Physics of casting, from fly to caster

Physics of casting, from fly to caster

Merlin | Tuesday, 19 April 2016

By Daniel Le Breton - Merlin

First part: overcoming the air resistance of the fly
The purpose of fly casting is clearly to send a fly into the distance. If we try to cast a fly attached to some monofilament line by hand, we immediately realize that this is an unrealistic way to do the job because flies are highly sensitive to air drag. However, fly fishermen found the solution to overcoming this air drag phenomenon a long time ago, likely by trial and error, by using a thick line. When you have to send an air-resistant object somewhere, one solution is to provide it with a “tank” of energy that will be consumed during the flight; a similar strategy is used to send a satellite into space with a rocket. The advantage here is that we can recycle the rocket (fly line) many times. Logically, then, we would choose a fly line depending on the size and air resistance of the fly.

Second part: the fly line is the energy carrier to move the fly

So the fly line is the energy tank which has the task of overcoming the air drag on the fly as well as the drag on the line itself (skin drag); all objects in flight are sensitive to drag. Air drag on the fly line affects both the loop and the moving leg of the line which is attached to the fly (fly leg); the larger the loop and the longer the line, the higher the amount of energy needed to land the fly in the distance. The loop is the most influential characteristic that you can control, and the smaller it is, the smaller the amount of energy that has to be used to cast the fly.  This is why it is important to learn to control the loop size, as far as we can. The skin drag on the fly leg can also be significant, as large as the drag effect on the loop, but you cannot control that.

The density of the line is also an important factor in reducing line diameter because fly lines are rated by weight and a denser line will be thinner for a given line weight and, consequently, will have less air drag. Thus we arrive at the key factor for skin drag which is the line diameter. It is quite difficult to cast long lengths of smaller sizes of fly line (below 5 weight) while bigger (and heavier) lines roll over easily. Heavier weight lines are often used with two-handed rods and these large lines make control of the cast easier because they are less sensitive to the effects of wind. 

A fly line can roll over at a nearly constant speed and it can also accelerate and decelerate. Basically, the line is moving thanks to the energy it carries, which depends on its mass and speed, and it is fighting against air drag to move the fly into the distance. Line diameter is the parameter, after loop size, which controls what the behavior of the line can be during its flight: acceleration, deceleration or constant speed.

It is quite amazing to realize that the first fly line scale was based solely on diameter. At that time, fly lines were made of natural silk and the simple logic, which is still valid today, was to adapt the size of the line to the size of the fly. Did our ancestors understand the key role of line diameter? We don’t know because we haven’t found an historical comment on this technical point. In the ‘60s a new scale was created by people experimenting with new materials for fly rods and lines. This new scale, which is in use today, was based upon the weight of the first 30 feet of line excluding the tip. Why such a change? Line weight incorporates both diameter and density, which is reflected by the variety of lines available today for most line weights, i.e. floating, intermediate-, fast- or slow- sinking lines, but also has a relationship with the rod, as we shall see later on.

To comprehend how a fly line rolls over, try casting a 6 foot length of rope by hand.  Hold it at one end (the rod leg) and give it some horizontal speed, then, as you stop your hand, the rope (the fly leg) will be seen to be rolling over. This happens because you are holding one end of the rope which is important because in order to roll over a line needs this tension. In this experiment, the tension is created by the fact that you hold one end of the line between your fingers (similar to the role of the tip of your fly rod) creating a high differential in speed in between the upper and lower leg, which in reality has a speed of zero. Rotation of the loop means tension in the loop. 

Repeat the experiment, and this time let the end of the rope go after the stop. Depending on the line speed, the loop may fully roll out or the line may fall down before reaching the end of the line. A loop that is frozen in its travel along the line means there is no longer tension in the line. For example, if you miss a cast with a shooting head for which you used too long an overhang (overhang is the distance between rod tip and the back end of the line, it is made of shooting line), this is because you failed to create sufficient tension in the line. Curiously, there is a mechanical limit to successfully casting a shooting head because you need to create a “rod leg” with sufficient energy from the start of the cast. Then the rolling speed of the loop slows down up to a point where it accelerates again. If you can’t achieve that acceleration, the cast is likely spoiled.

