Two heads cast at the same time

[James9118]

I'm not surprised you've been confused Will when you have two academic papers giving contradictory results as shown above. At least one of them must be a steaming pile of horse manure right? Was it Gatti-Bono that predicted my side casts should veer off to the right also - I can't remember.

I also agree, sterling work by Graeme and Lee here.

James.

[Torsten]

Thanks Lee, great video.

Results can depend very much on the initial conditions. I think many know, that a cast can be "underpowered" or "overpowered" (like Lee's) - so you need to compare these conditions carefully - both results could be plausible (acceleration / deacceleration of the line).

Gordy: Do you have actual measurements for the form drag / skin friction coefficients? These are useful for modelling.

[Merlin]

I agree with Torsten, comparisons are made out of the full context (assumptions). The information given by Josh and Bruce is fine as long as we neglect air drag, but this is the source of losses. And by the way I do not understand that sentence (Graeme in another thread).

Energy was dissipated in the taper because the mass of each dot was less than the one preceding it into the loop.

Where is air drag?

If one introduces air drag in the assumptions, then one can find that this phenomenon is related to the diameter of the line section. There are two limits in diameter which depends on various conditions (loop height, fly drag, etc.).

A first one (approximately 0.85 mm for a 2 feet loop and a dry fly) is the dimension corresponding to a rather constant fly leg velocity. Below this diameter the line decelerates, which explains why a #3 is not designed for distance and why the front taper can finally decelerate the line. A second one (approximately 0.45 mm in the same conditions, which may correspond to the butt of a trout leader) below which the deceleration becomes exponential.

This explains the final deceleration due to the leader, and also this remark from Bruce about the tip of the MED:

The MED is designed with a rather long front taper for smooth turnovers, but with an oversize tip to insure it does turnover at distance.

Hope this helps to understand the questioning about the final stage of rollover.

Merlin

[Graeme H]

G'day Lee,

Can I ask for another experiment please? Can you put both heads on the "right way", duplicate your leaders and tippets, but tie two very different sized "flies" on the ends please? I'd love to see the effect of the drag of the fly on the speed of the cast. Perhaps make one fly a little larger than you'd normally use for those lines and one a bit smaller. Maybe even a second set of casts with a fly that's well oversized for comparison.

Sorry to be a pest, but you have a great setup for doing this very test.

Cheers,
Graeme

[Lasse Karlsson]

G'day Lee,

Can I ask for another experiment please? Can you put both heads on the "right way", duplicate your leaders and tippets, but tie two very different sized "flies" on the ends please? I'd love to see the effect of the drag of the fly on the speed of the cast. Perhaps make one fly a little larger than you'd normally use for those lines and one a bit smaller. Maybe even a second set of casts with a fly that's well oversized for comparison.

Sorry to be a pest, but you have a great setup for doing this very test.

Cheers,
Graeme

One taper will deliver a pikefly good, the other will fail if we where to use lighter lines and less velocity (got it as a demo Kit) but would also love to see it with heavier lines

Cheers
Lasse

[Paul Arden]

Hi Merlin,

So understanding the effects of air resistance is a key here. How do you explain how the front taper of the fly line and leader slows the presentation of the fly? I think i understand it but not well enough to explain it! So probably not very well then

Many thanks,
Paul

[Merlin]

Ok Paul, let's take a simplified view for sake of understanding.

I keep the limit values I gave before. When the first limit diameter (0.85 mm) passes through the loop, somewhere on the front taper or at the tip level, then the rollover of the line starts decelerating. The deceleration increases up to the point when the second diameter (0.45 mm), likely located in the butt of the leader, passes through the loop. Then the deceleration becomes exponential (drastic).
PS: Linear density is on board for those who follow the technical intricacies.

I just measured a few tips on my lines and they go from 0.72 mm to 0.95 mm (#2 to #5, 3 brands). So I can imagine different conditions (fly, loop size, head wind, etc.), depending on the line I am using, for which the first limit will be reached somewhere in the front taper. The second limit (exponential deceleration), would likely still remain at the leader level.

My thinnest line is 0.88 mm at the belly level, so this one is very sensitive to drag conditions, and I use it with a fast short rod, tiny flies, and small loops (1 to 1.5 foot maximum), otherwise the rollover would always decelerate (like in the Gatti Bono example posted by Gordy). In fact it must just decelerate quite often

Hope this helps and not confuse anyone.

Merlin

[Paul Arden]

Hi Merlin,

Well actually confusion is my middle name right now and unfortunately that didn’t help In Graeme’s video there appears to be an acceleration - this is something I’ve seen quite often. An acceleration followed by deceleration. Obviously there is more than one factor at play here.

Sorry it’s probably a very basic question but if I don’t understand it then I’m sure Lasse doesn’t either.

Cheers,
Paul

[Merlin]

I see. That does not exclude the fact that there can be an acceleration before, due to the front taper. Assume there is a long front taper, there is an acceleration first but its intensity diminishes as the change in momentum becomes smaller. At some time (first limit), the acceleration comes to zero, then the line progressively decelerates, and when the leader enters the loop (second limit) the deceleration becomes drastic.

Merlin

[Paul Arden]

Ok but do we know why it sometimes accelerates and at other times decelerates

Paul,
It takes a complicated ODE to model the expected velocity profile of the fly even for a level line. It is much more involved for tapers since the taper impacts the change in momentum of the top leg and the drag forces on the loop and line as a function of the roll out of the loop.

Thus the net acceleration or deceleration at any given time point will depend on who wins the tug of war going on between the +acceleration tension produced by the momentum change vs the deceleration force resulting from all of the drag losses(line, loop, and fly). Changes in the loop size are also related to the linear mass density of the line going around the loop and its velocity that complicates things even more.

I have never seen a model of the expected fly velocity profile that includes the effects of the loop shape changes because I don't think loop morph is understood.

How you would model the loop shape changes that were observed in the double handed cast that Vince liked to show is beyond me. Even the relatively simple Dolphin nose shape that you get in so many types of loops (flyline, string, paper strips, dog leashes and bead chain for example) has not been explained although Dr. McMillen's model that includes the viscoelastic properties of the line is a notable start.


Knowing a definition that viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation is of no help to me in understanding how it affects loop morph.

and is there agreement between the physicists on this point?

I think the physicists would agree if the positive acceleration forces are larger than the negative acceleration forces then the velocity at a given time will increase. If the reverse is true then the velocity will decrease.

Dr. Gatti-Bono's simple model fits the data measured for a level line quite well so there is hope. Here is an example of model(green) vs measured(blue) velocity profile. This cast was made with a heavy bead chain so its rho_l*v.^2 momentum chain always produced positive acceleration. Long line with lighter line and it would probably decelerate until the line got really short at the end of the cast.


Gordy