Coam again

[Walter]

Starting a new topic.

This is a general question about conservation in general and angular and linear momentum specifically.

Given the following:

- tethered cast
- level line
- Casting 10 m of line
- semicircular loop
- 2 identical casts with respect to fly leg the same fly leg velocity at the point where the loop has just formed, i.e., the length of the rod leg is zero. I the loop in the first cast to be 1 m high and the other to be .5 m high.

I calculate the am for the loop of the first cast to be 4 times that of the second cast. The linear momentum of the fly leg in the first cast is 91% of the momentum of the second cast because the fly legs are both travelling at the same speed but there is more line in the loop in the first cast.

How do we think this will propagate in the two different casts? Anecdotal experience is that a cast with a smaller loop is going to go farther but if the am is a significant factor then the opposite should be true but this calculation tells me the opposite.

Another possibility is that the cast with the smaller loop begins with more KE as well as more momentum so, if there is some amount of conservation of energy or momentum happening then the cast will the smaller loop will end up traveling a bit faster as it unrolls and this could contribute to the smaller loop cast actually traveling farther.

I’m not trying to make any cases for conservation of angular or linear momentum. This is all strictly exploratory at this point. The values I derived for the two casts are based on calculation but the rest is conjecture.

[Paul Arden]

Hi Walter,

(I hope this is not a diversion in post 2 )

If the lines are tethered then they and both fully unroll then they will go exactly the same distance!

I think line speed only really matters once the loop has fully unrolled and is still shooting. I would argue that the same case can be made for loop diameter. In other words while the loop is unrolling, it doesn’t seem to matter if it’s high speed or low speed, wide or narrow, the shoot appears proportional (or very close) to carry (assuming that the loop fully unrolls), but then after this, what is left, can still shoot and float out.

I don’t know if this is right. But it certainly seems that way to me! Maybe it’s also that additional line speed finds diminishing returns?

Anyway to the point. In view of the above, wouldn’t it be better to have a much longer line that doesn’t fully unroll, or does it still always unroll under these theoretical conditions?

Cheers, Paul

[Walter]

You make a good point Paul, with a 10 m cast they’ll both fully unroll and there likely won’t be much difference outside of measurement error.

I just redid the calculation using an impossibly long length line of 1000m. (Hopefully, the response isn’t that neither one will unroll so who cares?) I can use any length you suggest. I can also change loop size as well. I can also provide actual values for am a linear momentum, I just think the ratio thing is easier to visualize for me. The idea is to give the line and loop as long as possible to evolve.

In this case the am in the larger loop is still 4 times that of the smaller loop but the momentum in the fly leg with the larger loop is 99.9% of the other. Do we think the two loop sizes will make any significant difference to the casts? We know in this case neither one will fully unroll but which one will go farther? Based strictly on am it would be the larger loop I think. Based on linear momentum and assuming that some amount of energy or momentum conservation is happening it would be the one with the smaller loop I think. Maybe am is a red herring for part of the cast but becomes significant when the length of line in the loop is similar to what’s in the fly leg.

I don’t think there are any right or wrong answers because I can change the scenario to wildly different loop sizes and different lengths of fly legs. Just trying to get people’s thoughts on this.

[Paul Arden]

Hopefully, the response isn’t that neither one will unroll so who cares?

Actually my question is do they both unroll? I assume it has to do if air resistance, or falling under gravity and hitting the floor, are included? Well-formed loops have an interesting ability to unroll, even in real world scenarios.

I really don’t know the answer to your question! It’s difficult because I often intentionally open the loops (170/distance casting) for… distance casting. But I also see examples of very tight loops travelling extreme distances too and is IMO the main reason we have gone to stiffer rods in the WCs. I can’t even tell you if an initially more open loop is better than an initially tighter one for distance with a long (but less than 1000m!) carry, from a practical sense. Which is really what you are asking from a physics sense.

Which I find really quite surprising by the way. But maybe that’s because we don’t have a semicircular loop but a top pointed one?

Cheers, Paul

[Walter]

I don’t know the answer(s) either. It’s not meant to be a trick question but an exploration.