There is a short taper on the end of a fly line followed by a leader which is attached at the end of the fly line in order to finish your casts properly. The end taper of the fly line is there to control the rate at which the energy you have put into the line is consumed as landing approaches by helping to reduce the line speed. Cast a level line if you can and you will see that it kicks down instead of turning the leader over. This is the clue that there is too much energy in the line.

The leader has the same function as the taper at the end of the fly line but it also must be fine-tuned according to your fishing goal and fly. There are many recipes to adapt the leader design to streamer fishing, nymph fishing, wet fly fishing or dry fly fishing. Many of these recipes have been determined by trial and error, but there are also technical leader recipes which have been determined by the mathematics of wave propagation in cables, however, you don’t need to know about that for fishing. The first rule of thumb for leader design is that the leader length depends upon the length of your fly rod. Today people often have a leader close to twice the length of the rod, whereas in the past it was closer to the length of the rod, and in extreme cases some modern leaders are up to three times the length of the rod. Your choice will come from experience.


Third part: the link with the rod

With a fly rod, the size of the loop is controlled by the path of the rod tip, and the rod itself has an influence on it as more flexible rods will create larger loops. You will also find that casting a longer length of line tends to enlarge the loop because the amplitude of rod bending increases accordingly. Here lies one of the subtle connections between rod and line: the mass of the fly line influences the behavior of the fly rod. By changing the basis of our line scale from line diameter to line mass we have demonstrated our improved understanding of the complex machinery of fly casting.

There is no precise fly rod scaling system which is a headache for users. The fly rod is resistant to a pure classification system because of the way it works and also of the way we cast it. Remember that our goal is to provide energy to the fly line so that it will carry the fly to the fish. This energy, as we already mentioned, is linked to line mass and line speed. Line speed will be created by casting the rod with the necessary mass (length) of line required to send the fly the desired distance. To do that, a caster accelerates the rotation (a simplification of reality) of the rod up to a stop, but what happens then?

The rod is a flexible lever and we use the mechanical advantage of the lever to create tip speed; the longer the rod, the larger the effect. But because it is so flexible, the rod tip can travel on a more or less flat trajectory which is interesting in terms of energy optimization because the flatter the trajectory of the tip, the more efficient is the transmission of energy from the caster to the line. The flexibility of the rod allows us to extend the duration and/or length of energy input and to minimize the energy left in the rod at the end of a cast.

As we start the forward portion of the cast and accelerate the rod rotation, there is bending in the rod due to the resistance of the line to movement (inertia). We then decelerate the rod and the rod returns to a straight position (nearly) as we come very close to the stop position as the line is launched, and then it flexes forward to create the loop in the line. The way the rod makes this flex backward - straighten - flex forward motion is linked to its stiffness and to the mass of the line on one hand, and to your casting tempo and arc on the other hand.  Simple, no?

This is where one speaks of the recovery speed of the rod or the tackle (rod and line). The technical term for that is rod loaded frequency: add more load to the rod and the frequency goes down. The spectrum is quite large, you can find from slow/soft rods to fast/stiff rods designed to cast the same line (or this is the assumption you can make as a customer). Each rod will vibrate with its own frequency and will react to the cast mass depending on its design. It will also respond according to the caster’s input, and common wisdom says you should adapt your casting rhythm to the rod; you may choose not to do so but then you may not capture the benefit of fine tuning your casting stroke to the rod’s rhythm. The rule of thumb is to adapt one’s casting style and rhythm to each situation, and this becomes instinctive with practice. However, different casters adapt differently to the same variations. There are just three secrets to achieving that: practice, practice, and practice.

