Most of us start our fly-casting careers with a floating line, but a lot of situations call for getting the fly much deeper than a floating line allows. A weighted fly does the job, to a point, but a weighted line does it better, and covers a wider range of variables.
Full-sinking, uniform-density lines have pretty much gone the way of the buffalo, and for good reason. The belly of a uniform-density line sinks faster than the end with the fly on it, resulting in a big sag in the middle. The fly, hanging above the line in the water column, is hard to manipulate, and sensitivity to the strike suffers as well.
Density-compensated lines are the rule of the day: Their forward section sinks faster than the belly and running line, preventing a sag in the middle.
Sink-tip or sink-head lines — 10 to 30 feet of sinking line backed by a floating or intermediate running line — find broader application, and handle much better than full sinkers.
Along with configuration, sinking lines are identified by their sink rate: for instance, from 1.5 inches per second (ips) for an intermediate line, and up to 6 or 8 inches or even 10 ips for a fast sinker.
These rates are a rough guide, not absolute. Sink rates determined in lab conditions seldom predict real-world performance. The actual sink rate depends on current or the drift of the boat, as well as line weight and type of fly (a big-eyed Clouser will bomb to the bottom, whereas bushy streamers will slow things down). Use the published sink rate as a guideline rather than an actual speed.
A line tagged with a 6 ips sink rate likely won’t sink that fast in a fishing situation, but certainly in those same conditions, it will sink twice as fast as a line rated at 3 ips.
Choose a sink rate based on generalities: slow sinking for less depth and fast sinking for deep water. Then learn how to make it work, which means slowing everything down.
Sinking lines cast like a bullet. Smaller in diameter and with more mass, they accelerate slowly during the false cast, but they really fly once you generate line speed and shoot a lot of line.
They also carry in the air less well than floating lines. Combine these characteristics, and the proper technique becomes obvious: Minimize false casting, and shoot for the desired distance with sinking lines.
Once the fly is on the water, patience comes into play. Give the line and the fly time to sink into the zone you intend to fish. Use the published sink rate, as well as the current conditions and the drift of the boat, to figure out how fast the line is really sinking. Bear in mind that a 12-weight or 600-grain line sinks faster than a 7-weight, 300-grain line. Weight trumps diameter.
Whether you are jigging the retrieve, swimming the fly or letting it swing in a current, there comes a time to make another cast, and floating-line technique won’t work. When the line is deep in the water column, you won’t be able to lift it up with a single backcast. The line must be at or very near the surface before you attempt a backcast. It’s necessary to strip in the sunken portion first, so when you raise the rod, the line comes off the water smoothly.
Best practice is to strip line in until the leader/line connection is at the surface, then make a roll cast, flopping the head and leader out on the water in front of you, and then, before it has a chance to sink, lift it into the backcast.
Once you get the feel, you should be able to make the clearing roll cast, pick that up into a backcast, then come forward and shoot line to reach the target.
The more false casts you attempt, the more likely you’ll develop problems in timing. Sinking lines have the innate ability to effectively cast with few or no false casts. Learn to utilize these strengths, and sinking lines can open up a new, third dimension of more-effective fly-fishing.