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Dolphin nose on the back cast?

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Mangrove Cuckoo
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Joined: Tue Jan 29, 2013 7:51 am

Dolphin nose on the back cast?

#1

Post by Mangrove Cuckoo » Tue Oct 01, 2019 1:57 am

Playing around this evening I saw what I think was a dolphin nose on one of my back casts. It immediately made me go :???:

I don't think I have ever done that before, or at least I do not remember seeing it. Tonight was a bit unusual as there was a pretty stout wind that was very constant in strength and direction. Because of that, my loops were veering which gave me an uncommon almost sideview of my loops.

After thinking about it, I realized that I do not remember ever seeing a dolphin nose on any back cast. Not on videos or photos.

Am I just unobservant? Or is there some fundamental difference between the FC and BC that makes them more likely in the FC?
“Very simple man. Catching fish makes me happy. Scaringly simple.”

Håvard Stubø

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Graeme H
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Re: Dolphin nose on the back cast?

#2

Post by Graeme H » Tue Oct 01, 2019 3:50 am

I make dolphin noses on command by introducing a sudden stop or a slight pull-back on a cast with the bare minimum required power. My default non-hauled BC nearly always produces a dolphin nose.

I think you may not have seen them on other people's videos because it's not common for people to concentrate on the form of their back cast.

Cheers,
Graeme
FFi CCI

Geenomad
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Re: Dolphin nose on the back cast?

#3

Post by Geenomad » Tue Oct 01, 2019 5:08 am

Graeme H wrote:
Tue Oct 01, 2019 3:50 am
I think you may not have seen them on other people's videos because it's not common for people to concentrate on the form of their back cast.
Spot on Graeme. General comment. It makes a big difference when we start thinking about making two forward casts one of which goes behind us. :) Well that's how I think about it now and it made a big difference when the penny dropped. :D

Cheers
Mark
"The line of beauty is the result of perfect economy." R. W. Emerson.
https://thecuriousflycaster.com

nicholasfmoore
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Re: Dolphin nose on the back cast?

#4

Post by nicholasfmoore » Tue Oct 01, 2019 1:09 pm

Very interesting topic, i do like the dolphin nose shape of loops. If i wanted to throw dolphin nose shaped loops (without hauling) i always think to myself "cast 6 inch wide loops as slow as possible, no.....slower.....SLOWER!" the shape comes from very low acceleration with the additional component of our world, drag! I can't remember what the scientific term is for this, it's .......number if i remember rightly, perhaps someone can point me towards the term?

Interesting you use pull back to achieve this, Graeme. Wouldn't this accelerate turnover and change the loop shape if you did it too much? I've never done that before for the dolphin nose shapes, i will have to try it.

All the best

Nick

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Graeme H
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Re: Dolphin nose on the back cast?

#5

Post by Graeme H » Wed Oct 02, 2019 12:22 am

nicholasfmoore wrote:
Tue Oct 01, 2019 1:09 pm
Interesting you use pull back to achieve this, Graeme. Wouldn't this accelerate turnover and change the loop shape if you did it too much? I've never done that before for the dolphin nose shapes, i will have to try it.
It's really only enough to stop the tip suddenly, and do so in close proximity to the line path.
the shape comes from very low acceleration with the additional component of our world, drag! I can't remember what the scientific term is for this, it's .......number if i remember rightly, perhaps someone can point me towards the term?
Reynolds Number is the term you want, but drag is not the cause. Have a look at the snap casts in this video, in which the dolphin noses form without any "forward" motion of the fly leg. In the very first instants this cast, you will see the beginnings of the dolphin nose and the fly leg has not even moved yet. (Also take a peek at the cast at the 1:50 time stamp.)



Cheers,
Graeme
FFi CCI

Dirk le Roux
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Re: Dolphin nose on the back cast?

#6

Post by Dirk le Roux » Wed Oct 02, 2019 7:46 am

A good example of a back cast dolphin nose can be seen in Graeme's cast analysed in this thread.

On that back cast-forward cast sequence, it was the narrower initial loop that had the dolphin nose.

Regards,
Dirk

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James9118
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Re: Dolphin nose on the back cast?

#7

Post by James9118 » Wed Oct 02, 2019 9:35 am

the shape comes from very low acceleration with the additional component of our world, drag!
Its very easy to get DNLs with T120 and T38 lines even when casting fast. This observation implies that line stiffness has some part to play.

