Rod deflection and acceleration

[Merlin]

Hi Lasse

I do not design virtual experiments to fit a particular pre concieved result, results are what they are. Incidentally I just selected two model rods with the same casting conditions, and guess what, the glass rod beats the graphite one in terms of launch speed (and counterflex of course). The main reason lies in the inertial effect of the glass rod which is pretty heavy by comparison to the graphite one. The glass rod is slow of course, even loaded it is slower than the graphite one, but the inertial effect makes the difference. So I can get different tests and different results, and it is likely that I could tune with patience one of the rods to make launch speeds match (just making the glass rod lighter would make it).

The message from your video could be "any rod can cast the same", but as I already told you, I guess that for long distance, you would not chose the glass one, and there are good reasons why. For fishing cast such a general statement could be acceptable within some casting range, differences being difficult to see. However, I think most casters can detect differences in between rods, even if they are small. I doubt you would find that my noodle (an old Conolon glass rod) can cast as well by comparison to a low cost graphite rod.

The comparison I made for Lou was using HT rod models (#6, #8 and #10). This answers Graeme's question I think. I have some data for the six which allowed me to derive 8 and 10 rod designs with my files. I used a long cast with a 24 grams mass of line, the model cast being derived from one of Paul's 170 cast. There is a haul incorporated, the same for the three cases of course. Thats looks like your proposal, but the difficulty will be to control that the casts are the same (arc, speed, translation & rotation), which the model can do of course. Anyway that would be fine, I can adapt the cast to actual records but that needs a minimum of data about the cast itself. I am not going to re model the HT rods (a pain), but maybe we can have some checks made (Paul has my test methodology) to see if the existing data are palatable. That would be very interesting anyway.

In the virtual experiment for Lou there is a correlation between stiffness and launch speed (that was the subject of another thread where I focused on input torque), but that does not mean that the rollover would give a different distance. I cannot model that part (line flight) correctly. By the way the speed differences I found are not huge (6%).

Merlin

[Graeme H]

Incidentally I just selected two model rods with the same casting conditions, and guess what, the glass rod beats the graphite one in terms of launch speed (and counterflex of course). The main reason lies in the inertial effect of the glass rod which is pretty heavy by comparison to the graphite one. The glass rod is slow of course, even loaded it is slower than the graphite one, but the inertial effect makes the difference.

This implies that lashing half a dozen ball sinkers to a HT will increase line speed. Is that the outcome predicted by your model Merlin?

Cheers,
Graeme

[Lou Bruno]

" Incidentally I just selected two model rods with the same casting conditions, and guess what, the glass rod beats the graphite one in terms of launch speed (and counterflex of course). The main reason lies in the inertial effect of the glass rod which is pretty heavy by comparison to the graphite one. The glass rod is slow of course, even loaded it is slower than the graphite one, but the inertial effect makes the difference."

Merlin,

Confused here! Not sure what "inertial effect" exactly is. The glass rod had greater launch speed, but, "The glass rod is slow of course, even loaded it is slower than the graphite one."

How can the glass rod exhibit greater launch speed and be slow?

Lou

[Merlin]

Hi Lou

Whislt I am writing an answer for Graeme, have a look at "the physics of the overhead fly cast" pdf file which is in SL database.

The inertial effect is typical of cane rods.

Merlin

[Graeme H]

Hi Merlin,

It shouldn't take too long to write an answer to my question. It's either "yes" or "no".

Cheers,
Graeme

[Merlin]

This implies that lashing half a dozen ball sinkers to a HT will increase line speed. Is that the outcome predicted by your model Merlin?

Hi Graeme

No, and I see that you do not know (yet) the technical subtleties of the physics for a fly rod. Let’s have a look. The main parameters governing line launch speed for a fly rod are its length, its loaded frequency, and its inertial effect. Considering rods of given length, we can focus on the two others. The loaded frequency of a fly rod depends on its stiffness (in other words and staying practical, on its line number), and on the equivalent mass at tip (a spring characteristic). The inertial effect depends on another spring characteristic which is also an equivalent mass (I call it intermediate, mid, transfer sometimes). Finally the self deceleration mechanism depends on a third equivalent mass at butt, and the sum of the three equivalent masses corresponds to the MOI of the rod.

Of course the caster’s input has its say so we assume that it is a given for sake of simplicity, and does not change with the rod.

