At the beginning of the front cast, the rod tip starts farther from the target than the butt of the rod. By the end of the front cast, the rod tip finishes closer to the target than the butt. During the cast, the rod itself also starts farther away from the target, and then for most casters, it ends closer to it.
Essentially, a fly cast consists of two kinds of motion: rotational and translational.
Although a fly cast is typically a blend of translational and rotational motions, let’s examine them individually. To make the following discussion simple, let’s consider the rod as a stiff stick that doesn’t bend.
Translational Movement
Translational motion means that the entire fly rod moves uniformly towards the target. This occurs if we were to push or pull the rod, without rotating it. See Figure 1.
Figure 1: Translational motion
If we move the rod forward in this manner, for every one centimeter we push or pull the rod handle forward, the rod tip moves one centimeter closer to the target. So, if we smoothly accelerate the rod forward as quickly as we can, without rotating it, the rod tip velocity will be similar to our hand velocity. Basically, the ratio of rod tip velocity to our hand velocity is 1.
The point is that translational motion is not very effective for generating rod tip speed. We are not taking advantage of the fly rod as a lever, and we are only using what little flex in the rod that we can create to help us to generate line speed.
Rotational Movement
Rotational motion is different.
When the center of rotation is at the midpoint between our casting hand and the rod tip of our fly rod, when we move (or more specifically) rotate the handle by one centimeter, the rod tip moves one centimeter. Since the duration of the rod’s rotation – up until the abrupt stop – is the same for all parts of the rod, the ratio of movement between the rod tip and the handle is equivalent to a velocity ratio.
Figure 2. Diagram showing that as the center of rotation of the rod approaches the caster’s hand, the velocity ratio increases.
Consequently, when the center of rotation is at the midpoint between our casting hand and the rod tip of our fly rod, the velocity ratio is one. As the center of the rod’s rotation moves down along the rod and approaches our casting hand, the velocity ratio increases significantly. See Figure 2.
When the center of the rod’s rotation moves up along the axis of the rod towards our casting hand from below, the velocity ratio also increases, but it increases even faster than that from the opposite direction. See Figure 3.
Figure 3. Diagram showing the velocity ratio of various locations for the center of the rod’s rotation.
The reality is that the rod bends during a cast. But, the bending of the rod stores energy that will be released when the rod is abruptly stopped. The concept of the velocity ratio may not be absolutely correct for fly casting, but it provides a better understanding of the physics of casting.
The key point is that the rotation of the rod will be more efficient as the center of the rod’s rotation is closer to the casting hand. The center of rotation may not be along the rod, but its proximity to the caster’s hand is critical.
Summary
If we understand the inefficiency of a translational motion compared to the potential efficiency of a rotational motion, it makes sense that, initially, we want to focus on the rotational aspects of the fly cast – not the translational.
The efficiency of a rotational motion increases as we make the center of the rod’s rotation closer to our casting hand: the nearer the center of rotation is to the hand, the greater the tip velocity we will be able to generate.
Let’s be clear. Translational motion is important – especially for changing the direction of the cast (or inertia of the line) and for removing slack.
But, translational motion is inefficient for generating velocity because translation is essentially a rotation where the center is an infinite distance away from the caster’s hand.
When we come to the realization that fly casting is a rotation, certain perspectives in this blog will be easier to understand.
Epilogue #1: Grip
We need a grip that allows all the torque that we must generate during the cast to be transferred to the rod without losing any energy.
If we think of rotating the rod as if it were a lever, the caster’s hand will be near the fulcrum. To move the rod tip, as a result, requires a huge amount of force to be applied at the handle – a force many times greater than normally required to accelerate the combined mass of the upper parts of the rod and fly line.
If you haven’t read my post on the best grip, here’s a link.
Epilogue #2: Equipment
If the rod is too long for our strength, then we won’t be able to rotate the rod quickly. If we can’t generate velocity and rod flex, we forfeit the benefits of rotating the rod.
The industry has convinced us that everyone should start with a nine-foot rod. Should everyone start with a size nine shoe, too? The notion that one size fits all is illogical.
Children and casters of smaller stature need shorter rods. In fact, most of us should be using shorter rods.
What we need to determine is what length of rod (with the mass of the line and reel included) that we can rotate the most efficiently or with the most comfort. And, as we build strength and skill, we can graduate to a slightly longer rod – as long as we can continue to generate at least the same tip velocity.
Another example is the importance of reel weight. Many people consider the reel as only a line holder. While the reel must hold line, it is also a counterbalance that plays a complicated role in the rotation of the rod.
At the beginning of the front cast, the weight of the reel will help the rod rotate forward. We might think that a heavy reel, working with gravity, would be good. However, after the rod reaches a vertical position in a front cast, the reel starts travelling up, against gravity. So as not to steal speed from the rotation of the rod in the latter parts of the cast, the reel can’t be too heavy.
For this reason, it is important to balance the weight of the reel (with line peeled off to represent a typical casting distance) with the rod (and line in the guides).
We will discuss equipment further in a future post.
Epilogue #3: Casting Stroke
In my next post, we will discuss various aspects of the wrist, elbow and shoulder to compare each joint’s ability to rotate the fly rod.