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The point of tapers in fly lines and leaders

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Paul Arden
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Re: The point of tapers in fly lines and leaders

That’s a good question. I really want to hear from James on the subject ass well as others. You may have noticed that from time to time there are disagreements in the Tech Analysis section? James isn’t a moderator but he is a physicist by profession so I thought it best to have any discussion in “neutral territory”.

Cheers, Paul
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Graeme H
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Re: The point of tapers in fly lines and leaders

As promised, here's the link to the post I put in the angular momentum thread. It concerns how that property might change the velocity of the fly leg.

As the tapered portion of the line enters the loop, the heavier bits are accelerating the lighter bits more effectively as per the ball canon video. Pulling back on the rod leg enhances that acceleration. Giving line into the loop at that point will deaden the delivery.

Cheers,
Graeme
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Paul Arden
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Re: The point of tapers in fly lines and leaders

It should be noted that the properties of the tear shaped loop that is propagating down a tapered whip is different than the u-shaped loop that is propagating down a fly line. Thus the effects of taper on a whip are much different than the effects of taper on a fly line.
Hi Gordy,

I’m struggling to follow this. I agree that I’m the case of the whip the loop is closed and in the case of a fly cast the loop is open or a u-shape. However they can both be made to crack.

Why would a tapered line, thrown as a pear shape loop, cause it to accelerate to a crack and a tapered line thrown as a u-shape, cause it to slow?

Tomorrow I’ll cast some pear shaped loops!

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

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gordonjudd
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Re: The point of tapers in fly lines and leaders

Why would a tapered line, thrown as a pear shape loop, cause it to accelerate to a crack and a tapered line thrown as a u-shape, cause it to slow?
Paul,
In the idealized case where only the tear shaped loop is propagating down a tapered medium, the speed increases because of the KE is nominally conserved and the velocity increases as the amount of mass in motion is reduced. That is the idealized case that Dr. McMillen analyzed in his whip wave paper.

I have never been able to do it, but expert whip crackers can get a u-shaped wave to crack. And of course a level length of string is even easier to crack with a u-shaped loop as demonstrated in the Smarter every day video.https://www.youtube.com/watch?v=3HABgm-UUi0. We are still waiting for the paper they are going to write on what they found with their ultra high speed videos, so we will probably find there are a lot of factors involved in producing a shock wave with a whip than the idealized analysis used by Dr. McMillen.

In the fly wave case the top leg is moving in addition to the propagation of the u-shaped wave down the line. Energy considerations show that because of the change in momentum in the top leg (or the loop due to a change in direction of the moving mass) the top leg is accelerated by the tension at the top of the loop.

Now there is a tug of war going on between the positive acceleration force from the line tension and the negative acceleration produced by opposing drag forces. At the start of the cast the drag forces on the long fly leg tend to dominate and the loop slows down. Near the end of the cast the positive acceleration forces are larger than the drag forces and the loop accelerates.

The effect of the line taper at the end of the line reduces the positive acceleration force and tips the balance in favor of the drag losses so the fly velocity is reduced as compared to the final velocity of a level line. Lingard explains the difference between the velocity history of a level line vs a tapered line as:
A distinct acceleration phase is encountered later in the flight which, in the case of the level line, is sustained to the end. The double taper line, however, attains a sharp velocity maximum and then decelerates finally as the effect of the taper is felt over the last few meters of the cast.
Gordy

Will
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Re: The point of tapers in fly lines and leaders

Still confused.

We seem to be saying that we can design a fly line to delay turnover (or slow turnover - there seems to be a distinction there that I don’t understand) by either making the tip thinner.... or thicker...

I understand Graham’s ball video. I also understand what Graham’s driving at on how tension in the rod leg can affect turnover. But I don’t understand what tension at the rod leg has to do with the front taper of a fly line?

Cheers

Will
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"The only advice it is necessary to give the angler… is to avoid any approach to foppery, as trout have the most thorough contempt for a fop…”
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Graeme H
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Re: The point of tapers in fly lines and leaders

Hi Will,

I'm sorry I didn't complete my train of thought last night. It was getting late and I forgot to put in some other information I had worked on previously.

I did a series of experiments last year, casting into a landing net to examine the force generated by the fly leg (see video below). The fly leg crashing into the net pushes the back of the net out from the hanging position, showing there is enough force generated by the fly leg hitting the loop to maintain tension in the rod leg of a cast.

The momentum of the fly leg hitting the loop keeps tension in the rod leg. At one end of the rod leg, the rod tip is holding the line, while at the other end, the fly leg is pulling the line away from the caster. While the mass of the belly of the line is colliding with the loop, tension is maintained pretty well. However, the mass of each unit length progressively decreases within the taper, so in a "normal cast", the tension in the rod leg progressively decreases as the loop occupies the taper of the line. With tension in the rod leg decreasing, velocity of the fly leg drops and fly touches down lightly. We can also see the rod leg begin to sag as tension fades.

