Fly Casting Physics: Casting Mechanics, What Do We Need To Know ?

the world of fly casting, and it’s subsequent contemporary research has by necessity (and thankfully) gone past the ‘touchy-feely’ to further understand and explain how all this actually works.
for sure, an angler doesn’t need all this to go out and have fun but the inquisitive fly angler will greatly benefit from having a few notions of basic physics because simply put and to reduce things to the core, everything about fly casting is about physics.
if anything, the article below will be of great benefit to understand how casting is explained throughout the different medias, books, articles, at shows or with instructors but also goes a long way in understanding the equipment we use: fly lines, rods, leaders, etc. non-neglidgable as well is this knowledge will also help sift through all the BS that’s randomly but unfortunately spewed right and left in so many ways on how fly casting works.

Mark Surtees, IFFF Master Certified Casting Instructor from England has kindly offered to share his wisdom with us here with what i personally consider to be a monumental reference work to be bookmarked and shared.  thanks Mark !

Mark Surtees TLC 15-10-13


There are parts of angling instructor examinations that require a candidate to have a basic understanding of casting mechanics.
What does this actually mean ?, What, after all, is Casting Mechanics ?
There seem to be as many interpretations of what constitutes casting mechanics, what makes a cast work, as there are authors, bloggers, magazine journalists, TV presenters or DVD producers out there in the Anglerverse.
In the world of the professional and the published, where cruel commercial reality dictates that personality and self promotion matters, differentiation from the rest is critical in order to sell the product, this is what puts bread on the table. So, no terms are quite the same, the explanations often in conflict, one with another, and there is an underlying, but perfectly natural, predilection to promote neat and highly personalised instructing techniques or concepts.
I think this is great…… I freely admit to the regular breach of the intellectual rights of these authors. I mercilessly steal and adapt teaching techniques and simple field fixes because they are better than any I can think up on my own.
But, if you look closely, some of these quick rules of thumb work by being pithily memorable rather than having any bearing on what may turn out to be inconveniently complex physical facts.
No-one disputes the benefits that can arise when we use these things from an instructional point of view. They can be a fantastically effective means of communicating a difficult concept and, in the hands of a good instructor, they are invaluable. However, being effective doesn’t make them “true”. They often turn up, on closer examination, to be completely at odds with what really happens in the world beyond the pages of the book in which they are written. Some, if published in more academic circles, might merit a Nobel nomination for the uncovering of entirely new and, no doubt very exciting, principles, which would turn the world of physics on its head.
Does this matter? Well, that’s up to you to answer…but it’s always mattered to me.
So, if we do decide to look beyond the one liners, how far do we go ?
Is there a limit on the things that an instructor should know about Classical Mechanics? When is enough, enough?
At what point does the subject become so complex that it becomes terminologically impenetrable for anyone but a trained physicist to understand?
For me at least, the simple answers are, in order…”its up to you”,.. ”no, not really”,… “don’t know” and “pretty quickly”.
Obviously, we set our own limits on our pursuit of knowledge, no-one is going to tell us when we have had enough. For most of us, who are untrained but nevertheless interested in these things, we set our own pace and plough through the arguments with a supplementary dip into the internet for explanations which in themselves can introduce confusion as a by-product of their generality. It is too abstract, out of context, mathematically testing and the unfamiliar terminology will trip us up early if we do not know how it even applies to the world about us, let alone how we might apply it to simple casts.
Sometimes, it’s difficult to ask direct questions of our better informed peers without appearing irredeemably stupid….almost no-one wants to be seen this way and so we don’t ask…the prospect of a highly public, id savaging polemic from an advanced theoretician is too much for the vulnerable soul of a simple instructor to take and, so, unravelling the terminological mystery of mechanics continues to elude us.
In order to try and help avoid these embarrassments, this is a basic glossary of terms with a vastly oversimplified explanation of what they mean and how they might relate to basic casting mechanics.
Most of the things we are going to look at have a “value”, they are quantifiable, calculable, measurable and they are often inter-related. Some relationships between terms can be represented by simple equations. In the case of the key relationships there is nothing of any challenging complexity, however, the connection between each term is explained in words so as to avoid the possible onset of an algebraic crisis.
For each term, its value is measured using an international system of units called SI Units. These units represent the amount of stuff something might have, a quantity. Where relevant I have put the SI units in but, mainly, I have left them out. I’ve started with “quantities”, gone on to “stuff” and then moved into “motion”
There are no equations anywhere…there are, however, some elephants.. If you can….please..

