“Dyneema/Spectra’s very high lubricity leads to poor knot holding ability, and has led to the recommendation to use the triple fisherman’s knot [rather than the traditional double fisherman’s knot] in 6mm Dyneema core cord to avoid a particular failure mechanism of the double fisherman’s, where first the sheath fails at the knot, then the core slips through.” http://en.wikipedia.org/wiki/Double_fisherman’s_knot#Security
That wikipedia entry recommends the Triple Fisherman where the Double Fisherman fails. I call B.S. Where the Double Fisherman fails, I’d be going with an entirely different bend approach.
I think you may be misinterpreting strength findings. Look at page 3:
For a double fisherman’s knot, Gemini and Titan share an interesting failure mode.
The sheath breaks at the knot and the slippery core unties, pulling through the sheath. When a triple fisherman’s
knot is tied, this does not happen. The strength gain for the triple fisherman’s is not large, but it is enough to
change the mechanism.
The failure mode they’re talking about is merely the mode of breaking with a small or “not large” strength change that one might expect anyway by using more rope. This is not a security issue with slippage, it’s what happens after the rope ruptures.
As I said before, the security issue with slick stiff rope and this knot is the possibility of things springing open, not slither or slipping.
I hear you, but a failure is a failure. The technicalities don’t matter to the user when it happens. I have to wonder if the same failure would happen if a Zeppelin Bend were used in the same test. That’s the important question.
It seems that that the Zeppelin Bend doesn’t put as much stress on a rope as does the Double Fisherman. The angles in the Zeppelin Bend are not as extreme. So, perhaps the rope failure wouldn’t happen in the first place. I don’t know, but again I’d be interested to know if the Zeppelin Bend would cause the rope to fail in the same manner. I understand that you’re not interested, and that’s fine. I am.
But the very report you cited shows on page 5 how the Double Fisherman’s Knot is nearly as strong as the Triple Fisherman’s Knot which is well respected for strength!
In the strength arena, it doesn’t make sense to say that you’d prefer a weaker knot (Zeppelin in this case) to a stronger knot just because the stronger knot has an unusual behavior after rupture.
How is the Triple Fisherman a stronger knot than the Zeppelin Bend? I haven’t seen any evidence.
I’m not even convinced the Triple Fisherman is more secure generally. I’m not referring to your shake test. I mean brute force pulling in slippery rope.
I’m right at the issue that I original brought up.
According to the test in the source I cited above, “The sheath breaks at the knot and the slippery core unties, pulling the sheath through.”
That is, the sheath breaks at the Double Fisherman Bend. I’m wondering if that same sheath would break at the Zeppelin Bend in the first place. That brought up my comment about how the Zeppelin Bend may be easier on the rope than is the Double Fisherman Bend. I don’t know, but would like to know.
I’d classify the issue in the report as being a security issue. The definition of security is the ability of a rope to resist slippage. Thus, there’s the link to your question about slippage. Anyway, the important thing to me is that there is a failure in the Double Fisherman Bend, as I explained above. The failure is a security issue and/or strength issue. Whatever you want to call it, there is an issue.
Again, I’m wondering if the Zeppelin Bend would have the same failure in the same test. You may continue to argue your point. However, I’m primarily concerned with that sentence I put in bold right there.
Not after rupture. After rupture, it’s game over. Once a rope ruptures and begins (in that split second) losing cross-sectional material, there’s no going back.
I’m done trying to think of different ways to explain this. Security issues only apply before rupture. It shouldn’t need to be said.
Well damn, if a person is going to use that type of rope and if it is known that the Zeppelin Bend won’t cause a rupture in that type of rope, I would think the user of that rope would want to know such information about the Zeppelin Bend.
This is the “Practical Knots” Forum, not the “The Theoretical Parameters for Knots” Forum.
All knots eventually rupture in any rope. No rope is indestructable. No knot can be stronger than the rope. These rupture issues are all questions of strength percentages, not security, not slippage.
I don’t care what label you put on it. Call it “all of the above” if you want. For about the third time, I don’t care what you call the issue. There is an issue. There is a rupture difference between the knots (Double Fisherman Bend and Zeppelin Bend). I would like to know about this difference. Sheesh, this place is impossible.
Repeating your secondary question on strength ad nauseum won’t answer it. The reports than analyze the strength of Double & Triple Fisherman Knots typically do not analyze the strength percentage of the Zeppelin bend in the same set of tests (or at all).
Someone did some very limited tests and found that the Zeppelin Bend was weaker than the Flemish (Figure Eight) bend:
Now in my other Zeppelin bend thread, Pulling a Zeppelin bend to failure… I did break it. Since I don’t have a way to break test non-knotted rope, I can only go by the manufacturer’s published MBS for 10mm PMI of 6070 lbs. That reduces the strength of the rope to about 63% of the rope’s MBS.
Since I couldn’t measure the non-knot break strength of the rope, I decided to at least pull an F8 bend for comparison. It broke at about 66% of the ropes MBS.
Since we’re comparing the Zeppelin Bend to the Double/Triple Fisherman Bend, we might as well be comparing the Double/Triple Zeppelin Bend instead.
Discussions about strength are always a bit hokie. There are always a few variables that are conveniently ignored for the sake of continuing the discussion.