non-flexible vs. flexible fly rod

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non-flexible vs. flexible fly rod

Post Number:#1  Postby Bernd Ziesche » Sun Jun 22, 2014 1:39 pm

Hi everyone,
let's assume I have two fly rods:

a) none flexible

and

b) flexible (stiffness like an avg. fly rod has it)

To keep it simple: Both rods are massless.

Which one would be more efficient* in order to create line speed?

*efficient in terms of energy input by the caster vs. energy output to the fly line at the rod tip.

Personally I think the none flexible one to be significant more efficient. Am I wrong?
Thanks a lot,
Bernd
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none flexible vs. flexible fly rod

Post Number:#2  Postby Graeme H » Sun Jun 22, 2014 2:05 pm

Good question Bernd. I'll be watching this one closely because I've often wondered about the same thing.

My own thoughts (probably wrong) are that you will get a higher line speed* with the inflexible rod, but it will be much more difficult to control the tip produce a straight line path. That means it would be harder to get a good cast out of it.

So, it would be "more efficient" in the terms you've specified, but less efficient for producing a cast with a tight loop.

Cheers,
Graeme

* Speed, but not straight line velocity.
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none flexible vs. flexible fly rod

Post Number:#3  Postby Merlin » Mon Jun 23, 2014 4:57 am

Hi Bernd,

Let's try to make you guess the answer. Given the assumption you have taken (massless rods), how much energy is there left in the rod as the line is launched in both cases?

You can deduct it from the input and then have an idea of the efficiency of the cast.

Merlin
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none flexible vs. flexible fly rod

Post Number:#4  Postby Bernd Ziesche » Mon Jun 23, 2014 6:26 am

Hi Merlin,
I think for a) the none flexible rod there wouldn't be any energy left and most energy I added to the butt section of the rod was transfered into the line. Instead for b) the flexible one there would happen counterflex 1 + rebound 1 + counterflex 2 and so on. Thus a higher percentage of the energy (than it was for a) ) I added would not go into the line.
Seemes like a) would be more efficient though.
Makes sense?
Thanks,
Bernd
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none flexible vs. flexible fly rod

Post Number:#5  Postby TrevH » Mon Jun 23, 2014 8:20 am

Borger and Lovell came up with some results and I believe there was no difference in mass between the actual rod and the hypothetical rigid rod.

Clearly the mass of the rods were a factor in those results, but if they were massless, would that work to the favour of the rigid rod or the flexible rod? Or maybe it wouldn't make any difference?

Could you consider the rod and line as a system, in which case there would be mass by necessity?
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none flexible vs. flexible fly rod

Post Number:#6  Postby LaMouche » Mon Jun 23, 2014 8:51 am

my .02 would be that a massless rod will not dissipate energy when (counter)flexing. but the waves in the rod leg will probably hinder loop propagation
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Post Number:#7  Postby TrevH » Mon Jun 23, 2014 9:07 am

As an edit to my previous post, to say there is no difference in mass in the rods in the Borger/Lovell results is to misrepresent them, but my question is the same.

I don't see why the results, in terms of whether the flexible or rigid rods wins out, should be any different.

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Post Number:#8  Postby VGB » Mon Jun 23, 2014 11:23 am

Bright but shite

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none flexible vs. flexible fly rod

Post Number:#9  Postby Walter » Mon Jun 23, 2014 6:19 pm

Bernd,

This is a very good question.

Here are some tradeoffs that will occur during the casting stroke.

Assuming the same casting stroke for both rods that consists of simple rotation around some point such as the elbow the tip of the broomstick is going to travel upwards and then downwards during the casting stroke (call this movement in the Y direction for brevity). This is going to result in larger loops for the broomstick and some loss of input vs output energy since movement in the +Y direction is somewhat cancelled out by movement in the -Y direction. For short casts this loss of energy is negligible because you use a limited casting arc and that results in minimal Y direction movement. You need a bit of overall movement in the +Y direction in your casting stroke to counteract gravity anyway.

As Merlin points out there are losses associated with loading and unloading the rod. These losses would be zero in a massless rod but you still have mass in the line that will resist rod unloading and result in lost energy and you have air drag. The big plus with the flexible rod is that it allows you to create a near straight line path of the rod tip and, if we continue to assume a casting stroke of rotation about the elbow only, the flexible rod will be more efficient for longer casting distances where movement of the broomstick path of the rod tip has a lot of +Y and -Y movement which means lost efficiency based on your definition of efficiency.

You could try changing your casting stroke with the broomstick to create a straighter path of the rod tip. I think with a 9 foot rod and a 3 foot arm (shoulder to hand) and trying to imagine how the shoulder, elbow and wrist would work together a casting arc of somewhere between 100 and 120 degrees could be done with slp or close to it. In that case the broomstick would be the winner. For casting arcs greater than that the flexible rod would have the advantage with the caveat that we expect some losses as the rod loads and unloads.
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none flexible vs. flexible fly rod

Post Number:#10  Postby Merlin » Mon Jun 23, 2014 6:45 pm

Thanks to all

You have a logic approach and good comments but the original assumption is irrealistic as Vince pointed out ironically. If we imagine we can apply Newton's laws to a massless object, we would end up in saying that both "rods" would be 100% efficient if I take away the issued linked to the real path of the tip and the real timing for the launch (e.g. assuming this is exactly RSP for the flexible rod).

Before entering more exhaustive considerations as illustrated by Walter, we have to come back to more realistic assumptions. In fact Bernd, when you said "to make it simple", you just made it awfully complex. Forget that and come back to our center of interest. I suggest we imagine it is possible to make a broomstick of equivalent mass to the flexible rod, although there is no material to make it happen. If you agree, then we can progress step by step and see how the picture goes on. Beware that the broomstick is not real at all, it is just a concept to see if flexibility brings an advantage or not.

I need another set of assumptions. Do you agree that the timing and casting arc must be the same or that the energy input must be the same which will lead to different casting conditions? It is up to you to decide. I would personaly recommend the first one, since we are looking for efficiency first (energy in the line divided by input energy).

Many thanks in advance for your comments.

Merlin
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