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## Covered topics /how it works

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Paul Arden
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### Re: Covered topics /how it works

Hi Gordy,

Maybe the force is stabilising the flight? I’m sure if the line was rolled into a ball it would travel a long way. The main issue is drag. Spaghetti loops have a lot of drag. Casting loops, well less anyway. So it’s definitely being pulled along, but principally it was already in flight.

Not COM. Just momentum. With a Tension force from the loop front. We can see it’s not pushing the loop because there wouldn’t be one dolphin nose but an accordion. So I 100% agree it’s being pulled. But principally we are pulling something that is already in flight.

Anyway that’s how I see it. Which is interesting because as far as I can tell most people seem to argue it’s one or the other Which is rather odd but maybe I just haven’t fully understood the arguments!

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

Flycasting Definitions

gordonjudd
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Joined: Sat Jan 19, 2013 11:36 pm
Location: Southern California

### Re: Covered topics /how it works

With a Tension force from the loop front.
Paul,
That is the key. It is that tension at the top of the loop that pulls on the fly leg and helps to overcome the negative acceleration force on the fly leg.

And where does that tension come from? Momentum is not a force (in this case an internal tension force) so momentum alone cannot be producing that tension.

If people understood the classic physics problem of calculating the internal tension in a rotating ring (there are any number of examples of this problem on the web) then it would be easier to see that is exactly what happens when the mass in the line goes around the loop. In this case the dp/dt (momentum change) is due a change in the direction of the moving mass, not a change in velocity.

Gordy

Dirk le Roux
Posts: 474
Joined: Thu Sep 19, 2013 5:09 pm
Location: Pretoria, South Africa

### Re: Covered topics /how it works

Gordy

The line is still the front 5m of a # 5 WF floater plus a 1,5m leader, this time without any weight added to the leading end. Casts for the previous video were made in both directions, as today there was a bit of wind about. Our "dog park" is bounded by high walls of adjacent properties and ideally aligned to take your pick according to sun position. Can't believe I walked past these each week without realising the potential! Oh, and Kubla had to be kept on leash today - all the false casting got him too involved.

Of course with that latest video I wanted to show what can be achieved, the best ones, some more mediocre casts for balance, and omitting the worst ones.

To find the differences with your very spaghetti outcomes, I don't think it worthwhile to compare minutiae. Rather look at the differences conceptually, and here I offer two observations.
1) As with any good fly cast, a good backcast (or a well devised back layout, which avoids shocks and other potential destroyers) sets the forward cast up for success
2) My best results had, or self-formed, a fairly slanted rod leg with a fly leg fairly even and fairly horizontal early on. This causes drag advantage to the fly leg's continued progress and the rod leg's continued impediment, which is good for turn-over (loop propagation) and tension. Note that the naughty dangle loops form at and are exasperated by horizontal sections of fly leg feeding into propagating bends, the more slanted line ahead of the bends not making such eased progress as the horizontal feeds.

The feed dependence on spatial travel of a loop's sub-bends explain a lot of loop morph too.

The tension at a propagating loop can be derived from more than one point of departure, and your momentum change one is as valid as working from centripetal acceleration, from wave behaviour or from energy conservation. The outcome relates the tension to the line's linear mass density and the square of the loop's tangential velocity, which also is the loop propagation speed or rate of the fly leg shortening and rod leg lengthening.
Paul Arden wrote:
Mon Aug 10, 2020 4:50 pm
So I 100% agree it’s being pulled. But principally we are pulling something that is already in flight.

Anyway that’s how I see it. Which is interesting because as far as I can tell most people seem to argue it’s one or the other Which is rather odd but maybe I just haven’t fully understood the arguments!

All the best,
Dirk

Paul Arden
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### Re: Covered topics /how it works

The tension at a propagating loop can be derived from more than one point of departure, and your momentum change one is as valid as working from centripetal acceleration, from wave behaviour or from energy conservation. The outcome relates the tension to the line's linear mass density and the square of the loop's tangential velocity, which also is the loop propagation speed or rate of the fly leg shortening and rod leg lengthening.
Thanks Dirk. That helped me. That’s the sort of reply I’m looking for.

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

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Lasse Karlsson
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### Re: Covered topics /how it works

gordonjudd wrote:
Sun Aug 09, 2020 5:48 pm
The Aitor clip in a like-for-like shoot-out does not count, as it appears to have been cast with practice yarn line, and had a good loop formed pre-release.
But it should be an apples to apples comparison my example where the line turned into spaghetti as soon as the running line left the rod.

