Line Weight

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Rod Design by Deflection

Considering Line Weight

The graph below shows the length of the fly line (X axis) in feet and the line weight in grain (Y axis). Both of Double Tapered line and Weight Forward line are compared by line weight numbers. The weight values in Y axis is the accumulated weight from the end of the line.

Source is the weight distribution of Coatland 444 line.

The reason why the Coatland line is used, is to have a confidence in generalizing the theory of rod design by referring to the existing high quality fly line. The silk fly line which I make for myself also refers to this line weight distribution in braiding and coating.

Very important considerations are hidden in this chart for designing and understanding a fly fishing rod.

The truth of Stress Curve
The truth of Stress Curve No.2
Introducing DynaRod
Rod Deflection in Casting
Rod Action
Line Weight
Designing the Rod Angle
 

Most of the fly line makers seem to follow the standard of AFTMA (American Fishing Tackle Manufacturers Association), that describes the weight for the 30 feet length from the line end. Either of Double Taper line and Weight Forward line is manufactured according to this standard. The Coatland data above also keeps this standard.

In the graph above, most of the line numbers has same weight for both of DT and WF from the end to the 30 feet point (to 40 feet point for #8, #9 and #10 in above case). WF line becomes less heavy after the 30 feet point since the line diameter is thin as running line. Thus WF line is less heavy than the same numbered DT line for the line length more than 30 feet.

In other words, the fly line standard just rules about the length of 30 feet from the line end.

Why I say the weight of the line is so important, is because almost of the existing rod design progrms use the line weight to calculate the load applied on the rod. Let's see what will hapeen when we fish with the fly line more than 30 feet (about 9 meter) long.

It can be read from the graph, between DT and WF for the same line number, the difference happens from 30 feet (or 40 feet for #8 and up). Assuming that we can cast a whole line (90 ft), WF#5 full line is just a little heavior than the full line of DT#3. This means that we can cast WF#5 full line by the rod designed for DT#3. (We are assuming that the rod can throw full line of DT#3 and the caster can do it.)

Let's see about this from the rod side. There is a rod which is specified as "Line Wt is #4". #4 rod is the rod which is to cast #4 line either of DT and WF (at least for 30 feet). This is true if we use 30 feet of fly line. Either of DT and WF can be cast with the same casting and feeling in the 30 ft length. If the rod is good to cast the full line of #4, though it depends on the ability of the caster, it can also cast the full line of WF#6 or WF#7 with the similar rod flexure.

Haven't you have felt something not "loaded" on the rod when you use #6 rod and WF#6 line? In a shorter range of line, rod hand would not feel enough line load is put on the rod, but after lengthing the line more, it became a good feeling while casting. Probably the rod might be designed for DT#6 line. If you often use WF lines and you are strict with the feeling which matches to the line weight, it may be better to have such rod which is specially designed for WF line. The indication on the rod seems vague sometimes such as "for #6".

The load of a rod in designing an order made rod

There is a similar consideration to the difference of DT and WF line in designing a rod. It is the consideraton of the lower limit, average and the upper limit of the load for a rod.

The lower limit of the loadimplies how the rod actions with the shortest line length. For instance, when you cast the rod with only the leader and the line taper part is out of top guide, how do you expect the rod to deflect? Should only top part of the tip section flex, or a little more part flex, or the entire rod deflect? In this case, we have to design the rod taper considering the weight of the rod parts and rod itself carefully since the effects of the rod weight becomes larger.

Average load is the load on the rod when you usually do fishing by means of the distance you would cast very often. This distance becomes the standard distance for you and your rod. For instance, if you usually fish at the upper part of the stream in about 30 feet distance, 30 feet becomes the average distance and the rod is designed assuming that the average line load is for 30 feet.

The upper limitis the load that the rod can cast the maximum distance, or the rod can endure with the weight of the hooked fish (or tree). Among fly fishers, there are such casters to say "I can throw full line length by any rod." Those are well trained peoples, so let7s group them as the exception from rod design view points. When you hook a fish, how much the rod can fight with the fish, is also the kind of the upper limit load. An well designed rod on the upper limit load, can cast the line in a good feeling and need not to worry about to land the fish.

We need to know that a rod does not always deflect in the same manner. It flexes with smaller rod angle and flexes deeper with a heavior load. But the rod is the same rod. The rod, even if it is noted as for DT#4, can cast whichever weight of the line. If a heavier weight line is used, it deflects deeper. It is the ability of a rod designer to design the rod considering how much degree the upper limit load should be involved.

It becomes the consideration of rod design how to assume the range of the load, among the lower limit, the average and the upper limit, in order to design a good rod. Too wide range of the load would result in a unconfortable rod to use. For instance, #3 stream rod, if the upper limit is too high, you may be forced to cast a heavy rod in a whole day. On the other hand, if the range of the load is too narrow, it might be convenient to use with the average load but it cannot cast a long distance. Some good casting rod might be hard to land the hooked fish.

As the custom rod may not carry those information very often, the fly fishers need to judge those matters by himself by flipping and casting the rod. But an order made rod, especially designed for yourself by a good rod designer, often involves those considerations in the rod desgign.

Max Satoh
May 25, 2007

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