Some quantities can stand alone, a “Kilogram” or a “Meter” or a “Second” for example. Others require a combination to produce a more complex value such as “Meters per Second” or “Miles per Hour”. These sorts of quantities have a simple magnitude and are called “Scalar” values, basically they have scale.

Quantities with magnitude only.

Some quantities have an added element of direction. This implies that whatever scalar value this quantity has, its size… it’s going somewhere or it’s pointed somewhere. These are called “Vectors”.

Quantities with magnitude and direction.

Vectors give us a means of placing, or describing the movement of, an object in a three dimensional world and explain its direction of motion. They can also be used to indicate the direction of a force and thus the direction something will move under the influence of that force.
To demonstrate the difference between scalars and vectors we can use two other common, inter- related terms.

The distance travelled by an object over time.

For our purposes speed can be measured in Meters per Second, or Miles per Hour…or Meters per Hour or Miles per Second, whatever we want. We measure the Speed of Light, Speed of Sound, Cars, Rabbits etc. in terms of distance and time. Speed is Scalar, that is, it is just a quantity with magnitude, but, rightly or wrongly, it is often used interchangeably with…

The speed of an object and the direction in which it is travelling.

Velocity adds a direction to the scalar quantity Speed and so represents Meters per Second or Miles per Hour perhaps, but in an easterly direction or south or up or down or left or right or a combination of these sorts of things.
Velocity is a vector. Velocity is a term that will re-occur as we move on. It is used to describe rod tip velocities, line velocities, angular velocities, linear velocities, loop velocities.
When talking about velocities, or any other vectors, force vectors for example, you will often hear the term “Component”.
In most casting contexts components are simply used to represent the amount of “upness” or “forwardness” of something, by displaying it graphically and using the X or Y axes of the graph to measure it. Usually, the X axis is the horizontal axis and the Y axis is the vertical axis, so when you hear someone talking about the X component of a vector they mean how much it is travelling forwards or backwards and the Y component is how much it is travelling upwards or downwards. Combined, these two “components” form a “resultant” which is the actual direction of the object in motion or the direction of the force being applied.
Imagine you’re on an ice rink, one person is pushing on an elephant in the X direction, in Fig 1 this would be to to the right, while another person pushes on the same elephant in the Y, upward, direction. The resulting motion, if any, is exactly the same as single person pushing the elephant in the direction that is upward and to the right,… none probably but you get the idea I’m sure.
Figure 1 shows how the X and Y components of a vector can be added to generate a resultant vector.



What about the inherent properties of the things we are trying to describe? The rod, the line and the fly attached to the end.
First and foremost …all these things have Mass

The amount of stuff in an object, how much “matter” it contains.

For all practical purposes this value is most commonly equated with, and, even though strictly speaking it shouldn’t be, it is used interchangeably with Weight.
Mass is measured in SI Units of Kilograms and its sub divisions, it is Scalar.

Is a measurement of the mass of an object under the influence of gravity
Weight is the mass of a thing multiplied by the force of gravity.

It is very important to understand that when we are standing on the earth, or any other planet you can stand on, things fall down. No matter what other forces are influencing an object there is always a force pulling down on it, and we will look more on this later.
Because it has a direction, Weight, Mass times Gravity downwards, is a Vector Quantity. What of this terminological confusion between Mass and Weight?
Well, just as with Speed and Velocity, most of us make no real distinction between the two terms. Weight is properly measured in SI units of Newtons. Because most of us don’t leave the surface of the planet, the force of gravity is treated as constant and we don’t weigh ourselves, or beans, or rice or anything else in Newtons….we save these units for something else and we blur the distinction further by using the SI units for Mass, Kilograms etc, when we weigh something.
No non physicists use Newtons for weight, in fact no one I know, physicist or not, knows their weight in Newtons, though I dare say they could work it out….however, these units are not wasted and become more relevant later in this discussion.

Just as a matter of interest, more massive things like the earth have more gravity than less massive things like the moon but less gravity than even more massive things like the sun….in a non fishing context this explains why exceptionally large tie knots attract more fluff and airborne detritus than small ones… .
Anyway… This leads us to…

The mass of an object by volume.

