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Simplified Casting Physics Document

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grunde
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Re: Simplified Casting Physics Document

#51

Post by grunde »

"Essentially, all models are wrong, but some are useful."
George E. P. Box

Always question the assumptions!

Flycasting Definitions
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Merlin
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Re: Simplified Casting Physics Document

#53

Post by Merlin »

Hi Tom

To my knowledge there was and still is no « cabal ». Some people contributed and keep on writing for a blog initiated by Mark Herron (see Grunde’s link) about this topic, and outside the SL Board, there has been from time to time some discussion in between people rather deeply interested in the subject (a handful), willing to understand what is lying behind the curtain of casting. A few years back Paul asked for volunteers to write something (the second reference in Paul’s post above), and this is the first time I see an official link for reading it on the forum. It was circulated among specialists at that time but I could not get the feedback to update it. Today I would write it in a shorter version with illustrations likely.

You pointed out the big quest: to have a proper assessment of how to cast properly. Well, I would ask in return if you expect that some physics knowledge can make casting easier for anyone. Learning to cast properly does not need a lot of physics, if any; the vast majority of casters and fisherman do not care about it. Let’s take an example with Bill Gammel’s five essentials and how they relate to physics.
• Slack line eliminated
• Timing (pause)
• Casting arc
• Power application
• Straight line path

It is necessary to eliminate slack in the line before any application of power; this is usually achieved by a correct prepositioning of the line and some translation of the rod. If you fail to do it the slack elimination will take place anyway but it might not be smooth and will likely disturb the rest of the cast. From the mechanical point of view one should ideally pull on a straight line with the rod tip in the direction of a defined target along the straightest possible tip path to maximize energy transmission efficiency. Any force application on a domed path (reality) will lead to some loss of energy. Any side deviation “off tracking” has detrimental consequences: energy loss, target miss, etc.

Power application is also related to tip path since too much energy too early creates a dip in the tip trajectory and tails in the line which spoil its rollover. From experience, casters start with a rather low speed and accelerate the rotation of the rod late in the cast.

Casting arc is related to the amount of line you need to cast: the longer the line is, the higher the launch speed has to be, and the longer the tip path must be to cope with a basic mechanical principle: energy is directly related to a force moved over a distance, so to increase the level of energy of the line you have two means: increase the force and/or increase the length of the path (consequently the casting arc). The easiest way is the casting arc, and for long cast you need both.

Timing is understood as the time necessary for the line to rollover; you have got to wait for it before starting to cast the other way around. If you restart too early you may crack the leader and loose the fly, if you wait for too long the line will start falling down on the ground and you might “tick” the ground with the fly, taking the risk to spoil your cast.

The five essentials are purely cast oriented; they do not make any link with rod and line mechanics. Once you master them you can increase your casting proficiency with other types of casts. If you read all the documents mentioned by Paul and Grunde, you will find more intricacies related to fly casting tackle and how it works. This is what the study of casting mechanisms aim at: understanding how the whole thing works with the hope to better design the tackle and why not better use it. There is an extra topic to consider which is biomechanics of human body for throwing purposes (base ball, javelin, etc.). From that you can learn how to improve your casting motions in specific conditions (e.g. competition). Going deep into mechanics relationship helps to understand how a loop can be nearly stalled by a snap cast or pullback, what happens during a shoot and when you should shoot, what are the mechanisms involved in a rod and in a line, why a haul can be tricky to tune and other things. But honestly, this kind of information may only improve your casting style a little bit and explain why you prefer such or such rod brand, they can’t teach you how to cast. Most of the mechanisms understanding just confirm observations. The satisfaction lies in the understnading itself.

If you have questions to help your understanding of casting, do not hesitate, you will find some answers on this forum, we are here to share, even if that can raise some controversies :D :sorcerer:

Merlin
Fly rods are like women, they won't play if they're maltreated
Charles Ritz, A Flyfisher's Life
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Paul Arden
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Re: Simplified Casting Physics Document

#54

Post by Paul Arden »

Trying to get physicists to agree on anything is virtually impossible but I do think we are very much closer to this than in the past. :p

I actually think that’s just how they work! Not unlike everyone else on this planet :cool:

I would like to go though the “how it works” questions from a physics standpoint that I mentioned a few months back. I think possibly the only thing that is in disagreement, somewhat ironically, is how a loop unrolls. We have waves, fly leg momentum, tension from momentum change, fly leg momentum and angular momentum combined, and magic. It’s an interesting package.

I do know that unrolling loops are pretty mesmerising but also understanding how they work is not the same thing at all as being able to throw them. I don’t think that matters either and it’s never actually been the point.

It’s for the same reason that we took things apart as a kid, and maybe still do: to understand how they work. I assume most of us here know how computers and televisions work, at least in a very basic sense, but most people who operate them probably don’t and don’t need to know. Understanding how a television works for example isn’t going to change how you watch it.

When I asked my wife just now how the computer worked she said “good”.

That works for me!

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

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Merlin
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Re: Simplified Casting Physics Document

#55

Post by Merlin »

I would like to go though the “how it works” questions from a physics standpoint that I mentioned a few months back. I think possibly the only thing that is in disagreement, somewhat ironically, is how a loop unrolls. We have waves, fly leg momentum, tension from momentum change, fly leg momentum and angular momentum combined, and magic. It’s an interesting package.
Paul,

Apart from the fact that there is a disagreement on the nature of the loop (wave, wavy thing, or not), the other elements are just parts of the application of Newton Laws (providing they are correctly written). Things may look magic sometimes (loop hovering for a snap cast for example) but they just follow mechanical laws. Contradiction arises when one compares incorrect equations with correct ones. As an example, the correct equations for tension at loop ends do not correspond to the classic transverse waves in a string equation.

There are several things remaining in the magic domain:
* the behavior of loop nose which can go up and down during the flight (there is a video showing that, from Dirk or Graeme, I cannot remember). To me it has something to do with what can be observed with string shooters (I’m waiting for comments from a specialist).
* loop morphing (although the phenomenon is included in the angular momentum of the loop equation in some way)
* DN which could come from a discontinuity in tension in the line if we believe in the work conducted by string shooters specialists.
Some day we shall find answers to those questions I hope, and then we shall have to find something else to argue about.

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