Can anyone give us an idea how fast in layman’s terms (mph) a fly line is moving during a normal say 30 to 40 foot cast?
And at the crack of a missed-timed backcast is the fly moving at the speed of sound (768 mph)?
Hair
Around 80 ft/sec (55MPH) would ball park it. This is at the start of loop formation. Depending on the drag of the fly, the fly line can actually increase in speed as the loop unrolls. Distance casters speeds are much higher sometimes approaching 200 or 300 ft/sec.
The crack in a missed time cast is a sonic boom. Same as in a bull whip. The direction change is such that the tip swings around and actually breaks the sound barrier. That’s why the fly sometimes parts company with the rest of your gear.
Bob
Now that we know that fact, is there anyone who questions how a beaded fly can destroy your fly rod when it strikes it on the forward cast!!!
Of course thats for a decent caster with a reasonably fast rod. For me with an old South
Bend boo it’s more like eighty feet a day. If someone will hold the rod I can take time out for lunch between the backcast and the foreward cast.
Fine, I’ll do it. I’ll also bring the salami sandwiches.
Hi Bob. Are you dug out yet? I think you should have left this snow with Marco on the wrong side of the lake.
Thanks Bob,
You’ve got me thinking now. The fly line actually speeding up went over my head. Is the reason this happens because the force remains the same but the weight of the moving top half of the loop decreases?
Been thinking about this while repairing my frozen plumbing. LOL.
HW
The line speed should be increasing because the casting stroke is a smooth acceleration. As one accelerates, the rod is going to bend and store energy (like a spring). When you get to the end of the casting stroke and stop, the rod is going to unbend and release that stored energy, causing the final acceleration.
Just think of sitting on a compressed coil spring. As it uncompresses, you are going to accelerate that whole time until it’s completely extended.
The reason distance casters can cast with so much speed is they can get the rod to bend a lot farther than most.
There is no acceleration of the line after the stop. Once you stop the rod you stop adding energy to the system and acceleration stops. The unloading rod just keeps up with the line and gets out of the way as the loop forms. There is some acceleration towards the center of the loop but that does not contribute to the the speed of the fly line that is behind the caster and moving forward.
fishbum
And that is what I thought too, fishbum. Got into the physics of it when I read some articles by Noel Perkins and Bruce Richards. Turns out, energy is not being added to the loop but it is already there. As the loop turns over, the kinetic energy of the moving line segments that turn over and stop is transferred to the part of the line that is still moving. This translates into a force at the end of the moving line ahead of the loop and is resisted by the aero drag on the moving line and fly. Add to this the fact that the mass of the moving line is also decreasing and viola, F=ma and if F is constant and m goes down, bingo. The physics is relatively complicated but if you like, I can dig up some of those articles. If you have someone stand out to the side and you cast a nice tight loop w/o a leader or fly, you can actually see it speed up by watching the loop which is traveling at 1/2 the line speed. I thougth it was nutty too until I started playing with the equations. There are some real physics mavants over on Sexyloops but you will have to deal with some (one in particular) real narcisists. I don’t go there anymore because of that.
Bob
I calculated about 40 mph on a 75’ on a double haul cast on my 4wt in still water situations Fro ( memory on timing). On a 30 or 40 ft simple overhead cast the mph be somewhat less. If you start your forward cast too soon your fly would be accelerated far beyond the 40 mph by centripetal force as the loop unrolls and you hear you dry fly snap into dust.
Just my opinion and experience I have never read any studies in literature
I agree Bob.
The fly line speeds up (accelerates) because the the fly line is tapered and Mass is decreasing. The only reason it would not is if the fly is so fluffy that the drag (which varies with velocity squared) prevents the acceleration.
Secondly, energy is added to the cast AFTER THE STOP. Therefore the line accelerates after the stop. At the stop, the fly rod is still bent backward and has potential energy which will be released into the cast as it unbends. As the fly rod unbends it does not “just keep up with the line” but is pulling the line forward adding kinetic energy to the cast. The rod tip that is ahead of the fly line is not being pushed by the line but is pulling the fly line.
If you doubt that allow me to stop my cast short of your face and allow my fly rod to “slap” you as it unbends. You will be convinced that there is energy and force behind an unbending fly rod.
Now here’s something that is also ignored about the stop. AFTER the rod unbends and straightens, it overshoots a bit and the uncoiling line is pulling on the rod tip. Newton’s third law of motion means that for every action or force, there is an opposite or equal force. So the pull of the uncoiling line pulls or bends the rod forward at the exact amount that the line is pulling forward. The rod must pull back according to Newton’s third law and this backward pull slightly removes energy from the line. Some of the energy goes to fight aerodynamic drag and some goes toppling against the rod tip.
The reverse pull of the rod tip is absolutely needed for the cast. It is this pull of the rod tip against the line that allows the line to uncoil. Said another way, if we did not have this opposite force of the rod tip pulling back on the line, no loop would form at all. Eventually as the potential energy and reverse pull stored in the bent rod tip becomes greater than the forward pull of the fly line and the rod tip moves backward. The backward pull is exactly like a line haul and adds velocity to the uncoiling loop. It is exactly like the the person at the end of the bull whip who pulls back rapidly on the whip to crack end of the whip.
I agree Bob.
The fly line speeds up (accelerates) because the the fly line is tapered and Mass is decreasing. The only reason it would not is if the fly is so fluffy that the drag (which varies with velocity squared) prevents the acceleration.
Secondly, energy is added to the cast AFTER THE STOP. Therefore the line accelerates after the stop. At the stop, the fly rod is still bent backward and has potential energy which will be released into the cast as it unbends. As the fly rod unbends it does not “just keep up with the line” but is pulling the line forward adding kinetic energy to the cast. The rod tip that is ahead of the fly line is not being pushed by the line but is pulling the fly line.