Each equivalent mass is related to the mass distribution of material along the rod shaft. If I change the material, then I am globally increasing more or less all equivalent masses. In terms of speed, increasing the equivalent mass at tip slows down the rod, and that generally drives the line speed down (it depends on the actual input). Increasing the intermediate equivalent masses increases the inertial effect and raises the line speed as the rod butt decelerates. The effects are antagonistic, and the final result depends on rod design.

If I add mass at the tip top level (or the mass of the carry), then I slow down the rod but that leaves that inertial effect unchanged, because a mass located at the tip top has no contribution to the intermediate equivalent mass. So the rod slows down and if I do not compensate by increasing rotation speed for the input, the cast is going to fail short.

The best place for increasing the inertial effect is the top ferrule of a four piece rod. Unfortunately, this affects the equivalent mass at tip so again we get antagonistic effects on line speed. It is not easy to predict the final result.

In my virtual cast with a graphite rod (a G2 type of rod) and a glass rod (an extrapolated Ritz Garcia model) of same length and stiffness, I did not expect the glass rod to win the contest. Since I can identify the major responsible parameter (inertial effect) of the rod which is on the extreme side of butt action, then there is a good chance that a tip action glass rod would not be in such a favorable position. The glass rod used by Lasse is of this kind (Berkley tip action rod) so it does not surprise me that the comparison show similar line speed in his case.

In practice, speed differences are limited to less than 10% (maybe 5% on average) for such comparisons, so it is
pretty hard to detect by sight. Making a demo to confirm that rod type is of little influence is thus relatively easy. Models are here to show us what is hidden behind that.

More in the next post about an experiment performed some 20 years ago.

Cheers

Merlin

[Merlin]

In 1999, Mike Hendry wrote his thesis “Measurements and simulation of fly rod dynamics” (UC Davis), comparing casting models to video records of casts performed by two people with 4 rods (9’#5, Sage RPL, RPL+, SP, SP+). The aim was to study the effect of rod design on performance with a model (based on Abaqus, state of the art software at that time) and check findings through experiments. Isn’t it what we are looking for?

Let’s see some sentences extracted from the discussion chapter:

“It is interesting and somewhat surprising that no strong conclusion regarding line speed differences between rods can be drawn from either caster”.

“Based on stipulations that the most desirable cast are those with the smallest loop height and highest line speed, there is no one rod that performs better than the others for both casters with all line lengths”.

“The fact that both casters alter their casting stroke with the different rods is suggestive of an attempt to modify rod inputs to produce a tip path and line speed that is desirable for that particular caster”.

Well that should make us humble and think twice before stating a kind of universal truth. A model can simulate a given cast and change one parameter at a time but this is not what casters do in fact, they adapt their stroke and I do not know if this is conscious or not. That brings us back to biomechanics and sensory motor learning.

When we change rod for a given line, it appears that we adapt our cast to the new rod, at least because we can capture a variation in MOI, speed and action and anything else I guess. Thus there is little chance to get nice comparisons between models and practice since the caster adapts his stroke to the rod. It is possible to retune the model using all important data describing the actual cast input, but this is a hell of a job.

Mike measured a change in line speed ranging from 1% to 8.5% in between rods, with an average around 3.5%. That is pretty small and hardly detectable in practice without a measurement system. Models are just here to indicate the basic trends “everything else being equal”, but that does not match the biomechanics of the caster.

Merlin

Thanks to Gordy for sending a copy of the thesis to me some time ago (in dust we trust).

[Graeme H]

So the answer was no?

[Graeme H]

Of course the caster’s input has its say so we assume that it is a given for sake of simplicity, and does not change with the rod.

And after your last post, it seems like an assumption we should not make.

By which I mean that the single "constant" in this discussion is the skill of the caster. If he or she is skilled at adjusting to the rod and can pull every last bit of line speed from the rod, can he/she get more line speed out of a stiff light rod or a soft heavy rod?

The Casting competitions suggest a light stiff rod is the best option for maximum line speed. Regardless of the mathematical models.

Cheers,
Graeme

[Merlin]

Agree somehow with your last point Graeme, but this is not what Lasse’s video suggests. Any rod could make it. But of course this is not completely true, competition casts represent a specific domain.

Models help to understand, like it or not.

Merlin