Rod (loaded with line) response to the energy input to the cast is what drives line speed. Now it is impossible to really separate what is due to the lever mechanism from what is due to the spring mechanism, they both work together. It is often asked if the energy stored in the rod as it is being flexed backwards is transferred into the line.  

The answer is yes, nearly all the rod’s elastic energy goes into the line (there is some possible loss during the loop shaping phase), and at the end of the unloading (when the rod is straight), there is just kinetic energy left, mainly in the tip, and this kinetic energy is then transformed into elastic energy as the rod is counter flexing.

If we want to distribute the magnitude of the typical energies, we have to consider the following: the elastic energy placed in the rod (typically 30% of final line energy), the kinetic energy placed in the rod (contributing to something like 20% of final line energy and to the remaining energy left in the rod, mostly in the tip, when it straightens), and the energy directly coming to the line from the lever effect (about 50% of final line energy). In fact these figures vary with several parameters including the casting tempo and the line weight. The leverage effect can represent 30% to 70% (e.g. light load); the elastic part 15% to 35% (heavy load), and the kinetic part 10% to 30% (intermediate load). I let you imagine the number of combinations.

The difficulty is in separating the different kinetic energy components of the rod. Here is some explanation: this kinetic energy is due to the swing weight of the rod as it rotates and deflects, and to simplify the situation we can consider that it has two components: one is linked to the butt and the other one is linked to the tip. As the rod starts to bend, most of the kinetic energy in the rod comes from its butt, the tip having little motion, then the kinetic energy transfers up the rod shaft to the tip where some of it is transferred into the line, and, once the rod is straight, the remaining kinetic energy stays in the tip which has got speed while the butt is stopping. If we consider the rod kinetic energy alone, some 10% to 30% (intermediate load) can be transferred to the line, the main part 50% to 80% (light load) stays into the tip and creates the counter flex which the caster has to eliminate one way or the other, and the remaining part, 10% to 40% (light load), is used to stop definitively the rotation of the rod.

We can say that the spring effect is such that we can increase the line speed by up to 40% with the spring function, but there is a price to pay which corresponds to the energy needed to bend the rod. Typically, if we add one unit of energy to bend the rod, we can expect to get four units at the end, taking most of them from the kinetic energy placed in the rod. In other words, pay a little bit more and you will be highly rewarded.

Note that the mechanical system is rather tolerant, you can miss perfectly matching your rhythm to the rod’s frequency and the fly line will still cast nicely; in reality, few people are able to get this extra efficiency which corresponds to a so called “perfect” cast.


Fourth part: the caster

The most important point in casting is that you must accelerate the rotation of the rod up to the point where you decide to decelerate quickly. (This critical point applies equally to the back and forward casts but this discussion is focused primarily on the forward cast.  If you were to graph the positions of the rod, wrist, elbow and forearm over time through the forward and back casts you would see that the motions are essentially identical). The rod will start unloading within milliseconds of the time at which you decide to decelerate. Almost everyone’s timing is flawed, but you would only realize that this imperfection exists if you had significantly different tackle (e.g. casting very, very slow tackle very quickly). Fortunately for us, the design of rods does not allow us to experience this millisecond difference which means that a rod can start unloading slightly before or slightly after you start decelerating. So, while this phenomenon exists, it is usually not discernible in practice and you may just forget it for normal fishing.

Concentrate on smoothly accelerating the rod rotation up to the point at which you decelerate. How quickly you decelerate is a point of discussion and, again, only practice will help you to determine that. In theory, for a given casting arc (representing the physical limitation we are facing) it is desirable to smoothly accelerate as much as possible and then decelerate as quickly as possible. Now unless you are at the extreme limit of the casting arc, you may decelerate over a slightly longer period which means you will have a larger arc. This is another key element in learning to cast properly: you have to tune your arc to the length of line you have to cast. 

