Server - I'm not sure I understand your description of what happens in
the snap T. Are you saying that, given a stationary
rod, if excessive torque is applied at the butt the rod tip will actually
move in the opposite direction to the rest of the
rod resulting in the line kicking back (i.e. doing a snap T)?
----- Original Message -----
Sent: Wednesday, March 29, 2006 10:48
PM
Subject: Re: High Speed Data for
Straight Rod Tip Trajectory Cast
Hi Walter and Gordy,
For purposes of keeping to "single effects" I called the cast I was
talking about a baseline cast somewhere but I didn't go into details.
This is a cast where the hand essentially doesn't translate (i.e., move
forward//backward) during the cast and the cast is implemented largely by a
wrist motion. Granted there will always be an earlier phase where the
hand and arm move to position themselves for the cast but this phase doesn't
add much energy to the system. So after arriving at the "casting
position" the caster generates the casting phases by rotating the rod
butt. This is a good prototypical cast to analyze as a
baseline because it will be a part of any more energetic cast.
For example you can add a line haul to this or superimpose a base motion to
it (translation velocity of hand) each of which change the energetics of the
casting interaction. Walter, the work done by the (torque)x(casting
rotation) in this scenario (that I defined in the earlier E-mail) is the
work the caster performs on the rod-line system. If one is trying to
generate maximum rod-line energies then the goal is maximize this work (work
done = energy gain of rod-line).
With a system where the motion has
a limited range then the energy which can be gained is limited to the
"integral" of the torque against the angular displacement (a number like
60-70-80 degrees). This work is increased if one can apply a larger
torque throughout the displacement. I didn't discuss the flip side of
the relationship -- if we could we'd all dial-in a large torque and apply it
throughout the range of motion and impart lots of energy to the
flyline. But you can't just request a large torque -- the torque which
results from a particular casting motion depends on a lot of factors which
are at the control of the caster -- ultimately limited by natural physical
ability. A novice caster can seek a large torque but end up with low
torques because of numerous faults. The things you mention about
"point of impact", etc address these aspects. How to eliminate faults
and have all the factors favorable to allow the generation of larger torques
(things like a line configuration without slack, orientation of the line
relative to the casting direction, preload on the rod when performing the
"cast", the amount of line in the upper leg of the line, etc).
Assuming all these factors have been optimized then the torque on the rod
(you can think of this as a resistance to rotation of the rod butt) can be
increased by reducing the time duration for rotating the rod butt. We
all know from experience that everything being equal (and not initially in a
faulted state) that demanding that the rod rotate through its range as
quickly as possible raises the resistance to the rotation because the
flyline and the rod interact to generate larger tensions in the flyline
(associated with larger line momenta rates of change) and the "reactions"
(basically an equal and opposite tension acting against the tip of the
rod) acting on the rod tip add to the torque which can be applied by the
hand onto the rod butt. Without the forces at the rod tip we cannot
generate large hand torques and can't do much work on the rod. So a key
for casters is to know how to maximize the torque during casting. You
think what I've said to this point is long -- well the discussion of how to
setup this interaction to generate large rod tip forces (and hence greater
resistance at the rod butt yielding increased work) is rather a lengthy
discussion. But I''ll just mention a few facts so you can start
getting used to them. You need for the line linear momenta to be
changing to produce tension in the flyline. For a straight flyline
this is proportional to the acceleration of the line. If the flyline
has a top and bottom leg then tension is generated by the line passing from
one leg into the other as the loop progresses. Acceleration and line
tension are cheap//easy when things aren't moving and they become more
expensive//hard as the velocity increases. Moving at a speed which
isn't changing means there are no tensions/forces and you are back to having
no resistance (only resistance is from things you don't care
about like aerodynamic resistance against moving rod). If
the flyline isn't lined-up straight in the direction of rod tip travel it
will try to gradually line itself with that direction. If the
direction of rod tip motion isn't in a straight line the line is always
being asked to change its direction. Any changes in line direction are
caused by the tension in the line and if there isn't much tension (casting
defects) the changes won't happen or will take a long time. For a good
basic cast you want good angles -- (to achieve the tip trajectory
you seek), the flyline in the proper position in the air, proper preload,
and a wrist rotation quickness correlated with the energy you seek to
generate. All things being equal the most energy generation occurs
with the quickest rotation because this yields the largest accelerations,
the largest resistance (again this is resistance is at the rod tip), and the
largest torques at the hand. This all assumes a cast which starts
without faults -slack line, etc.
I think, perhaps you in particular as well as possibly other board
members may be interested in I what I feel is the most common and
important casting fault for experienced casters. It relates directly
to your remark about the difference between BC and FC in terms of generating
bad loops. I mentioned this on the other board but strangely it didn't
raise any comments -- seems to have flown under the radar. It is a
description of the same situation that Gordy mentioned but provides
detail in understanding it and correcting it. It stems from a
structural effect that fly rods demonstrate and was described to me by a
past colleague (Al Harral). He told me to hold a fly rod with tip near
a ceiling and to snap the rod down (but carefully) to see the tip move in
the opposite direction and touch the ceiling (if you haven't done this
yourself you should). Of course today we have a name for this and it
is a favorite casting move of Gordy's - the Snap-T. Well the Snap-T
effect is a primary casting fault for many experienced casters -- most
experienced casters with casting faults are probably running into
inadvertent//unwanted Snap-Ts. This is the condition we also might
call "overpowering the rod". Basically it is a casting fault to try
and exert a large torque if the rod is not suitably preloaded because the
tip, worst case, will actually move backward as the rod loads, throw slack
into the system, and bang the line when it unloads or the rod tip begins
actually moving forward and make it physically impossible to maintain the
rod loading. The rod almost immediately unloads, sends a flaky
wave down the line (can be a tailing loop) and then the casters continues on
with the cast with all kinds of various results. As Gordy
indicated this isn't common with the backcast motion because its not as
natural thing to do as it is during the FC. What you do to solve
it is have a planned casting phase before the "cast". This is the
preload phase -- move the hand rather deliberately timing the motion to have
significant preload when arriving at the casting location. With enough
preload it seems that the Snap-T effect is eliminated -- experiment with how
much preload you need. To give you an idea of preload, one of the high
speed images in a earlier E-mail clearly shows the end of the preloading
phase and the rod preloaded. Walter let me know if my WAG about Snap-T
is valid for you.
Best regards - Server