, if well done.

On a related issue, an article on the impact of water on strength of climbing ropes shows that wet nylon has only about 30% of the strength of dry nylon when subject to abrupt shock loads, such as a falling rock climber.  I suspect the situation is not quite so bad in the less abrupt shock loads seen in anchor rodes.  Data on tests would be very interesting.
Hi Neil,
It is nice to have sources for this data and to be reminded of the limitations our ropes have when knots are tied in them.
Your admonition of splices needing to be well done is, I believe, very well taken when it comes to braid. I taught myself how to splice braid years ago and then decided that if I did not do them regularly, they would not be done well enough to distribute strain equally. And there is so little call for splices that there would be no regular practice. I then hired riggers to do the splices who I knew did them regularly.
I have figured a 50% hit on rope strength generally for most knots used on a boat: this allows for a certain safety margin. On cruising boats, we generally have a fair amount of wiggle room when it comes to rope strength as we like lines big enough to be comfortable in the hand.
For me, bowlines, rolling hitches and buntline hitches are the knots I use that see the most strain. I tend not to use knots unless I use them with some regularity, so I do not use the fisherman’s knot nor sheet bends of any sort, although I see them as good knots. Do you use them regularly? If so, for what application?
As to the strength loss reported for wet nylon, my experience with nylon makes me somewhat skeptical of this. Shock loading always stresses rope: was the break by any chance at the knot or where the line was going over a bend? Luckily, sailing does not subject lines too often to shock loading, tethers being one exception where this needs to be taken into consideration. My nylon line that is often wet for much of its length and gets loaded hard is my snubber, which is fairly light as these things go (7/16 inch) and has weathered squalls and storms and aggressive backing up to set the anchor.
Interesting discussion: thanks for initiating.
My best, Dick Stevenson, s/v Alchemy

​The info I have on resistance of wet nylon to shock loads is from a climbing oriented study.  It is all over the place, with one source being       http://www.paci.com.au/downloads_public/PPE/12_Wet_rope_dangers.pdf
My thinking is that anchor rodes and safety harnesses experience less abrupt shock loads than those caused by a falling climber.  Given the dramatic loss shown in the reference, I am assuming that being wet causes something like 50% loss of shock resistance for sailing.​
Simon Currin
Simon Currin
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Neil
Was it a figure of eight that failed on the Series Drogue that led to the loss of a Golden Globe Boat?
Simon

neilm - 5/15/2019
There is some good data on the extent to which various knots weaken ropes at
http://caves.org/section/vertical/nh/50/knotrope-hold.html 
​To summarize, double and single sheet bends give only about 50% of the rope strength, and then only if backed up by half hitches to resist pulling out.
The fisherman's knot gives 75% strength, and is safer with half hitch backups

For loops, a bowline is about 65% of a splice, and double figure eight loop about 75%

Splices have the full strength of the rope, if well done.​​​​​​

On a related issue, an article on the impact of water on strength of climbing ropes shows that wet nylon has only about 30% of the strength of dry nylon when subject to abrupt shock loads, such as a falling rock climber.  ​I suspect the situation is not quite so bad in the less abrupt shock loads seen in anchor rodes.  Data on tests would be very interesting.​



Dick
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Simon Currin - 5/16/2019
Neil
Was it a figure of eight that failed on the Series Drogue that led to the loss of a Golden Globe Boat?
Simon

neilm - 5/15/2019
There is some good data on the extent to which various knots weaken ropes at
http://caves.org/section/vertical/nh/50/knotrope-hold.html 
To summarize, double and single sheet bends give only about 50% of the rope strength, and then only if backed up by half hitches to resist pulling out.
The fisherman's knot gives 75% strength, and is safer with half hitch backups

For loops, a bowline is about 65% of a splice, and double figure eight loop about 75%

Splices have the full strength of the rope, if well done.

On a related issue, an article on the impact of water on strength of climbing ropes shows that wet nylon has only about 30% of the strength of dry nylon when subject to abrupt shock loads, such as a falling rock climber.  I suspect the situation is not quite so bad in the less abrupt shock loads seen in anchor rodes.  Data on tests would be very interesting.



Hi Simon,
I believe you are referring to the pitchpoling of Susan Goodall's boat in the Golden Globe Race, I believe the jury is still out on exactly the failure point on the bridle, although I believe most think it was at the knot, cow hitch or, as you say, the figure eight.
Dick
neilm
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Simon Currin - 5/16/2019
Neil
Was it a figure of eight that failed on the Series Drogue that led to the loss of a Golden Globe Boat?
Simon

neilm - 5/15/2019
There is some good data on the extent to which various knots weaken ropes at
http://caves.org/section/vertical/nh/50/knotrope-hold.html 
​To summarize, double and single sheet bends give only about 50% of the rope strength, and then only if backed up by half hitches to resist pulling out.
The fisherman's knot gives 75% strength, and is safer with half hitch backups

For loops, a bowline is about 65% of a splice, and double figure eight loop about 75%

Splices have the full strength of the rope, if well done.​​​​​​

On a related issue, an article on the impact of water on strength of climbing ropes shows that wet nylon has only about 30% of the strength of dry nylon when subject to abrupt shock loads, such as a falling rock climber.  ​I suspect the situation is not quite so bad in the less abrupt shock loads seen in anchor rodes.  Data on tests would be very interesting.​


The failure was at a point where both legs of the bridle (terminated in eyesplices)  were cow-hitched ​to a loop made with a figure of eight. Seems that the figure eight failed.   My info is mostly  from John Harrie's article at  https://www.morganscloud.com/2019/04/15/susie-goodalls-series-drogue-failure/     
Two cow hitch/splices were sharing the load, whereas the figure eight was on its own.
​​
Dick
Dick
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neilm - 5/16/2019
Simon Currin - 5/16/2019
Neil
Was it a figure of eight that failed on the Series Drogue that led to the loss of a Golden Globe Boat?
Simon

neilm - 5/15/2019
There is some good data on the extent to which various knots weaken ropes at
http://caves.org/section/vertical/nh/50/knotrope-hold.html 
To summarize, double and single sheet bends give only about 50% of the rope strength, and then only if backed up by half hitches to resist pulling out.
The fisherman's knot gives 75% strength, and is safer with half hitch backups

For loops, a bowline is about 65% of a splice, and double figure eight loop about 75%

Splices have the full strength of the rope, if well done.

On a related issue, an article on the impact of water on strength of climbing ropes shows that wet nylon has only about 30% of the strength of dry nylon when subject to abrupt shock loads, such as a falling rock climber.  I suspect the situation is not quite so bad in the less abrupt shock loads seen in anchor rodes.  Data on tests would be very interesting.


The failure was at a point where both legs of the bridle (terminated in eyesplices)  were cow-hitched to a loop made with a figure of eight. Seems that the figure eight failed.   My info is mostly  from John Harrie's article at  https://www.morganscloud.com/2019/04/15/susie-goodalls-series-drogue-failure/     
Two cow hitch/splices were sharing the load, whereas the figure eight was on its own.

Hi Simon,
Interesting. And I am very glad that I am not doing the R&D necessary to fine tune the JSD's use. Dick
neilm
neilm
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Something strange happened to the link in my post above on impact of knots on strength of ropes
I think it is fixed now

On a related issue, an article on the impact of water on strength of climbing ropes shows that wet nylon has only about 30% of the strength of dry nylon when subject to abrupt shock loads, such as a falling rock climber.  ​I suspect the situation is not quite so bad in the less abrupt shock loads seen in anchor rodes.  Data on tests would be very interesting.​
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