Fly Line Stretch and Viscoelasticity

[Graeme H]

(Keeping in mind we are in the "Flycasting Physics" forum here ...)

So if stiffness is not a problem, how much effort are we going to put into establishing the elasticity of a fly line is influencing a cast?

I know that with a long line out, setting a hook on a fish is easier with less stretch in the line: The "in-touch" lines from Rio come to mind here. I suspect there might be a similar effect when initiating a cast with a long carry in action, but as a caster, I'm going to adjust naturally for it in a similar way that I'd adjust for line lengths, wind conditions, rod "feel", etc. . I doubt I would be able to pick a difference between a low-stretch and a normal line in a blind test of casting them.

I'm agreeing with the premise of your FP though Bernd: Fishing with low stretch or normal lines is best done on a case-by-case basis.

Cheers,
Graeme

[Walter]

What’s the best line?

What’s the best rod?

What’s the best leader?

What’s the best fly?

Only physics has the answers …

[Merlin]

I keep on working the subject, and losses can come to the level of skin drag losses if the modulus is in the 1 to 3 MPa range. This is a low level but this same level appears in the scientific paper I mentioned earlier in post #40.

Consequently it would be possible to meet a sensible level of loss but at the same time, the line would stretch significantly. That may be part of the feel. I remember when I cast a barstool rigid line for the very first time, the feel was different, strikng was more efficient for long casts.

Merlin

[Paul Arden]

That’s probably because they are overweight and polyurethane

[gordonjudd]

losses can come to the level of skin drag losses if the modulus is in the 1 to 3 MPa range. This is a low level but this same level appears in the scientific paper I mentioned earlier in post #40.

Merlin,
Were the low modulus values you referenced in that paper related to the measured modulus of PVC or some other type of coating material?

Based on Torsten's and my measurements of the spring constant for fly lines in the range of 300-1100 N/m, I would think the tension modulus of the the core materials used in fly lines would be much higher.

His example in a previous post derived an estimated value around 400 MPa as noted below: Matlab gives a value for 353*1/(pi*(.001/2)^2)=449.4 Mpa

If you know the cross-sectional area, you can compute the elastic modulus from the following equation:

E=k∗L/A

A is the cross-sectional area
L is the length of the object

for above example, if we assume a fly line diameter of 1mm:

A=π∗r² where r is the fly line radius, thus

E=353,16N/m∗1m/(π∗(0.001m/2)²)=401274175N/m²=0.4GPa

The slight challenge is here, that a fly line consists of multiple materials; the core has a different elastic modulus than the coating. You could assume an average cross-sectional area, but I'd say that choosing the stiffness@1m as property makes more sense.

You can also get expected modulus values from the stretch information given in the German data base at https://www.fliegenfischer-forum.de/riotarpon.html
They found the stretch produced with a 17.6 Newton load on a low stretch Rio 10wt Tarpon line having a diameter of 1.77mm was less than 1%. That gives a tensile modulus value of.
E=17.6/(pi*.00177^2/4)/.01=715 Mpa.
That value used the measured diameter of the line for the stress calculation. Using a core diameter of 1 mm the tension modulus of the core material by itself would be even higher at 2250 MPa.

Thus I would think the modulus of fly lines would be well in excess of of the 1-3 Mpa range you expect would be required for the loss factor in a line to approach the drag loss values.

Gordy

[Merlin]

Gordy

It is a question of ambiant conditions, since the elastic modulus of viscolelastic materials varies with temperature and falls down in the "transition" period. I suspect that some lines are affected by hot conditions like 40°C /100 F. I do not think it is a general rule but it might happen that some losses may be noticeable sometimes. Let's wait and see what James can find in his DMA tests.

Merlin

[VGB]

Thus I would think the modulus of fly lines would be well in excess of of the 1-3 Mpa range you expect would be required for the loss factor in a line to approach the drag loss values.

Are you still expecting there to be zero losses, as assumed in every published casting model?

Vince

[Bernd Ziesche]

I doubt I would be able to pick a difference between a low-stretch and a normal line in a blind test of casting them.

I'm agreeing with the premise of your FP though Bernd: Fishing with low stretch or normal lines is best done on a case-by-case basis.

Hi Graeme,
I am pretty sure I couldn't either.
For me the biggest factor is memory. Memory dampens the takes and can be tricky for setting the hook.
Low stretch remains worth little, if memory goes up...

Also with some big fish on distance we want some stretch!
Regards
Bernd