The flyrod acts as a lever and a spring but there is a third mechanism that is involved in generating line speed. It is linked to the swing-weight characteristics of the rod, and it is amplified by the rate of deceleration that can be achieved. This extra boost is advantageous for casting short lengths of fly-line because the conditions are not favorable for using the spring advantage. For long distances the spring has more effect together with the lever and the swing-weight effect is cushioned by the amount of fly-line that is cast. There is just one drawback with the swing-weight effect at the beginning of the cast: if the acceleration is abrupt, the rod tip tends to move backwards and may create a casting problem (tailing loop). 

In practice, our tempo does not change very much under most fishing conditions (typically 0.45 second, as recorded from many experiments in normal casting conditions with fast cameras or special devices); instead we play with the casting arc to get more speed when needed. The normal casting arc varies between 60 and 120 degrees (beyond that arc you enter the distance competition casting area). Remember that the length of the arc influences your style and, for a short to medium distance cast, you may not need such an abrupt stop of the rod. Be certain, however, to master the casting arc since it is extremely important for controlling the loop size.

You need to cast a mile away to the other bank? Then you must be strong enough to rotate your rod at light speed on the largest possible arc, about 180 degrees. You must also use a massive shooting head to get the energy necessary to carry your fly to the target. Your rod needs to be unusually stiff/fast to make the cast, or you will have to resort to a two- handed one. As a fisherman, I would suggest you do your best to move closer to the spot (e.g. cross the river somewhere).

Now, if you decide to take up fly fishing, get ready to face a big question: what rod should I get? Think about the river, the fish and the types of flies that you will probably be using. From that information you can deduce the line weight that you will need, the rod suitable to this line and to the length of line you will be casting, which is a function of the river size. Is there an all-rounder? For trout fishing, which is the usual starter rod, pick up a 9 foot rod for a number 5 weight forward line. Which one among the 200 models offered? Take advantage of the experience of your casting teacher because you will need a good rod to get quickly and properly started fly fishing. That will save much of your time and a great deal of frustration.





Special report about line flight and the loop


First, let’s say that the loop is a length of line shaped in something like a half circle or ellipse with an upper leg, which we will call the fly leg because it connects directly to the fly, and a lower leg which we will call the rod leg because it ends at the tip of the fly rod.

As we launch the line in the forward cast, there is a large difference in speed between the legs since the rod leg is at a standstill and so the loop rolls over as the fly leg moves forward. Our “energy tank” is the fly leg and it fights the air resistance to bring the fly to the desired place. The energy from the tank is consumed progressively and we may ask ourselves if the speed of the fly leg increases or decreases during its flight. That depends on the amount of energy in the tank and the amount necessary to drive the fly and line the required distance against air drag. Basically, the energy required is proportional to the mass of the fly leg and to the square of its speed so, as the whole thing moves forward, the mass of the fly leg is being reduced and the remaining energy concentrates in the fly leg. But the actual change in speed of the fly leg depends on the amount of air resistance it has to face. So, depending upon the conditions, e.g. wind speed and direction, the fly leg speed can increase or decrease. At the end of the cast it will be decreased by the design of the fly line and of the leader to allow the fly to land at a reasonable speed. The basic mechanism that drives the fly leg speed is the management of energy: if you generate too much line speed for a cast there may too much speed remaining in the fly leg at the end, and if you do not generate enough speed the fly could fall short of the target.

Shooting line helps to increase the distance achieved and that requires a higher line speed at the start of the cast. The extra energy input at the start of the cast is used to extend the length of the rod leg and thus get more distance. Even in that case the loop rolls over. In some cases, the loop may stop rolling before the end of the cast because too much energy was spent in feeding the rod leg. Shooting line generates speed in the rod leg so the difference in speed between the two legs starts by decreasing the speed of rotation of the loop as you release line in the rod leg. Then there is a competition between the two legs to get or keep energy and usually the fly leg wins so, the rotation speed of the loop recovers and the rollover can be completed. In particular conditions, when shooting line (e.g. low launch speed, light line, wind, small rod leg by comparison to the fly leg), the rotation speed of the loop never recovers and the fly falls short of the target. Even if you shoot a heavy line at high speed, beware of releasing line in the rod leg after the loop has been shaped or your cast could be spoiled. Shooting line requires control and there is a right time to do it; a late shoot is risky since it takes energy when the energy tank level is low.