Here's that video from last year:

Cheers,
Graeme
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Will
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Re: The point of tapers in fly lines and leaders

Cheers Graeme, think I’m getting there.

So, a specific taper appears to have two impacts on the cast:
1) it impacts the speed of the fly leg in flight BEFORE it reaches the loop face: thinner line behind thicker line speeds up, and vice-versa. This is the effect Bruce was trying to manage in the MED, thickening the tip to maximise flight-time on long casts.
2) it impacts what happens when the taper hits the loop face: as thinner line goes into the loop face the tension decreases and the line slows, and vice-versa. In which case the thicker tip on the MED would then give the final few feet a little kick to help final delivery (assuming it hadn’t lost too much speed in flight ).

Am I right?

And no mention of air drag anywhere?!

Cheers

Will
Lineslinger
Barrio Pro-team
SGAIC
AAPGAI

"The only advice it is necessary to give the angler… is to avoid any approach to foppery, as trout have the most thorough contempt for a fop…”
WC Stewart

Paul Arden
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Re: The point of tapers in fly lines and leaders

I have never been able to do it, but expert whip crackers can get a u-shaped wave to crack. And of course a level length of string is even easier to crack with a u-shaped loop as demonstrated in the Smarter every day video.
So is the taper on the bullwhip making it harder to crack? It would be nice to see them throwing a level bullwhip to see the difference!

Cheers, Paul
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Paul Arden
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Re: The point of tapers in fly lines and leaders

I wonder what would happen if they reversed the whip?
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Graeme H
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Re: The point of tapers in fly lines and leaders

Will wrote:
Mon Jun 17, 2019 8:14 am
So, a specific taper appears to have two impacts on the cast:
1) it impacts the speed of the fly leg in flight BEFORE it reaches the loop face: thinner line behind thicker line speeds up, and vice-versa. This is the effect Bruce was trying to manage in the MED, thickening the tip to maximise flight-time on long casts.
Maybe. I am not sure I understand your question, but I'll clarify, just in case.

I don't think the front taper has any design function during flight until the line within that taper hits the top of the loop.

Referring to the above image here, line traveling right to left. My understanding of how these tapers and bulbs work is that as the rear taper enters the loop (A) the running line in front of it (already in the loop) has less mass per unit (lets call that "MPU" for a bit) than the line at A does. It can't transfer its energy as efficiently into A so the line in the rod leg (labelled "Running line" above) gets tighter and rotation is slowed a little. The same thing is happening as B then C enter the loop. Lighter line trying to turn over heavier line, rotation is delayed, rod leg gets tight even though there is now a LOT of line being carried.

When D (The Bulb) is in the loop, the opposite starts to happen. The heaviest part of the line is in the loop, maximum tension is being placed on the rod leg by the fly leg and is pulling the rod leg tight. The next line to enter the loop (E) will have an MPU less than the line already in that bend. Because it's lighter than the line of "The Bulb", it begins to (a) lower the tension of the rod leg AND (b) it begins to accelerate in the same way the ball canon worked. Each successive inch of line in E is lighter, so any angular momentum pulling on the fly leg has more effect than it did before, and there is also much less total mass of the line in the fly leg to pull. The fly leg and leader speeds up a little.

Depending on what we do now, it can either;
• Maybe fail to turn over (if we push the rod tip into the cast)
• Alight softly on the water in a straight line (if we do nothing), or
• Crack like a whip (if we pull the rod tip away from the cast)
2) it impacts what happens when the taper hits the loop face: as thinner line goes into the loop face the tension decreases and the line slows, and vice-versa. In which case the thicker tip on the MED would then give the final few feet a little kick to help final delivery (assuming it hadn’t lost too much speed in flight ).

Am I right?

And no mention of air drag anywhere?!
I think I answered 2 above ...

Regarding air drag: It's there, for sure, but these principles work whether we cast on a calm day, against a strong head wind or (more telling) with a strong tail wind. If drag were a defining factor, we'd expect vastly different results casting into a wind vs with it.

As an aside, Berlin (one of the earlier members) noted that casting with a strong side wind made it very easy to turn over nearly every cast. He was/is a strong believer in keeping tension in the rod leg, and a strong side wind makes that tension a given.

Cheers,
Graeme

(PS: Please excuse my diagrams. I've got great design tools on my work computer, but at home, I'm forced to cobble stuff together in Google's version of PowerPoint. Nothing is to scale.)
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