It is all about models. Unfortunately, but perhaps inevitably, models for describing fly casting and fly rod behavior have been dominated by both the precedent and predominance of trout level fly fishing. The traditional light rods, small flies, and short line casting of fresh water trout fishing contrasts sharply with the heavy rods, large flies, and double-hauled distance and wind-fighting casting of modern saltwater fly fishing.
Ask any Florida fly fishing guide who gets booked by visiting trout fishermen who “want a go” at the big boys and buy, for the first time on their lives, a 9 weight rod. Two different worlds, connected by similar physics and principles, but applied quite differently.
Out of the “trout” world models were developed to describe, clarify, and teach the “magic” that occurs in a fly cast. They also proposed to clarify, regulate, and code the continuum of line weight and rod stiffness, …..and simplify matching the two, a very worthy goal! Unfortunately, based on a singular knowledge of trout casting and trout equipment, the model developers did not even understand, initially, how inadequate and off the mark their models were.
Let me clarify the differences…and why they seriously change the physics.
Throwing 35 feet of a 3 weight line involves a one handed flick of the wrist barely utilizing a fraction of hand/arm/shoulder power. A 100 foot SW cast with a 9 weight involves BOTH arms and for many stresses the limit of their upper extremity power(s). In heavier weights strength becomes the limiting issue.
The shortening effect of the rod-as-lever is trivial dealing with light line and minor power needs. But when trying to maximally accelerate a heavy load, the shortening effect of rod-as-lever becomes a drastically critical issue.
I The Fly Rod as a Hammer....and as an Earthworm
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