Stealth mode - New Generation Ropes

[Dockhead]

Originally Posted by Seaworthy Lass

One other thing I am puzzling over is why are we using minimum breaking strain to select line diameter, not safe working load?

The recent loss of a drogue and resulting pitchpole is inlikely to have occured (even with the Flemish loop) if SWL had been observed.

Edited to add: Maybe when polyester and nylon were the only options, line diameter and weight would have been phenomenal if any attention was oaid to SWL rather tham MBS, maybe making the set up unworkable. In this case you just hoped you never encounted the worst case scenario.

With UHMWPE this is no longer an issue. Why don’t we now select line based on figures for SWL, which is generally taken as 20% of MBS for UHMWPE?

I was wondering the same thing the last time I re-read Don Jordan's design materials.


What he writes are that the normal loads in a bad storm are 10% of the "once in a lifetime worst case" loads. That is the only clue I can find about how he came up with the specs.


But in defense of his design -- it seems to be sound. There's enough experience now with them to know that they are not breaking due to insufficient strength.


BTW, I don't think there is any such "generally taken" % of MBS, which is SWL, for UHMWPE. The whole idea of SWL assumes that there is one margin of safety which suits all purposes -- I guess that this is not a sound concept. And perhaps that justifies Jordan's approach, after all. If he really knew what the once in a lifetime load was going to be, and that was the limiting factor, the biggest demand on the safety factor, then he didn't need any concept of SWL anyway.


In any case, I think sizing the cordage so that 20% of its MBS is equal to the once in a lifetime load, would be overkill. That's two safety margins packed on top of each other -- if anyway the normal "really bad storm" is producing only 10% of the design loads.

[Seaworthy Lass]

Originally Posted by s/v Jedi

I've been contemplating every option presented and think this drawing (I attached it again) from SWL comes closest to the simplest, strongest and most proven setup. I don't think this is an area for experimental knots, splices etc.

That said, It worries me that when all the force is on one leg of the bridle, the other leg could drag all the way up to the throat of the leader splice. Even when you think it would be above water, with waves you can never tell and we must consider this is being violently dragged and whipped through water.

So, I would want an insurance policy and in this case that would simply be a lashing around the legs of the leader splice that keep the two bridle legs nicely in place at all time. Even though there should never be much force on this lashing, I would still use that 1.8mm Spyderline

I am clueless regarding the exact importance, but I agree it would be nice to keep the legs from flopping.

One problem with any lashing is that it creates a tight throat with a high tearing force at this point. I wonder of some kind of “loose” lashing that avoids these tearing forces can be used?

I played around with coming up with a design a few years ago that left the throat open and Dockhead actually used this on his loops with low friction rings when he found whipping was just tearing apart. I think he had moderate success with it.

Let me put my thinking cap on .

[s/v Jedi]

Originally Posted by Seaworthy Lass

I am clueless regarding the exact importance, but I agree it would be nice to keep the legs from flopping.

One problem with any lashing is that it creates a tight throat with a high tearing force at this point. I wonder of some kind of “loose” lashing that avoids these tearing forces can be used?

I played around with coming up with a design a few years ago that left the throat open and Dockhead actually used this on his loops with low friction rings when he found whipping was just tearing apart. I think he had moderate success with it.

Let me put my thinking cap on .

There is no tearing force: you just demonstrated that the bridle loops stay tightly together. This means it's just the flopping around that moves them which is no force at all.

Also, I don't think the lashing reduces the strength of the splice. You must have something else in mind than what I envision

[Dockhead]

Originally Posted by Seaworthy Lass

. . .



One problem with any lashing is that it creates a tight throat with a high tearing force at this point. I wonder of some kind of “loose” lashing that avoids these tearing forces can be used?

I played around with coming up with a design a few years ago that left the throat open and Dockhead actually used this on his loops with low friction rings when he found whipping was just tearing apart. I think he had moderate success with it.

Let me put my thinking cap on .

Yes, I was baffled why the whipping on my strops was exploding, and many thanks to SWL for figuring it out. Was that when you starting studying throat-tearing forces?