For a given mass, the more space the mass occupies the lower its density, the less space for the same mass the higher the density.
It is differences in density that explain why a floating line floats and a sinking line sinks when they both have the same mass or weight. A #5 floater floats because it has less density than water, and a #5 sinker sinks because it is denser than water, even though the two lines weigh exactly the same.
As our lines get denser, for any given weight, the volume decreases and so too does the surface area. The less the surface area, the less water resistance the line will have and the quicker it will sink. Fly fishers call this the sink rate of the line.
(A physicist would call this the lines “terminal velocity in water”, but, if you speak to a physicist who fishes they will know what you mean when you say “sink rate”… in fact, anyone who uses “terminal velocity in water” instead, should, in all probability, be repeatedly hit with a heavy skillet until they agree to stop.)
Rods are tapered, this usually means that there is a regular reduction in mass towards the tip. Uneven or irregular mass through the taper may result in an odd action or tip wobble. This is usually referred to as Mass Distribution and is most commonly met with when discussing lines and line profiles.
We can see just by looking, that, for rods and lines, the mass and or density is not evenly distributed. A weight forward line has more mass towards the tip while a double taper has its mass evenly distributed for most of its length. A 5# floating line has less mass than a 7#.
We will see later that it is changes in mass and density, along with a few other simple concepts, that explain why a sink tip can kick like a mule, or a shooting head turns in to spaghetti, or a roll cast sometimes just won’t roll. The relationship between the mass in the line, the air or the water and the motion of the rod is often crucial to understanding how these things occur and how basic casts actually work.
Any object with mass has a ….

Centre of Mass
This is just like the centre of gravity.

In a sphere or a cube where mass is evenly distributed, the centre of mass is slap in the middle. Because we know that rods and line don’t have an even distribution of mass, the centre of mass isn’t necessarily going to be anywhere near the centre of the object. This is going to affect its balance, how it moves and how easily or difficult it is to actually move it.
In addition CoM is commonly used as a reference point for measuring purposes. When discussing forces and motions we can think of objects as being imaginary points, i.e. they have no size, so no matter where we push on the point object we are always pushing on its centre of mass.
Obviously, in real life, rods and lines are not imaginary points and we don’t always apply force directly against the centre of mass. In fact, we most commonly don’t, and, as a result, the rod may turn or twist in addition to moving away from the force we are applying.
If you know the relationship between an object’s centre of mass and any forces applied to the object you can determine if that thing will simply move away from the force in a straight line, turn, twist, or any combination of these things. We will talk about straight line motion as, “translation”, and turning through an angle as, “rotation”, later.
You can locate the centre of gravity of an object like a rod by finding the point that it balances on your finger. This is not so easy for a length of fly line…but it still has one..

Of course, things are never quite as simple as we would like, just describing the properties of an object isn’t enough.
Because we want to move these objects around, we have to take in to account certain rules that relate to getting an object to move from A to B. We have to get our rods and lines and flies moving and we may want to change the direction, or speed, that they are moving in once we have managed to get them in motion. If we are completely nuts, we also might want to be able to record or measure how we have made this movement happen so that we can repeat the process.
Three basic rules were outlined by Sir Isaac Newton approximately 325 years ago.
Just in case you’ve missed them, here they are… they are sometimes referred to as N1, N2, and N3. I have paraphrased Sir Isaac, he won’t mind I’m sure…

Newton’s Laws 1/2/3
1- An object will remain in motion, or not, at the same speed and in the same direction until acted upon by an external force.
2- Force equals mass times acceleration.
3- For every action there is an equal and opposite reaction.

Application of these very simple rules enables us to describe the processes involved in moving our rod and line.
Sir Isaac Newton should not be confused with Isaak Walton.
Isaak Walton was author of “The Complete Angler” and never mentioned Newtons rules mainly because he knew nothing about physics and he died before the other Isaac published his “Principia Mathematica” and this also probably why he never became an SI unit like Newton. (I would however like to recommend that forces used in angling contexts are from now on measured in SI units of “Waltons”.)
This disconnect between the great angling literature and basic mechanics has existed down the ages and whereas Isaak W..had a pretty good excuse on the basis of his prior demise…no-one else since can honestly play the same card…
Anyway, we can see from Rule One that nothing is going anywhere until we apply a force to it…

A pressure, a push or a pull, something which causes an object to move or deform.

