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Walter & Group....
[GH] Mark Surtees replies to Ally Gowans. Great thinkers often "have to have the last word" in a discussion. Whether or not this is the case with this string, I don't know. To make it easier for you to follow, I have taken the liberty of placing [MS] before Mark Surtees' comments and [AG] before each of Ally's. Walter Simbirski follows providing his usual assistance with the understanding of physics terms.
Unlike the CHECKMATE of chess, there may never be a "final word" on a subject as complicated as this. Some commonalities, however, are beginning to emerge as the differences start to eclipse :
[MS] Hi Gordy
I realize that you would like to bring this topic to a close but these are my replies to Ally's questions anyway….
[AG] "Thanks for your message, obviously my teaching skills not always up to the task!"
[MS}My learning skills are occasionally a bit dodgy too Ally…
[AG] How can any of the line pass the rod tip without having greater velocity than the rod tip and therefore being launched?
[MS] [This is precisely what puzzles me too….if we look at it another way… how can we consider the line to have been launched if none of it has passed the rod tip?. I’m assuming that there is a conflict between having a “location of line launch direction” where you have indicated it and having it at the point where the line passes the rod tip at the same time. They can’t both co-exist as definitions of when the line is launched.]
If you define line launch to mean that point when the line has sufficient momentum for just part of it to extend forwards then this is very likely to be very early in the cast. I’m struggling to see how this concept could be practically useful.
[AG] Where did this definition of line launch come from?
[MS] [I made it up so it’s a bit of a straw man really…..it doesn’t work for me either though J
I was trying to explore the argument to explain the counter position to the one that says that the point of line launch is that point where the effective mass of the line has sufficient momentum to complete the cast. For me it doesn’t work either way because although there is a point where this exists it is variable from cast to cast and from a teaching perspective not terribly useful.]
[AG] Line launch occurs when rod tip velocity component in the direction of the line momentum is less than the line velocity.
[MS] [Lost me again here… sorry….what is “the direction of the line momentum” ?
In a curved tip path the velocity of the tip is constantly changing because of its change in “direction”. The velocity of each bead in the line chain is also constantly changing for the same reason. The momentum of each bead, being dependent on mass and velocity is also therefore constantly changing. The rate at which the velocities change is acceleration or deceleration, which will occur at fractionally different rates at fractionally different times during the cast. Even when the force on the line is reducing, provided there is a forwards element in the line of force of any magnitude then the line will, bead by bead, continue to be moved in the direction of the cast. When a part of the line ceases to be moved forwards then the rest of the line will continue forwards until it has a downward force applied to it sufficient to change its momentum. It is worth pointing out I think that the first part of the line to be launched is the bead next in line to the one right at the rod tip not the beads in the “effective mass”. When that second bead advances past the first then the line will have been launched..in my book anyway.. J ]
[AG] It is impossible to prevent this.
Note that the energy possessed by the line at that time and the final layout of the line is irrelevant.
If we define line launch as occurring at the point at which the line passes the tip these problems don’t arise.
According to this definition factors affecting line launch will include, how long the rod is, how the rod is held, how far the casting arm is extended etc. Explain the logic please!
[MS][These factors will affect any cast in much the same way however we define line launch. Other factors will include rod stiffness, Casting Arc, Stroke length etc etc. ]
[AG] "Line launch is “the CAST”. RSP is just a state of the rod that has somehow been elevated to importance, in the same way that rod tip speed has."
[MS] My “cast” is over when the fly hits the water, launching the line is just a part of it. RSP is just used as an analytical point of reference common to all casts, it is not in itself a “useful” part of the cast. Without some rod tip speed your line won’t move, this justifiably elevates it above RSP to a position of some importance IMO.
[AG] I’m confused by this comment but I think we can agree that all casts are over when the fly hits the water. Most instructors hold the opinion that the cast is over much sooner because energisation of the fly line is complete at line launch.Like throwing a rock or firing an arrow after it is launched you can’t thereafter change the direction or increase the distance.
[MS] [It’s one way of looking at it I suppose, maybe they don’t teach presentation casts.]
[AG] Perhaps another case of two fishermen and three opinions?
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[GH] Mark says he made up his particular definition of line launch. The terms "line launch" and "launch angle", however have been used for several years. I thought perhaps I had introduced these terms a few years ago in these Group deliberations.....but others may have preceded me.. I can't recall if or when these terms have actually appeared in the published fly casting literature. If any of you can find them in print, I'd like to know.
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[GH] From Walter Simbirski ( I believe that CoM refers to Center of Mass. )(Remember the ordinate and abscissa of the X - Y graphs where the X line is horizontal and the Y line is vertical) :
Hi Gordy,
A few comments:
- It seems that people are getting more familiar with the concept of vector vs scalar quantities which is good.
In the past we have said that the loop forms (or the cast is over, or the line is launched) when the line speed is greater than the speed of the rod tip.
Now we have stepped beyond that and look at X and Y components of line and tip velocity.
If we consider that the line is moving in the X direction then the cast is over (or line is launched) when the X component of the rod tip velocity is less than the X component of the line velocity. In terms of the scalar value, speed, the rod tip can actually be traveling faster than the line when the line is launched since the speed of the rod tip includes both its X and Y components while the line continues to travel largely in the X direction.
The change in direction of the rod tip determines when the line is launched and the size of the loop. Since we have added a Y component of acceleration to the mix the rod may still be bent for a significant period of time after the line is launched.
- The line cannot accelerate by itself (unless we look at line stretch and the resulting contraction as tension is removed but that is best left for a separate discussion). The rod tip, on the other hand, is always accelerating as long as the rod is bent but it is not necessarily accelerating in the direction that the line is traveling. The rod hand is not constrained to move in an arc or a straight line and as a result we can significantly alter the path of the rod tip at any time during the stroke.
- The line is flexible so it may not be obvious that it reacting to external forces according Newton's laws.
After the rod tip changes directions it appears that the line continues to move in the same direction even though there is now a Y component of force acting on the line.
To understand what is happening we have to look at the line's center of mass. When the line is stretched straight its CoM will be located on the mid point of the line. When the line begins to bend and form a loop the CoM will be located at some point in space not directly located on the line. When we say "line follows rod tip" it would be more accurate to say that the line's CoM follows the rod tip. In addition, there are significant losses in kinetic energy due to internal resistance as the line forms a loop and unrolls.
Walter
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