Podcast Show Notes: The Mechanics of Fly Casting
Introduction
Fly casting is a unique form of casting in which a relatively massive line is used to cast a very light fly.
Unlike other throwing objects, such as a baseball or golf ball, the fly accelerates during the throw.
This podcast will examine the physics of fly casting, including how the line and its design affect the motion of the fly.
Basic Principles
In a fly casting, the fly is attached to a line that allows the fly to be cast and retrieved.
Air resistance is a critical factor affecting the speed of the fly.
To cast a light fly over a longer distance, a heavier line is needed to overcome air resistance and give the fly momentum.
The analysis focuses on the motion of the line from the moment the line is stretched backward after a back cast.
The Model for a Fly Cast
The most common type of cast is an overhead cast, which is analyzed in this model.
The cast starts with a line in an arc shape, where there is a moving and a stationary part of the line.
The moving line forms a loop that rolls down the line and transfers energy to the fly.
The loop moves like a wave down the line until the line is stretched and the cast is completed.
The Work-energy method is used to calculate the speed of the moving line and the fly.
The line loses energy to air resistance during the cast.
The line is modeled with several segments to calculate the air resistance: the loop, the moving line and the fly.
The loop is modeled as a cylinder in cross-flow.
The moving line is modeled as a long cylinder parallel to the flow.
The fly is modeled as a sphere.
The Effect of Air Resistance
Without air resistance, the fly’s speed would increase steadily during the cast because the mass of the moving line is reduced.
However, air resistance has a major impact where the speed does not increase steadily, and is instead characterized by acceleration and deceleration.
A level line tends to decelerate at the beginning of the cast and then accelerate towards the end.
A long taper line will accelerate at the beginning of the cast and then decelerate.
A double taper line, which is widely used, exhibits a more complex motion with both local acceleration and deceleration, creating a “kick” that helps turn the leader (pre-bait).
An experimental taper line has been developed to achieve a more constant speed at the end of the cast.
Line Design and Tapering
Different line designs (tapers) affect the fly’s speed history.
Level line: Uniform diameter along the entire length of the line.
Double taper line: Popular line with a compound taper at both ends.
Long taper line: The line gradually increases in diameter along its entire length.
Experimental taper line: Similar to long taper line but with a less steep taper.
Tapering the line affects the mass of the moving line and thus the acceleration of the fly.
Importance of the loop
The air resistance on the loop is the dominant factor in the loss of mechanical energy.
A smaller loop means that less energy is lost to air resistance.
It is therefore important to have good loop control to achieve an efficient cast.
Conclusion
The acceleration of the fly during a cast is a result of the reduced mass in the moving part of the line.
Air resistance significantly affects the flight speed, especially on the loop.
Choice of line type affects the speed profile of the fly.
The diameter of the loop has a great impact on how much energy is lost during the cast.
References
Spolek, Graig A. (1986). The Mechanics of Flycasting: The Flyline. American Journal of Physics, 54, 832-836.
These show notes provide an overview of the main points of the article and can be used as a starting point for a podcast on the mechanics of fly casting.
Source file:
https://typeset.io/pdf/the-mechanics-of-flycasting-the-flyline-3xgpbqo33h.pdf