A force, in a casting sense, is just something needed to make an object move. We apply a force to the rod, the rod applies a force to the line, and the line pulls the fly.
With objects that are elastic, a rod for example, a force is also necessary to make only part of that object move i.e. to deform it, like bending, and it is force that causes the line to stretch as we cast.
Forces are vectors, they have magnitude and direction and, like weight, are measured in Newtons too.
We know from experience that it’s not just any old force that is going to make an object move. Anyone who has tried to pull an elephant around an ice rink or push one out of the bathroom will know that it takes a lot of force to get it to move at all. How much force is dependent on the mass of the elephant, small elephants are easier to shift than large ones.
Based on the first rule, if an object is already in motion we need to apply a force to change its current speed or direction otherwise it will just carry on and that the more mass the object has, the more force we will need to apply to it to make it change its speed or direction.
This resistance to moving, or having a change in motion, is a property called…

The propensity of an object to resist a change in motion.

We already know from our experience of elephants in the ice rink that, when something is stationary we won’t be able to move it until we have applied a force big enough to overcome its inertia. This is not the same as overcoming friction or Gravity. Inertia is closely linked with mass, lots of mass, lots of inertia. We think of inertia usually in terms of things that are stationary and subject to other frictional and gravitational forces, but objects in motion also continue to have inertia. A rod or a line have the same inertia when they are moving as they did before you made them move and to change their speed or direction the force you apply must still be big enough to overcome that inertia. Of course the mass of a rod or line isn’t very big so you don’t need much force to overcome their inertia when they are moving…an elephant on the other hand is a different kettle of fish.
What we also know from Newton’s second rule is that, once we have overcome that inertia, if we continue to apply the same force then the object will accelerate.

The rate of change in velocity of an object.

We have to be a bit careful here because we are dealing with velocity and velocity has two elements, speed and direction. So, the force we apply can do one, or both, of two things. It can either change the object’s speed or it can change the object’s direction of motion. Either of these things would change the object’s velocity and so acceleration isn’t just about making something go faster, in physics an object undergoing a change of direction is also accelerating.
This is sometimes a bit difficult to grasp and it is further complicated by physicists referring to decelerations as negative accelerations. For the most part however, non physicists use acceleration when we are talking about increasing speed and deceleration when decreasing speed. It’s not right, technically speaking but it is what we mean.
If an object has mass and a velocity then it is said to have

The mass of an object times its velocity.

Momentum is about mass moving…a 5000kilo elephant in the bathroom has mass but no motion, so it has zero momentum. Because it is linked to velocity, momentum is a vector quantity, it has direction. Momentum is often confused with inertia but they are not at all the same thing.
A 5000kilo elephant shoved out of the bathroom at 10 miles per hour has a momentum of 5000kg x 10mph and is quite likely to be pretty hard to stop. Of course, if you get in the way of the elephant as it comes out of the bathroom it will, in turn, thanks to Newtons third rule, apply a force to you. Quite a big one probably, which will cause you too to either move, move and deform a bit, or, if you’re up against the opposite wall waiting your turn in the bathroom, deform quite a lot.
The more momentum an object has, the harder it is to slow it down. So, in the case of a fly line for example, the greater its momentum, the harder it is to stop and the farther it will go.
When discussing fly line trajectories, the fact that mass is commonly not distributed uniformly throughout the fly line means that the momentum of the fly line is not uniform either. For a given velocity, the parts of the fly line that have the most mass have more momentum than those with less mass and it takes more force to change their speed or direction.
This explains why the line doesn’t really follow the path of the rod tip, why a weight forward is easier to shoot than a Double Taper, the hinging effects of overhang, hooked line layouts, dangling leaders and the kick of heavy flies and sink tips as mentioned above and many other phenomena.
So, we have applied force to our line, overcome its inertia, accelerated it and it now has momentum…what stops the fly line carrying on indefinitely once we have stopped applying force to it?
If Newton’s first law is true then some sort of force or forces must be slowing it down and causing it to fall to the ground. The most obvious candidate is gravity. Gravity operates uniformly over the line and, irrespective of the magnitude of the line’s mass or forward velocity, left to its own devices, (which it isn’t, bear in mind we are holding one end up with the rod) it will all fall to the ground at the same rate. Obviously, the more velocity it has, the further it will go before it hits the deck. In this respect, if distance is your goal, velocity is king.
Also working on the line is air resistance, this is called…

The force exerted on an object in motion as a result of friction in air or water.

