Snap cast anatomy

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Snap cast anatomy

Post Number:#1  Postby Merlin » Wed Nov 14, 2018 10:02 am

Hi all

I took some time to model a snap cast. The idea behind modeling was to identify the most sensitive parameters for this type of cast. I used data produced by Dirk from Graeme’s record as a guideline, the aim being to find something corresponding nearly to the guideline. Graeme’s record is illustrated with dotted lines, and the model is illustrated with solid lines. I took a set of best guess characteristics using a picture in the video as a guideline (snap angle, line length, initial loop height above the ground, etc.). The fly leg is supposed to be hanging entirely just as if the snap cast was made from a balcony, this allows to simply simulate some friction on the ground for the fly leg, which is likely a pessimistic view. I can play with the various parameters and fit more closely to the record, but this is not the purpose of the model, the purpose is to look after important casting parameters.
Here are two graphics, one for speeds, and one for acceleration:

snap speeds.JPG
snap speeds.JPG (48.91 KiB) Viewed 388 times

Snap accelerations.JPG
Snap accelerations.JPG (50.68 KiB) Viewed 388 times

I tried various approaches and finally chose to define the main input by taking inspiration from rod behavior, and the best simulation is obtained with a sine signal up to max tip speed, followed by another one down to the ground. This assumes that the caster lowers the rod tip during counterflex and the return to RSP, until the line hits the ground with the rod tip just above. You can see that there are discontinuities because the changes in accelerations are not smoothed; they are more brutal than in reality where things blend together more gently. The point I was after here was to identify a sign on the curves corresponding to ground hitting, and there is one corresponding to an inflexion in most speed curves. You can notice that the model is unable to catch the observed variations in the fly leg curves (both speed and acceleration, their frequency correspond approximately to a second vibration mode for a rod fitted with some line).

In this model there is some drag added for a small loop during the last phase, after ground hitting. The model is rather crude with a level line/leader and a tiny loop, which allows simplifying equations. The energy corresponding to the collision with the ground is supposed to be entirely lost. I tested a step change in the linear density for the last meters of the line/leader, just to see what was going on in that case:

Snap speeds leader.JPG
Snap speeds leader.JPG (51.35 KiB) Viewed 388 times

Snap accelerations leader.JPG
Snap accelerations leader.JPG (52.21 KiB) Viewed 388 times

As one can see, the trend is a flattening of the fly leg speed curve, with acceleration close to zero. It may explain the trend observed in Graeme’s cast because there is a taper for the line and the leader in real conditions.
Now it is interesting to identify the most sensitive parameters. If you look after higher speeds in general, then you must make a large downwards acceleration to maximize the downwards speed of the rod leg, but more than that, you should do you best to give a first upward input to the fly leg before snapping down. I do not speak of m/s here, but of tenths of that. It is really amazing to see the effect of 0.1 m/s on both max input speed and initial fly leg speed, the latter being the most sensitive one. If we assume that the illustrated cast is fine, then what happens if we reduce the snap maximum downwards speed but compensate that by a larger initial upward speed for the fly leg? If I reduce the downward maximum speed by 20% (about 4 m/s), then I have to compensate by adding 2 m/s to the initial upwards speed for the fly leg (e.g. from 1m/s to 3 m/s). So if your snap is somehow weak in the downwards direction for any reason (e.g. a slow rod), get prepared to give more upwards momentum to the fly leg before you snap down. Now snap casters can tell us if this makes sense along their own experience.

The third main parameter is the line/leader length, the shorter it is, the better is the final upward fly leg speed, but this is intuitive. It is more difficult to snap a long line.

A lot of work for few results, but the interest was in the modeling challenge. I hope I did not open a new can of worms.

Merlin
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Snap cast anatomy

Post Number:#2  Postby Paul Arden » Wed Nov 14, 2018 12:23 pm

Hi Merlin.

I find the last part interesting because I can snap more line than I can pickup! In fact it’s my most effective way of clearing the line from log jams and sailing boats.

I agree that the line must be moving for the Snap to work at all.

I’ll have to have a play with softer or heavier lined rods. But I certainly think so.

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Snap cast anatomy

Post Number:#3  Postby gordonjudd » Wed Nov 21, 2018 11:42 pm

Merlin,
Your model's correlation with the measured velocity data is impressive.

Since Graeme's snap cast example was nearly vertical did you include the 9.82 m/s.^2 magnitude of gravity in your acceleration curves?

Gordy
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Snap cast anatomy

Post Number:#4  Postby Merlin » Thu Nov 22, 2018 9:04 am

Yes Gordy, gavity is included.

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Snap cast anatomy

Post Number:#5  Postby Mangrove Cuckoo » Thu Nov 22, 2018 4:04 pm

I frequently use a snap to check my leader or fly when casting on grass, but have found little use when fishing.

If there is a fly of any significant size on the end of the leader and the fly is under the water, attempting a snap cast will either fail if the fly is a tad too deep, or the pick up will rocket back so fast and with so little control as to be potentially dangerous!

Two questions come to mind:

Paul, when you say you use a snap to clear flyline from a logjam, etc.. I assume you mean the hook is not caught, just the line is over something??? Doesn't the leader break if the hook either has or finds a home?

And on a more technical plane... how does a snap cast differ from pull-back? Are they the same phenomena, with one being just more pronounced?
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Snap cast anatomy

Post Number:#6  Postby Paul Arden » Thu Nov 22, 2018 4:31 pm

Hi Gary,

Yes just to clear the line away before it fouls or gets run over. When fishing heavier flies like you are using I use circles instead of snaps. With dry flies I frequently use snaps on my Spey casts.

Interesting question on pull back vs snaps. Obviously they are tied together. It will be interesting to see what the physicists wrote.

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Snap cast anatomy

Post Number:#7  Postby Merlin » Sun Nov 25, 2018 9:57 am

And on a more technical plane... how does a snap cast differ from pull-back? Are they the same phenomena, with one being just more pronounced?

A pullback is a complementary action on a forward cast, usually, and a snap is a dominant forward action on a tiny backcast, but the physics are the same. However, a snap is followed by a collision with water and that changes the dynamics by comparison to an aerialized pullback.

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