If anyone doubts that, allow me to stop my cast short of your face and allow my fly rod to “slap” you as it unbends. They will be convinced that there is energy and force behind an unbending fly rod.
Now here’s something that is also ignored about the stop. AFTER the rod unbends and straightens, it overshoots a bit and the uncoiling line is pulling on the rod tip. Newton’s third law of motion means that for every action or force, there is an opposite or equal force. So the pull of the uncoiling line pulls or bends the rod forward at the exact amount that the line is pulling forward. The rod must pull back according to Newton’s third law and this backward pull slightly removes energy from the line. Some of the energy goes to fight aerodynamic drag and some goes to pulling against the rod tip.
The reverse pull of the rod tip is absolutely needed for the cast. It is this pull of the rod tip against the line that allows the line to uncoil. Said another way, if we did not have this opposite force of the rod tip pulling back on the line, no loop would form at all. Eventually as the potential energy and reverse pull stored in the bent rod tip becomes greater than the forward pull of the fly line and the rod tip moves backward. The backward pull is exactly like a line haul and adds velocity to the uncoiling loop. It is exactly like the the person at the end of the bull whip who pulls back rapidly on the whip to crack end of the whip.
I think that although the physics formulas may be too much for many people, I hope when the action and reactions are explained in common terms, it is possible to visualize what is happening. Like someone said before, you can actually see the fly accelerating as the line unrolls. Then the question is why does this occur. I hope the above description is not too technical and confusing.
What about the line you are adding (shooting) in a backcast or forward cast, which is increasing the total mass of the line WHILE the loop travels? The energy is fixed at the point of the loop traveling to or fro, but quite a bit of it is being used to pull line through the guides. The physics are fairly simple with a static length of line out the rod tip.
I’m sure some eggheads have it all down to the numbers… but there are about ten billion and one variables in each cast which means that the numbers don’t really matter.
It is as much art as science. That is why Lefty Kreh can cast an entire fly line WITH NO ROD AT ALL, and I can cast 80 feet with a good rod on a good day when I’ve got enough sleep the night before and my wife’s not bugging me about something and I don’t have to go to work or cut the grass. He’s an artist with a fly line, I’m a hacker standing in front of a copier with pictures of great casts… The physics are the same for him and me.
As you said, any line that is shot (added) into the cast increases the mass in motion. Therefore velocity must decrease.
To explain this in terms that we have actually experienced think about shooting a weight forward line and a double taper line when there is 30 feet of fly line outside the tip top guide. For any given casting energy, when you shoot the DT it will not go as far as the WF. Why? Because you are adding less mass for a given length of WF running line than the same length of DT line. Less mass means greater velocity preserved, which means more distance.
Having said the above I do agree with you in that we don’t need to understand the physics behind a cast to do it well. Our minds and bodies do it for us through experience. I doubt that either of the two quarterbacks in the Super Bowl understand the physics of a prolate spheroid, but they are the pretty good at throwing it.
In my practical experience, based on the pain factor experienced when a bead-head wooly bugger contacts the back of my head on a windy day, I’d say “pretty darn fast”. Sometimes I mis-time the “duck” part of the chuck-n-duck equation. 
I like the idea of getting something for nothing. That is what you get if you have acceleration after you stop adding energy. I can slow down my cast and still reach the target because the line is speeding up at the stop. I like that. The slower I cast the faster the line should go.
There is indeed acceleration in the loop. It is a vector quantity that is towards the center of the loop. No loop would form without it. Works that way with any object on a curved path.
Fact is you add energy in the form of potential energy and kinetic energy and at the end of the cast all of that energy is used up. All the pluse and minus equal a sum total of zero.
That’s my story and I’m sticking to it.
fishbum
What I like about this post is that there are people interested in the physics of casting. I became interested a number of years ago and wrote a series of articles which you can find on my website. I didn’t have any help writing this and I had never heard of Bruce Richards or Noel Perkins but I forged ahead anyway. And there are some mistakes in my reasoning - surprise - surprise. I got butchered when I started talking with the self appointed real experts on another site but I learned a lot.
One of the most important things I learned is that words do not work well in explaining math and physics. Two people will say the same thing in different ways and argue all day over who is right. Much that has been said so far on this thread is spot-on. There a few ideas that need a little Kentucky windage to get to zero but many have the right idea. I don’t want to get into a discussion as to who is wording things the most correct way because that helps no one.
I have not seen a good mathematical explanation, even by the experts, on what happens when you shoot line. Also, even the pro’s do not all agree about what happens when the rod unloads or even when the loop forms. The matematics of a propagating loop are reasonably well understood and are explained in the Gatti-Perkins papers. I can send them to whoever is interested if you PM me.
You can find those old articles I wrote on www.hatofmichigan.org and go to the e-book page and look for the Mechanics of Flycasting articles. But be aware, understanding the physics will probably not make you a better caster. It should be approached as a hobby unto itself with no practical use for the average fisherman.
Bob
BTW Silver Creek - nice grasp of the concept. Nice job on the words too.
Bob
The old bamboo rods for smallmouth fishing were described as having enough time between the forward & backcast to go get a cup of coffee…not quite enough time for lunch; but odds are if you can’t hear the flyline cutting thru the air casting to tarpon with graphite there’s not going to be any jump ! Try the old bamboo casting strokes at 7:30 am for tarpon & the guide will wonder what’s wrong with a new graphite rod; then have a try casting with it & you’d swear there’s a zip-line really close to your ears or some new form of electricity similar to lightning out on the water !