I just crossed the North Sea (arrived earlier today in Brunsbuttel, nonstop from Cowes) using those same old strops, which are going strong after years and thousands of miles of hard sailing. They even went to the Arctic with me! They are not your exact design (I added loops of whipping to capture the ring) but the concept is certainly yours -- bulking up the whipping between the legs (that sounds vaguely perverted, doesn't it) in order to reduce that force. It works! It works bloody well, in fact.



Then you came up with your superior Bullseye Strop. I keep waiting for my old strops to die, to have an excuse to make up a bunch of Bullseyes, but the day hasn't come yet. Now that I have a few miles of Acera on board, of different sizes, maybe I'll just have a play.

[Seaworthy Lass]

Originally Posted by Dockhead

BTW, I don't think there is any such "generally taken" % of MBS, which is SWL, for UHMWPE.

Aussies think they can give recommendations on what SWL should be taken as for rope :
Cordage and Splicing - Break Strength Comparisons : Sydney Rigging Specialists

Below is the table they present with a 20% figure.

Regarding other items, it varies between manufacturers, but 33% is a common figure for chain, I have found roughly 20-50% for low friction rings. 20% doesn’t seem unreasonable for rope, even if it is a vague general estimate.

Working on minimum breaking strain to hold me in the worst case scenario where I am likely to lose my boat if not my life just doesn’t sit comfortably with me. If we are going to all the trouble and expense of carrying a series drogue, why not work on more than MBS?

[Seaworthy Lass]

Originally Posted by Dockhead

Yes, I was baffled why the whipping on my strops was exploding, and many thanks to SWL for figuring it out. Was that when you starting studying throat-tearing forces?



I just crossed the North Sea (arrived earlier today in Brunsbuttel, nonstop from Cowes) using those same old strops, which are going strong after years and thousands of miles of hard sailing. They even went to the Arctic with me! They are not your exact design (I added loops of whipping to capture the ring) but the concept is certainly yours -- bulking up the whipping between the legs (that sounds vaguely perverted, doesn't it) in order to reduce that force. It works! It works bloody well, in fact.



Then you came up with your superior Bullseye Strop. I keep waiting for my old strops to die, to have an excuse to make up a bunch of Bullseyes, but the day hasn't come yet. Now that I have a few miles of Acera on board, of different sizes, maybe I'll just have a play.

Yes, that is when I started looking at tearing forces. A long and painful process.

Did the Bullseye I posted never arrive?
It was returned to me from Finland as “undelivered”, but I then sent it to Cowes.

[Dockhead]

Originally Posted by Seaworthy Lass

. . . Did the Bullseye I posted never arrive?
It was returned to me from Finland as “undelivered”, but I then sent it to Cowes.

Of course it did, did I never thank you? Shame on me.



It is a beautiful piece of work, or rather piece of ingenuity, and it hangs in my salon, waiting to be used as the model when I start making my own. Everyone admires it greatly, and I think CF has gotten a few new members from people following the link to your thread, which I send them.

[Dockhead]

Originally Posted by Seaworthy Lass

. . . Working on minimum breaking strain to hold me in the worst case scenario where I am likely to lose my boat if not my life just doesn’t sit comfortably with me. If we are going to all the trouble and expense of carrying a series drogue, why not work on more than MBS?

SWL is just a safety margin, more or less arbitary according to the application.


I think Don intended that his approach provides an adequate safety margin -- just not expressed as SWL. I wouldn't get hung up on that concept.


That being said, I DID oversize mine, compared to the spec. In "worst case scenarios where I am likely to lose my boat if not my life" safety margins are good. I think that this device is a bit like a parachute harness -- something we really may be hanging on to for dear life.



I would not have oversized the cordage for my bridles and leader had I been juggling some other important parameter -- say the drogue was getting too big to carry or it would have cost another €5000. But the cost in bulk, weight and money to go to 14mm vs 12mm was moderate, so I went for it.


I was not so much concerned about breaking the cordage -- I think there have been enough JSDs used in enough storms to have statistically valid data that the specified cordage is strong enough. But I wanted some extra margin against chafe, flaws in connections or splices, etc.

[Seaworthy Lass]

Originally Posted by Dockhead

SWL is just a safety margin, more or less arbitary according to the application.


I think Don intended that his approach provides an adequate safety margin -- just not expressed as SWL. I wouldn't get hung up on that concept.


