Duncan Loop in lieu of eye splice or bowline? - SailNet Community
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post #1 of 23 Old 04-05-2011 Thread Starter
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Duncan Loop in lieu of eye splice or bowline?

When one does not have time to make a proper eye splice, common practice is to use a bowline; but, a bowline halves the strength of the line and is thus suboptimal, however convenient.

There is a popular fishing knot, called the duncan loop, which is purported to preserve 95% of the line strength, and I'm wondering if anyone has used, or has any opinions about its use in lieu of a splice, as it can be quickly tied without tools, and can even be used with a thimble to prevent chafe.

The key difference that I can see between a duncan loop and an eye splice or bowline is that the loop will almost surely constrict with use over time, and as such, under significant load it's really more of a bend than a loop, but if a thimble is used, that could be a non-issue.

Though that raises the question of whether a thimble might reduce the strength of the knot, and perhaps a perfectly round thimble might be better than the typical teardrop shape.

Here is one tied with 10mm line:



Given that it is as strong as an eye splice, and can be made more quickly than a splice and without tools, one might wonder why splice at all? Cosmetic arguments aside...

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post #2 of 23 Old 04-05-2011
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If that is tied as I understand, the eye will slip closed under load. That is impractical to get it back off in many circumstances.

I'm unfamiliar with the declaration that the bowline halves the strength of the line. Any references?


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post #3 of 23 Old 04-05-2011
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I do believe that it greatly weakens the line, but by 50% is a bit of a stretch. I'd like to see some break testing to back that up.

This knot looks like a slip knot. Getting very tight with a load and very hard to untie. It will also not work for a halyard unless the sail wasn't at full hoist, and it looks bad. Doesn't offer much. It would hang up on shrouds as much as a bowline with being harder to untie. And most jib sheets are much larger than they have to be, so breaking one is rare.

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post #4 of 23 Old 04-05-2011
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been through a court proceeding last year with a rope expert, who opined that a bowline takes about 25% of the strength off a poly mooring rope. 50% seems high.

And the bowline not only won't slip, but is much easier than a round turn and two (or more) half-hitches to "break" and untie afterwards.
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post #5 of 23 Old 04-05-2011
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Am a retired Boatswain Mate; Now that may or may not make me an expert on anything, but I'am a student of most things and still learning.
Have seen it referenced on the Bowline that it reduced the strength of the line to about 54%. Checked with another retired BM and he said about the same thing.
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post #6 of 23 Old 04-05-2011
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Very interesting.

Knot Break Strength vs. Rope Break Strength


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post #7 of 23 Old 04-05-2011
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Quote:
Originally Posted by Minnewaska View Post
Minnewaska's data is acceptable to me on this... 63% is a tad better then the 54% that I've quoted.

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post #8 of 23 Old 04-05-2011
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Interestingly, there seems to be some disagreement as to know the Duncan Loop is tied. This version clearly shows a slip-knot:



Yet I just watched a video that shows the initial loop being made in the standing part, which of course would prevent the knot from slipping:

http://www.videojug.com/film/how-to-...bJQ36w&cad=rja

However I don't know if this knot has the same strength as the slipping version. Actually I don't know if the slipping version has the strength that is claimed in the OP, but I have heard 50% for the bowline many times.

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post #9 of 23 Old 04-05-2011
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So what we should gather from all of this discussion is that rope selection should be based on the knotted breaking strength as opposed to the single line breaking strength.
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post #10 of 23 Old 04-06-2011
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Quote:
Originally Posted by patrickstickler View Post
When one does not have time to make a proper eye splice, common practice is to use a bowline; but, a bowline halves the strength of the line and is thus suboptimal, however convenient.

There is a popular fishing knot, called the duncan loop, which is purported to preserve 95% of the line strength, and I'm wondering if anyone has used, or has any opinions about its use in lieu of a splice, as it can be quickly tied without tools, and can even be used with a thimble to prevent chafe.

The key difference that I can see between a duncan loop and an eye splice or bowline is that the loop will almost surely constrict with use over time, and as such, under significant load it's really more of a bend than a loop, but if a thimble is used, that could be a non-issue.

Though that raises the question of whether a thimble might reduce the strength of the knot, and perhaps a perfectly round thimble might be better than the typical teardrop shape.

Here is one tied with 10mm line:



Given that it is as strong as an eye splice, and can be made more quickly than a splice and without tools, one might wonder why splice at all? Cosmetic arguments aside...
That is the equivalent of a Prusik Knot. We use this at work under extreme loads. We use it for many purposes but quite often it's used as a sacrificial line (sac line) in order to take the weight off the main line. In other words we will tie it on another line which is under extreme load, then release the load and this sac line will take the full weight without slipping. It's a very useful knot but I always put in one more turn (4 in total). Your photo shows 3 turns prior to the half hitch. From experience, that knot if rubbed against a hard edge or corner will definitely cause it to slip and in your case, constrict but with a thimble that wouldn't happen. I would question that without a constant load, the half hitch may just work it's way loose....just a thought.


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