---

“Look, if you just let your arm drop by effect of gravity, you send the fly ten meters away effortlessly and accurately.”

That is my way of showing how easy it is to make a pick up and lay down cast—elbow forward style— at the most usual trout fishing distances. However, something comes to my mind in those instances, something that I never say to my student:
“If it is so easy, why did it take me so much time, effort and frustration to do it properly?”

Intuitive motions don’t work

Let’s spare the backcast altogether by placing the line on the ground straight behind us, and let’s ask our beginner student to make a forward cast. A windscreen-wiper motion with its consequent non-loop is almost guaranteed.
All would-be casters find intuitive that you need to rotate the rod for it to be useful — after all every lever works by rotation, therefore, no rotation, no advantage. Achieving a near-straight motion of the rod tip from a curved motion of the rod butt seems to be too much of a novelty, though; a hard-to-grasp idea, indeed.
But, is that straight motion within a curved one actually that new for us? Don’t we all throw a dart to its target with a straight motion of the hand? Doesn’t our forearm rotate, however our hand travels in a straight line? Don’t we do it just instinctively, without any instruction whatsoever? Somehow we manage to figure it out from the start. Take a fly rod in your hand and that natural response seems to dissolve itself. Crazy!

Similes do work

In fact I am very fond of the dart-throwing metaphor when teaching: to throw a dart the hand goes straight, so the dart flies straight. In the same way, the rod tip should move in an ideal (not real, though) straight line, and the line will follow, flying straight to the target and forming a narrow loop.

But fly casting is a very special case of throwing, as the very same rod flexibility which allows us to form pretty tight loops with straight legs demands a very precise control of that flex. Since rod flex is a result of the force applied to it, if you apply force erratically you are in trouble; a progressive increase in rod-butt speed is key, then. This is one of the most difficult aspects to get right. Getting the line as well aligned as possible with our target at the end of the backcast is another, as we will see later.

So “if the rod tip goes straight, the line will follow” is a very useful mantra for teaching casting fundamentals, mainly because it shows beginners—and reminds false beginners— that waving their rod hand in a windscreen wiper fashion just doesn’t work.

—Your hand has traced a curved path so your rod tip was moving downwards at the end of the backcast; the line has followed that direction and almost touched the ground behind you.

The sentence above is something I have to repeat very often. So far so good, but is that all there is?

Some Physics

Two concepts are essential to understanding line behavior: inertia and momentum. Don’t worry, They are very basic and easy to grasp.

• Inertia

Inertia is a property for which every object tends to keep its state of motion. If the object is still, it tends to remain still, resistant to any force trying to move it. If the object is in motion, it will resist any attempt to modify that motion; that is, it will try to avoid any change in its speed or in its direction.

• Momentum

In Physics, momentum is the resulting value of multiplying mass times velocity, and is expressed by the formula p = m.v —in which p is momentum, m is mass and v is velocity. At first sight that formula doesn’t convey anything meaningful to a layman caster; let’s try to translate it into something digestible.

What the concept of momentum reflects is the amount of resistance that an object opposes to any force trying to change its state of motion; also known by the more graphic expression quantity of motion, momentum is like a way of measuring the amount of inertia of a body, so to speak. In practice this means that the heavier the body (i.e., the bigger its mass), the more force is needed to change its motion; on the other hand, the faster the body is moving (the higher its velocity is) the more force is needed to change its motion. Finally, we must remember that “change in motion” not only refers to a change in the speed of the body, it could be just a change in its direction while keeping the same speed. Inertia of a body resists changes in both parameters speed and direction.

That is all we need to know.

Metaphors work… up to a point

Our fly line is, obviously, an object, and as such it has inertia and, when in motion, momentum. But unlike a body whose mass is concentrated —a spinning lure for instance— a fly line is a long and flexible body, so every part of it can move semi-independently  from the rest. And if different parts of the line are moving in different directions each of those parts has a different momentum —even if all parts have the same speed— because momentum is defined by direction of motion apart from speed. This semi-independence of the different parts along the length of a fly line can lead to a very peculiar behavior. Let’s study it on the following video:

---

A bead chain —the “line”— is attached to a rod. The rod tip traces a pretty straight path… and the line goes a totally different way. Exactly the opposite of what we normally teach! How is that?

