Author Topic: Properties of #1019 and #1021 loops?  (Read 907 times)

mcjtom

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Properties of #1019 and #1021 loops?
« on: January 18, 2020, 07:51:07 AM »
There are two simple loops: a slip knot + half-hitch on either the standing part side (#1019) or the loop side (#1021).   I kind of dismissed them as being too, I don't know, unsophisticated...?  But they seem interesting, adjustably-lockable...

What is known about them?  Which one may be better?  Do they hold or come apart easily on a stiffer rope?  Do they jam easily?

Thanks!

p.s. There is also an interesting issue of chirality (if that's the right word) of the finishing half-hitch for both of those knots.  For example, it feels better to me to reverse the direction of half hitch in #1021 compared to Ashley, but I'm less keen about doing that for #1019).
« Last Edit: January 18, 2020, 08:56:08 AM by mcjtom »

roo

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Re: Properties of #1019 and #1021 loops?
« Reply #1 on: January 19, 2020, 10:09:00 PM »
There are two simple loops: a slip knot + half-hitch on either the standing part side (#1019) or the loop side (#1021).   I kind of dismissed them as being too, I don't know, unsophisticated...?  But they seem interesting, adjustably-lockable...

What is known about them?  Which one may be better?  Do they hold or come apart easily on a stiffer rope?  Do they jam easily?

Thanks!

p.s. There is also an interesting issue of chirality (if that's the right word) of the finishing half-hitch for both of those knots.  For example, it feels better to me to reverse the direction of half hitch in #1021 compared to Ashley, but I'm less keen about doing that for #1019).

I have noted in my Ashley Book of Knots that #1019 jams.  I don't bother with the #1021 as the method of securing the free end is just too prone to popping open from motion or eye flexure.
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mcjtom

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Re: Properties of #1019 and #1021 loops?
« Reply #2 on: January 15, 2021, 12:50:39 PM »
I've been trying a double version of 1019 - it seems to jam less than a single version, but still.  Is there a better loop/knot with similar properties, i.e. whose length could be adjusted while tensioning it and then locked at the maximum length?

Long story application-wise (I can explain), but the idea is to pull on the rope, which should lengthen as it is being pulled, then, when reaching a certain maximum length, should stay at this maximum length (even if slacken) and then could be permanently locked at this length, though it would be good if it didn't jam when locked and under greater tension (so that it could be later unlocked and its length could be readjusted and locked again).
« Last Edit: January 15, 2021, 02:46:38 PM by mcjtom »

roo

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Re: Properties of #1019 and #1021 loops?
« Reply #3 on: January 15, 2021, 09:55:50 PM »
I've been trying a double version of 1019 - it seems to jam less than a single version, but still.  Is there a better loop/knot with similar properties, i.e. whose length could be adjusted while tensioning it and then locked at the maximum length?

Long story application-wise (I can explain), but the idea is to pull on the rope, which should lengthen as it is being pulled, then, when reaching a certain maximum length, should stay at this maximum length (even if slacken) and then could be permanently locked at this length, though it would be good if it didn't jam when locked and under greater tension (so that it could be later unlocked and its length could be readjusted and locked again).

I'm not 100% certain on envisioning your application, but you might explore the HFP Slippery 8 Loop:
https://knots.neocities.org/slippery8.html
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Dan_Lehman

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Re: Properties of #1019 and #1021 loops?
« Reply #4 on: January 16, 2021, 12:47:33 AM »
That slippery 8 is going in the wrong direction,
working when shortening the eye.  One might muse
about some friction hitch (a clove h. e.g.) that doesn't
grip so well initially, AND THEN gets set to hold;
but I'm not seeing a happy time at this, even.

The crabber's eye is intended to do this sort
of work, but it might not grip so well at first?!
And might not be so good when set, either.

Could you explain in greater detail about this
application need --about what cordage & forces,
et cetera, are involved?!  We might need to just
solve the problem (if...) for a narrow field.