Why such a difference? When did the running line he was holding on to when the loop was formed actually get free of the guides and totally exit the rod? Was he casting with the wind with a heavier line as opposed to my going into a slight headwind with a floating line? We should be able to come up with reasons as to why his loop continued to turn over when my line turned into spaghetti. After all I don't think the physics of loop propagation in Spain are any different than they are in California.

Gordy
The Aitor clip is a mpr line thrown handcasting.

Tracking? Timing? Acceleration? Trajectory? Ticking?

All things that makes the flyline behave less optimal in the air.

We've all seen the high speed clip of Niklas Ericssons cast in Tampara, Finland, and what happens with a mistimed misaccelerated, and mistrajected cast that clips the caster.

Why is it so important to use light yarn that can't be handcast with good loops to prove things go south?

I also thought your flying spaghetti line was a light sinker thrown into a headwind?

Cheers
Lasse

http://www.karlssonflyfishing.com

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

gordonjudd
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Location: Southern California

### Re: Covered topics /how it works

Note that the naughty dangle loops form at and are exasperated by horizontal sections of fly leg feeding into propagating bends,
Dirk,
That is an excellent observation that I missed. Now when I look at my released clip above I see the same thing.

It is the propagation of the dips related to small transverse waves (what produced them I have no idea) that turns the line into spaghetti.
The tension at a propagating loop can be derived from more than one point of departure, and your momentum change one is as valid as working from centripetal acceleration, from wave behaviour or from energy conservation.
Agreed. I have made the same point before and regardless of how you approach the problem you get the same rho_l*v_tangential$$^2$$ result. What come first is a chicken and egg type problem.

Gordy

gordonjudd
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Location: Southern California

### Re: Covered topics /how it works

The Aitor clip is a mpr line thrown handcasting.
Lasse,
I thought he was using a 1.5 m length of mono attached to the end of the MPR line that he released after the loop was formed. By the time the mono left the rod the rod leg was longer than the fly leg and thus the added force on the bottom of the loop from drag was able to get the loop to propagate down the fly leg and turn over the loop.

I think Dirk has shown that when the fly leg is much longer than the rod leg the resulting tension difference at the top of the loop can cause the loop to propagate down the rod leg and turn over the loop. That early release is hard to get with a rod since it the friction of the running line from the guides as it exits the rod is sufficient to keep the loop rolling out in the normal direction.
Why is it so important to use light yarn that can't be handcast with good loops to prove things go south?
It was easy to see and illustrates the factors that are impacting loop propagation. Without tension on the rod leg the propagating loop soon turns into spaghetti due to its small rho_l and large fuzzy diameter that results a much larger drag factor. That higher drag force soon kills the lines initial momentum at the point of release.
I also thought your flying spaghetti line was a light sinker thrown into a headwind?
It has been a long time since this was done but I suspect it was a #4 floating line.
Tracking? Timing? Acceleration? Trajectory? Ticking?
Those factors were nominally the same for a tethered cast under the same conditions which resulted in a normal roll out of the loop.

Gordy

Dirk le Roux
Posts: 474
Joined: Thu Sep 19, 2013 5:09 pm
Location: Pretoria, South Africa

### Re: Covered topics /how it works

gordonjudd wrote:
Tue Aug 11, 2020 2:02 pm
I think Dirk has shown that when the fly leg is much longer than the rod leg the resulting tension difference at the top of the loop can cause the loop to propagate down the rod leg and turn over the loop.
I did? Where?

gordonjudd
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Location: Southern California

### Re: Covered topics /how it works

I did? Where?
Dirk,
You are right. I lost track of which leg was which since they were overlay-ed at this point. Maybe you could highlight where the end of the fly leg is at this point.

I should have seen the fly at the end of the roll out and realized the loop did completely propagate down the fly leg in that cast.

I have no explanation as to why such narrow loops tend to continue to propagate. There has to be some line tension for a transverse loop to propagate V=$$\sqrt(T/Rho_l)$$ but aside from drag forces I do not see why the rho_l*v_tangential$$^2$$ tension at the bottom of the loop is maintained when there is no opposing force on the other end of the rod leg.

Wider loops tend to stop propagating as shown in your other released casts, so I cannot explain the difference.

File it under the "seeing what you want to see" category.
Gordy

Merlin
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### Re: Covered topics /how it works

Gordy

For an untethered cast, you have 2 unknowns (legs speed) and two equations (energy, momentum). To make things simple, the rollover is a question of mass of legs. As I said before, the rule of the thumb is that the rollover goes on if rod leg is about 25% of fly leg. The size of the loop has no influence on tension by the way.

Merlin
Fly rods are like women, they won't play if they're maltreated
Charles Ritz, A Flyfisher's Life