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## Conservation of Momentum

Moderators: Paul Arden, stesiik

Graeme H
Posts: 1940
Joined: Fri Jan 25, 2013 2:54 pm
Location: Perth, Western Australia

### Conservation of Momentum

Okay, here's that demonstration of how a tight (sexy) loop can accelerate the fly leg relative to the rod leg.

The video shows that an open loop exhibited no force on the fly leg when I released the fly, but a tight loop flung the fly forward when I released it. It wasn't a lot, I admit, but I believe this effect is cumulative during a cast.

I believe a caster can make the fly leg accelerate relative to the rod leg if they can propagate a loop that is tight enough to make the line spring back from the curvature created by the loop. This will only happen if the line is stiff enough to recoil from a tight bend and the loop is sexy enough to make that line recoil.

Line subjected to an open loop or a cold-water line used in tropical temperatures will not spring back into a straight shape, and this acceleration does not occur.

My explanation of why this works is that each segment of the line acts as a miniature fly rod (as you can see in the video), flinging the subsequent segment a little bit faster than the preceding segment.

In the tapered section of the line, the segment in the loop is heavier than the one immediately following it. We can use F=ma here to show that while the force in the line remains constant* and the mass of the line is decreasing, the fly leg of the line accelerates. I believe these tiny accelerations on each segment can accumulate in the fly leg, speeding up the fly.

One complicating factor to take into account is that thicker line (i.e. the thickest part of the taper) is generally stiffer than the tip end of the taper. A loop that maintains a constant radius may be tight enough to "fling" the line at one point, but be too wide to have any accelerating effect on the tip end. The taper can help dissipate energy in the cast in this manner.

I know this is simplistic, and it may well be wrong, but this is one way that Paul's observations can be accounted for (he's a sexy loop caster) without discounting the fact that the fly leg will carry on with its own velocity until the rod leg terminates its velocity through tension when a common, open loop is observed.

Cheers,
Graeme

* we aren't adding extra energy to the line at this point, so the forces aren't increasing
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Marc Fauvet
Posts: 826
Joined: Wed Jan 09, 2013 11:00 pm
Location: Pyrénées, France
Contact:

### Conservation of Momentum

what do you tape the rod leg to when you're casting ?

Graeme H
Posts: 1940
Joined: Fri Jan 25, 2013 2:54 pm
Location: Perth, Western Australia

### Conservation of Momentum

Skyhooks.
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Graeme H
Posts: 1940
Joined: Fri Jan 25, 2013 2:54 pm
Location: Perth, Western Australia

### Conservation of Momentum

Sorry Marc,

That was a little flippant.

I know what you're worried about: if there is an equal and opposing force, how can the rod leg propel the fly leg without going into compression?

I haven't had time to video this, but imagine the same thing but with only a short segment of line with flies tied to both ends. If I bend the line and release it so that it recoils, both flies will move forward. There is tension on both ends of the "C" shape when the line recoils.

Since the rod leg is already in tension and we won't be able to stretch it, the force is transferred to accelerate the fly leg.

Cheers,
Graeme
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Lasse Karlsson
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### Conservation of Momentum

Hi Graeme

That's a very very very tight loop you need for that spritny action, who can throw so tight?

Cheers
Lasse

http://www.karlssonflyfishing.com

***Bring Mark back!!!!!! ***

Marc Fauvet
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Location: Pyrénées, France
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### Conservation of Momentum

hi Graeme,
firstly, i like the concept, specially the written part. i have no idea if it's correct or relevant or not but good on you for introducing another perspective to this stale thread... and also for demonstrating your ideas using a fly line, 'leader' and fly instead of the usual unrealistic symmetric/non-tapered, non-leadered, fly-less bead-chain...

my problems (for lack of another word, but rest assured, it's not a problem... ) with the video is that even though a rod leg may be considered temporarily inert at that point of the cast it most certainly isn't attached in mid air. there's no fixed 'hinge'. as such, the 'spring' force you demonstrate created by the fixed rod leg would most certainly be inexistent in a real cast.
other issues are that not all lines are of the same stiffness and profile proportions and even if they where, their stiffness will increase with colder temps and decrease in higher temps. results would vary enormously !
would this work with a braided silk fly line ? nope !

cheers,
marc

Walter
Posts: 1498
Joined: Thu Jan 10, 2013 7:06 pm

### Conservation of Momentum

He did say the effect was a minor one (and it also isn't a conservation of momentum thing ) but stiffness of the line definitely affects how the line behaves. Lasse's bead chain video is a good example of what extreme stiffness would do.
"There can be only one." - The Highlander.

PS. I have a flying tank. Your argument is irrelevant.

PSS. How to generate a climbing loop through control of the casting stroke is left as a (considerable) exercise to the reader.

Graeme H
Posts: 1940
Joined: Fri Jan 25, 2013 2:54 pm
Location: Perth, Western Australia

### Conservation of Momentum

Thanks guys, but as Walter has noted, I do believe this effect is only a minor component of the cast for most casts. Conservation of momentum in the fly leg is the main reason the fly reaches our target.
Lasse Karlsson wrote: ...who can throw so tight?
Not me - most of the time. Sometimes it happens though, and when it does, the fly does "seem" to accelerate. It's just a perception though - nothing I can say I have measured and I can rarely reproduce it anyway …

Please remember too, that that video was made using the very tip of a fly line, where there is no taper to speak of. The thick part of the taper - which recoils when a less drastic bend is applied - was out of play: I wasn't going to cut up a perfectly good line to access it for the sake of science.

I do have another video sequence in mind which I believe will demonstrate quite clearly that the loop is nothing more than a transition zone and has no "life of its own" in most casting situations (that is, it does not cause itself to unroll nor cause the fly to progress forward.) Best of all, the experiment I'll film is entirely reproducible by any fly fisherman without need for a camera or even any casting skill.

Cheers,
Graeme
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Unregistered
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Joined: Wed Jan 09, 2013 10:22 pm

### Conservation of Momentum

Paul Arden wrote:So it's a secret?
No, apparently the secret is the document that you mentioned Lee sent you instead of sending it to the study group that you arranged.

Some of the nonexistent "secrets" have been written in this same thread a good number of times.

Unregistered
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### Conservation of Momentum

Merlin wrote: If you consider shooting line, you may simplify the picture by saying that all happens as if you cut the rod leg at tip level: then everything moves forward at the same speed, the center of mass follows some kind of parabola but even if the loop goes forward it does not roll over itself anymore so it will land as such. Fortunately there is still some tension at rod tip level because of friction in the guides and the rod leg ends up in a standstill again allowing the loop to roll over itself and the line and leader to extend forward. Not sure it needs 20 pages to agree on that point.

Merlin
Really?
Have you taken a look to the bunch of videos showing that an untethered line has a propagating loop at the front?
Maybe reality is wrong. It must be if the technical forum says so.