The greater the surface area of the thing in motion, the higher the drag will be as a result of friction. As velocity doubles the effect of drag quadruples – the higher the speed the greater the drag will be.
Also, the more line is in the air the greater the surface area and so the greater the drag.
However, remembering density, if we can squeeze the mass into less volume then we can reduce the surface area and thus reduce the drag. This is why a #5 sinking line will cast further than a #5 floating line… less drag.
Sometimes drag is referred to as “lift”. This is because drag opposes the force of gravity as an object falls to the ground. Since “drag” or “lift” increases with speed, something in freefall will continue to accelerate until its lift is equal in size to the force of gravity acting on it. At that point the object will have reached its “terminal velocity”. (It’s OK to use terminal velocity in this context so don’t reach for the skillet.)
Since there is more drag when an object falls through water than when it falls through air a sinking line will have a much higher terminal velocity in air than in water. This, if you are casting for distance for example, may have an effect on the trajectory you choose for your particular cast. A sinking line will go forwards faster than a floater because it is denser and has less drag but it will fall quicker because it has less lift and thus a higher terminal velocity in air than a floater.
You may have heard of Galileo’s experiment dropping cannon balls of different sizes from the Leaning Tower of Pisa. In Galileo’s experiment the two cannon balls fell to the ground in exactly the same time. If he had replaced one of the balls with a feather he would have had an entirely different result.
Since drag is a result of friction with the medium the object is travelling through, would a feather fall at exactly the same rate as a hammer if dropped in a vacuum?
In fact, this experiment was demonstrated during the Apollo 15 moon mission and the hammer and feather did fall at exactly same rate. So, even if you are one of those who believe the Apollo missions were faked, you can take comfort in the fact that “they” must have built a massive vacuum chamber in order to also fake the hammer –feather drop experiment and so the money wrenched from the pay packets of hard working US taxpayers was spent in a way that genuinely represented real value for money….for people reading this in the US this must be a huge relief. (Sorry about the weight in Kilograms thing earlier too)
In this context, Drag is about the amount of surface in contact with the air or water, it is not the same as…

Form Drag
The force exerted on an object in motion in air as a result of its shape.

A ball with a given surface area will have less form drag than a cube with the same surface area simply because of its more aerodynamic shape. If Galileo had flattened one of his cannon balls out before dropping it he would have found that the differences in form drag would have changed the outcome of his experiment.
The line only has Momentum with which to work against the effect of drag….for a given mass, more velocity means more momentum, velocity is king…but sadly for the tournament casters, drag always wins in the end.
A fly line in motion is under…..

For our purposes tension is a force that attempts to stretch the fly line.

For tension to exist there has to be a force at one end and a resistance, or an opposing force, at the other end.
If you pull your finger it will go into tension. The tension in your finger will be uniform all the way from the end that you are pulling on, to the joint. This is because both ends of your finger are to all intents and purposes, fixed…this is not true of a fly line because only the rod end of the line is fixed (unless you’ve hooked a 5000 kilo elephant in your bathroom that is).
Discounting the effects of drag for the moment…, because only one end is fixed, we rely on the inertia of the line to oppose the force applied to the line at the rod tip, but, the mass and inertia become less and less as you approach the leader end…why is this we wonder ?
Imagine the line as a series of, let’s say, a thousand tiny interconnected balls and give each ball an equal mass. During a casting stroke the ball at the tip has to pull 999 balls along behind it, the next ball 998, the next 997 and so on until the last ball which is pulling on nothing but the fly and leader, so the tension on the last ball is 999 times less than the tension on the first. Basically, tension in the line is greater at the rod tip end than it is at the fly end and this fact will help us when we look at “waves” later.
For the moment let’s leave lines and look at the rod.
An ordinary single handed fly rod is both a spring and a lever. It is what is known as a “third class” lever which is, in this case, a device that goes faster at one end than the other and we use it to increase speed.
To make this happen we rotate the rod, as we do this the tip will travel a much greater distance than the butt in the same amount of time i.e., it has travelled faster.
We describe the motion of the rod in terms of…

The angular change in position of the rod.
The linear change in position of the rod.

To make the rod move we apply force to it at the butt end. To make the most of the lever effect, that is the magnification of velocity at the tip, this force needs to make the rod rotate. This is not a force which acts in a straight line, a linear force like the one needed to push the elephant out of the bathroom, or the tension in a straight fly line, this is a force which needs to act through an angle. This sort of angular force is called a torque.

An angular force, the force required to rotate an object.

