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James' front page on fly and line

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Merlin
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James' front page on fly and line

#1

Post by Merlin »

Thanks to James, this front page is an interesting insight of the necessary link between the line and the fly. The topic is difficult but we can make some approximations and have a look about the main parameters which are the weight of the fly and its “size” (aerodynamic properties).

Using information given by James I could see that the order of magnitude given from experience is pretty good, but for that one needs to choose aerodynamic characteristics (SCx) for the fly. They appear to be realistic and if I make a parallel with trout tackle I get also realistic correspondence between a typical trout fly and the adequate line (#5).

In order to find a realistic limit between the line size and the fly I compared the (horizontal) speed history given by a fly line and the one for a free flight for the same fly. We assume that they both fly horizontally or so and that their vertical fall is limited by vertical drag characteristics. That’s a bit optimistic if you want to consider a weighted streamlined nymph.

As the line is launched the fly is also launched at the same speed, so what you expect is that the line gives at least the same speed history (and even better) than the one for a free flight of the fly. This is a crude approach indeed but it is not completely stupid. I used a level line for sake of simplicity and defined the limit when both speed profiles are close together.

As can be expected the main fly parameter is its size, the mass is secondary at the beginning of the flight. However, at the very end of the flight, as the line taper and the tippet enter the loop, the speed level diminishes faster for the line by comparison to the free flight. We all experienced casting a rather heavy streamlined nymph and usually the nymph takes over the leader at the end, because the line is slowing down but the momentum of the fly itself remains large enough to keep fly speed at a higher level. This type of cast is not a real pleasure for sure. So there is also a link with the weight of such streamlined flies, but it is pretty difficult to establish. To limit the phenomenon, an option is to take a larger line since it better withstands aerodynamic forces and can maintain fly leg speed at a sufficient level, and also to shorten the leader and try to maximize its weight but this is contradictory with fishing constrains sometimes (e.g. long tippet for a nice drift of the sunk nymph). Heavy nymph casting remains awkward anyway, and there are good reasons for that. There is no such issue with a pheasant tail.

The rule of the thumb indicated by James (fly weight may be about 20% of the weight of the carry) is fine for a reasonably non aerodynamic drag characteristic of the fly in saltwater fishing conditions. If for example I take a fly of similar mass but having half of its drag effect, then I could cast it on a #7 line instead of a #10. This is just to illustrate the fact that drag is the dominant factor (at the beginning of the rollover). If I cut the weight of the fly by 2 but keep the same level of drag for the fly, I must stay with the #10 line. In general conditions, there is some link between the mass of the fly and its drag characteristics, so it should be possible to use a “fly weight scale” to choose the corresponding fly line as a rule of the thumb. When considering a trout fly I find a 5% ratio instead of 20% in James’ example. Thus the ratio varies with the weight (class) of the fly. This lower ratio for smaller flies / lighter lines is due to the fact that such lines have poorer aerodynamic characteristics by comparison to heavier lines (the larger the diameter of the line is, the easier the rollover is).

Here are some illustrations. We start with two lines, a level one, and a TT one to show the influence of the taper on speed as the line rolls over. Both carry have the same weight. The first graphic corresponds to 16 grams of carry (12 m, #7 approximately for the level line) and the fly rig weights 5 grams. The free flight is illustrated in red.
speed histories 2.JPG
speed histories 2.JPG (24.73 KiB) Viewed 4086 times
No doubt that the fly will overtake the line and we shall quickly lose control. Note that the TT profile induces more speed loss at the end of the roll over because the line gets thinner and too thin for the fly in fact at some stage. Now we change for 25 grams of carry (#10 for the level line).
speed histories.JPG
speed histories.JPG (24.12 KiB) Viewed 4086 times
That’s much better, unless for the TT line at the end of the rollover. Trends indicate that a somehow “reversed” taper before the final drop for the leader would help casting such a big stuff.

All that is more qualitative than quantitative but I thought it would be interesting to show the logic behind the fly and the line. You know by experience that you can cast a midge and a large sedge with a #5 line, so there is no perfect relationship to expect.

Merlin
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Re: James' front page on fly and line

#2

Post by Mangrove Cuckoo »

Merlin

Thanks... that is quite interesting.

I believe the "reverse" taper you speculated is already in production for lines specifically designed for permit fishing. Crab flies are quite heavy and have poor aerodynamic properties... a lot like bomb nymphs on a trout flyline... only on much bigger lines.

One thing stood out for me: when you said cutting the fly weight by 2 but kept the same aerodynamic properties you need to go up to a heavier line.

That seems contrary to my experience.

In fact I often add weight to make my larger flies turn over better... on a certain line.
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Mangrove Cuckoo
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Re: James' front page on fly and line

#3

Post by Mangrove Cuckoo »

Umm...

I may have to retract that second statement!

After thinking about it I see that my experience is not contrary, but might support your theory?

Oops!
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Merlin
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Re: James' front page on fly and line

#4

Post by Merlin »

One thing stood out for me: when you said cutting the fly weight by 2 but kept the same aerodynamic properties you need to go up to a heavier line.
Hi Gary

I said that if I cut the fly weight by two I have to stay with the same line (#10 in that case), not to take a larger one, whilst if I cut the drag by two I can switch for a lighter line. The weight of the fly becomes influential at the very end of the rollover, when the leader is involved. At that stage it is as influential as drag. So it can happen that if the fly is too light for its drag chracterisitc, you should add some weight to help the leader rolling over, or take a thicker/shorter leader if possible, allowing it to deliver higher speed.

If I could make a raw statement I would say that the line is fighting against drag, and that the leader is influenced by the weight of the fly. For trout fishing drag is the only thing given the very small weight of flies, but it is not the same story in saltwater conditions (or pike fishing).

Merlin
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Re: James' front page on fly and line

#5

Post by Mangrove Cuckoo »

Merlin wrote: Wed Jul 15, 2020 8:07 am
Hi Gary

So it can happen that if the fly is too light for its drag chracterisitc, you should add some weight to help the leader rolling over, or take a thicker/shorter leader if possible, allowing it to deliver higher speed.

Merlin
Merlin,

Yes... that is exactly what I have found.

As I have gone to larger and larger flies, I found that they need more weight to balance their drag. In fact, I now tie a new pattern unweighted and then try to throw it.... by hand... then wrap lead wire to the shank until it flies well on its it's own.

Turning it over on the end of a leader is then just a matter of matching the line to the cast I need.

I think your raw statement is pretty well done!

Thanks.
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Re: James' front page on fly and line

#6

Post by George C »

Mangrove Cuckoo wrote: Wed Jul 15, 2020 11:31 am
I now tie a new pattern unweighted and then try to throw it.... by hand... then wrap lead wire to the shank until it flies well on its it's own.
Turning it over on the end of a leader is then just a matter of matching the line to the cast I need.
Brilliant!

The idea of testing by just throwing the fly makes things a whole lot simpler.
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Re: James' front page on fly and line

#7

Post by Lasse Karlsson »

What lineweight would you guys choose for something like this?
20210211_173326.jpg
Weight is 59,4 grams.

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Lasse
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Re: James' front page on fly and line

#8

Post by James9118 »

I'd put it on a French leader, it will go further that way :-)
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Re: James' front page on fly and line

#9

Post by Lasse Karlsson »

Won't it overtake the line then? ;)
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Re: James' front page on fly and line

#10

Post by Boisker »

The line doesn’t matter, you can just float it downstream :D
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