Torsten
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Re: Dolphin nose on the back cast?

#8

Post by Torsten » Wed Oct 02, 2019 10:23 am

Hi Mangrove Chuckoo,

Mac Brown has written a good article about this subject, read [1].

Are you referring to the "Dolphin" or the "Wedge" (leading edge, Fig. 3/4) ?

The wedge shape is usually a result of loop morphing, such loop shapes begin with a roundish loop and morph into a pointy loop shape.
Major factors that promote in my opinion the change of shape are the trajectory of the line, tension and aerodynamic forces (especially the drag force). If the line speed is high enough even large rounder loops morph into this shape.
After thinking about it, I realized that I do not remember ever seeing a dolphin nose on any back cast. Not on videos or photos.
The wedge shape is quite common for tournament distance casting and seen often for both back-/forward cast.
Am I just unobservant? Or is there some fundamental difference between the FC and BC that makes them more likely in the FC?
Many use an asymmetric casting stroke and thus its more likely that they get a roundish loop (backcast); an example is the "classic
english" style of casting, with early rotation on the backcast.

--

[1] MORPHING LOOPS, WHAT CHANGING LOOP SHAPES TELL US ABOUT A CAST, Mac Brown - THE LOOP - SPRING 2014,
https://flyfishersinternational.org/Por ... 095224-000

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gordonjudd
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Re: Dolphin nose on the back cast?

#9

Post by gordonjudd » Wed Oct 02, 2019 4:50 pm

Mac Brown has written a good article about this subject, read [1].
Torsten,
Thanks for finding that article. Very interesting observations but I did not understand his reference to Reynolds number affecting the tension in the loop.
Low acceleration yields lower tension on both fly and rod legs (low Reynold’s numbers, for the scientists out there)
That line tension is related to the linear mass density of the line and the square of the tangential velocity of the line going around the loop and thus has nothing to do with the Reynolds number value that will impact the skin drag coefficient. Also, as given by White when the length/diameter factor of the line is taken into account the resulting Re_l factor can actually be quite high. For instance a 1meter length of line having a velocity of 20 m/s Will have a Re_l number that is greater than a million.

I still don't think we understand all of the factors that are involved with producing the Dolphin nose loop shape, but I think most high speed video's show that the loop shape that is formed at MCF is elliptical in shape and then morphs into a DN shape after it propagates and is influenced by aerodynamic effects.

The Dolphin nose shape shows up in so many mediums (dog leashes, paper strips, yarn, chain fountain, etc.) that it probably is affected by the line's stiffness, visco-elastic, and density factors in addition to specific loop size and line tension factors.

No doubt numerous aerodynamic effects come into play as well. In that regard I find it telling that the key factor that was observed in producing the DN shape in some string shooter loops has never been mentioned as an important cause of the DN shape in fly casting.
Another peculiar behavior is the following. At the right extremity, where the string turns
back in direction of the wheels, one sees the string rising before going down again. (see the red
circle on the setup picture 1). As surrounding pressure decrease, this little loop slowly disappear.
Image

Other teams in that International Physics Tournament competition saw a similar effect on some of their loops and thus attributed the dip in the DN shape to some kind of an aerodynamic effect.

In further discussions with Maxime he noted:
Further analysis by the Ukrainian team using a smoke generator to view the air stream near the top leg end of the loop showed the existence of vortex shedding at the very top of the loop that could explain the shape by the gradient of pressure.

These vortexes were produced when the boundary layer of air being carried along the top leg of the loop was shed as the direction of the line changed in going around the loop. That could well be a source for a DN shape to show up in fly line loops as well, especially when the tension in the line is insufficient to offset the pressure gradient associated with the vortexes at the top of the loop.

That does not explain what may cause the perturbed wave that shows up in DN shape observed in the chain fountain however as the heavy chain should not be impacted in any significant way by drag forces since it is so heavy. Thus I would expect that vortex shedding could be an important, but likely not the only effect that produces the DN shape in fly casting loops.

Gordy

nicholasfmoore
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Joined: Tue Jul 16, 2019 1:41 pm

Re: Dolphin nose on the back cast?

#10

Post by nicholasfmoore » Thu Oct 03, 2019 10:05 am

Thank's all for the scientific explanation, especially enjoyed your video, Graeme!

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