As we know rod tip path has a main role in the resulting trajectory of the fly line, but it isn’t the only aspect to have into account. There is another —capital— key to consider: the angle between the fly line at the start of the casting stroke and the rod tip trajectory during it.

Let’s study that video above again:

When the rod tip accelerates, it forces the part of the chain closer to it to move in its same direction… but only that short part actually moves in that direction.

It is useful to think of the chain as a set of individual beads. Each bead —apart from those very close to the rod tip— follows a different path from that traveled by the latter. The chain’s flexibility itself allows for each part to move independently, up to a point.

We already know about inertia’s whims, so now we are aware that each bead moving in its own direction wants to keep moving in that same direction, resistant to any force trying to change it. The amount of resistance it opposes to the mending of its divergent trajectory is given by the own momentum of each bead—that is, by its mass and its velocity. Only a few beads are actually moving in the direction of the intended cast, and those have the task of bringing back to the “right” path the rest of the astray beads; a hard task indeed, as the combined mass of the “good beads” is much smaller than that of the “bad beads,” which, as a result, have a much bigger momentum. So what we can see on that video is a spectacular tug-of-war among the different sections of the line. The final lay out of that cast is the result of the fight between the good guys’ and the bad guys’ respective momentums; a surprising result indeed as, for once, the bad guys win.

Yes, a bead chain and a fly line are different… but not that much. One difference is that the chain doesn’t have a taper, so its free end has a bigger percentage of the total mass, while the tip of a modern fly line is thinner than its belly—and therefore comparatively lighter. But just think of that bead chain as if it were a level fly line and the example remains valid. The main difference lies on gravity. That bead chain video pretends to represent an overhead cast. On a real cast, for the line to climb upward, it must oppose the force of gravity pulling it downward. The chain on the floor doesn’t have to fight gravity, so the effect of moving away from the rod tip path is more exaggerated. In summary, the dissimilarity between that experiment and real world casting is just one of degree: a fly line is subject to the same physics principles and behaves in the same way, just not in such an exaggerated one.

Real world

At first sight you could think that my example isn’t a good one, that the extreme angle between rod tip and line isn’t found in real world casting. Well, guess what? In fact it is a pretty common occurrence; sometimes made on purpose:

• Oval cast

---

You can see that by making my oval backcast low and parallel to the ground and then raising the rod tip high before making my forward cast stroke almost parallel to the ground, I break the 180 degree rule because my line starts angled sharply upwards and a very wide safe loop results. 

The first nine meters of a #5 line weigh around nine grams; now think of the effect of a very dense four gram tungsten beaded nymph instead of the very light piece of yarn used on that video above: it would fall faster during the overhead forward stroke, and it would climb up higher during the oval cast forward stroke. Oval casts keep rods and human heads safe due, precisely, to line and rod tip following different paths. 

• Hooked line

Forward stroke perfectly straight but a hooked layout? A momentum tug of war is at play:

---

• Line dangle

Dog-legs speak of some momentum challenge as well:

---

Line dangle in depth:

---

That the line doesn’t necessarily follows the rod tip path is crucial to analyse spey problems:

---

Corollary

The fly line follows the rod tip path perfectly… only if it is perfectly straight and pointing exactly in the trajectory that the rod tip is going to describe.

Are these things relevant? For a casting geek as myself just discovering how casting works is a pleasure in itself. From a more practical standpoint, I think that this knowledge gives a casting instructor a broader perspective to solve a more complete array of problems.

Nothing new under the sun!

I re-read the fly casting books in my library with some regularity. Strangely enough, when I was about to finish this article I came upon an interesting insight by Lee Wulff on his Trout on a Fly book (bold is mine):

Many fly casters in making their back cast stroke move the rod butt in a convex arc. If they continue this arc beyond the point where the line leaves the water on the pick-up they will be throwing their back cast “down” behind them… they are surprised and disappointed when the following forward cast drives up into the air instead of out toward the target because it wants to go 180 degrees from where the back cast started.

That book was published in 1986 and I bought it 15 years ago. It took me a long time to understand what Lee already guessed decades ago. There is nothing new under the sun.