How much physics do we need to know to understand and improve casting?
I can imagine five levels of understanding, the scale being not linear. This is arguable but this may be useful as a starting point.
First level: efficiency of the cast
Let’s start with Bill Gammel’s five essentials. They all turn around keeping a straight line path: no slack, straight line before casting, proper power application (avoiding tailing loops) and finally adapting arc size to line carry.
Why do we need a straight line path? For line control for sure but mechanically speaking, this allows an efficient transmission of energy from the caster to the line. The energy is given to the line by moving a force at the tip of the rod over a suitable distance. To be efficient the force must be in the same direction than the line. This force is following the tip path and if this path is curved significantly, a part of the work will be spoiled and not transmitted to the line to give it speed. Think about pushing a wagon on rails: you will never try to push perpendicularly to the rails.
If you increase the carry there is more energy to transmit to the longer line which is more difficult to roll out in the distance, so you have two options like increasing the distance over which you move the force, or increasing the intensity of that force. The fact is when you increase the arc; both the path and the force are increased if you keep the same overall timing for the cast, meaning you have to make your cast faster on a larger arc.
The most difficult point to understand is “proper power application”. There is no exact explanation of how to get it (not easy for sure). If you put power too soon you will spoil the tip path (tip moving down), so it is recommended to increase speed progressively, and to delay the wrist rotation at the very end of the cast (use with moderation for fishing casts).
The acceleration phase is in fact responsible for something like three quarters of the final line speed, so it is very important to apply acceleration properly. The deceleration phase is also important since it controls loop size (short deceleration, small loop), but also the degree of ease to stop the rod (long deceleration, easier stop). Mastering the whole process needs training, especially if you change your #4 outfit for a #12 one.
If the students can understand these points, they have the basics.
Second level: how does the line works?
I have explained that in the paper I wrote for answering to Paul’s challenge. Let’s try to make it even simpler. Since a fly cannot cast itself, our ancestors discovered that it was smarter to attach the fly to a “massive” line that would be the carrier and will be able to place the fly in the distance. You cast the line and the fly follows. The trick is to roll over the line which is possible through a rather simple mechanism. As you give speed to the line and stop it to shape a loop, the part of the line which is holding the fly can overtake the part which is stopped. The energy given to the line when launched is transferred to the moving part, and is used to fight air resistance until the fly can land in front of the line. Since the mass of the moving line diminishes it may accelerate if air resistance is not too large. Air resistance being related to the area of line facing the air under speed (you can compare with your own experience if you walk with a front head wind), you have to minimize the size of the loop to use as little energy as possible. Air resistance uses to be the winner and the fly can land softly if it is launched with sufficient speed. If this can be understood, that’ fine for a beginner. Do not use a mechanical principle that the beginner cannot understand or he is not familiar with, avoid any technical word at this stage, e.g. “this is due to conservation of momentum”: nice to impress the girls at the bar but pretty wrong.
Third level
Up to this point, we have not mentioned rod characteristics or the way a fly rod works, and said nothing about what are the real important parameters of the fly line. Does one need to know about these things and how much?
To make a long story short, a fly rod is a “harmonic oscillator”, in other words, it is as if you were pushing a marble on a table in a straight line (e.g. in a V block), but there is a spring (the rod) between the marble (the line) and you, which is attached to your finger. As you start pushing, the spring is contracting because the marble is resisting the motion (imagine a bowling ball instead), but after some time, the spring decides to unload itself and launches the marble. You can imagine that things will vary depending on the weight of the marble and the stiffness of the spring. It will take more time for a soft spring to react and unload. Things will even change if you change the weight of the marble. The characteristics managing the launch speed of the marble are the stiffness of the spring, the weight of the marble, and your input. It is ever simpler than that (for a specialist), the actual mechanical characteristic involved is the frequency of the loaded spring (attach the spring vertically with the marble and make the marble go up and down, then measure the frequency of this motion). It combines both the spring stiffness and the weight of the marble. We are speaking of the role of the “speed of the tackle” here. The “harmonic oscillator” can tolerate a reasonable variation of this speed characteristic and give a suitable speed to the line, this is important to know. Only few casters are able to adapt their cast to the tackle, for most this is not necessary. With experience, rod makers have adapted their tackle to our physical capability to cast. Now it would be fine to find another wording than “harmonic oscillator”.
The key parameters for a fly line are its density per unit of length and its diameter. The first one is easy to capture, a denser liner is thinner and you can test yourself the impact with a sinking line. For a given type of line, the mechanical rule may not be straightforward: the larger the diameter, the easier it is to roll out. You can compare a #2 with #8, which should be convincing
Fourth Level
If you understand quite well what I explain in my large paper (Flycasting 2014), you have this level of expertise.
Fifth Level
If you can understand what modeling is and the principles governing mechanical equations, you have achieved this level of expertise and you can argue against your peers. A good subject is (there are many other subjects of course): what is the use of the elastic energy placed in the rod, for example. Of course, be ready to justify your argument by numbers if you want to minimize the duration of the dispute, but there is no guarantee it will not last.