I read McClane a long, long time ago, and decided then his description was gobbledygook.
More new words doesn't simplify the communication - always abused in fly rod marketing (even new glass rods), and we're still talking about two opposite ends of the taper spectrum.

Where it gets confusing is long rod, short rod, fast rod, slow rod, something blended in between - that's where people begin to disagree about what's what.
Thin tip/ thick butt Progressive.xXxxxXX......X...x....X.......X.x.....X......x.X....xxx.....X.......x.......X.....xXXX.....Para Thin butt/ thick tip
throw in different lengths, different line weights, different load rates, actions, MOCs, both from each end and every point in between.

As far as casting goes, every rod is benefited by following the taper and loading with haul, but a para is most sensitive and may not work with aggressive timing leading the rod taper.

JoJo's in a place to see many more long fast paras than the rest of us. Most of the paras we see here are small niche trout rods, and blended taper to give them friendly tips.

I think you're spot on Corey. And quoting Vic Johnson in his book on FFR:

Computers were used by Berkley to design the Para/Metric's curved taper so that as the rod bent there was equal stress at every point along the length of the rod. Equal stresses eliminated the kickback problem so common in most rods and allowed casting characteristics that were identical regardless of the length and weight of the rod.

Thanks for getting this discussion recentered with descriptions that try to define the term for fly rods. The constant in such discussions from the time when it made a marketing/mechanical transition from bamboo to tubular composites is that no one agrees. Believe it or not, there was a time when the math was not over my head and I studied it all out. I still have a book with a good chapter on it as to fly rod design, but I don't want to muddle my brain anymore by looking at it.

My own conclusion, though, was similar to Carlz's. Well, I still find it a interesting to study technical/logical arguments on the topic, but long ago I concluded that it isn't worth disputing about.

I guarantee--and if this directs a dispute to me, I can take it--that every well reasoned question and description above puts far more thought into it than Charlie Ritz did when he dreamed up/popularized the term.

Parabolic has long been more of a marketing term than a technical term, so the best we can have is any one maker describing the action of that maker's parabolic "models."

A strict definition for fishing rods has never been agreed upon and never will, or else the Eagle Claw 8 1/2' 'glass model I looked at yestereday would not say "parabolic" on it. It wasn't a bad 'glass utility rod, especially for less than $30.

Reviewing the argument since the term became rampant, from that time forward, few agreed on whether it applied to the taper itself, static flex, flex curve, and so on. Charlie Ritz had no idea other than the more generic stiff mid, softening point near the grip. He knew it made good magazine and advertising copy.

That's partly because "taper" doesn't translate in analog dimensions from bamboo to a tubular composite, whose flex is determined by wall thickness and diameter, not to mention materials with different stiffness and load-bearing properties.

To exaggerate the point just a bit, a section of a 'glass blank, without an intricate layup impractical for production(or strategic sanding or placement of sleeves, also labor intensive), can't be made to increase in stiffness, and then soften significantly again. To do this would mean increasing its I.D gradually--the norm in rods whose stiffness increases tip to butt, and then to decrease its I.D. Try to slip that one off a mandrel, which would be thicker at each end then some point between where the "softer" butt begins.

Parabolic rods never interested me to use since I don't fish where their casting nuances would seem advantageous. I never saw a pic of Arnie Gingrich or Charlie Ritz fishing where they might be either. Yet I get from the excellent casting descriptions above that they could be a lot of fun to use. Still, I like a rod to tell me when it is reaching its limits, and I'm not sure paras do that as well because of the softer section down into the grip.

Probably, thinking of AJ McClane's description, a great hobnobber and angler like Ritz, we could do better with fiberglass starting with several good progressive action--Fisher--blanks, lets say 8'. We will lay down pairs of butt sections and cut them, let's say at 10", another pair at 12.5", and another at 15". Now we have joints/sectioins that will spigot ferrule nicely when assembled backwards. The short cut off section can be turned around(thus softening into the grip and sipgoted, or it can be spigoted to the butt of yet another uncut butt section, easy since the I.D.s match and getting us a longer parabolic rod.

That's right. I just completely ruined several progressive action blanks, but by golly, I have a rod that is parabolic as Charile Ritz's bank account. I think I won't try it.

Hat's off to a builder and caster who can make a distinct taper/action of this type and get three or four guys to try it and agree--even though they may not agree with three or four others trying another maker's "para."

If I ever finish up the one Fisher blank I still have, maybe I'll write "pendular progressive" on it.

Oh, forgot, "wet fly" action: C-shaped relatively uniform bend, thus "slow." Naturally casts an open loop so as to reduce tangling of a gang of wet flies. Absorbs shock of hard strikes to swung wet flies.

Have been following this with furrowed brow. It appears to me that there is no "industry standard" or commonly accepted definition, or consensus on what exactly a "parabolic" rod is. A term pretty loosely used?

Labelling a fly rod as "parabolic" or anything else for that matter, to me, is unimportant. I know what I like, when I try it. I think cabin fever is setting in and we all need to go fishing.

BobHa, are those measured stress curves at ~full load, or calculated?
I would have expected a smoother curve with out the kinks.

answering out of turn, it's the grass in grass - nodes, imperfections affecting the individual measurements the curves were plotted from.
If you measured a sampling of rods, the curves would smooth out.

The top three are probably rated at 40' of line out. The last one which is mine is rated at 30' of 4wt line out the tip. The middle two are classic tapers which where probably measured over varnish plus had some error which would lead to some of the Kinks. That said kinks can be good in bamboo. Those ones you see at 25" and 60" are there for a reason. The 25" helps with loop control and the 60" let's it hinge into the butt quickly. You would see kinks at the ferrules in a tubular rod if a means for an accurate stress curve was available.

Ps. The stress curves are just where the rod is most flexible per given static load. If you hang weight on the tip that is not the curve you will see but looking at those curves for the fast action you would see a lot of tip defection with little butt deflection and looking at the para curve above you would see little tip but a lot of butt deflection. After you cast many rods and look at their strees curve or build and tweak many by their curve. You can have a pretty good guess how a rod will feel in hand.
Bob

The longest one

I’ve owned a lot of 8’ 6wt 3pc and 2pc Berkley Parametric rods over the years. I buy them whenever I see them on ebay and give them to local kids.

Jason, I assume you are asking about the number of models of the Para/Metric. The Johnson book, Fiberglass Fly Rods, states that the first P40 came in 8' and 8'6" in 1967. They expanded the line in 1968 to five rods from 6'3" to 9'3" including a 3-piece 6'3" PD40. The Curt Gowdy Para/Metric Signature series began in 1972 in two-piece and included these models: ( I believe this series was made in Taiwan.)

PG40 6'3" #6
PG40 7'0" #6
PG40 7'6" #6/7
PG40 8'0" #6/7
PG40 8'6" #7
PG40 9'3" #9

Is the "40" significant, are all the "40" rods parametric or is the "40" just show it's Berkley?
It would seem that using the same mandrels and blanks for different finishes/price levels would be economic.

All the PG40s I’ve handled were made in the USA.