Sinking Fly Lines and Tips Sink Rates- Fact and Fiction

via Fly Fishing Research

if you too have ever had the strong feeling that sinking lines and tips don’t get down as advertised this article’s for you.
of course, given the myriad variables encountered in real-life fishing situations as opposed to lab environments like:
– current or it’s equivalent on stillwaters: wind
– and water temperature
– and tippet diameter and length
– and fly size and it’s buoyancy
– in the case of sink-tips if it’s attached to a floating line or sinking main line
– how hard or delicately the lines/tips land on the water
and other goodies like whether different parts of the line or tip’s diameters sink at different rates (something i didn’t see in their findings but i suspect is highly relevant)
yes, some manufacturers make density compensated lines, meaning the front of the fly line will sink faster than the back with the goal of keeping the complete line straight during the retrieve, but most sinking lines are single density.

anyhow, to spice things up even more, add to all of this a quasi-consistently ever-changing environment and probably a few other bazillion other things i’m not thinking about at the moment and it would basically be impossible for manufacturers to give us exact sink rates, but then, they could at least do those tests with the exact same things we’re buying in the package instead of shortened lengths and other non-realistic methods.

so, in the end we’re left with nothing very concrete sink-rate wise but is this really a problem ?
no, but since most of the variables mentioned above are about slowing down the sink process we’ll have to take them all into account and react accordingly instead of blindly relying on what’s written on the package, most often selecting lines or tips of a higher sink rate to eventually get the fly to what we hope is the right depth. hopefully…

if the article below tickles your funny bone be sure to click the links for descriptions of their studies, fluid dynamics studies, realistic charts, the science of sink rates and equations and other goodies. enjoy !

Sink Rate
Fly fishermen and manufacturers have long used sink rate (also called “type”) as a standard by which to compare sinking lines. Simply, sink rate is the speed at which a straight, horizontal section of line sinks in still water. For example, a type 3 sink tip sinks at 3 inches per second; a type 6 sink tip sinks at 6 inches per second.The manufacturers with whom we have spoken (Airflo, Rio, and Scientific Anglers) all measure sink rates the same way. They drop a short (1″-2″) piece of line in a tank (or tube) of water in a laboratory setting, and use sophisticated laser technology to determine the exact amount of time required for the line to sink a given distance. The advantages of this method are that it’s simple, transparent, and reproducible (doing the same test multiple times for the same line segment yields nearly identical results). However, we have found that the sink rates determined by this method overstate the true sink rate of a longer section of line — as would be used for fishing. We are grateful to Bruce Richards of Scientific Anglers (SA), who helped us test this in a sensible way. In his labs, Bruce measured the sink rates of 1″ – 2″ lengths of three of SA’s sink tips. He then shipped these same three sink tips to us. We meticulously measured the sink rates of long (10 ft) segments of these lines in a still-water swimming pool. We found that the long segments sink more slowly than the short ones by 8% to 17%.That short line segments sink faster than long ones is also predicted by fluid dynamics theory. The principle is the same in the design of aircraft wings, where theory and experiment have shown that very long, thin wings provide more lift than shorter ones. For short line segments, water flows rapidly around the ends of the line, reducing the vacuum on the high side of the line that contributes to its drag (…more on why short segments sink faster).For long (e.g. 10 ft) line segments, this “flowing around the ends” effect is negligible. In actual fishing situations, it is totally negligible because both ends of the line are connected to another line. So, while the manufacturers’ test is simple and reproducible, their measurement method itself leads to overestimation of sink rates. Jim Havstad independently reached this same conclusion — that short line segments sink faster than long ones — in his seminal study of fly line sink rates.In addition, we have noticed that in some (but not all) cases, manufacturers “type” designation deviates further from actual (long segment) sink rates than the difference in measurement methods would suggest. For example, a 109 grain, 15′  tip we tested sinks at 5.6 inches per second, even though its label says “Type 8.”To support more accurate determination of sink rates, we explain the science of sink rate. Multiple independent experiments have validated the accuracy of it’s predictions within a few percent — over many years, over many line types, and across several different experimenters. The results are simple sink rate lookup tables and also a sink rate and rule number calculator, which we publish here for the first time. For any given line diameter and grain weight, you can simply look up its sink rate in a table or calculate it with an on-line calculator. Such calculations are exact, in the sense that they are determined entirely by known laws of fluid dynamics and known physical constants. (Working through the sink rate equations is not for the faint of heart, but we provide them for those who wish to understand the theory or to program their own sink-rate calculator.)

3 thoughts on “Sinking Fly Lines and Tips Sink Rates- Fact and Fiction

  1. Nice Marc….
    Lots of factors for sure to consider for river and loch style angling!
    My loch setup reels are up to like 16 I think now! The surface film is most elastic with cold water and high barometer….favorite for emergers trapped right below surface film. Nice read bud….
    All the best,

    • glad you liked it Mac. i wonder if we can sue SA for false advertising ? 😆
      my stillwater kit has taken a major reduction over the last few years:
      1 full floater
      1 floater with tip cut back for the whole range of sink tips
      1 intermediate
      1 S3
      1 S7
      like with the floater w/sink tips, the S3+7 accept different tips to get different results. a hot combo has been the S7 with a floating (and well-floatanted) tip to slowly skip nymphs just above the bottom, sorta booby style. give it a try !
      btw, i just got notification from Coch y Bondu that your book is on it’s way. can’t wait !

Comments are closed.