A linear force, one which makes the rod translate and, an angular force, torque, one which makes the rod rotate can, and do, act on the rod at the same time. By combining these forces we are able to move the rod through a vast range of positions at various rates and it is the combination of these two simple processes that enable us to cast at all.
The use of our springy lever has the effect of converting the angular force, torque, applied by us at the butt into a linear one applied by the rod tip on the line.
Up to now we have used terms that are largely linear, the directions in vectors are unchanging. When rotations are involved, the directions in vectors change, the relationship between the x and y components changes constantly but the terms we use are very similar to those described above. If we are able to understand the terms as they apply to linear forces then it helps enormously when we try to deal with their rotational, or angular counterparts.
So from Velocity we can generate the term Angular Velocity.

Angular Velocity
The speed and direction of an object as it rotates.

In rotation, direction is measured relative to the axis or point about which the object rotates, e.g. clockwise, counter clockwise, positive or negative degrees per second. Speed is measured by the amount of arc travelled in a given time, e.g., degrees per second or revolutions per minute.
Changes in Angular Velocity are Angular Accelerations.

Angular Acceleration
The rate of change of angular velocity.

Because we are using a flexible lever, it gets lively with this one. If the rod were to be completely stiff then the rate of acceleration at the butt of the rod would be the same as the rate of acceleration at the tip. Even though the tip is travelling faster than the butt, the rate of change is the same, as the speed at the butt doubles, the speed of the tip doubles.
The rod isn’t completely stiff though, it bends when we apply force at the butt as we try to overcome the inertia of the line and the inertia of the rod itself. This means that even if the acceleration at the butt is constant the acceleration at the tip won’t be. If the rod has inertia which is its resistance to changes in linear motion then it also has a resistance to angular motion. If we were being consistent, this should be called Angular Inertia but it isn’t.. it’s called…

Moment of Inertia
The propensity of an object to resist a change in angular motion.

The mass distribution in the rod will determine how hard or how difficult it is to rotate it.
A rod doesn’t just act as a lever, it also operates as a kind of spring.
There are a huge number of reasons why a springy rod is easier to use than a rigid one which we won’t go into here, but, springy things have unique properties too and act in a regular and predictable way.
Springiness, that is, the way that the rod bends and unbends is a function of the properties of the material that it is made from and how that material is distributed through the rod.
The relevant material properties are described by using…

A quantity that numerically expresses the degree to which a substance possesses a property, such as…

Modulus of Elasticity
Modulus of elasticity is commonly used to refer to stiffness in fishing rods, it is the property of a material that describes how much it deforms and how it recovers from a deformation to its original state, its “elasticity”, so, the higher the modulus, the stiffer the material.
Elastic or springy things have been studied for centuries and one of the basic relationships is described by…

Hookes Law
In a spring, or elastic material, the extension of the spring, is proportional to the load.

So, for a rod, when it behaves as a spring, the bigger the load the more it will bend. This seems very obvious, but what, in physics terms, is a load…?

The forces that are working on the rod.

Load is commonly associated with the bend in the rod due to the weight of the line that we are trying to move and there is nothing inherently wrong in looking at things in this way. But it is also related to the other forces at work on the rod. The amount of torque and how we apply it at the butt will also influence how the rod bends.
The weight of the rod itself and the mass profile of its taper, its moment of inertia, its surface area and the effects of drag all influence the way the rod behaves when it is in motion.
Where there are multiple forces at work like this we can add them all together using the term…

Net Force
The sum of all the forces acting on an object.

This is like the resultant force we described earlier. Since force is a vector quantity we can sum all the forces acting on an object to determine the net force. In turn, this tells us which direction all of the individual forces acting on an object will tend to move the object. For example, if we look at the line the net force acting on it will be a combination of forces applied by the rod tip, gravity, and those caused by air resistance.
By the nature of springs they have a propensity to boing, that is, the load forces the spring to extend and then the spring boings back to its original shape and so on, there’s a whole lot of boinging going on here. And, if it needs a force to make it extend then it must also need a force to make it go back to its original state, this force is called a…
Restoring Force
In a spring or elastic material it is the force that works to return the spring or material to a state of equilibrium.
Since we have introduced the topics of tension and restoring forces it might be a good time to talk about waves

A wave is a disturbance that travels through a medium and transports energy from one place to another without moving the medium itself.

