Leaders & Sinking Lines
As you probably know, all modern fly lines are tapered… and for good reason. The taper allows the energy contained in the leading edge of of fly line to dissipate as the line turns over the leader, tippet and fly. Without the taper, the line will retain energy for a longer period of time. This creates a tendency for the end of the line to "kick", due to insufficient dissipation of energy prior to the leading edge of the loop reaching the poorly chosen or poorly designed leader.
To get a good idea of what this looks and feels like, remove the leader and fly from a fly line and then cast it on the lawn. As the line turns over, you will feel a bump as the line turns over and you will notice that the end of the fly line is directed in the opposite direction of the upper loop (down). The effect can also be imitated by overpowering a false cast. As the line turns over you will feel a bump as the fly turns over. In this case that bump is the fly being kicked, as opposed to the line. If you make an overpowered hook cast to place the fly around a rock in the stream, you have taken advantage of kick to do so. Now consider using a leader that is considerably lighter than those you would normally use for that line and you can see that the fly line will still kick and take the light leader with it.
What is apparent visually is the end of the line, the leader, and the fly, not turning over. Instead they will pile up as much as 10 to 15 feet short of the intended target. What causes this "kick" is that few if any single strand leaders are designed properly for these sinking lines, even those
you find in your local fly shop. They have insufficient mass in the thick "butt" end of the leader to match the mass in the tip of the fly line. Unless the mass in the two materials is similar, the energy in the line will not effectively transfer to the leader.
Because the energy has to go somewhere, it causes the line to keep moving in the direction it was last sent, down. And if the leader is too light it will go where the end of the fly line goes.
Why isn't this effect seen with floating lines you might ask? Because floating lines and leaders are properly matched in most situations. Leaders are manufactured to match floating lines and are based upon the line weights they are most likely to be used with. Take a look at some of the leader specs for light trout leaders (7 foot, 6X or 7X, used on the lightest weigt rod), heavier trout leaders (9 foot, 4X and 3X, used on the common medium weight rods), and then some saltwater leaders (1X or 2X, used on heaveir weight rods). You will see that the butts of each leader class is different, and is in line with the 2/3 diameter rule (see below) for the most common line weights used in each type of fishing. This was not by mistake but deliberately designed that way. Most people are not aware of this.
The rule of thumb for matching a butt section to a floating line is to use a mono line that is approximately 2/3 the diameter of the line it is attached to. As it turns out, the tips of floating lines are about 50% larger in diameter than a piece of mono line of the same mass. that makes the mono 2/3 the size of the fly line. This is how the 2/3 rule was developed, and most shop personnel should know this rule. But beware of fly shop personel. Remember that many of them don't actually know the how and why of lines and leaders.
Some improperly feel this 2/3 rule applies also to sinking lines, but it doesn't. With a sinking line, if you use the 2/3 rule, you end up with a mismatch. The fly line will not only kick, but the junction of the line to leader will hinge, making the problem worse.
When using an intermediate sink line or tip, use a leader that has a butt that matches the diameter of the end of the fly line. Both the line and the leader are made of similar materials in this case and therefore have similar mass profiles.
The Problem with Sinking Lines
Matching the mass from a fast sink rate line to a leader butt can be challenging. For a fast sinking 400 gr. shooting head with a tip diameter of about .035 it will take a large diameter mono of maybe 80 to 100 pound test to match the mass of the tip of the line. At this point the diameter of the mono is greater than the diameter of the end of the fly line. Any attempts at tying on a knotted connection will cause failure of the connection. Any time you bend a monofilament line in a radius near to, or smaller than, its own radius, the knot is prone to failure. This is caused when the outer surface of the mono fractures from being stressed while bent in too sharp of a turn. Basically the mono develops a split in its outside wall and the line fails.
Ultimately, trying to go directly from a sinking fly line a to mono butt is an exercise in frustration.
So how is the problem solved? First, use a Braided Loop at the end of the fly line and attach the leader to it. Video instructions for braided loops can be found on the Braided Loop page.
Second, use a leader that is properly designed to handle the difference in mass between the materials while effectively transferring energy into continuously smaller diameter sections. This is how to get an efficient turn over without the kick.
This brings us to Twisted Leaders. A twisted leader is made from a continuous piece of flat diameter mono or fluorocarbon line (fluorocarbon is best for salt water) that is about 21 feet long, making a 7 foot leader. The tippet is added later so it can be designed to the needs of the angler. Video instructions for making a twisted teader can be found on the Twisted Leader page.
