Here's one I've been wanting to "put to bed" for a while, and I finally got around to testing it the past few days. You've probably heard something like this before, "Fluoro and mono stretch at different rates, so at low pressure levels, like 'bites', fluoro is very low stretch. This is what makes fluoro more sensitive". Part of the problem I've had with this statement is that in every instance I've run across it, there has never once been any link to or posted data to support such a statement. It's just there for the reader, take it at face value and believe it. I can't accept that. I need at least some anecdotal support for a claim. In addition, I had touched on this issue in part via my earlier testing, and wasn't seeing where the little data I had would support the claim.
As part of my testing and research for my initial article on fluorocarbon, I had tested several lines for stretch at levels starting at just 1/2-lb. of pressure and moving up in 1/2-lb. increments. I also did a single static test at the 1.5-oz. rate, and saw no measurable difference between mono and fluoro. Still, to truly compare these low pressure rates, I needed to test a range of low rates of force and then adjust them so the lines were compared based upon diameter, a true stress-strain test.
So I grabbed my spools of 6lb. line, using 2 fluoros, 2 copoly's and 2 monofilaments, and went to testing. I used 9 different force calculation pts., starting at 1.3 ounces and going up in increments to 12 ounces, or 3/4 of a pound. Then all the gory calculations to calculate actual force per unit area and converted to kilopascals (kPa), the/(a) generally accepted unit of pressure measurement. Of course, this was plotted against % elongation to create the following stress-strain graph (click to enlarge).
So, the simplest answer to the question seems to be, "yes, sometimes, but not necessarily". For example, at any given pressure, Stren original (mono) and P*line CXX (coploy) were nearly identical and always stretched the most. In the middle of the pack you have three almost identical curves for Trilene XT (mono), Berkley Sensation (copoly), and Stren 100% (fluorocarbon) - practically no differences between those 3 lines at any given force measurement. The best of the bunch, that line with the least amount of stretch per unit area/force in this test turned out to be Berkley Vanish Transition (fluoro). I actually ran this test twice over two separate days, in one case using regular Vanish and in the other, Vanish Transition, and came up with basically identical results, so I'm comfortable with the results in that regard.
I also went back and ran the calculations for some previous testing I did that encompassed these exact same lines plus a few more, all tested at the 3lb. static load level, equivalent to half the rated breaking strain, and got the following results. Again, after digesting the data, the simple answer is that the different line types are all over the board at this higher pressure testing, and not necessarily the same outcome relative to their lower pressure results.
One other comment relative to the testing is that these lines were all tested "dry". it is well known that monofilaments will absorb water after a good saturation period of a couple hours, and their respective amount of stretch will increase. However, it turns out that most all copoly's also exhibit this same phenomenon, and surprisingly, several of the fluoros (which arguably shouldn't if they are close to 100% pure fluoro), including Vanish fluorocarbon as used in this test, do too. And the percent increase in stretch turns out to be almost identical regardless of line type, typically in the 1.0% - 2.0% range. As such, retesting with these lines "wet" would basically be a wash. Still, if you use a flurocarbon that doesn't exhibit this increase in stretch when wet tested (see Tackle Tour test results for some specifics, along with my previous article linked at top), your line would perform a little better in such a test, and a greater distinction might be noticed.
Finally, there is the whole argument around what degree of difference in stretch would there need to be in order for an angler to detect the difference in feel? If two lines have a 1 or 2 percent difference in strain at a given stress level, is that even discernable? What about a 2X difference between, say 4% and 8%, like the 'high' and the 'low' at a given force in this chart? That becomes a qualitative issue, as opposed to a quantitative one, where the perceptions or pre-conceived beliefs, i.e. from marketing, might erroneously skew results.
So the bottom line to me is that it still comes down to the fact that individual lines are truly "individual" in their properties, and that to make an across the board declaration as relates to fluoros "stretching less/being more sensitive due to less stretching" in any capacity against mono's or copoly's is to put yourself at high risk of being wrong. Until someone can provide some test data using more sensitive/technical levels of equipment (Instron testing, etc.) to show otherwise, I'll still stick by the density factor as the primary reason for the increased sensitivity with fluorocarbons, with perhaps, the sinking/straight-line theories a secondary but lesser component of that basis. As always, you need to pick the line you use based on how it performs for your exact scenario, and probably evaluate several of its attributes when making that decision. Just be careful of the claims others make in regards to this issue.
- see also, "Connecting to Fluorocarbon"