Paul Arden wrote: ↑
Tue Oct 20, 2020 6:33 am
Hi Graeme, I’ll post this here.
So I understand what you are saying. The flycasting loop wave is actually travelling vertically upwards from the rod tip and at 90 degrees from the casting direction. So for you the loop straightening is it’s amplitude and the wave length is half the height of the loop.
I say half the height because a full wave cycle would be both back and forward casts. I suppose however that because the wave doesn’t travel vertically upwards and instead the loop straightens then I’m not sure how exactly to measure the wavelength? Is it twice the height of the loop above the rod tip when the fly passes the rod tip? I’m not sure. In fact I’m not sure how to measure the amplitude either because normally a wave rebounds on its own accord as it propagates and doesn’t need to be rethrown in order to start the process anew.
So it’s not a full transverse wave cycle that propagates as a transverse wave, that you mean, rather it’s half a transverse wave that finishes at loop straight? And then we start another half wave cycle with the next casting stroke and over and over again?
Sorry If I sound a bit confused, but I’m getting there right?
Yes, you seem to be following along so far.
Yes on the loop straightening being the amplitude. The exception is an underpowered cast in which the line fails to straighten - in this case the amplitude is the distance the loop landed away from us (in the same way the amplitude of a mend is the distance it deviates from the straight line to the target.)
No on the wavelength. Wavelength is calculated from the period of the wave and the speed of propagation of the wave, which itself must be calculated. Here are the diagrams of each (period and wavelength):
From this, we can see that the Wavelength = Period x Velocity (of the wave*)
The Period is the time between the start of each cycle, e.g. the completion of each forward cast.
The Velocity (of the wave) is the average velocity of a falling line during the cycle, calculated from the distance an object will fall during the Period divided by the period. (Minus the time it's being lifted by the rod ...)
The full wave cycle is when it returns to the place it started from and starts the next movement. For example, a pickup and lay down cast or a full false cast (back cast and front cast.) You could time the cycle by the time it takes for the fly to pass the caster on the front casts of each false cast it you like - it doesn't matter at which point in space the fly passes.
If you are only including a front cast OR a back cast, you can't calculate wavelength or period and you break the rule about the wave not transporting the medium. (Returning the fly to its starting position in space is the same as the going from peak to peak in the diagram above.)
I know this is all somewhat counterintuitive and I don't blame anyone for not grasping the concept. I certainly don't teach this sort of detail to any student but I do often tell them the loop is a wave and tension in the line is important to make the wave move along the line. They easily grasp that concept.
* Note: the velocity of the wave is NOT the velocity of the line moving across the ground.