As the wave travels through the medium there is some displacement of the medium but the medium returns to its original position after the wave passes by.
If we drop a pebble in to the centre of a pool we create waves on the surface of the pool. The waves transfer the energy from the falling pebble to the edge of the pool. Obviously, the water from the centre of the pool does not end up piled along the sides of the pool so we know that, although we can see the
waves moving the water up and down, the water itself does not move laterally with the waves. As the wave moves the water is initially displaced upwards and then restored downwards to its original state.
When a wave travels through a medium in one direction and causes the medium to be displaced sideways or upwards or downwards the wave is called a transverse wave. Examples of transverse waves can be seen when we cast, in tailing loops, some types of mend and the irritating wobbles we occasionally get in the rod leg of the line.
If the medium is displaced in same direction that the wave is travelling then the wave is called a longitudinal or compression wave. For an example of a compression wave simply listen for that bead or fluff whipping past your head…sound is made of compression waves.
At this point you are no doubt wondering what tension or restoring forces have to do with waves.
If you have a guitar handy, pluck a string. You will hear a certain note. It doesn’t matter how hard or soft you pluck the string you will always hear the same note.
What do we have to do to get it to make a different note? The answer is to adjust the tension. By turning the tuning peg for the string we change the amount of force being applied to the string and this changes the tension on the string. Add more force and the tension increases and the note becomes higher because the wave moves faster. Reduce the tension and we get a lower note because the wave moves slower. Remove all tension and we hear nothing. Waves simply won’t travel along the string any more. Pluck the string and it just stays plucked. The wave just stays where it is. We have removed the tension and this removes the restoring force.
Why does this matter ?
Well, if you remember the discussion on tension in the fly line, there is more tension at the tip end than the fly end. We know that the greater the tension we have on a line the faster a wave will travel along the line. And without tension there is no restoring force to bring the material the wave is travelling through back to its original position.
So, a wave in a fly line will travel fast at the beginning and slow down as it approaches the fly end. Here it is now going so slowly, or the restoring force is so weak, that it can appear to get stuck. This is a wave that can collide with the rod leg during a casting stroke and produce the classic tailing loop.
That’s enough about waves for now. Let’s get back to our discussion on forces…
So, what do we want all these forces to do? Essentially we want them to do work to the line and the fly to get these things from one place to another. In physics work has a particular meaning…

Work is force applied to an object, times the distance over which the force is applied.

This term is closely linked with another….

Impulse is force applied to an object, times the time over which the force is applied.

Between them, these two concepts are crucial for understanding how we manage the variables, force, time and distance, to make the most basic of casts function in our favour.
Interestingly, work is measured in SI units of Joules. Joules are also used to measure energy and so work can also be used to describe the change of energy in an object. So, notwithstanding the effects of drag, we can work out the velocity of an object if we know what its initial energy level was and how much work we have done to it.
Managing the three variables using the concepts of work and impulse, we can say that a high force applied over a short distance, i.e., in a short time, will have the same final result as a low force applied over a long distance, i.e., a long time. In a casting situation we choose how we will mix this up to achieve the desired line velocity that we believe we need for the circumstances we find ourselves in or just to fit our own biomechanical preferences.
We briefly touched on energy as we looked at the concept of work. In this context we can look at energy as the ability of a thing, in quantitative terms, to do work on another thing. Energy is most commonly described in two forms…

Potential Energy
Energy stored in an object.

A rod that is bent has potential energy or PE, stored energy, and has a capacity to do work on the line. When that energy is released it is converted into….

Kinetic Energy
The energy of an object in motion.

Also called KE by people who have a hard time spelling, or saying, kinetic. The energy of the line when it is in motion for example. This is the stored energy being used.
We often talk about the combination of Potential Energy and Kinetic Energy as Total Energy.

Total Energy
The combined kinetic energy and potential energy of an object.

Most people can spell total so total energy is never referred to as TE. Usually totally energy is just referred to by physicists and clubbers alike as E.
Energy is said to be “conserved”, that is..

Conservation of Energy
In a closed system energy in = energy out.

A closed system is a construct used for analyzing interactions between objects and forces. In a closed system the amount of stuff or matter within it does not change. And, similarly, in a closed system, energy is neither created nor destroyed ie it is conserved.
If we are juggling elephants, as we throw our elephant in to the air we give it a certain kinetic energy. As is rises it goes slower and loses kinetic energy. The amount of kinetic energy lost is identical to the amount of potential energy it gains as its height increases. As the elephant peaks and begins to fall the potential energy is again converted to kinetic energy and by the time it reaches our hand again, it is going the same speed at which it left.
Whilst we are elephant juggling away in our closed system there is another property that we met earlier also being conserved…

Conservation of Momentum
In a closed system momentum is conserved.

