This article was written by my friend Daniel le Breton and I did the illustration.
All of the energy in the cast comes from the caster but we are going to discount the energy needed to move the body, joints and muscles of the caster and we are going to concentrate on the energy applied to the fly rod, with the aim of transferring the maximum amount into the fly line. The figures come from my model so should be considered as order of magnitude and guidelines (not a universal truth).
Two kind of energy are involved. The energy associated to speed (kinetic energy for scientists) and the energy associated to the flexibility of the rod (elastic energy, a “potential” kind of energy for scientists).
We have to move three objects: the line, the rod, and the reel. Let’s start with the reel: this is a sort of dead weight for the caster, it has to be accelerated with everything else and mostly all the energy needed to achieve that will have to be produced again to stop it. Generally speaking it is always better to keep the weight of the reel to a minimum, but unfortunately for us, it is nearly always a little bit too heavy. It plays a role in the balancing of the outfit and that is also why its weight is important. This is because of a potential vibration problem, and to make things simple, when holding your rod horizontally with the reel attached, the rod should tend to rotate forward just above where your hand is, around the center of mass of the rod & line system. It must not balance just under your hand. Under ideal conditions, a vibration node will occur under your hand; this will allow you control vibrations and avoid undesirable side effects. Now, leaving the reel to one side we can concentrate on the rod and the line.
Most of the energy produced by the caster is transferred to the rod and the line. The caster generates kinetic energy for both, and at the same time generates the elastic energy that is stored through the deflection of the rod. During the acceleration phase, all of these energies are increasing in parallel, there is no transfer from one to another, but when the caster decides to stop the rod, things change.
The energy budget during deceleration will vary with the usual difficult-to-determine parameters: casting style, casting arc, timing and the mechanical characteristics of the tackle, including the mass of line that is cast. There is also an influence of these parameters during the acceleration but they increase more independently if you like. Let’s start by a well-executed and timed cast. The budget is approximately 75% in the line and 25% in the rod.
Like it or not, we always have a major part (e.g. 80%) of the kinetic energy of the rod (of the 25% from above) to absorb at the end of the cast but it is concentrated in the rod-tip (the energy of the counter flex). And in the meantime, we can introduce extra energy converted from elastic form (typically 25% of the overall energy budget, so 1/3 of the final energy of the line). The rest is kinetic, energy generated either during acceleration or deceleration in approximate equal parts. Deceleration of the butt causes most of the kinetic energy of the rod to go into the rod-tip and the remainder goes into decelerating the fly-rod and transfer to the fly-line.
Now what happens if we deviate from a good cast (proper timing and casting arc)? Typically, this corresponds to a lack of generated energy during deceleration. Most of the energy of the line comes from the acceleration phase (e.g. 2/3 instead of 1/3), and the caster has to make an extra effort to completely stop the rod, something corresponding to nearly 20% of its kinetic energy. When the self deceleration mechanism (SDM) is not fully used the caster has to eliminate the resulting surplus of rod-butt kinetic energy by decelerating the butt. He has to substitute the SDM for stopping the butt of the rod.
We can say that we are inclined towards the “broomstick” side of the cast under such conditions. In practical terms, the timing is too long and the casting arc may be a bit wide. The timing is crucial; this is typically a situation that occurs when casting a short line and reduction of the casting arc only is not sufficient to compensate for the bad efficiency of your cast. So the solution is to use a more compact and quicker cast.
The situation is easier for a long line, increasing the arc is the correct and easy solution because the match in frequency between the caster and the tackle is adequate.