Strange effects of rod dynamics

A new contribution from my friend Daniel……

Fly casting can be analyzed by using various models and I use is the simple spring and marble one. When studying mechanics, it is usual to start from a simple set of equations and repeatedly refine them. Initially, the equations I used were rather crude and caused me some concern when I looked for a coherent representation of the whole process. Today I use a set of equations that give good (if not perfectly accurate) and consistent results. To achieve this I had to describe the non-linearity of the spring of the fly rod. It is, in fact, a hard spring and the more it is loaded the more reluctant it is to bend. This stiffening effect brings a benefit because it prevents the rod from fading away too soon; in other words not to slow down too fast under load, which is a characteristic that is beneficial for long casts. With light loads the spring of the rod is rather linear and this phenomenon is difficult to detect.

Another important point is to understand what comprises the swing weight of the rod. It is something that changes slightly during the cast because of the rod bending and is not very significant; say less than 10% variation and most of the time less than 5%. If it were large then “loading the rod” would be easier but the rod would unload with less stamina. It is important to realize that the swing weight has three main components: one is related to the butt rotation, another to tip rotation and a third one, which is linked to both the tip and the butt. The effect of the tip component is well known; it is a major parameter of rod speed. The higher this component’s intensity the slower the rod. Conversely, a smaller butt component makes it easier to decelerate the rod but there is a major effect due to the third one. This component explains why there is a natural tendency for the rod tip to move backwards at the beginning of the cast. This does not refer to the effect of a sharp acceleration inducing a higher harmonic, with its visible node in the tip. No, even with a moderate acceleration, this backward motion exists but is hidden by the inevitable translation that is used to rotate the rod. In other words, this intermediate swing weight component counteracts our motion, slightly, as we start the cast. However, this effect has a reward, just as we rather sharply decelerate the rod: in that the intermediate swing weight component boosts the acceleration of the tip! This phenomenon is particularly significant for light loads. You may have noticed that when you use a short line with a timely and sharp stroke, it appears as if some of your energy is thrown directly into the line. This is the inertial effect of the intermediate swing weight component, which is boosting tip acceleration. As the line is lengthened the increasing line weight hides more and more the inertial effect coming from the rod because there is more mass to accelerate with a limited amount of kinetic energy.

Additionally, the proportion of rod kinetic energy used to improve tip speed is taken from the butt, which further decelerates. This intermediate component is part of the phenomenon, which I describe as the self-deceleration mechanism. Therefore, when the line length/weight is increased the self-deceleration mechanism of the butt is amplified. This combines with the effect of the non-linear stiffness of the rod.

So we have two favorable effects on line speed, an inertial one for short casts (inertia) and one for long casts (non linearity), and an extra positive effect from butt deceleration.

The good news is that both main effects combine for intermediate loads, thus maintaining casting effectiveness. Finally, since these characteristics depend on rod design it is understandable that their prevalence varies. Needless to say that maximizing both gives the best performing rods.

About Daniel le Breton

My background is mechanical engineering. When I was about 30 years old (32 years ago), I had the chance to meet Harry Wilson, who founded Scott fly rods who became a good friend of mine and inoculated to me with a severe form of rod design virus. Trying to understand how fly rods are working brought me in touch with a number of people involved in fly casting analysis, mainly in USA. I am still investigating this subject and keep in touch with people that are active in the fly rod industry. You may have met me on the Sexyloop's web site, where I am known as Merlin.
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