That being said, I DID oversize mine, compared to the spec. In "worst case scenarios where I am likely to lose my boat if not my life" safety margins are good. I think that this device is a bit like a parachute harness -- something we really may be hanging on to for dear life.



I would not have oversized the cordage for my bridles and leader had I been juggling some other important parameter -- say the drogue was getting too big to carry or it would have cost another €5000. But the cost in bulk, weight and money to go to 14mm vs 12mm was moderate, so I went for it.


I was not so much concerned about breaking the cordage -- I think there have been enough JSDs used in enough storms to have statistically valid data that the specified cordage is strong enough. But I wanted some extra margin against chafe, flaws in connections or splices, etc.

Yes, I am going to follow your lead and at least upsize my bridle and leader. I can find other uses for the substituted bits so they won’t go to waste.

I think there is negligible data for drogues constructed using UHMWPE. It does not behave like other fibres. We may well be the test subjects.

By the way, that Bullseye is useless hanging in the salon, or are you are actually using it for some other purpose there? My thoughts are running wild

[Seaworthy Lass]

Originally Posted by s/v Jedi

There is no tearing force: you just demonstrated that the bridle loops stay tightly together. This means it's just the flopping around that moves them which is no force at all.

Also, I don't think the lashing reduces the strength of the splice. You must have something else in mind than what I envision

It is not the tearing force on the bridle I am concerned about. It is the tearing force on the eye of the leader if you are planning to lash it to stop the bridle legs flopping about.

I think the tearing force (F) expressed as a % of the total load is
F = (50 tanθ) % where θ is half the throat angle:

Edited to add: It is not just an issue of the lashing tearing. If you manage to secure it well, the UHMWPE lashing will exert a high cutting force on the eye of the leader, inevitably causing damage to the leader and reducing its strength.

[thinwater]

First, SWL is an obsolete term that is not used by ISO. Rated capacity or WLL are now used. The common explanation for the change is legal risk, but it is more complex that that.


There are two ways of looking at it:

• What load gives an acceptable fatigue life?
• What load results in a breakage rate below some sigma factor (more than 6 as a minimum, but generally far more)?

With carbon steel there is a fatigue limit below which fatigue is not a factor. But this does not apply to all metals (aluminum or stainless) or any organic fiber. Thus, one way of setting WLL is an acceptable fatigue life. That is how nylon is typically designed, with about 8-12% being common. The fatigue limit of steel is about 50% BS, but 20-25% is generally used as the WLL to include variation and minor flaws.


The case of drogues is more like climbing rope; a lot of low force use and always the possibility of a worst-case strike (and we're not sure of what "worst case" is). With low-pull drogues, like a Shark or Seabrake, we can estimate the worst case based on maximum speed through the water. Perhaps the same logic can be applied to a JSD. The drag is about pounds = 25 x cones x (knots/10)^2. I think this might be a little high at higher speeds due to turbulence from the previous cone. This is about 11,000 pounds at 20 knots, for 107 cones, or just slightly more than the 10,000 pound figure Jordan speced for a 15000-pound boat. (Please check my work--no pride in authorship here.)


But the above assumes that the boat reaches 20 knots (almost certainly less) and that the rode is unused (which it is not). In fact, we have a mix of normal surges from waves, larger waves, and a few real hits. We may have some chafe. With Dyneema, we may get slack and snatch loads (polyester stretches). This means that the max load on nylon, polyester, and Dyneema may be different, but we're not really sure how or how much.



So now we are firmly in the realm of experience and judgment. I'd love to see some manner of overload indicator, such as used on slings. Something like this might work.

http://www.dcl-usa.com/wp-content/up...ucts_Part2.pdf

[Seaworthy Lass]

Originally Posted by s/v Jedi

I've been contemplating every option presented and think this drawing (I attached it again) from SWL comes closest to the simplest, strongest and most proven setup. I don't think this is an area for experimental knots, splices etc.

That said, It worries me that when all the force is on one leg of the bridle, the other leg could drag all the way up to the throat of the leader splice. Even when you think it would be above water, with waves you can never tell and we must consider this is being violently dragged and whipped through water.

So, I would want an insurance policy and in this case that would simply be a lashing around the legs of the leader splice that keep the two bridle legs nicely in place at all time. Even though there should never be much force on this lashing, I would still use that 1.8mm Spyderline

I have a possible solution.
There will be chafe guard on the eyes. Why not simply sew on a piece of additional chafe guard at the top of the eye of the leader, allowing the throat to stay open? If this tears then only the chafe guard tears. The underlying eye of the leader remains undamaged.

