Fly Fisherman magazine has recently published a portion of Simon Gawesworth's book Single-Handed Spey Casting
The Fly Cast - Fly Fisherman
I wanted to add a quote from his book because it illustrates another important difference between rod actions for identical line weight fly rods.
Before I can explain the difference, I need to explain that when a fly rod bends, the "effective rod length" or what Simon would call the "lever length" of the rod shortens. In the illustration #5 And #6 below, it is the dotted line. In geometry this line is called the chord
. A chord is the line that joins any two points on circle or an arc. Since the rod bends in an arc as we cast, we can define the effective rod length as the length of the chord from the rod tip to our casting hand.
It is obvious that if we are to cast so that the rod tip moves in a straight line for a tight loop, we must compensate with our casting stroke for the rod shortening.
Given the facts above, we can see that substituting a heavier fly line to slow the timing of the cast, does not make a fast rod into a slow rod. The timing might slow but the degree of rod shortening of the fast rod over time (graphed during the rod stroke) will not duplicate the shortening of the slow rod over time. So the caster still must modify his casting stroke if there is to be tight loop formation. Going up in line weight does not make a fast action rod into a slow action rod.
For rods of identical length and identical line rating but different rod actions, the faster action rod shortens less for a given casting stroke. Because it shortens less, the faster action rod is a longer lever and can accelerate the line faster for a given rod stroke.
"An aside note is that two rods of the same length don’t necessarily have the same leverage. If you took a nine-foot rod that was very fast and a nine-foot rod that was very soft and slow and you loaded them up with identical line weight and length, the fast rod would actually be a longer lever. My dad, who was a teacher of math and physics before he became a fly-fishing instructor, showed me this one day while trying to give me an understanding of fly casting. The easiest way to see this is through illustrations #5 and #6."