Viewed the other way around, with a more technical approach


After a top down view of the fly cast, let’s have a bottom up approach with a little bit more technical insight.

We start with the caster this time and focus primarily on the forward portion of the cast. To rotate and stop the rod we need to coordinate our muscles, and we are not all equal at this game (just as a very few can run 100 meters in 10 seconds). For a long distance cast, the torso, shoulder, elbow and wrist come into play in a complex orchestration of joints and muscles. Let’s take a simpler example: with a “wrist locked” cast only the forearm is rotating. How do we achieve that? Begin the rotation of the rod by extending your forearm forward and the triceps (the muscle at the back of your upper arm) enters into action. Then, unconsciously, the biceps (the muscle in the front of your upper arm, the one you sometimes flex to impress someone) takes over as the triceps is deactivated by the brain which allows the deceleration of the rod. The biceps decelerates the rod up to the point where the forearm is stopped. If the biceps were not acting to stop the rod we would break the elbow because the triceps would be trying to rotate the joint further. Since you hold the rod firmly to lock the wrist, the main sensation that you have is the contraction of the muscles in the forearm and you have little consciousness of the triceps/biceps machinery in action. 

While you rotate the rod, a force is created at its tip since the rod and the line are reluctant to move (we are speaking about their inertia). Your rod being flexible, the magnitude of the force is increased by the spring effect. Now the objective is to make that force increase and travel along a path that must be as straight as possible. The flexibility of the rod tip allows that path to flatten and you to make it as long as possible. Energy in the line is created by the force travelling on a straight path. If the rod was a broomstick, it would be difficult for our joints to make the line travel over a long, straight path. So there are two phenomena involved: one is that the flexibility of the rod allows us to generate a straight path for the tip relatively easily and the second is that the spring of the rod allows us to generate a higher force at the tip which imparts more energy to the line. As the forward cast is progressing, the intensity of the force at the rod tip increases and the speed of the line goes from zero to its maximum when the rod is very close to its straight position. Our ability to coordinate the whole thing is, of course, linked to the characteristics of the rod which translates into the tip path and the force exerted on the line.

The deceleration that is used to stop the rod butt generates a positive effect on line speed, which is significant for short casts. This is derived from the swing-weight of the rod as the kinetic energy is transferred from the butt to the tip of the fly-rod (the butt loses speed whilst the tip just gets increasingly faster). The softer and heavier the fly-rod, the greater the “inertial” effect that is achieved for short casting distances.

As the rod is straightening before the line is launched, it decelerates its butt up to some point, depending upon the elastic energy it has stored just before. This “self deceleration mechanism” (sdm) helps the caster to decelerate and stop the butt. This phenomenon contributes to the feel of the behavior of the rod, and to the perception of the line fit with the rod. You need some line length before you can feel this mechanism, which may let you think that the rod is “working” for you. If you do not think that the fly line is suitable for your rod, it is because the sdm effect is too small. Change the line for a heavier one and this will solve the problem (or change the rod for a more flexible one, but that is more expensive). With a short length of line the sdm effect can be undetectable and you have to make a significant effort to stop the rod: you are close to a “broomstick” situation and you have to eliminate a significant part of the kinetic energy of the rod by yourself since it will never go into the line under this condition. If you use too much line length for the rod, the sdm is large but the rod behaves like a “catapult” as the line is launched after the stop.

