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Square snaps

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Dirk le Roux
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Square snaps

#1

Post by Dirk le Roux » Fri May 08, 2020 8:11 pm

What I came to call the “square snap”, is one of a few fun casts I have been playing around with during backyard practice these days. Well, the result is not an exact square, but the idea is to form a shape with square-ish corners. The result is essentially a vertical snap with the 180° loop at the top split into two 90° bends and separated by a “mid-leg”. What prompted me to photograph and analyse the cast was, during the run-down/aftermath post-stroke, the apparent levitation of the mid-leg and brief periods during which the shape appeared frozen in space. The footage and bits of analysis shared here may be useful to illuminate some aspects of fly casting line dynamics.



Starting with more or less a pile of line lying on the ground under the rod tip, you make this cast by drawing three legs of a square/rectangle with the rod tip, ending with a fast downward-leg stroke. What we think we do and what we do often differ, the image below shows more of a squared circle rod tip path and the resultant line shape when “frozen”.

rod-tip-path_1.jpg

Here are time-lapse “fingerprint” composites of the line configuration at regular intervals during the aftermath period of the four casts correspondingly numbered in the video (Colour coding relates to whether the rod leg bend is moving down - green, up - yellow, or down-and-sideways - blue. Generally in that sequence.):

1:
composite---square-snap-_-1.jpg

2:
composite---square-snap-_-2.jpg

3:
composite---square-snap-_-3.jpg

4:
composite---square-snap-_-4.jpg

I made a fingerprint composite of the similar period of a cast made with a marked level line as well:

composite---marked-level-line.jpg

And here is a plot of markers’ measured speeds over that period:

SQUARE SNAP MARKER SPEEDS.jpg

The speed of all markers, be it in whichever leg of the cast shape at a time, generally decreased, with the exception of the last few (no's 10 to 12) markers while these were part of the upward accelerating fly leg. This fly leg upward accelerating phase, from roughly 0.15s to 0.45s, coincided with a less steep reduction in speed of the non-fly leg markers, followed by all markers slightly picking up speed again (now freely falling, I think) from around 0.5s.

Regards,
Dirk

Dirk le Roux
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Re: Square snaps

#2

Post by Dirk le Roux » Fri May 08, 2020 8:14 pm

Here is another reel of "square snap" casts.


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gordonjudd
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Re: Square snaps

#3

Post by gordonjudd » Sat May 09, 2020 1:42 pm

Dirk,
Square indeed.

Can you get an idea of what the line velocity around the bends for the forward going bend that you initiated with the first upward movement of the rod and how that compares with the line velocity going around the rearward bend that you produced with the snap after the first bend starts going down the line?

As you know an internal tension (rho_l*v_tangential.^2) is produced when a distributed mass goes around a rounded path irrespective of the the bend radius. Thus it is a matter of which bend has the higher tangential velocity that determines who wins the tension tug-o-war produced at the two bends.

Waves in ropes are facilitating. Dr. McMilllen calls them the "gift that keeps on giving".

Gordy

Dirk le Roux
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Re: Square snaps

#4

Post by Dirk le Roux » Sat May 09, 2020 5:15 pm

Hi Gordy

I can't get an idea of what you are really asking in that first paragraph. Can you try and rephrase it using "fly leg bend" (the one on the left in all the images and footage) and "rod leg bend"?

Regards, Dirk

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gordonjudd
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Re: Square snaps

#5

Post by gordonjudd » Sat May 09, 2020 7:45 pm

Can you try and rephrase it using "fly leg bend" (the one on the left in all the images and footage) and "rod leg bend"?
Dirk,
Can you get an idea of what the line velocity around fly leg bend (the forward going bend) that you initiated with the first upward movement of the rod and how that compares with the line velocity going around the rod leg bend ( the rearward going bend) that you produced with the snap after the fly leg bend starts going down the line?-

Without a u shaped loop I don't know if the tangential velocity of the line going around the bend is still equal to 1/2(v1-v2) as it is with a fly loop.

Gordy

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Graeme H
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Re: Square snaps

#6

Post by Graeme H » Sun May 10, 2020 4:46 am

Cool stuff Dirk!
FFi CCI

Dirk le Roux
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Location: Pretoria, South Africa

Re: Square snaps

#7

Post by Dirk le Roux » Sun May 10, 2020 8:42 pm

Hi Gordy
gordonjudd wrote:
Sat May 09, 2020 7:45 pm
Can you get an idea of what the line velocity around fly leg bend (the forward going bend) that you initiated with the first upward movement of the rod and how that compares with the line velocity going around the rod leg bend ( the rearward going bend) that you produced with the snap after the fly leg bend starts going down the line?
I could not measure accurate line speed/velocity through the bends, as none of the marked line casts (level line) footage had smooth corners such as I sometimes seemed able to cast with the tapered line.

As your question appears to relate to occurrences around the snap, I measured what I could during and after the downward snap, leading into the deceleration phase, which occurs at the previous chart's beginning. The graphs below show the fly leg and mid-leg speeds during two casts different from that chart's cast (call that one cast A).

SPEEDS DURING DOWN STROKE.jpg

What stands out in my view is that there is lag, as might be expected, between the down stroke's acceleration and that of the mid-leg and fly leg. Following the lag divergence, speeds in all legs broadly equalise (the small differences we can discuss later), at least up to the fly leg's final upward acceleration illustrated in the first chart.
gordonjudd wrote:
Sat May 09, 2020 7:45 pm
Thus it is a matter of which bend has the higher tangential velocity that determines who wins the tension tug-o-war produced at the two bends.
During the downward snap and the lag effect on the mid-leg and fly leg speeds, it should be fair to say that the rod leg bend's propagation speed and therefore its tension during that time is higher. When leg speeds have "equalised", tension at the two bends should be of similar intensity.
gordonjudd wrote:
Sat May 09, 2020 7:45 pm
Without a u shaped loop I don't know if the tangential velocity of the line going around the bend is still equal to 1/2(v1-v2) as it is with a fly loop.
I'm thinking on a universal approach which would serve any permutation...

Regards, Dirk

Dirk le Roux
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Joined: Thu Sep 19, 2013 5:09 pm
Location: Pretoria, South Africa

Re: Square snaps

#8

Post by Dirk le Roux » Sun May 10, 2020 8:42 pm

Graeme H wrote:
Sun May 10, 2020 4:46 am
Cool stuff Dirk!
Thanks Graeme!

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Paul Arden
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Re: Square snaps

#9

Post by Paul Arden » Mon May 11, 2020 2:19 am

Very interesting Dirk, particularly from an analysis POV. I often make two or three back-to-back aerial snaps when playing around. It’s not uncommon to hear multiple soft Whip-cracks as the line unfurls.

Cheers, Paul
It's an exploration; bring a flyrod.

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hshl
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Re: Square snaps

#10

Post by hshl » Mon May 11, 2020 9:15 am

Hi Dirk,
interesting stuff - I just wonder about the practical benefit ? :blush:
Cheers,
http://www.passion-fliegenfischen.de/_en
All in its proper time ...

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