Bucket Science Report - Horizontal Snap Casts

[Geenomad]

Found a concrete apron outside a local school. Not as smooth as I would have preferred but good enough for bucket science.

Laid out the tape and line as previously described but for convenience here it is again. The tape is laid out on a level, low friction surface and the fly line is positioned over it. Line end at Zero. Loop front (LF) at 10m. Side casts are made back toward the Zero end of the tape and then both the distance travelled by the line end (LE) and the distance travelled by the loop front (LF) are measured. These casts are made with a guesstimated force of a cast using 10m of line with complete turnover.

The original design called for a series of casts all made with the same 10m of fly line but with progressively longer rod legs and shorter fly legs. Thus in the first series the loop front was positioned at 10m, the second series at 5m and the final series at 2.5m.

Having achieved LE movement in a couple of dry runs there seemed no point in repositioning the loop front to a shorter distance from Zero – ie 5m and 2.5m. I also decided it wasn’t worth removing my leader which was loosely coiled behind the Zero tape end for each cast.

Before testing I made about 10 casts with complete turnover of the 10m of flyline to get a feel for the force needed in the casting stroke. That’s as close I could get to standardising the force applied.

During testing I used a small loop c.100mm wide and as short a rod leg as I could manage – again about 100mm long.

With apologies for the crappy layout because I couldn't work out how to image my spreadsheet, the results were recorded as follows:

Cast# : LE Position: LF Position: LFM(ovement): LFM/LE
1: 1.95: 2.04: 7.96: 4.08
2: 1.74: 5.4: 4.6: 2.64
3: 3.88: 5.81: 4.19: 1.08
4: 1.5: 4.16: 5.84: 3.89
5. 0.5: 5.4: 4.6: 9.2

My reflections on the results are:
1.The data suggest that the force I applied varied significantly.

2.That said, the results for LF and Loop Front Movement (LFM) aren’t too shabby. Two LFMs were identical at 4.6m, another varied by about 0.4m and another by 1.2m

3.Contrastingly, the LE movements varied considerably. Ignoring Cast 1, which showed unusual LF and LFM distances, the other 4 casts gave LE movement of between 0.5m and 3.88m.

4.The two LE movements that were the most similar (#2 1.74m & #4 1.5m) were produced by somewhat different LFM movements namely, 4.6m and 5.84m respectively. However note that in these two casts greater LFM did not result in greater LE movement. In fact greater LFM was associated with lesser LE.

5.Some of the lumps in the data set might even out with a much greater number of casts.

6.Alternatively, the relationship between LE and LFM is somewhat vague, irredeemably so.

7. Clearly there is some tension in the loop which results in the rod leg moving the fly leg in an essentially opposite direction. ie The rod leg accelerates the fly leg via the loop.

8.It is possible that surface friction varied a small amount between casts – but I doubt by enough to fully explain the variation in LE. Compare the smaller variation in LFM as a group and in individual casts.

9.It is even more unlikely that any variation in the inertia of the fly leg combined with friction can explain the variation in LE movements.

10.These casts are an extreme example of pullback, far greater than would occur in standard overhead and roll casts with pullback executed after loop formation.

11.We can conclude that snap casts work and that tension between legs can result in a fly leg being accelerated by a rod leg.

12.However, the inconsistency in the results points to a somewhat vague relationship. Some acceleration occurs but it does not seem to occur predictably.

Comment
What would I take away from the experiment for standard overhead and roll casts? The short answer is keep your rod legs tight and your loops narrow.

The longer answer is not much and here is why. There is a fundamental difference between snap casts and overhead casts. They neither look alike nor dynamically function alike. In overhead casts the fly leg is the repository of the kinentic energy with which the cast is completed (post loop formation) and the line is turned over. It is the engine once the casting stroke is finished. In Newtonian terms the fly leg is the action and the rod leg is the reaction. The rod leg pulls back as a reaction to the fly leg pushing forward and being diverted into the rod leg but only to the extent of the force required to do that. The rod leg is tethered to the rod tip at one end and connected via the loop to the fly leg.