If we consider the line, the force exerted on it by the rod tip is often called its “tension”. This is the same word usually applied to strings, but it’s just a question of terminology. As the line is reaching its maximum speed in the forward cast it overtakes the rod tip and its tension drops below zero abruptly, just as if the tip of the rod and the line were disconnected for a millisecond, followed by a quick compression that occurs somewhere between the rod’s straight position at the launch and the maximum counter flex position. From this counter flex position back to the next straight one, the tension increases again and is slightly erratic up to the point when the rod tip becomes still as the line starts to roll over it towards the target. Now the tension in the line is no longer controlled by the tip of the rod, but by the rotation of the loop over itself. Although surprising, the compression sequence can explain why using too much overhang for a shooting line creates some slack followed by a tug as the rod leg is restrained by the tip when the slack disappears. Although limited, the stiffness of the fly line can withstand this fugitive compression without creating slack, while a very flexible shooting line cannot.

This tension is equilibrated at the tip of the rod: the line pulls on the tip and the tip pulls on the line with the same intensity. This tension results in a small deflection of the rod tip because its intensity is quite small. It is just as if a small mass, e.g. 20 grams, was pulling on the rod under gravity (so you could imagine using a pulley and a string with such a mass attached at the end to reproduce the deflection). As the fly progresses towards the target, the tension in the line decreases because air drag is slowing the entire fly line and thus reducing the rotation speed of the loop.

If you don’t shoot any line, the rod leg is immobile and equilibrated by the tension forces at each end: one produced by the rod under limited bending and, at the front end, the tension created by the rotating loop. If you shoot some line, the tension in the rod leg will be reduced because there will be no more force from the rod tip and, as the speed of the rod leg increases, the rotation speed of the loop decreases, reducing the tension at its level.

The rotating loop pulls on both legs which in return pull on the loop to keep it under rotation. This is again an equilibrated system. Remember that this rotation is due to the fact that, from the beginning, the fly leg has a significant speed while the rod leg is immobile. The loop is rolling over itself which means there are forces (centripetal forces) acting on each part of the loop directed towards its center to maintain the nearly semicircular shape. These forces are exerted by the legs which keep on pulling on the loop. As the rotation speed of the loop over itself changes during the cast due to air drag, the tension on both legs changes accordingly.

Air drag acts differently at each end of the loop because the top of the loop, the fly leg, has a greater speed than the rod leg, which has zero speed. The travelling speed and rotating speed of the loop are equal but opposite at the bottom leg while at the top leg they are in the same forward direction. This means that air drag influences the tension at the top of the loop which has consequences on the fly leg. It is also suspected to be a cause of the loop morphing during the flight.

The fly leg is pulled by the loop during rotation, but it has to face not only the air drag on the loop but also the drag on itself, the skin drag, and, on the fly, the frontal drag. To understand what happens to the fly leg we need to consider the relative importance of energy loss due to drag forces and the concentration of energy in the fly leg. The fly leg decelerates if its change in kinetic energy due to its shortening is more than counterbalanced by the loss of energy due to drag forces. Simplifying the actual situation, we may say that if the drag forces are large enough and overwhelm the effect of the tension value, the fly leg decelerates. This is the general case, although sometimes during flight the situation can change, for example when air drag diminishes as the fly leg speed decreases. 

You may ask yourself what is making the whole thing work. Is it the loop or is it the fly leg? Just ask yourself where the “energy tank” is. By far, most of it is in the fly leg, although the loop also has some energy due to its rotation and forward travel which can become relatively important at the very end of the flight. Is the fly leg pushing the loop forward? Not really, it just travels in equilibrium with the loop. Why is tension pulling on the loop or on the legs? These are internal forces that maintain the integrity of the line: the legs pull on the loop as it rotates and the loop pulls on the legs as well. They all move together, just like dancers holding each other while they move across the dance floor.

At the end of the flight, the energy in the line has been consumed and, if the cast is well under control, the speed of the fly as it lands will be exhausted. Tension has disappeared from the line as it lands on the water, pulled down by gravity, and slowed down by air drag again. And it all started from our body.