It was brought about by the idea that for some people the loop is pulling the fly leg and for others the loop is more of an artifact caused because you have a moving fly leg that is constrained by the rod tip. I can think of scenarios where both are true. If we imagine making a wheel out of line and spinning it up to the point where internal tension is holding it in a circular shape and suddenly attaching a fly leg to it what do we imagine happening? Suppose that wheel has a total of 10 m of line in it (it’s a bit over 3 m in diameter) and we hook a 1000 m fly leg on that has no initial velocity the wheel is going to come to a screeching halt. But if we hook on 1 or 2 m of fly leg it will slow down but in the absence of other forces it will continue spinning although somewhat slower. If that 1000m fly leg had initial velocity it could actually force the wheel to speed up. So, both scenarios could be right. We could do all sorts of calculations but I’m more interested in understanding the underlying mechanisms.

We know that in the tethered cast without a strong head wind that the main force acting on the fly leg comes from the rod tip but I don’t think we (at least not from any of the conversations I’ve seen) understand the mechanism for how that force interacts with the fly leg. We know the loop is involved but it’s not a squishy pulley or lever. I have my own view of it but I’m holding on to that for now. We’ve seen the example where a dropped loop with one end tethered falls faster than one where both ends fall freely so we know things happen at loops that are counterintuitive but I personally don’t have a good handle on that and its confusing because gravity is also major factor in that case where it is much less of a factor in the unrolling loop of a fly cast.

What constitutes a well formed loop for you and, in your experience, how much difference does it make in your cast?

[Graeme H]

I'll follow this but stay away from posting many contributions.

The only thing I'll say is that I don't think there will be a significant difference in the casts made with the two loop sizes for normal casting distances and line speeds. I don't think the AM of the line inside the loop is a significant factor. If it were, we'd readily see that in our observations of casts.

The linear momentum of the line inside the loop and fly leg is important though.

Cheers,
Graeme

(My reason for not contributing significantly is due to my stance on what the loop represents in the cast. I don't want to make this thread fly off from its intended topic.)

[Walter]

Graeme,

I hope you will join in. I have to give credit where it’s due and say that the “fly wheel” concept came from you.
I think it’s a great piece of critical thinking which is why I shamelessly stole it.

I think what the loop represents in the cast is probably best suited for its own topic. I’ll try to participate if you start one.

Fwiw I’ve seen many clever ideas generated in this forum but some time now they seem to get mired down.

For now I’m hoping to get a better understanding of the forces in the line, both internal and external.

[Graeme H]

Thanks Walter.

I’ll watch closely from the sidelines though. I don’t have much to say that isn’t just critical without also being constructive on COAM.

If I can contribute I will.

Cheers, Graeme

[Paul Arden]

It was brought about by the idea that for some people the loop is pulling the fly leg and for others the loop is more of an artifact caused because you have a moving fly leg that is constrained by the rod tip. I can think of scenarios where both are true.

This is what I think too. An unrolling loop is both a fly leg flying through the air, as well as being pulled. Most of the time, because people like to ask which it is most, it’s mostly flying through the air. But that’s irrelevant because if it’s not also being pulled, even just a little bit, it will stop being a loop and will turn into a squiggle. Other times there is a lot more pulling going on, aerial snaps, check hauls and so on. Other times there is less - shooting monofilament.

So for me the arguments over whether it is one thing or another have always been quite bizarre, because if it’s not both then it’s not an unrolling loop; instead it’s a collapsed mess.

What I personally would be interested in is how these things balance out, because it’s quite amazing that they do. I always found the dropped loop chain video that Vince posted quite fascinating although he said it didn’t apply.

Cheers, Paul

[George C]

Apologies if this is naive but I’ll learn by asking.

Why can’t a loop be considered just the transition zone between compression and tension?
The fly leg is colliding against the slowing line in the loop, which would produce compression in the medium of the line. The rod leg is pulling on the loop front moving away from it which should produce tension. The medium between must be in transition between the two. Because that medium has stiffness the transition must be gradual rather than sharp. That gradual transition will be oriented (I’d think) in a curve with neither compression or tension at the apex of the loop.
George