How does a fly rod work? It is amazing machinery which seems to escape common understanding, and there are good reasons why. Fly rods have been developed over more than 100 years, and space age materials and computing science have contributed to improve their design. Impressive modeling of a cast has been performed but the complexity of this approach does not help us understand the underlying basics quickly.
Viewed from the bridge, a fly rod is both a lever and a spring, sometimes we use it as a pure spring (bow and arrow casting), sometimes as a nearly pure lever (leader casting only), and most of the time we take benefit from both functions. The lever allows us to generate a high speed using a simple rotation, but what about the spring?
Casting instructors are familiar with the casting essential of SLP, or straight line path of the rod tip, but the path of the rod tip is only part of casting efficiency. Mechanically speaking, it is most efficient to move an object along a trajectory by pulling along that trajectory. This is why to cast a line; it is desirable to generate speed on the flattest possible trajectory and to align the line with the path of the rod tip, aiming at the smallest possible loop. The flexibility of the spring allows us to achieve that goal with more or less success, and we can say that this property of the spring is complementary to the advantage of the lever.
But there is more than that coming from the spring function. There are hidden properties that I shall try to show you, and you will realize, I hope, that there is some kind of magic in a fly rod.
Let’s start by the beginning: the spring is also a speed amplifier (the lever also has this property, but it works differently). Casting a line with a fly rod is similar to launching a marble by pushing it with a spring along a flat table. Depending on the way we push on the spring (along a straight line), the speed of the marble can be increased up to 40% by comparison to the maximum speed of the push. It can also have no amplifying effect if you do not push properly on the spring: it is a question of timing. On top of that, it is a question of spring and marble characteristics, which goes along with the push. So let’s concentrate on that “spring & marble” system, which is our “rod & line”.
The single characteristic which is important for the speed amplification is the frequency of oscillation of our system. It depends on the natural frequency of the spring and on the mass of the marble. The heavier that mass, the slower our system becomes. So we have to adapt our timing, and this is why DH rods are different from SH rods in terms of frequency (which we usually call speed). DH rods are slower to adapt the tackle to our physical capabilities.
The frequency of the tackle drives its response to our input, and there is nothing straightforward here since we are under what we call “transient conditions”: the spring oscillates just one time up to the launch of the marble. It is rather tolerant. If your timing is not perfect (acceleration followed by deceleration to a stop), then the speed amplification is still significant. You must cast a noodle (extremely slow rod) or a broomstick (extremely stiff or fast rod) to lose speed.
The speed of the marble is also determined by the distance covered during the push (the casting arc): the greater that distance, the greater the marble speed.
The spring function is able to explain the importance of adapting our casting arc to our objective. Most of the time we use a more or less constant timing (longer for a DH rod), and thanks to rod designers the frequency of the tackle we are using is matched to our physical capabilities. This simple approach of spring function allows defining a line scale if you need one. Have we found the basics here (tip trajectory, casting arc, timing)? I think we can say yes, but there is more than that.
Fly rods are hard springs (non linear), which means that they resist more and more to bending as we increase the load or bend: you begin at the tip but you face the butt after, and it is really stiff. This increase in stiffness occurs as you lengthen the line, and this allows us to cast a longer line, because it maintains the frequency of the rod & line system on the high side. Consequently the line speed is also maintained on the high side, which is needed if we want to cast for distance. This is not a small effect; you may get something like 40% again by comparison to a rod having little hardening of its spring, and this comes on top of the speed amplification due to the basic spring effect that we described before. So if you look for distance you will have to choose a “hard” rod, but this may not be useful if you fish at short distance a small stream. It is worth knowing some basic rules: short rods are more “non linear” that long rods. A steeper taper has a non linear effect but it is difficult (it depends on the material) to have a long non linear rod and a short linear one.
If you fish at short distance you may wish to benefit from another strange aspect of the lever. This aspect is due to the spring somewhere, since it is linked to the fact that the tip and the butt have very different speeds during the cast (at the same time). The swing weight of a fly rod (its inertia for a mechanic) is relevant to the difficulty (resistance) to rotate that rod. During the cast the rod deflects and its swing weight diminishes during the acceleration of the butt, which helps reduce the effort for the caster, and it increases again as the rod is decelerated and the rod returns to straight position, which contributes to that deceleration. Swing weight variations are limited most of the time, mostly less than 10% (if the distance between tip top and butt end is reduced by 10%, the swing weight is reduced by 5%; this is just a rule of the thumb). This is why “slow butt action” rods are more comfortable to cast than “stiff fast action” ones, but this is a general statement, not a universal law.
Swing weight seems to be a single number but in fact we can consider that it is made of three components that all have a role to play during the cast. One is linked to the tip and is a major parameter of its natural frequency. The second one is linked to the butt and is important for casting comfort, and the third one is an in between parameter linked to both tip and butt, and it is responsible for the (magic) effect I am going to describe now.
This intermediate component of the swing weight manages the kinetic energy between tip and butt; it tends to counteract the influence of these parts of the rod to our advantage. Let’s describe some phenomenon coming from its influence. As we start casting forward, the tip begins moving backwards for a short while. This is not to be confused with the potential effect of a higher vibration mode (the second one, with a node in the tip), we are accelerating progressively here, and we do not use sharp wrist acceleration. Later in the cast, as we decelerate the butt, the resulting effect is a boost of tip speed, something like 30% for a short line. You get that on top of the 40% given by the spring function, but you get little from the non linearity of the spring under such conditions. So if you are casting a short line, you will appreciate the benefit of this phenomenon. It is the designer’s job to provide this faculty into his rod.
Casting at intermediate distance benefits from both the non-linearity effect and swing weight transfer effect. All in all you may get an additional 30% there too, and that’s good news. If one could make a rod with maximum transfer effect and high non linearity that would be a good one (in fact there are already some on the market).
And this is not yet the end of the story. The intermediate component of the swing weight, by providing more kinetic energy in the tip, takes it from the butt, so it helps to decelerate the butt of the rod just when we need it. At some stage (line length), this deceleration phenomenon is amplified by the effect of the mass of the line being cast. This deceleration effect could take place even if we could stop driving the rod (letting it go at the end of acceleration), this is why I call it the self deceleration mechanism. The mass of the line limits the tip speed boost during deceleration, because there is more mass to move for a limited quantity of kinetic energy available, but then there is a transfer in the deceleration of the butt to our advantage. I am convinced that this is key for tuning a line to a rod, but as usual, the various possibilities in casting style can affect that tuning. It can also explain why we think to cast “stop less” a long line. The weight of the line is helping us to decelerate the rod, up to the point that we do not need to make any effort to stop it if that line mass is large enough.
To conclude, a short summary of the lever and spring functions:
- Lever: amplifies speed by rotation, allows (because it is flexible) a kinetic transfer between tip and butt to the caster’s advantage
- Spring: basically a speed amplifier which can be reinforced by a hardening stiffness as load increases.
To me it is not possible to separate these functions completely, they are linked together. At the end of the day, the fly rod has visible and (nearly) invisible properties than can be used by a designer to maximize a desired effect. It has been fascinating me for more than 35 years now, and I think it will still be a subject of investigation for understanding its complexity. I did not describe other properties here, a fly rod has several natural frequencies, the three first being the most influential, and it has some consequences on rod behavior too.
Enough for today!
Daniel, 4 Dec 2015
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