In a snap cast the rod leg becomes the engine and the repository of kinetic energy. The fly leg pulls back. The fly leg is not tethered at one end like the rod leg in an overhead cast. Once the inertia of the fly leg is overcome it begins to move. Tension in the loop enables the rod leg to power the fly leg movement forward as the rod leg continues its drive in the opposite direction.This relationship between the legs is far from fixed and predictable - at least as shown by my results. In an overhead cast the equivalent outcome is tension in the rod leg and not movement of it.

To my simple mind it makes no more sense to speak of the rod leg in an overhead cast somehow powering the fly leg than it would to speak of the fly leg in a snap cast somehow powering the rod leg. I simply can't see where any or any significant KE would come from to do otherwise. I'm trying to keep an open mind but until some significance for casting is demonstrated I'm left with the conclusion that tension in the loop might be interesting but is not in itself something I can manipulate to my advantage in presenting a fly to a fish.

Cheers
Mark

[Dirk le Roux]

Hi Mark

Thank you for conducting this experiment and sharing the interesting outcomes from it. You have successfully done Wong and Yasui's Einstein Equivalence Principle Experiment with a length of flyline.

I assume you did the experiment with a tapered line. Would it be possible to repeat it with a level line, if you have one?

A few remarks on your comments:

I agree with the idea of "engine switch" between horizontal casts and snap casts and which leg's side the "engine", for the most part, sits on. Regarding the force exerted by the rod tip (tethered case) in opposition to the fly leg direction being only to the extent required to keep the fly leg becoming stationary rod leg - I agree with the description but it may be hard for us to form a concept of that "only"'s magnitude if we don't know what it relates to. A simplified way of stating what it relates to is to consider the difference in leg velocities. The velocity difference's contribution is exponential in a way but it is safe to say the higher that leg velocity difference, the higher that force. This can be related to by the difference in intensity of shoot (once the rod tip restraining force is cancelled) between casts with low leg velocity difference (weak shoot) and high velocity difference (strong shoot).

You described the snap cast dynamics quite well. Though I would say the relationship between the legs' dynamics is predictable. This is where Vince's jerk comes into play. If you had a machine do the five casts, applying the same amount and rate of acceleration every time, the results should be quite consistent. I suspect your different results have a lot to do with rate of your acceleration (jerk) not being consistent. In my own similar experiments I found that jerky actions result in quite unruly dynamics.

The "powering" relationship is not always what it seems and the linked Einstein equivalence principle article describes that well. To summarise, the effect on fly leg's acceleration in your experiment (where the rod leg moves in the opposite direction) is equivalent to the effect on the fly leg's acceleration if the U-setup was to travel through the air in the normal horizontal cast direction and the rod leg was then restrained. From the fly leg's point of view the rod leg starts moving in the opposite direction and from the rod leg's point of view the fly leg starts moving in the opposite direction.

I agree with the limited significance on how we are able to cast, just as well as we are generally able to conduct our lives quite well not knowing that the earth is round. If we desire to understand how this all works, however, it is quite useful to understand the interaction of the legs' velocity difference, or alternatively stated as loop rotation rate/angular velocity, with tension and the impact of that on the dynamics of a cast. Personally it helped me better understand controlling slack, what helps to get full turnover on an all-out let-go shoot, why I so easily lose control over the cast with long carries on my weight forward lines, why my shooting head casts routinely end up on the water with the fly behind the head, why I can see the fly end accelerating especially towards the end of a cast's roll-out and why the fly sometimes kicks over strongly at the end of the cast (and how to control this to my advantage).

Regards,
Dirk

[Geenomad]

Hi Dirk
Thanks for your thoughtful consideration and comment on the experiment and my comments. I should say at the outset that as a general rule I support pure science work, rather more than is presently fashionable. However, when it comes to casting I am an applied sort of guy.

