Before discussing vertical drag, I want to introduce the concept of "Effective Drift Length (EDL)."
We all unconsciously apply this concept when we dry fly fish. Because we can see where the rise is when dry fly fishing, we instinctively know that the fly must be in the right current seam, on (adult) or in (emerger) the surface film, AND drifting drag free. Only when all of those three things occur do we have a chance of catching the fish.
The identical principal applies when we are nymphing. The fly must not only be in the correct current seam laterally (1) but vertically (2) in depth as well, and the must be as drag free (3) as possible. Only when these three criteria of lateral and vertical location and drift speed are correct is the drift effective in catching fish. The fishing method that gives keeps the fly at the correct location at the correct drift speed for the longest length has the best "Effective Drift Length".
With this concept in mind we can examine "vertical drag."
The definition of drag is tension on the fly that causes it to deviate from a path it would take if it were unattached. When vertical drag is used in nymphing it means to me that the fly is being pulled in the vertical direction by the leader/line drag. Vertical drag can be up or down but since we use floating fly line when nymphing, vertical drag is almost always in the upward direction. I don't know if Duane uses the term "vertical drag" in the same way I do.
The surface current is faster than the mid-current, and the mid-current is faster than the bottom current. A routine straight up stream cast places the leader and fly line downstream of the "splash point" of the nymph. However, as the nymph starts to sink, the leader and fly line are in a faster current seam, and there is both a downstream and upward tension on the nymph. So there is both DOWNSTREAM and VERTICAL drag.
Both types of drag limit the "EDL."
Downstream Drag causes the fly to speed downstream above the holding water of the fish and shortens the available length of the drift that remains when the fly does reach the level of the fish.
Vertical drag slows the sink rate of the nymph. By slowing the sink rate, there will be less EDL left in the drift.
To defeat vertical drag, the cast must place slack tippet on the splash point of the fly. To defeat downstream drag, the cast must place the leader and line upstream of the splash point of the fly.
So the cast must place the nymph downstream of the leader and fly line, AND place some loose tippet on the fly. When both things occur, the nymph can sink WITHOUT vertical drag AND downstream drag. This allows the nymph to sink quickly and delays the downstream drag of the faster traveling fly line. Immediate upstream mends of fly line are then placed to negate the downstream drag of fly line to maximize the EDL.
The correct cast for this is the Tuck Cast which results in the vertical mend and upstream position of leader and fly line. The tuck cast has the advantage of driving the fly into the water and this causes the fly to sink even faster than normal cast. This means a longer EDL because the fly reaches the level of the fish faster.
http://stevenojai.tripod.com/tuck.htm
http://www.flylife.com.au/library/casting/tuck.html
Regards,
Silver
"Discovery consists of seeing what everybody has seen and thinking what nobody has thought"..........Szent-Gyorgy