The Physics Of Fly Rod Systems 21Oct20

"What you measure will change." But, will it really, if it's all just unadulterated physics and behavior is not involved? I regularly track stuff and results change but humans, often I, are involved. Here measurement needs to be fully objective, allow comparison and is pursued to document and understand. We may cast differently but we can find a common measure in spite of our style differences. "If you cannot measure it, if you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind." - Lord Kelvin.

I agree with Lord Kelvin and have been frustrated as a fly fisherman and custom fly rod builder that buying and specifying a fly rod, reel and fly line as a system can not be more objectively constructed. Aren't you too? It's not about what "feels" best for you really - it's what a rod will optimally cast - there is a balance, pace, and power in an assembled system which you as the caster must accommodate... or assemble something different, but it is objective and determinant when optimal. Let's go through this. Every fly fisherman has seen this classic "graph" (never posted with actual measures - just notions and marketing - WTF is "power factor?"). The 2nd image is from Orvis who somewhat characterizes the design/flex of their rods - but not with disclosed objective measurements.

So, we need to get down to real physics, and real empirical measurements, to determine how a given rod, loaded with a given line, will perform. Mainly that is to say, find what line will optimally load a specific rod to allow its longest cast, if you are able. Sure, buy the rod (with specs) and line (recommended for your rod) that meet your needs, but we should not have to guess how the combo may work. I wish I could demystify rod-line system performance for commercial manufacturers but they will obfuscate with marketing forever - or maybe they don't really know what I, and now you soon, know. The shape a rod makes under load is determined by two related factors: the stiffness of the body, its Intrinsic Power, IP (which is set by material and taper design) and the flex of the tip, its Action Angle, AA (also set by material and tip taper design). Here are the secrets/measurements...

1. IP - casts are longest/optimal when the weight of the first 30 feet of fly line (one doesn't cast a fly, one casts the line) is equal to the weight needed to deflect the tip of the rod 1/3 of its length.

2. AA - the angle struck by the (non-flexing) tip top at the deflection point above determines the "action" of the rod (very fast >70, fast 66-70, moderate fast 63-66, moderate 59-63, or slow <59). [Dr William Hannemann invented this most righteous measurement].
   

It is as simple as that - all rods should be marked with these two measurements (completely objectively measured) and we'd all cast better and choose set ups more wisely.  I've measured my entire quiver of rods, commercial and custom, and present them below. I use Effective Rod Number (basically real IP) for the measurement I took by actually measuring the weight required to deflect the rod tip 1/3 it's length.

Some things jump out.... Especially for very fast rods, weight designations are understated regularly; my six weight Orvis Helios measures out as optimally loaded by a 7 wt line and my "5 wt" Sage X could bear a 6 or 7 wt line. Even my 2 wt is more like a 4 wt in strength, despite it being the slowest rod in my quiver (though a half inch was snapped off the tip and that may have changed it's performance from the original). Fly lines are held to comply with AFTMA standards and if a rod manufacturer says their rod is a 5 wt, then a mid-spec AFTMA 5 wt line would weigh 140 grains and implies an Intrinsic Power (IP = 10*line weight + 60*AFTMA #) of 1700 grains for the rod. A rod of lower measured IP would be under-powered to throw the 5 wt line and conversely a rod of higher measured IP could be cast more efficiently with a heavier line. My measured ERN could be similarly stated in grains and is a comparable, but observed measure. Since rods (and blanks) are marked with line weight, I used that as my Y axis. Rods are not marked with objective measurements to my knowledge, except for my Rodfather series henceforth. Here's my Rube Goldberg setup to take measurements. I'm using Hanneman's Common Cents approach - I know how much a penny weighs (for strength, IP, or ERN) and if my rod tip top is level when undeflected then the angle struck at 1/3 length indicates measured action (AA).

Mates know I cast the shit out of any given system I wield... so aggressively in fact that I'm believed not capable of a Euro-nymphing day ~ frankly thus far that's proven true. I find the power curve of a rod - that is for what they are designed ~ even when cranking a 1 weight on remote creeks.