Casting definitions 10 Convex vs. Concave

Walter & Group....

[GH] Troy miller comments on the drawings from the attachment to the Casting definitions 7 message. This was a diagram showing the stylized outlines of the convex/compound and concave/compound flyline forward tapers. They were taken from Modern Fly Lines by Bruce Richards. *

As an engineer, Troy has an eye for detail. Even though these drawings are taken from the corrected edition and show the proper labels of CONVEX/COMPOUND and CONCAVE/COMPOUND, he points out a discrepancy with respect to relative taper lengths. I must admit, it slipped by me because I had tunnel vision with respect to the curvatures alone!

So you don't have to back pedal to that past message, I'll include the diagram again in an attachment.

Peter Minnick wrote to remind me that I had given him one of the old copies of that book some years ago. He checked and noted the incorrectly labeled drawings in that copy. He promised to scan it and provide it for us in a few days. I had told Bruce that one of those early copies would likely make great collectors items and go for an even higher outrageous price than the corrected ones.

Gordy

* MODERN FLY LINES, by Bruce Richards, 1994, pp. 79,80.

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From Troy Miller:

And the labels are still wrong! The top one should say “12’ Straight Taper vs. 12’ Convex Compound Taper” and the bottom one should say “12’ Straight Taper vs. 6’ Concave Compound Taper”. The top one may even be a 14 foot compound, if the straight taper is 12 feet…

Hope you’re doing well Doctor Hill. Take care

Troy

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>From JF:

Hello Gordy,

Glad to read your response, cause it now confirms that I have one of those books which had the drawings reversed.

Thanks for clarifying this for me.

Cheers

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>From Ivan Streif:

Hi Gordy,

I have always found the concepts of "concave" and "convex" difficult to talk with students (and sometimes other instructors) about because they are relative to a person's point of view.

For example, if you are sitting under a trampoline and someone gets on, pressing the mat down toward you, it would appear that the fabric is bulging out, relative to your position. I would call this outward curvature convex, while the person on top on the trampoline would see the same deflection of the fabric as an inward curvature, or a concavity.

Similarly, the curvature of a fly line loop may be interpreted differently by the student looking up at his loop, compared to an observer from the front, back or side.

I don't think the concept is difficult to understand, it is just a matter of making sure that student and instructor are using the same words to describe the same action. I have used the line "curves away from the tip of the rod" or "curves toward the tip of the rod" which may be accurate regardless of casting plane, but it doesn't give the student a very good visual image, so it is not ideal descriptive analogy.

I used to be pretty comfortable describing a convex tip path as "moving like a windshield wiper" until one person showed me that the wiper on the back window of his car actually swung down!

Ivan Streif, CCI

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>From Jonathan Walter:

Hi Gordy,

I, too, have gotten away from the terms concave and convex and only use them when I have both verbally and visually defined them. For concave my favorite word is dipping, as in a dipping rod tip path. For convex, it is arching, like the tip of a windshield wiper. As I describe these I also pantomime with my finger what the tip of the rod will do. I also carry a small 8x11 whiteboard and markers to draw on to describe these. Seems to work for me(and hopefully the students).

Other reasons I have gotten away from using concave/vex is that I often mix them up when using the terms---always bad---and even when I get them right students new to the terms have to stop listening and think about what it means. I think that happens less with dipping and arching.

Thanks,

Jonathan Walter

P.S.--As I proof read the above I found I had indeed reversed the terms---see what I mean?!---JW

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[GH] Jonathan,

Yes, indeed. That is why with the stress of college exams some students mix them up and come up with wrong answers.

"DIPPING" = CONCAVE "ARCHING" = CONVEX. I like that.

Gordy

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>From Scott Swartz:

Gordy,

I am glad to hear that I am not the only one who thinks concave and convex are not particularly descriptive terms for teaching (okay for instructor communications).

Most commonly we see new students with a convex tip path. We describe them as a “windshield wiper” casts while we demonstrate to them with arm and finger.

For the student who is dipping the rod tip, correction is often achieved by addressing speed (or power) vs. tip path.

In both cases Bruce’s 6 steps help the student discover how to make the desired change along with drawing it as Tom mentioned. A neat trick to drawing diagrams on the casting field is dry erase markers and a Plexiglas clip board.

Best,

Scott Swartz

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[GH] Pat Peterman explains differences in relating curved lines to different things as seen by engineers, laboratory folks, and fly casters. I think this explains the confusion :

This concave-convex path discussion is amusing to me. If you look at Webster’s or some other English dictionary, you will find something mumbled about the shape of a circle or sphere looking at it from the inside (concave) or looking at the same circle or sphere from the outside to define convex. I am a Civil/Structural Engineer (depending on the State) and would get into discussions about this issue with my mentor. In all the physical sciences, i.e.. engineering, surveying, geology, soils engineering, etc. the terms concave and convex are used consistently and opposite the way they are used in our casting jargon. The perspective is always the observer. If the “shape” has its center away from you (forms a ‘cave’), it is concave and if it has its center toward you it is convex. That is how all technical writhing in the physical sciences is done.

In the lab, where most things are in jars and beakers, the sky is the reference (like we use in casting). The shape of the meniscus (curved up or curved down) can be a telling physical property and this exceptional usage was developed accordingly. * That is the only place in technical writing that I have found, other than our casting definitions, where the sky, not the observer, is the reference. I don’t know how we went down this path, unless the original definition writers were lab rats or had their definition based by someone from that environment.

To this date, if I use those terms, I hesitate to assure that I am using the casting board’s (or lab sciences) definition and not the more universal definition that I have become so comfortable with over the years, and which is more generally applied to the physical world. For me, if the middle of the shape goes away from you, the observer, and forms a cave, it is best described as concave. If I apply that rule, everyone in the world, but for casting instructors and lab techs, will know what I am talking about. If we would make the caster or observer the point of reference instead of the sky, the confusion about usage would quickly fall away. Can anyone think of anything else in our lexicon where we do not write and talk from the perspective of the caster?

Pat Peterman

MCI

* [GH] I placed a simple drawing in the second attachment.

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Tim Kempton comes in with a "closer" ! :-

Thanks Gordy

To me this simply gets too complicated

Luckily fish don’t read books or our emails.

Kind regards

Tim

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[GH] Tim,

Well we must not tell them else they get so smart we'll never catch them !

Gordy

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