Fly appearance, physics and hydrodynamics

[William Anderson]

Okay, so this topic might be mislabeled from the beginning, but if I already had the answer I wouldn't pose the question. This one probably straddles the area between fishing soft-hackles and tying them. But I'll press on.

There is a physical phenomenon evident when you lift a wet soft-hackle (most flies for that matter) out of the water and the materials, especially the hackles collapse, cling together, and form an impression of what a fly looks like wet, but has almost no relevance to what the fly looks like in the water, much less how the fly behaves.

I've done it, we all have. You want to show pics of what the fly looks like wet because the looks of the fly while dry are misleading, but some materials show better wet. It's the hydro-tension, for lack of an accurate term, that pulls all the materials together misrepresenting the fly's behavior.

So, from both a physicists perspective and or a layman's understanding, what is the best term for this phenomenon?

Thanks in advance.

W

[hankaye]

Howdy All;

So, ... I took dub-ya's question and Googled it. First I Googled his guess about "hydro-tension"
and found this; https://www.google.com/search?q=hydro-t ... 8&oe=utf-8
and selected this for that search; https://www.google.com/search?q=hydro-t ... ric+stress
Not such a good answer So then I searched for WiFlyfisher's "hydro-static pressure" and found this;
https://www.google.com/search?q=hydrost ... 8&oe=utf-8 found that the item
needs to be IN the fluid for this to be considered. Then I just asked "what causes a material to attach itself to another when wet?"
and found the following. It may or may not be the correct words but appears to fit the bill.
https://www.google.com/search?q=what+ca ... 8&oe=utf-8
So, that's my input for this query. Always intrested to read what others may think or know.

hank

[letumgo]

William,

That phrase you are describing is "surface tension". Surface tension is due to water molecules being more strongly attracted to other water molecules (cohesion) than they are attracted to the gas molecules in air (adhesion). The forces of cohesion and adhesion seek the state of lowest surface energy, by drawing the surface film together kind of like the skin on a balloon. When a soft hackle wet fly is taken out of water, the water molecules draw themselves together to form a surface film, because they are more attracted to themselves than the surrounding gas molecules. The attractive forces are strong enough to bend the flexible fibers of a soft hackle, resulting in a characteristic nymph-like teardrop shape.






However, once the fly is submerged below the surface, the cohesive forces are now acting to in all directions at once (not just directed inwards), resulting in zero surface tension force. The forces are now balanced and allow the soft hackle fibers to spring back, even though the fibers tend to be very weak.


I am convinced that this is one of the key elements of the life-like appearance of wingless wet flies. The moment a wet fly penetrates the surface film, the fibers spring to outwards, giving the impression of life and a struggling insect. I am a firm believer this is why stikes often happen the instant a fly hits the water.

These surface tension forces are also key in the behavior of Tenkara flies. The cohesion force helps hold a fly on in the surface, pulling it downwards. It also helps spread the hackle fibers, as the fly is pulled up against the surface film. The forces are small, but definitely strong enough to bend delicate hackles.
http://www.flymphforum.com/viewtopic.ph ... ank#p50279










Here are a few links to some prior posts which may be useful.
http://www.flymphforum.com/viewtopic.ph ... t=20#p7918
http://www.flymphforum.com/viewtopic.ph ... hilit=Tank
http://www.flymphforum.com/viewtopic.ph ... hilit=tank
http://www.flymphforum.com/viewtopic.ph ... ank#p50279

[tie2fish]

What Ray said.

[daringduffer]

What did Ray say?



You aren't a lazy bastard, Ray.

dd

[Trifly]

I wonder how the hackles react in current? After moss has been cleaned off from them? After hours of being water logged and beaten from continual casting? I wonder if fish slime can have an effect on the hackles. Do the hackles remain in "full blossom" (for lack of knowing another term) after minimal/maximum use? I have seen marabou collapse around it self with minimum effort. Will soft hackle?

[Old Hat]

I was going to say "water cohesion", but Ray beat me to it. Water cohesion is what happens to create surface tension. I think labeling it "surface tension" might be a bit confusing to some as the term implies or often portrays the idea of a flat surface not a 3D surface.

With that I will add that the same cohesive appearance is achieved when the fly is being swung downstream against the current, the #1 reason flies are generally not as effective on the swing. They loose the lively appearance. Currents thus can be both positive or negative to the attractiveness of the fly dependent on the strength of the current and the direction in respects to the fly's presentation. The chaotic currents found in front of boulders midstream or on high grade mountain streams will toss and turn the soft hackle, working the hackle and give it a very lively struggling appearance much like any insect caught in the same turbulence. Unidirectional current working against a soft hackle will decrease the effectiveness of the fly by removing the ever important "lively" appearance. Does this mean there is no reason then to offer a soft hackle on the swing? No. However, if you do, the fly should be tied purposefully for that presentation. I have found a couple things that "help" out my swinging flies. 1. Go larger #6-#10. 2. Tie patterns that will look like something within that cohesive profile (small baitfish, leaches, large caddis etc).

[DUBBN]

Get a little fish slime , decaying moss, or dirt on your wet fly. Especially after it is water logged. I bet those barbules press against the abdomen at the slightest current caused pressure.

[redietz]

Get a little fish slime , decaying moss, or dirt on your wet fly. Especially after it is water logged. I bet those barbules press against the abdomen at the slightest current caused pressure.

Not so much in my experience. I used to believe that, but have looked often enough to believe otherwise. It takes a fair mount of current pressure to collapse them. Of course if you're pulling the fly hard upstream quickly, they will collapse, but as soon as you let up a bit, they fluff right out.

[DUBBN]

Get a little fish slime , decaying moss, or dirt on your wet fly. Especially after it is water logged. I bet those barbules press against the abdomen at the slightest current caused pressure.

Not so much in my experience. I used to believe that, but have looked often enough to believe otherwise. It takes a fair mount of current pressure to collapse them. Of course if you're pulling the fly hard upstream quickly, they will collapse, but as soon as you let up a bit, they fluff right out.

It takes very little current pressure to compress them. In my experience.