Author Topic: The "Front-Back Bowline"  (Read 9132 times)

DerekSmith

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Re: The "Front-Back Bowline"
« Reply #15 on: July 11, 2010, 12:43:21 PM »
I said it before, and I'll say it again:
JCS

Hi JC,

While repetition (i.e. 'chanting') can induce a trance like state of belief in the chanter, it rarely makes the chant any the more true, and does very little for communication which is I believe the primary purpose of this forum.

In your haste to rush to the defence of your knot and the path of innovation, I think you might have blindsided yourself to the point that I was trying to make.  So rather than simply 'saying it again' I will try to explain from a slightly different tack.

Spectra and its ilk are quirky materials - very little stretch or compression and vanishingly small grip friction.  Very few knots will hold in Spectra.  I was not suggesting that because your knot will not hold in Spectra that it is a bad knot.  I was suggesting that Spectra could be used as a tool to help us see how a knot worked under load and perhaps identify any weaknesses it might have.

Loopknots are special cases.  Of the five parts - the loaded line, the knot, the loaded line leg, the end or tail leg and the most often ignored part - the end or tail.  In use, the loaded line carries 100% of the load, and the general misconception is that the two loop legs each share ca 50% of the load, and of course, the tail is unloaded.

In reality, whatever the loop is 'containing' spreads the legs and so vector forces come into play and can massively amplify the load delivered to them by the loaded line.  On top of this, the loop wraps around its contained object and friction allows a disparity in the forces in each leg to build up.  Finally, load shifting means that the knot can be cyclically loaded across the three lines.

A good loop knot will be able to handle variability in load and direction without changing its structure.  The Eskimo bwl is particularly poor in this respect and will open and close if the load alternates between the two loop legs and in doing so, the tail will be ratcheted progressively into the knot, and unless it has a stopper, will eventually fail.

Likewise, a good loopknot will increase its grip on the tail end as the load is increased, it is this aspect that your knots structure fails to provide.  The loaded line leg cannot 'go' anywhere, it is attached to the loaded line, but the tail leg has only the grip of the knot to hold it in place, so it is important that, as the load increases, so this load is utilised to increase the grip on tail leg to stop it sliding through the knot.  In your knot, the extra load grip is 'wasted' by gripping the load line leg which cannot 'go' anywhere.

Derek

jcsampson

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Re: The "Front-Back Bowline"
« Reply #16 on: July 11, 2010, 07:22:44 PM »
Quote from: jcsampson
"I said it before, and I'll say it again."

To Derek:

Yeah, I have my favorite expressions. I can come off sounding very threatening sometimes. I got that from my father.

Your defense and explanations are very good; you deliver explanations very well.

There's just one thing bothering me: My tests of this knot have failed to verify the various complaints that have been made. I therefore have reason to believe that those who have complained haven't been doing what I've been doing to make the loop work.

However, considering shifting loads and Spectra's poor grip, try this one. I suspect that it has what you're looking for:

- http://igkt.net/sm/index.php?topic=1927.msg13313#msg13313

JCS

knot4u

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Re: The "Front-Back Bowline"
« Reply #17 on: July 11, 2010, 08:17:13 PM »
I've tested this Front-Back Bowline in various materials, yanking and pulling it various directions. While I may not be a knot expert, I do know what I like.  I like this knot, and I don't like all the knots that JCSampson has posted.

A few points:

-I'm not sure why, but the FB Bowline seems to have better weight distribution than the Bowline.  The Bowline may not have a 1 diameter turn on a leg, but it's so darn close to being a 1 diameter turn that it might as well be called such.  Further, the 1 diameter turn on the FB Bowline may make the FB Bowline more secure.  Who knows?

-100% of the load is NOT placed on that 1 diameter turn.  That's because both legs are not going through that 1 diameter turn.  The 1-diameter turn is somewhat off to the side, and the load at that point seems to be shared at least somewhat with the load on the other leg of the loop.  I'm having serious doubts that the knot would break at the 1 diameter turn.  So, the point about the 1 diameter may be a moot point.

-I don't see how a 1 diameter turn is so horrible anyway, and they're practically unavoidable.  For example, a trucker's hitch and a versatackle use 1 diameter turns.  These two hitches are probably my most useful hitches of all.  Also, some useful knots have 1-diameter turns.  An overhand knot has two 1-diameter turns in a small area, right?  When testing knot strength using a machine, stopper knots are typically used at the ends to hold the rope to the machine. Those stopper knots must be stronger than than the knot being tested.  I know of at least one test, involving testing Fisherman Bends, where the person was using overhand knots on the ends for rope connection to the machine.

-The following point is my personal preference, and so please nobody get too defensive.  Theory should take a backseat to experimentation on knots.  The replies that include actual testing should dominate the thread.  I remember some mathematicians in my undergrad applying their theories to knots.  I recall a really smart guy who realized high-level knot theories were nearly useless for predicting real world knot behavior.  Theory is important, but let's keep it in perspective, especially when it comes to the practical usage of knots.
« Last Edit: July 12, 2010, 05:47:30 AM by knot4u »

Dan_Lehman

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Re: The "Front-Back Bowline"
« Reply #18 on: July 16, 2010, 05:39:33 AM »
Since no one complains about the interlocking of single [eyes], then no one should complain about the use of a single-diameter [turn] within a knot, which is similar to the interlocking of single [eyes].

Interlocking eyes is an old mechanism, and it is different from
turning around a single diameter -- consider the respective tensions.

--dl*
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Dan_Lehman

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Re: The "Front-Back Bowline"
« Reply #19 on: July 23, 2010, 11:58:34 PM »
A few points:
-I'm not sure why, but the FB Bowline seems to have better weight distribution than the Bowline.

Odd conclusion:  this peculiar knot puts a lot of knotting on one side
of the axis of tension; the Bowline is much better balanced.

Quote
-100% of the load is NOT placed on that 1 diameter turn.
That's because both legs are not going through that 1 diameter turn.

Even after setting the knot by pulling hard on the tail bight
(i.e., either/both tail & tail-side eye-leg), on loading the knot
the SPart straightens right into that 1dia turn -- there is no
off-loading of force to another part.

Quote
-I don't see how a 1 diameter turn is so horrible anyway, and they're practically unavoidable.  For example, a trucker's hitch and a versatackle use 1 diameter turns. ...
 An overhand knot has two 1-diameter turns in a small area, right?  When testing knot strength using a machine, stopper knots are typically used at the ends to hold the rope to the machine. Those stopper knots must be stronger than than the knot being tested.  I know of at least one test, involving testing Fisherman Bends, where the person was using overhand knots on the ends for rope connection to the machine.

For one thing, some cordage won't make 1dia turns readily
-- caving kernmantle ropes can be (even new) resistant to
bending even to 2 diameters(!).  Lack of closure spells trouble
in security or just getting a knot tied.  Now, if your cordage
doesn't suffer such stiffness, it's not a concern.
The 1dia turns of the Trucker's hitch are seeing less tension
in the line at that point.  And for testing, rope is wrapped
several times around a capstan or pin or like structure before
it is knotted or clamped, so far as I know -- so that tail of
that securing is seeing only reduced force.

Now, in re security (when slack/shaken), this knot seems less
vulnerable to loosening, as the 1dia turn doesn't so quickly
*unturn* (in my in-hand small nylon solid braid), and the tail
bight is being gripped by another turn of rope not loosening
with SPart feed.

(Hmmm, try crossing the tail --from knot as tied fully-- under
and out through the 1dia (now become 2) turn!?  Gains
*balance*, IMO; loses some of that security; the 2dia turn
isn't greatly oriented.  hmmmm)

--dl*
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