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Silk Vs Plastic Lines
Casting and Line Splash, A practical Analysis

By Ralph Shuey

Two of the aspects of using silk lines vs synthetics with bamboo rods have always been the reputed ease of casting and the "stealth" capability of silk. My limited experience with the medium confirms these claims, and being an engineer I started looking for reasons behind these characteristics. Now you can look at these things objectively (hard facts), and subjectively (feel). I had experienced the subjective, the line felt easier to cast, and control; as well as it seemed to cause less 'commotion' when it landed on the water. I now wanted to confirm those observations and feelings with some real 'facts.'

For the uninitiated, there was a great revolution in fly line in the late 50's and early 60's with the advent of new synthetic floating lines. The period was very dynamic, with new materials and manufacturing processes seemingly being tried daily. In the end, not only did the materials and methods of manufacture change, but also more importantly, so did the line's size. The old lines that were made of woven/braided materials required "greasing" to float (thus the old term, "greased line technique"). The new synthetic lines trapped microscopic 'bubbles' in a plastic sheath to facilitate the same end. The new manufacturing technique was cheaper, and far easier to control, especially for mass production and required less care in the field. But the new technique came at a price. . .bulk.

Being good marketers the industry boys decided to put a good face on things and change the then existing system of line size designation. The old system was based on line diameter, so thus a silk line of diameter 0.045 inch was designated a C size (today's 6wt), a diameter of 0.035 inch was designated D size (5wt), 0.030 was E (4wt), and so forth. Industry "borrowed" a system in use in England at the time and converted from letters (A,B,C,D. . .) to numbers (1,2,3,4. . .) and tied the numeric system to line weight rather than diameter for the new synthetic lines. The new system required the first 30 ft of synthetic line to be the same weight as the old silk line designated as equivalent. And, as Reed Curry, a gentleman well versed in silk lines, so succulently put it: "Diameters were thus free to expand . . .and they did."

"So what's the big deal?" you ask? Simple. Two major aspects of line use, casting and presentation (i.e. line splash), are tied directly to line diameter. "Pardon me?" you say, "How so?" Well to exaggerate a bit to make my point. . .What could you throw further? A stone that weighs a half-pound, or a ball of cotton that weighs a half-pound? Obviously a stone. Why? Something that airplane/automobile/etc. designers call 'drag' which retards distance moved for the same effort expended. What would make a bigger splash? A half-pound steel ball or a half-pound wooden ball? The wooden ball; it's bigger! In this case the important function is 'displacement' (the amount of water moved). Mathematical equations describing both 'drag' and 'displacement' share a common factor. . .size. And in these cases, size is directly proportional to diameter, and in this case, size is known as 'area.'

Oh oh! "Things are starting to get hairy!" you claim? I'll try to keep it simple, trust me.

All of us know the equation for the area of a circle; pi (3.1416) times D (the diameter) squared (i.e. times itself) divided by 4 (or pi R squared). The thing that jumps out at you, as far as fly line is concerned, is that term D, diameter. "Isn't that the dimension the manufacturers of fly line let 'float,' you might ask. Yes, it is, and the horrible fact of the matter is, the effect of an increase in diameter is not linear (like a straight line) but exponential (a steep curve), because it is squared! Let me see if I can explain it simply.

The folks that manufacture silk line today (2 firms that I know of. . .) both agree that a silk line is on average 30% thinner than an 'equivalent' synthetic line. That 30% is very important to you the fly fisherman. A circle that is 30% greater in diameter than its supposedly equal companion has an area 69% greater in cross section! [Since all the other terms cancel out you only need to compare the effect of diameter [or radius] squared i.e. (1.3 x 1.3) - (1 x 1)/100 = 1.69 - 1/100 = .69]. That means 69% greater drag, or 69% greater splash between a silk 6wt line and an 'equivalent' synthetic line.

That means up to 69% less effort to cast a silk line a given distance, or, that means that a silk 6wt hits the water like a synthetic 4wt (roughly equal diameters)! WOW! So objective numbers confirm my subjective experience fishing silk fly lines vs. synthetic lines with a bamboo rod. There is a real reason that silk line "feels" so good. That distinct difference in drag, is why we, the bamboo/silk community, cast with a subtle wrist action. Whereas, the synthetic/graphite folks, must practice a full arm "high line-speed" school of casting. Also this may explain why rods cast so differently with silk vs. synthetic line. You literally 'load' the rod differently with a 6wt silk and a 6wt synthetic. (And now you have the reason those Orvis bamboo rods marked GBG or GBF are commonly used with 6wt synthetic lines.)

We in the bamboo rod community, have long recognized that a dearth of lightweight vintage production rods is available. Here is one of the reasons. With 69% less splash yesterday's silk 6wt could fish as quietly as today's synthetic 4wt. Thus, there probably wasn't much call for 4wts (About the same proportionate number as 2wts in use today?). This may also help explain why 7/8wt rods were proportionately more popular in the old days. An old 8wt silk would splash and cast about like today's 6wt synthetic and yet give you more beef and reach for those big bass and pike as well as the occasional salmon or steelhead.

So numbers confirm, "feel" and also answer some old questions. The old silk lines tended to cast and present better than their supposed synthetic equivalent because of smaller line diameter. But, of course, this analysis opens up some new questions as well:

    (1.) How much compensation was made by the old manufacturers (in tapers) to accommodate the new lines?

    (2.) How did the old time builders compensate for the new lines and when did they start?

    (3.) Is there a true correction factor one can apply to determine which synthetic line to use on an older bamboo rod?

    (4.) What are the optimum casting methods for bamboo rods/synthetic line and graphite rods/silk line?

All these questions and more beg answers. ~ Ralph Shuey

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