I assume you did the experiment with a tapered line. Would it be possible to repeat it with a level line, if you have one?

Closest thing I have, sadly, is a double taper or two and some running line which would be a bitch to wrangle. The experiment is, of course, available to anyone with the time and interest to conduct it. A collection of data from repeats might even out some of my bumps.

I agree with the idea of "engine switch" between horizontal casts and snap casts and which leg's side the "engine", for the most part, sits on. Regarding the force exerted by the rod tip (tethered case) in opposition to the fly leg direction being only to the extent required to keep the fly leg becoming stationary rod leg - I agree with the description but it may be hard for us to form a concept of that "only"'s magnitude if we don't know what it relates to. A simplified way of stating what it relates to is to consider the difference in leg velocities. The velocity difference's contribution is exponential in a way but it is safe to say the higher that leg velocity difference, the higher that force. This can be related to by the difference in intensity of shoot (once the rod tip restraining force is cancelled) between casts with low leg velocity difference (weak shoot) and high velocity difference (strong shoot).

I understand your points about velocity difference but I find the relationship more useful in understanding loop propagation and travel than in force derivation and application. Different frame of reference, perhaps.

You described the snap cast dynamics quite well. Though I would say the relationship between the legs' dynamics is predictable. This is where Vince's jerk comes into play. If you had a machine do the five casts, applying the same amount and rate of acceleration every time, the results should be quite consistent. I suspect your different results have a lot to do with rate of your acceleration (jerk) not being consistent. In my own similar experiments I found that jerky actions result in quite unruly dynamics.

You may well be right. In the case of rolls and overhead casting one might theorise that pullback or hauling post loop formation could produce some jerkiness but probably not to the same extent as commencement of a horizontal snap with both legs static.

From the fly leg's point of view the rod leg starts moving in the opposite direction and from the rod leg's point of view the fly leg starts moving in the opposite direction.

Perhaps but for me it leaves unanswered the question of kinetic energy, which leg is the provider/engine and what that means in practice.

I agree with the limited significance on how we are able to cast, just as well as we are generally able to conduct our lives quite well not knowing that the earth is round. If we desire to understand how this all works, however, it is quite useful to understand the interaction of the legs' velocity difference, or alternatively stated as loop rotation rate/angular velocity, with tension and the impact of that on the dynamics of a cast. Personally it helped me better understand controlling slack, what helps to get full turnover on an all-out let-go shoot, why I so easily lose control over the cast with long carries on my weight forward lines, why my shooting head casts routinely end up on the water with the fly behind the head, why I can see the fly end accelerating especially towards the end of a cast's roll-out and why the fly sometimes kicks over strongly at the end of the cast (and how to control this to my advantage).

I understand what you are saying but my interest is concentrated on how I can cast better . That is my KPI for significance. Which is to say that Earth being round is very significant if my daily life included long range navigation or even if I just wanted to comprehend the nature, origin and significance of my home planet. Fly casting might be compared with the former as a common pastime - albeit a little less critical to the preservation of life.

The greatest insight provided to me by physics and the most significant for improving my casting is the big picture that it paints. That's the one named "Efficiency".

Cheers
Mark

[Dirk le Roux]

Hi Mark

All good

I don't have a level line either, which is why I asked.

I understand your points about velocity difference but I find the relationship more useful in understanding loop propagation and travel than in force derivation and application. Different frame of reference, perhaps.

Only slightly different frame of reference, I suspect. Mine is that the interrelatedness provides me a feel for what goes on and applying this understanding to what I can manipulate, and what I can't, to cast better. That inerrelatedness is simply: Half the difference in leg velocities = loop propagation speed; Square of loop propagation speed x linear mass = tension at the loop.

If the second essential is to eliminate slack, I can focus on manipulating/managing the difference in leg velocities/loop rotation to better maintain tension.