I have no idea how much the bridle not under load will flop about, but I doubt it is much. Cruisers do not report the boat yawing violently when the series drogue is deployed. So the above solution may work well.

SWL

[Seaworthy Lass]

Originally Posted by thinwater

First, SWL is an obsolete term that is not used by ISO. Rated capacity or WLL are now used. The common explanation for the change is legal risk, but it is more complex that that.


There are two ways of looking at it:

• What load gives an acceptable fatigue life?
• What load results in a breakage rate below some sigma factor (more than 6 as a minimum, but generally far more)?

With carbon steel there is a fatigue limit below which fatigue is not a factor. But this does not apply to all metals (aluminum or stainless) or any organic fiber. Thus, one way of setting WLL is an acceptable fatigue life. That is how nylon is typically designed, with about 8-12% being common. The fatigue limit of steel is about 50% BS, but 20-25% is generally used as the WLL to include variation and minor flaws.


The case of drogues is more like climbing rope; a lot of low force use and always the possibility of a worst-case strike (and we're not sure of what "worst case" is). With low-pull drogues, like a Shark or Seabrake, we can estimate the worst case based on maximum speed through the water. Perhaps the same logic can be applied to a JSD. The drag is about pounds = 25 x cones x (knots/10)^2. I think this might be a little high at higher speeds due to turbulence from the previous cone. This is about 11,000 pounds at 20 knots, for 107 cones, or just slightly more than the 10,000 pound figure Jordan speced for a 15000-pound boat. (Please check my work--no pride in authorship here.)


But the above assumes that the boat reaches 20 knots (almost certainly less) and that the rode is unused (which it is not). In fact, we have a mix of normal surges from waves, larger waves, and a few real hits. We may have some chafe. With Dyneema, we may get slack and snatch loads (polyester stretches). This means that the max load on nylon, polyester, and Dyneema may be different, but we're not really sure how or how much.



So now we are firmly in the realm of experience and judgment. I'd love to see some manner of overload indicator, such as used on slings. Something like this might work.

http://www.dcl-usa.com/wp-content/up...ucts_Part2.pdf

Many thanks for all this information. It explains why many manufacturers are not listing SWL currently.

It does also reinforce how many unknowns there are. We are really just winging it here. At least the series drogue has been tested by cruisers in polyester reasonably well. There has been little experience with UHMWPE in this application.

Regarding the figures given for MBS, no one has mentioned that in Susie’s case where nylon was used (I presume that means double braid with a polyester cover), there would have been an additional loss of strength with the nylon being wet. No additional margin was allowed for that.

[s/v Jedi]

Originally Posted by Seaworthy Lass

It is not the tearing force on the bridle I am concerned about. It is the tearing force on the eye of the leader if you are planning to lash it to stop the bridle legs flopping about.

I think the tearing force (F) expressed as a % of the total load is
F = (50 tanθ) % where θ is half the throat angle:

Edited to add: It is not just an issue of the lashing tearing. If you manage to secure it well, the UHMWPE lashing will exert a high cutting force on the eye of the leader, inevitably causing damage to the leader and reducing its strength.

Well yes, it needs to be done right just like all rope work. Lashings have proven themselves for centuries, even completely replacing splices and still today the method of choice for the heaviest loads on Dyneema applications.

I have never even heard of a lashing cutting the line of the eye, let alone seen it. I don't believe it until you show me the picture

Some pictures of lashings done right attached

[conachair]

Originally Posted by Seaworthy Lass

Regarding the figures given for MBS, no one has mentioned that in Susie’s case where nylon was used (I presume that means double braid with a polyester cover), there would have been an additional loss of strength with the nylon being wet. No additional margin was allowed for that.

More to it than that according to morganscloud, smoking gun is the knot rather than a splice. Plus the line was small for a very heavily loaded boat compared to unladen weight. Then by the sounds of it an absolute monster southern ocean wave made everything go quiet before all hell broke loose and she got pitchpoled..

As for the bridle, I'm tending towards cast thimbles in dyneema then a few wraps in a spanset/shackle to join to the main line.