My friend Daniel le Breton made the major contribution to this article.
The term “rod loading” is used quite widely for describing how the action of the caster causes the fly-rod to flex and often the expression “I could feel the rod loading” is used to describe the feedback that is felt, at the casting hand, by the caster, when force is applied at the rod-tip. But is this meaningful and is it a correct way of explaining what is happening: or is it misleading?
During a typical cast the rod-tip is accelerated and this causes a force to be applied to the rod-tip due to the mass of the fly-line (Newtons second law; Force = mass x acceleration). This force causes the fly-rod to flex and because it is a spring, potential energy is stored within the rod blank. The fly-line is not the only load that is applied to the fly-rod because there is the mass of the rod itself (carbon fibre cloth and resin) and the fittings, such as rings and whippings. These will also result in potential energy being stored in the blank through flexing. You will know from experience that a fly-rod can be flexed without a fly-line attached. So there are a number of masses that are “loading” the fly-rod but how much useful potential energy (energy that goes into the cast) is transferred from the fly-rod?
After flexing, the fly-rod will return to its straight condition (rod straight position or RSP) and at this point, there are no forces acting on the fly-rod, otherwise it would not be straight. This is a simplification of what happens in reality but is an acceptable explanation. This must also be the moment at which to fly-line is launched, if no force is being applied, and so after this point no forward energy can be added to the fly-line.
The aerialised fly-line gets most of its energy from during the loading phase (swing) of the fly-rod, created by leverage from the caster’s hand, and it is kinetic. The amount of kinetic energy (Ek) that is transferred into the fly-line depends upon the rod-tip speed (velocity) because Ek = ½mv², where m is the mass of the fly-line and v the velocity of the rod-tip. There will be some kinetic energy in the fly-line, which is derived from the rod unloading (spring) but potential energy remains in the fly-rod because it flexes in the opposite direction (counterflex) and so there is quite a lot of energy that is wasted. The potential energy represents about 20% of line kinetic energy, most of it being expected to go into the line, but this topic is still subject to investigation. In particular, the potential may contribute to help stopping the rod (we intend to cover this subject in a later post). Many with an interest in casting will be familiar with the two casting experiments that have arrived at this conclusion; “The Dynamics of Flycasting” – The Flyfisher 1980 by Ed Mosser and William W. Buchan, “The Rod & The Cast” – Fly Casting Institute by Grunde Lovoll and Jason Borger.
So we are loading the rod but more importantly we are accelerating the rod-tip, and there is a force required to do this and this is what gives us the feedback at the casting hand. And why do we need a flexible rod? Well it seems that the combination of swing and spring actions have a beneficial interaction, which means that when they act together they develop an exceptionally high rod-tip velocity and because the velocity component is a squared function this has a significant effect on the total kinetic energy in the fly-line at the point of launch. If indeed this is required to carry the fly-line over a long distance. Alternatively, it means that we can cast with less effort!