Walter & Group......
From Dan Storaska:
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Dan...
When you drift you place your back cast loop farther behind. If you combine this with a line shoot, the back cast loop unrolls a lot farther behind you. In the time it takes to do all this, gravity is acting to have your line descend. This places it in the same angular position with respect to the ground as if you had made a back cast directed back and down (of course, not enough to hit the ground). Since all this takes place after the STOP, as the rod tip is moved back during drift as some line is shoot back, there is very little if any real loading of the fly rod.
Now: If you follow with a high trajectory (Launch angle) forward delivery cast, the back cast and the forward casts will now be 180 degrees from one another ...... a good situation, indeed for maximum distance. ( "The 180 degree rule").
Some refer to this concept as, "Joan Wulff"s SEESAW". Check out her text on LONG CAST TRAJECTORY and her diagrams, A. and B. on p. 121 of her book, FLY CASTING TECHNIQUES.
# 7.) Was my comment on OVERHANG. As you probably know, overhang is the length of line between the rod tip and the back of the fly line head .... usually a combination of rear taper and running line.
When you drift back and shoot line back, you can increase overhang. Most casters can't handle a great deal of overhang, since light line can't control heavy line in front of it very well. By increasing this overhang on only one cast: the final back cast prior to the delivery cast, some of these casters can control and use more overhang.
Why do this ?
1.) This allows the caster to carry more line. Since you can shoot somewhere about 50% of the length of the line carried (depending on the distance of the cast), it follows that you can make a longer cast altogether because you are unrolling more line as you shoot more.
2.) This increased overhang results in the line head turning over at a slower rate (since you have provided more line to turn over on the forward cast.). Now, the loop is only travelling forward as long as it is unrolling ..... so the longer it takes to unroll, the farther it can go. This increases the distance of the cast.
The best description of this scenario is found in Jason Borger's, THE NATURE OF FLY CASTING, pp. 240 - 241 including his quotations of Steve Rajeff. They are talking about casting with shooting heads, to be sure, but the same principles apply with wt. forward fly lines.
Gordy