A familiar example of this is Newton’s cradle. When two objects collide and bounce off each other, the momentum lost by one of the objects will be precisely equal in magnitude, but opposite in direction, to the momentum gained by the other object.
This has, historically, been used to explain the phenomena of a fly leg appearing to speed up toward the end of the cast but the jury is definitely out on that one. So, as a lay person, I think it’s best to watch the argument unfold slightly away from the bathroom door…just in case I get involved in conserving the momentum of a rapidly moving, entirely metaphorical, elephant.




As I re-read these things, garbled as it all seems, I wonder if it is particularly useful, from an instructional point of view, to be anything other than dimly aware that these concepts even exist.
After all, no-one is going to be explaining to a complete novice the concept of conservation of momentum or the value of KE in a line with a non uniform, mass distribution. In fact it is massively unlikely that they would be discussed with anyone other than like minded instructors.
However, for an examination candidate to express themselves properly and correctly to questions that might arise on these matters, they must have a clear grip on the basic terms and how they fit together to explain how a cast actually works without resort to those pithy one liners that we mentioned in the intro. Having said this, I still frequently use these teaching tools myself but, where necessary, I amend them in order to better fit the facts.
To do this properly I have tried myself to understand the relationships between Force and Work, Levers and Springs, Speed and Velocity, Momentum and Inertia and the choices made by us, the casters, in how we manipulate and manage them to our advantage because, without us, nothing happens at all.
Just for the record, I am no physicist and it has never crossed my mind to actually work out the values associated with these things on a cast by cast basis. From a teaching perspective I can see no useful purpose in trying.
There are others out there, however, for whom this quantitative analysis is a source of constant fascination…sadly, perhaps, it is not for me.
Thanks, Mark

© Mark Surtees

NOTE- originally published Oct. 15 2013, this article has since been revised by Mark Surtees: curent version Nov. 11 2016

13 thoughts on “Fly Casting Physics: Casting Mechanics, What Do We Need To Know ?

  1. it may well be totally devoid of teaching relevance / usefulness. but as a collection of conceptual devices the human mind has built in order to tackle the most vexing and urgent problem of them all (ie: movement) it’s absolutely brilliant.
    thanks A LOT for sharing, Mar(c/k)!!

    • it’s far from irrelevant for quite a few students. as an example, in Sweden, a lot wanted tech talk. i don’t teach that way but i want to know this stuff for myself.

      does anyone wanna take on his ‘conceptual’ comment. i most certainly won’t ! :mrgreen:

  2. This article is great! Reading your posts about fly casting really does makes me rethinking casting itself!

    • glad you liked and specially that this and the other articles can be of help. many of us are constantly rethinking our approach to casting so, welcome to the club !
      don’t know if you noticed the article section on each page’s top bar but you’ll find a lot more HERE.
      thanks for stopping by,

  3. Marc

    I’ve just re read it and there is a bit missing. James rightly questioned whether I actually meant to write that Potential energy is the energy of an object in motion…obviously it isn’t.

    This bit is missing from the article you have published…

    “Potential Energy

    Energy stored in an object.
    A rod that is bent has potential energy or PE, stored energy, and has a capacity to do work on the line. When that energy is released it is converted into….

    Kinetic Energy”

    Then the stuff about objects in motion.

    Dunno why that has happened, its in the original piece.

  4. Not sure why Marc but the article is missing a crucial paragraph, The bit after Potential energy should read :-

    Potential Energy

    Energy stored in an object.
    A rod that is bent has potential energy or PE, stored energy, and has a capacity to do work on the line. When that energy is released it is converted into….

    Kinetic Energy

    Then the stuff about an object in Motion.

    Thanks to James Evans for pointing that out.

    Sorry bout that…

    • since it’s your masterpiece would you like to add whatever it is you seem fit the way you like it, send it to me and i’ll reprint the whole shebang ?

  5. Thanks for sharing this, Mark (&Marc)! With a large portion of humor, and covering most physical concepts involved in fly casting, it was pure joy reading it. For me the English names for universal concepts were most useful.

    • hej Staffan ! glad you liked and yes, while i hadn’t thought of it before, i’m sure having all this clearly defined in english must help all non-english natives. it’s all good.
      hope you and yours are doing well,

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