If I want to make a better tuck cast, I can focus on manipulating tension by arrest/pullback at the right times to better control loop propagation at the end of the cast, and consequently kick-over.

If I am frustrated by slack and lack of control limiting my weight forward carries, I can understand that maybe it is because I can't do much about mass distribution (loop propagation and tension relate to linear mass too) of the line I have and rather should get a distance/double taper line if getting longer carries is really that important to me.

Of course I can ignore this understanding or not have it at all and "perfectly well" learn these things from experience and practice, but for me the understanding enhanced my big picture and extended horizons in useful application. This is what I aimed to convey with the round Earth reference.

Perhaps but for me it leaves unanswered the question of kinetic energy, which leg is the provider/engine and what that means in practice.

Out on a limb, may receive heat for this and I know it's simplistically and roughly, but what the :

With the snap cast after stroke input, rod leg kinetic energy is transferred to fly leg kinetic energy. The rate of transfer relates inter alia to loop propagation speed. Towards the end of the cast, as can be read from plots of the data from Graeme's snap, the rod leg remains with almost only potential energy of its remaining weight suspended in the air, while the fly leg (mostly leader) at the time exhibits kinetic energy in its final climb-out.

With a normal overhead cast after stroke input, fly leg kinetic energy is transferred to rod leg potential energy (in case of tether, which manifests in tension exerted on/by the rod tip) or to rod leg kinetic energy (in case of shoot). The rate of transfer relates inter alia to loop propagation speed.

The greatest insight provided to me by physics and the most significant for improving my casting is the big picture that it paints. That's the one named "Efficiency".

YEP!

Regards,
Dirk

[Geenomad]

With a normal overhead cast after stroke input, fly leg kinetic energy is transferred to rod leg potential energy (in case of tether, which manifests in tension exerted on/by the rod tip) or to rod leg kinetic energy (in case of shoot). The rate of transfer relates inter alia to loop propagation speed.

Et voila. Not much energy left to turn the loop into the engine that powers the fly leg eh?

Energy transfer is fly leg to rod leg.

As an applied science sort of guy, that's game over for me.

Cheers
Mark

[Dirk le Roux]

Hi Mark

Who said the loop is the engine that powers the fly leg, eh?

Regards,
Dirk

[gordonjudd]

Who said the loop is the engine that powers the fly leg, eh?

Dirk,

The fact that in a tethered cast the tension that comes from the rho_l*v_loop.^2 momentum change of the line going around the loop provides an acceleration force on the fly leg that helps to overcome the drag forces on that leg is why the loop travels as far as it does.

I would say the "engine" was the caster since his work input is the source of the initial KE in the fly leg, but it is the kinetics of the loop propagation that then makes the best use of that energy and provides an offsetting acceleration force to the negative drag forces on the fly leg is the key to why the loop propagates as far as it does.

Try using a rubber band to launch 60 feet of straight line where the line detaches from the rubber band at the launch so no loop is formed and see how far the fly end of that line will go. I doubt it would even make it to the launch point.

Gordy

[VGB]

an acceleration force on the fly leg that helps to overcome the drag forces on that leg is why the loop travels as far as it does.

Hi Gordy

What do you believe this force does that helps overcomes drag?

Regards

Vince

[Geenomad]

Who said the loop is the engine that powers the fly leg, eh?

Hi Dirk
Not sure if your question still stands but the short answer is that my typology of [loop as consequence v.] loop as engine summarises endless pages of debate in this section. Any further engagement in the contest would be pointless.

In the other snap cast thread we didn't quite get to the final phase of the cast in following the KE trail but I think I know how it ends.

Cheers
Mark

[Merlin]

Hum, not sure to agree about KE and PE.

The engine is the leg which is directly impacted by the input of the caster. The loop is the gearbox (with the clutch), and the software for the powertrain is designed by sir Isaac Newton.

Merlin