How to choose a sinking fly line? Yikes!
Sinking lines are not quite as widely-used as floating lines, but they can very often be a requirement. Whether fishing a river, lake, or the wide-open ocean, if you need to keep a fly down in the strike zone the most efficient way possible, there's often no better way to do it than with the right sinking line. Floating lines are pretty simplistic and easy to purchase, but the fact that sinking lines combine some additional factors and ratings can cause confusion. For those who might need the help, I wanted to break some of this stuff down into basic easy-to-understand sections....read on!
Grain Weight: In fly fishing, you cast the weight of the fly line which carries the fly out to the water. To do this, a fly line needs to have a certain amount of weight to it. A fly line's weight is measured in grains. Grain weight is typically measured in the first 30 feet of a floating fly line (minus the short level tip at the very front), but with sinking lines the measured section that's specified can vary depending on head length.
The grain weight of a floating or sinking fly line allows it to be given a corresponding line weight rating (like 5 weight, 6 weight, etc) to easily match to your rod. Some sinking lines, however, are specified with just the grain weight and no line weight. If a chart is not shown to help you find the right grain weight for your rod, you can refer to our chart found on this page. Although manufacturers often stray from those numbers when designating line weights, it at least provides a baseline.
The part of this that can often be really confusing and sometimes misconstrued is that the grain weight has nothing to do with sink rate. You'd think that a heavier grain weight automatically means a line will sink faster, but that's not the case. Remember, floating lines come in all sorts of different grain weights ranging from light to super heavy, but that's simply so they can be matched up with different rods; a 380 grain floating line floats just the same as a floating line weighing 140 grains! Just like floating lines, sinking lines also come in different grain weights so they can be casted on all sorts of different rod sizes, but the next section explains why some lines sink faster than others.
Density: Let's say you have two sinking lines. Each line displays the exact same grain weight, but one of them sinks quite a bit faster than the other. How can that be? That's because the faster sinking line has more density than the other. Manufacturers design lines like this so you can own multiple lines with varying sink rates for a single rod size. Pictured below are the labels from two different Scientific Anglers SONAR sinking leaders, which are basically like little detachable/portable sink tips. As you can see, both weigh 50 grains but they each sink at different rates due to different densities.
If I took a basketball (which is not dense) and tossed it into a pool, it will float. Now, if I took a rock that weighs the exact same and tossed it in that pool, it will sink because it's dense. Similar to that basketball being full of air, floating fly lines are made with little hollow air pockets in the coating to keep them buoyant. On the other hand, sinking lines incorporate material like powdered tungsten in their coatings to increase density to make them sink.
Sink Rates: Based on density, sinking lines are usually given a name/number rating to show their sink rates. The inches-per-second (ips) at which the line sinks is often shown as well. Starting out, a line labeled "intermediate" sinks very slowly at about 1–2 ips. Beyond intermediate, lines sink faster and will often be classified by a name such as "Type" along with a number. For example, a Type 3 line may sink at around 3 ips, a Type 5 around 5 ips, and so on. The naming and related sink rate of the line can vary, however, so look for the manufacturer's listed ips to know exactly what you're getting.
To confuse matters further, sometimes a certain line may also be specified to sink within a range. Let's say as a random example a manufacturer makes one Type 5 sinking line that sinks at 4ips–6ips and comes in different line weights (aka grain weights) for use on 5–9 weight rods. In this case, the higher the line weight rating the faster the sink. This line purchased for a 5 weight rod might sink at around 4ips (the low end of the range), while that line in a 9 weight might sink around 6ips (the high end of the range) because that bigger line simply has a little more density. Also of note, notice I never said I think a given sink rate is absolutely exact. I always consider the listed sink rate to be approximate since many factors affect how a line sinks, but at least it gives me a rough idea of about how quickly my line is getting down.
I hope this helps shed light on some of the basic technical aspects of sinking lines. They don't cast quite as fluidly as floating lines, but they can be crucial to you catching fish in some situations. Give them a try!
Also, be sure to have a look at one of our older articles RIGHT HERE explaining Density Compensated Sinking lines!