--dl*
====

mcjtom

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Re: Properties of #1019 and #1021 loops?
« Reply #5 on: January 16, 2021, 01:47:39 AM »
You may laugh, but this particular one is about calibrating the slingshot.  When you shoot it normally, you have something called a 'draw' - a length to which the rubber bands extend before the pouch is being released.  For experienced shooters, and I'm hardly one of them, the draw is quite constant and consistent.

One way of estimating the initial velocity of the ball bearing ball used as ammunition is to shoot it vertically and using a little complicated model, which is a combination of SUVAT equations modified by two different version of air drag (up and down - two because the initial velocity when shooting up is usually higher than the terminal velocity of the ball coming down).  Then by using this model and the time of the return of the ball back to the surface (you can hear it striking the ground when lucky), you can estimate the initial velocity of the ball, and thus, its kinetic energy and momentum.

The problem is that when shooting vertically, the draw length is significantly different from the normal horizontal draw, and also, it is difficult to measure in either case or keep the same.  One idea was to double the rubber bands with strings (I was going to use a thin 1 mm radio dial cord, which I believe is made of Dacron, but it doesn't stretch much anyway), tie the strings so that they would 'record' the length of the horizontal draw, and then lock them so that the draw is the same (i.e. this time the locked strings will limit the vertical draw to the same length as horizontal thus keeping the elastic energy stored in the extended rubber bands about the same as in horizontal shooting, thus resulting in about the same initial velocity of the ball).

The best I could think of was [1019] or a double version of it, which kind of works, but it tends to jam, so I need to cut it and redo it for each trial.

So yes, it is not a life-critical application (and probably would not inspire as much interest, controversy, and emotions as e.g. the Scott's Bowline saga I recently followed :-), but I was hoping that someone would know a better, less jamming option.   If you think that [1019] can not be really improved further, that would be quite useful too as I will stop looking.

Cheers!
« Last Edit: January 16, 2021, 01:51:41 AM by mcjtom »

roo

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Re: Properties of #1019 and #1021 loops?
« Reply #6 on: January 16, 2021, 04:02:28 AM »
The problem is that when shooting vertically, the draw length is significantly different from the normal horizontal draw, and also, it is difficult to measure in either case or keep the same.  One idea was to double the rubber bands with strings (I was going to use a thin 1 mm radio dial cord, which I believe is made of Dacron, but it doesn't stretch much anyway), tie the strings so that they would 'record' the length of the horizontal draw, and then lock them so that the draw is the same (i.e. this time the locked strings will limit the vertical draw to the same length as horizontal thus keeping the elastic energy stored in the extended rubber bands about the same as in horizontal shooting, thus resulting in about the same initial velocity of the ball).
I suppose a Slippery 8 Loop or two could be used by trial & error to measure & limit the draw length, but I would be afraid that the cord mass might affect the ballistics if it was connected to the pouch.  If the cord was merely used as a backdrop to serve as a witness (not connected, but only pinched behind the pouch), that would keep it from affecting things too much.

If you have an assistant to help you adjust the ends of the cord as you size your V-shaped witness cord, that would speed adjustment quite a bit.  Adjusting a couple of Slippery 8 Loops by pulling the end with your toes might take some skill.   ;)
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enhaut

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Re: Properties of #1019 and #1021 loops?
« Reply #7 on: January 16, 2021, 03:13:27 PM »
@mcjtom
This is a very interesting problem more complicated than it seems because you need to arrange two states of draw length who don?t inhibit each other as Roo pointed out.
If you search this site and are lucky enough, you will find a number of adjustable loops susceptible to meet you task.

Are you using a round tubing of a flat rubber?
Different shapes call for different solutions.

Posting an image of the material utilized would be ideal if you want  a precise outcome.

mcjtom

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Re: Properties of #1019 and #1021 loops?
« Reply #8 on: January 17, 2021, 03:04:15 AM »
Semi-flat bends (i.e. thicker flats).  I'll try to post the image later this week.  Thanks!