It's difficult to say without a video clip or first hand observation, of course we can formulate some theories.
Most likely it's a matter of things being out of alignment, as this most often results in a curved line. The d-loop and casting direction could be skew, the forward stroke could be curved, or the position of the anchor relative to the d-loop could be too far apart causing a 'flipping' effect when powered through.
What you can do regardless of cause is:
a) make sure your initial movement produces a nicely shaped 'candy cane' shape with the belly of the line curved nicely upstream leaving the end of the line where it meets the leader about where you could touch it with the rod tip on your right side and out in front (45 degrees out to the right in this case).
b) make sure your second movement (the sweep) pulls the line around into the backcast at an almost level motion but with a slight incline (like a screw-top jar lid) as it comes back to the d-loop...
c) then make sure that the rod does not swing too far around behind you - in other words stop short of straight back leaving an angle in the rod and let the top half of the d-loop tighten up backwards into a straight line opposite the target. Put your left foot back and watch the top half of the loop with your eyes. It should tighten up in line with your target. If it isn't in line with the target, you can't force it over... you will have to obey the angle of aim and go where that top half is lined up.
d) make sure your anchor is on the right side about a rod's length out and to the right. Having it be too far up or down river causes it to jump up irregularly and flip to one side.
e) make sure you have enough power, but smooth power to energize the line through it's full course. Another way to get a curve in the line is to under-power the loop.
If I had the time I would draw some diagrams... but just read Simon Gawesworth's book for the most thorough descriptions of the double spey you could dream of.
Also, if you had video clips you could get some super diagnosis. Feel free to send them privately in the event you don't want them posted publicly but on the other hand we've all been through the same thing and it could be education to others.
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Last edited by juro; 02-08-2005 at 10:42 PM.