Of course those truly advanced students having trained to cast a troubled loop for 20 years may need longer to heal a cause. Again as a fly casting teacher it'll be your part to offer proper exercises to support your student getting rid of every small move causing trouble in his fly casting. Even for the experienced fly casters it shouldn't take long to get significant better!
Having said this it took me a long way (teaching many students) to get to a point at which I think to be able to identify nearly every single movement in fly casting causing trouble for the cast. Trouble shooting the fly cast is really easy once you understand the cast over it's full life cycle in all details. Again to get to this point isn't easy at all!
I remember to have had a fair number of students when I started teaching fly casting more than 20 years ago that I couldn't really help because I didn't find the cause for some of their trouble. Even after having taught fly casting for more than 10 years I still had a student here and there causing trouble in a way that I wasn't able to fully identify (understand).
When I started to prepare for different fly casting instructor certifications I often came over an old concept for a better understanding of fly casting:
"The line goes wherever the tip goes."
That seemed to be a fair concept to teach like:
"When you move the rod tip in a big circle path, you get a wide (rouded) loop shape."
"When you move the rod tip along a straight path, you get a tight loop."
Looking back at this concept today - it helps to understand some things. But as soon as the line carry (line outside the rod tip) gets longer a lot of things may cause trouble in your fly casting that you can't explain with that concept.
In fact I was teaching distance fly casting during the past weekend in Switzerland. Some truly advaced casters entered the lesson. Several times I was asked: "Why don't my distance casts turn over nicely into a straight line in the end?"
I know that this is a heavy - not easy to answer - question for many fly casting instructors around the globe. In fact most of us have the same problem when casting highest possible distance. Today I can answer that question pretty well. It was my good friend (and fly casting instructor) Aitor Coteron from Bilbao (Basque country) who put my nose into a great concept for a better understanding of fly casting details:
"Every part of fly line can only follow the direction the part of fly line in front of it has."
This is independant of the direction the rod tip will be moved. In other words the old concept mentioned above doesn't hold true in many aspects. Still it may help for a quick start, but that doesn't change the fact the the line end doesn't follow the rod tip in most of the loop's life cycle.
I believe that if you want to teach fly casting it is worth studying the fly cast in slow motion watching all details frame by frame. One of the drawings to support an easy understanding I made after watching nearly 3000 slow motion videos is this one:
I hope it will help some of you out there to get into a better understanding of trouble shooting serious distance casts. To those of you not being interested in distance casting I shall add that all details happening in distance fly casting happen in short casts as well. Just not that pronounced and even harder to be seen. ;)
For those of you watching your fly casting in slow motion for a first time: Don't worry too much about what you are seeing. In fly casting there is no straight! Even the best fly casters don't have a straight fly line when slomoing all important details our eye can't see in real fly line speed!
Great week to all of you!
All my best
P.s.: If you are keen to get more details about trouble shooting the final turn over in distance casting you may find a lot of valuable information in this Sexyloops board thread.
My last days...