A thought experiment

[Mangrove Cuckoo]

Imagine there are three investigators on a long bus. One sits in the front seat, another in the back seat, and the third is free to stand in the aisle somewhere in the middle. The person in the front seat extends the tip of a flyrod out the window. In the back seat, the second person holds a not insubstantial fly, attached to a leader, that extends through a full head-length of flyline to the tip of the flyrod.

While the bus is at rest, the rear person pulls on the fly to put a deep bend in the flyrod tip. It is important to imagine that the extended section is only the tip, not a full rod. Then the fly is released, and the tip propels the line, leader and fly forward for some distance. It is not important how far you imagine the fly moves, but anyone who has played with Bow & Arrow casts will imagine it does not go very far, because even a fully bowed static flyrod can seldom propel even a short section of flyline very far.

The middle person measures how far the fly travels and sits down for a reference point.

Now, imagine the bus is moving. It doesn’t matter how fast you imagine, but lets say something like a constant 30 mph. Same as above, the tip is out the window, the same line, leader and fly are extended, and the rear person pulls on the fly and bends the rod tip. And the experiment is repeated, but now at constant bus speed.

Most everything is the same, although now the rod tip is being retarded by the by the apparent wind of 30 mph, as is the flyline and the fly. All three now have drag to overcome. When the fly is released by the person in the back seat, the middle person watches closely and again sits to indicate the distance traveled. Does the middle person indicate that the fly has traveled further than in the first case above?

It is important to “see” the second experiment at a constant speed.

Now, as a third experiment: imagine that just before the fly is released, the bus collides with an immovable wall. Much like the three flycasting “scientists” in the bus, the rod tip, flyline, and fly will initially continue to move forward at 30 mph, but then the tip unloading will again have some additional influence.

How far will the fly travel in this third case?

Like the unfortunate person standing in the aisle… only to the wall, of course.

[Graeme H]

Relative to an observer standing outside the bus, the fly should travel the same distance for versions 2 and 3, both of which are further than the first cast.

Relative to the seated scientists: if they were still able to observe, the fly would go further on the third try (until the fly hits the wall).

That is one hell of a stop for that cast though!

Cheers,
Graeme

[George C]

Cool question. Thanks for asking it, Gary.
And surprising (to me) answer. Thanks for giving it, Graeme.
The answer seems obvious in retrospect but prospectively, after being conditioned by repeated focus on how important the stop (or deceleration) is, I must have been unconsciously thinking the stop magically added energy to the cast. It is amazing how the mind plays tricks on you, and how easily it can lead you astray. This is a great site!
George

[Merlin]

I find the analogy misleading and not representative of a cast. There is energy added to the line during deceleration to a stop (moving force at rod tip).

Merlin

[Mangrove Cuckoo]

Merlin,

Thanks.

And yes, I am not trying to make an analogy of an entire cast, only what is the input from the tip of the rod. I am really just trying to figure out what is going on by looking at things in a way I think I can envision.

There seems to be a lot of focus on the tip unloading to and through RSP and into CF as if something extra is added during that sequence because the tip moves fastest then, but I cannot understand where that extra is coming from.

If the tip section of a flyrod cannot propel an entire front section of a flyline when performing a Bow & Arrow cast, how can it do so to that same section of flyline during a cast when drag should be higher?

I imagined the rod out the window to eliminate any torque or leverage gained from the lower section of the rod. It seems to me that most (or all?) of the energy that has been imparted to the flyline has already been done before the tip unloads. It is easier for me to envision that the tip moves fast at the end because the flyline is overtaking - not that there is something extra that has moved up through the rod.

What am I missing?

[Graeme H]

Hi Gary,

There is very little being added to the forward motion of the line at RSP.

It takes force to deflect the rod from the straight state it naturally rests at. The further the rod is from being straight, the larger the force required to keep the rod bent. So deflecting the rod through the first centimetre from straight take less force than deflecting it through the hundredth centimetre.

That means that in a bow-and-arrow cast, the most acceleration the line experiences is in the instant we release the fly. At every point in time after that, the accelerative force is diminishing, but it's still there. At RSP, the rod is no longer bent, so there is no force accelerating the line (ignoring the mass of the rod tip and the inertia it has that is somewhat maintaining its forward motion.)

Once the rod begins to counter flex, there is a force bending the rod forward that is produced by the fly leg pulling the rod leg and in turn pulling the rod tip (again, ignoring the tip's inertia.)

So I agree: most of the work being done on the fly line has already occurred well before the rod tip straightens. I don't think you are missing anything.

Cheers,
Graeme

[Merlin]

Hi Gary

It seems to me that you refer to the “big spring” theory by using an example based on the bow and arrow cast in the framework of the rod. Let’s put the haul aside.

The spring of the rod is used to store energy for some time then release it for launching the line. As such it is not that large but without the spring, we lose some 40% of line speed. The price to pay for the caster is to produce the energy stored into the rod, about 20% of the total energy captured by the line. In a bow & arrow cast you are just left with those 20%, and this explains why a bow & arrow cast is so inefficient.

There is energy introduced into the system as long as the rod is rotated / translated, and this is something which does not take place for a bow & arrow cast since the rod butt is immobile. Drag forces apply in both cases, they are larger for an actual cast by comparison to a bow & arrow cast, and they do not correspond to a constant speed like in your example.

At the time a caster decelerates more than 50% of the final line speed is achieved, but there is still a lot to get back from the rod by several means. The major part comes from the unbending of the rod which takes place as its butt is still moving forward: you move the force at tip and get more energy than the stored one (which is the only part recovered in the bow & arrow cast). The second part comes from the variation of inertia of the rod. The butt unbends first, then the middle, and finally the tip. That kinetic energy concentrates (not completely) into the tip which increases its recovery speed at the very end, before RSP. That is the “inertial” or “whip” effect. All that allows to get a line speed something like 40% larger by comparison to a fully rigid rod.

It seems to me that most (or all?) of the energy that has been imparted to the flyline has already been done before the tip unloads.

Certainly not, only half of the final energy has been introduced into the line at that time. And at the very end, the line overtakes the tip a few ms before RSP (without hauling). This has no measurable impact on tip speed.

I know the mechanics of the fly rod cast are tricky and far from being straightforward.

Merlin

[Mangrove Cuckoo]

Thank guys!

I will have to think about this for a while, which is good as today the weather is poor for actually casting outside.

I will have to do my casting inside my head. (Where the knots are much harder to untie! )

[Walter]

Gary,

I’ve actually done the first two parts of the experiment and can tell you what I observed. In my case we used tossed ball rather than a fly and we sitting in the bed of a pickup truck. With truck at a standstill the ball went the same distance as it would for any standing throw. With the truck moving at a constant velocity the observes in the truck saw something interesting. The ball would at first move forward, slow down, come to an apparent full stop in mid air for a second and then appear to reverse direction. Of course there was also the normal vertical movement but the horizontal movement to observers in the truck was as I described. Air drag acting on the ball slowed it down in flight while the truck continued to move under it.

In your scenarios the fly released on a moving train, without the addition of a stop, would not appear to move as far to the observers on the train.

You are probably thinking that a bow and arrow cast is different from throwing a ball but it actually is. The difference is that the rod tip pulls the fly forward until RSP and then it goes on its own when the rod tip reaches RSP. With the ball the arm pushes it forward until the hand releases it. I could actually pull the ball forward to make the throw.