Posts Tagged ‘energy conservation law’

the meaning of the translatory motion in fly casting

Freitag, März 16th, 2018

Many articles about fly casting tend to focus on the rotary motion of the fly cast, since the rotation generates the velocity of the tip most. The meaning of translatory motion is often neglected for that reason. Discussions about the translatory motion are often focused on the way how the translatory motion should overlap the rotary motion: rather uniform over the casting stroke or more at the end of the casting stroke.

With the exception of shorter accuracy cast I almost always recommend a delayed dominated rotary motion “at the very end” of the casting stroke, which leads to a dominated translatory motion at the very beginning of the casting stroke.

This translatory motion, especially as it dominates the very beginning of the casting stroke, basically causes the following:

1.)   Possible slack fly line is removed and the whole fly line pulls on the tip of the fly rod, at lastest when the rotary motion prevails.

2.)   The path of the tip is elongated.

3.)   The mass of the fly rod is set in motion, so the fly rod is not situated in a defined condition.

The translatory motion precedes, “introduces” respectively the rotary motion so to say, and the better this introduction is the better the benefit for the fly cast could be. Both findings are very important to complete the casting stroke successfully with the following dominated rotary motion.

Removing slack line is always a very good idea as well as to elongate the path of the tip for longer fly casts, but there is a further aspect. Since the translatory motion is setting the mass of the fly rod in motion, the dynamical energy transfer property of the fly rod can be improved (redistribution effect). A dominated translatory motion at the beginning of the fly cast enables a deeper deflection of the fly rod, whereby the lower mass elements of the fly rod are able to gain a higher angular velocity than the upper mass elements during the earlier fly cast. This is the moment the energy transfer along the fly rod shaft begins, which leads to an efficient fly cast[1]. This relation I worked out in my “Experimental investigations on the fly rod deflection” as well as in some videos – especially these two: http://vimeo.com/226547073http://vimeo.com/187846392

So an optimal interrelation between the translatory and the rotary motion is the key of a good fly cast. Without the translatory motion the rotary motion will miss a very important partner, who provides required properties to perform an efficient, power minimized fly cast.

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[1] In contrast to a rigid fly rod the lower mass elements of a flexible fly rod have a higher angular velocity ω than the upper ones during the early phase of the cast, rotation respectively. This leads to a „butt dominated distribution“ of angular momentum L (L = J * ω; J = moment of inertia). Since this angular momentum distributed along the lower mass elements just can’t disappear due to the energy conservation theorem, it must be transferred towards the upper mass elements during the later phase of the fly cast (energy transfer / redistribution).

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angular velocities of the mass elements on the fly rod shaft

Mittwoch, Juni 28th, 2017

Playing with the GIMP tool (GNU Image Manipulation Program) I superimposed some pictures taken out of a casting sequence of me. I liked the result since it visualizes how the angular velocities of the mass elements on the fly rod shaft vary over the entire casting stroke.

The angular velocity ω is determined by the angle φ divided by time t (ω = φ / t). Due to the deflection of the fly rod during the earlier phase of rotation the lower mass elements are covering a larger angle than the upper ones (shown by the violet lettering, see picture 1). According to the relationship shown above they have the highest angular velocity ω.


During the later phase of rotation it is the other way round. Now the upper mass elements are covering a larger angle (shown by the red lettering, see picture 2), for which reason they have got the highest angular velocity ω.


So what can be detected is a shift of the highest angular velocity from the lower mass elements towards the upper ones over the duration of the fly cast (see picture 3 – visualized by the black arrow), which correspond to the varying contribution of the angular velocities.

The angular velocity (ω) multiplied by the moment of inertia (I) leads to the angular momentum (L). L = I * ω. Taking the modification of the moment of inertia caused by the deflection into account, this relationship points to a contribution of angular momentum, which shifts towards the upper mass elements like the angular velocities.

Due to the energy conservation law the energy can’t just disappear, thus the energy of the lower mass elements must contribute the accelerate the upper ones, resulting in a higher angular velocity, angular momentum respectively during the later phase of rotation (energy transfer).

The towards the tip of the fly rod shifting contribution of angular momentum equals the shift of the center of the rotating mass shown in my “Experimental investigations on the fly rod deflection” (rev. 2.0, November 2014 – section F1) and indicates, that some kinetic energy could climb up along the fly rod shaft towards the tip. This behavior benefits an efficient fly cast (ratio of the output and input energy).
It is obvious that the energy transfer from the grip towards the tip of the fly rod depends on the way the fly rod is deflected. The varying contribution of the angular velocities of the mass elements is a good indicator for that.
The pictures above are taken out of a video, which I produced in order to explain what I wrote before:


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To me the following picture is a good summarizing illustration about the ‘energy flow’, transfer of angular momentum respectively taking place along the fly rod shaft.