Of all the hype and advertisement lingo generated by bass fishing tackle, perhaps some of the most misunderstood relates to fluorocarbon line. We're going to take a look at some of the claims, and set the record straight on much of what you hear or read, "Big Indiana Bass" style. That means they'll be lots of science and/or math involved, or actual experiments used to make the points. We wouldn't have it any other way on this site.
To start, let's tackle one of the two worst abused properties of fluorocarbon, that being the claims of "invisible", "nearly invisible" or "virtually invisible". This is widely touted based upon a term called refractive index. You can read about it at the link above, but the numbers you hear thrown about are the following:
- Water has a refractive index of 1.3330
- fluorocarbon has an index of 1.42
- and nylon monofilaments tend to range from 1.53-1.62
All this means is that fluorocarbon's number is closer to that of waters, but it doesn't necessarily translate into any form of "invisibility" because of it, simply because there are so many other variables involved. Back in 2001, Jeff Thomson posted a great piece that refuted scientifically any claims as to whether fluorocarbon line was "invisible". That piece has since disappeared from the Net, but being the science junkie I am, I immediately saved a copy when I first came across it. I've tried to contact Jeff to get permission to use the piece on this site, so far unsuccessfully, so this may or may not stay up depending, as my intent isn't to in any way risk copyright infringement. It has been 5 or 6 years since I've seen Jeff post on any of the message forums, but I've added his presentation to the reports section of this site, just because such nice pieces of fishing related science shouldn't be lost to the world :)
So, to once and for all crush this idea of fluorocarbon invisibility, see Jeff's piece entitled "Mathematical Theory of Fishing Line Visibility" at this link, or in the reports section of the site, lower left sidebar.
Next, we'll look at another of the egregious claims bestowed upon fluorocarbon line by anglers and manufacturers alike, and that is one of less stretch or low stretch. First though, we need to learn a little more about just what stretch is, and to do that means we need to know stress-strain curves. Keep in mind I'm not a materials engineer, but I'll try and do justice to the terminology as best I can while still getting the point across.
Many of you have probably heard the term tensile strength when talking about fishing line. Advertisers frequently reference it when trying to convince you how tough their fishing line is, often in relation to it's breaking strenth. Tensile strength is actually a measurement of ultimate breaking strength (a strain) divided by the cross-sectional area of the line in question. But material testers also use what is termed a tensile test, also commonly called a stress-strain test. Basically, a material has a strain placed upon it and a measure of that material's stretch or elongation relative to its original length for that given strain is documented. When plotted out you end up with what is known as a stress-strain curve like in the above pictures. A typical curve generated for monofilament is on the left, while on the right, a similar type curve representative of what you might see for a thermoplastic such as polyethylene or polycarbonate. You'll notice the differences (forget the specific units for now), and that brings us to another needed explanation.
Stretch actually involves two different components; elasticity and plasticity. Elasticity is typically at the intial loading of the curve and encompasses the ability of a material to immediately return to it's original state (length when referring to stretching a fishing line) upon release of that load or strain. Elasticity is usually a linear relationship whereby as strain increases, % elongation increases proportionately. But many materials after reaching a certain level of strain start to deform to the point of not being able to return to original length once the load or strain is released. That part of the stretch equation is referred to as the plastic phase.
In the thermoplastic curve above, that is represented by the curve flattening out across the graph after the initial peak. At this point, a small increase in strain creates a disproportionate increase in elongation. With brittle materials, little or no plastic deformation occurs and the material fractures near the end of the linear-elastic portion of the curve. But with various thermoplastics, there is a gradual transition from elastic to plastic behavior, and the exact point at which plastic deformation begins to occur is hard to determine.
All that brings us around to what is starting to finally be disseminated to the angling public, and that is that fluorocarbon has as much total stretch as a percentage of starting length (elongation) as monofilament when tested off the shelf. It's just that after all the stretching is done, fluorocarbon will typically be left at a longer length than original due to permanent deformation, while most monofilaments will be close to their original length as measured prior to testing. And since most bulk spools of fluorocarbon have some type of "trade formulation" to achieve specific attributes such as suppleness or decreased memory, it becomes very difficult to translate the strain curves in "real life".
Rick's test was probably the first I saw that documented this permanent deformation of fluorocarbon after stretching using as little as 3 to 5 pounds of stress, and more recently TackleTour's published study documented this phenomenon at only 3 pounds of stress. Which made me wonder about even lighter amounts of pressure, and whether a simple stretch test could capture this limit where the elastic turns to plastic in fluorocarbon. Could this difference in the stress-strain curves account for the supposed increase in sensitivity people claim fluorocarbon has?
So I carried out my own little stretch test using two different pound tests of line, and comparing fluorocarbon to monofilament. To try and help detect this lower stress curve, I increased the length of my test lines to approximately 15' from the typical shorter lengths used by others. The thought being that during the elastic phase with fluorocarbon, it might stretch less than monofilament as theorized by anglers based upon sensitivity and the like. My results are below.
In the first diagram are the results for the 6 pound test. As you can see the mono stretched further at all load levels, though they were all within 2-4% of each other all the way up to the 14-16% range under the 4 pound load. They probably would have stretched more before breaking, but that wasn't the point of the test. The loads started at just 0.5 pound, but there is no difference in the overall stretch pattern of the two.
In the second diagram was a test between .016" fluorocarbon and .015" monofilament. In this case, the same patterns emerged at all levels again, with the exception of the fluorocarbon stretching slightly more than the mono. The reason for the two fluorocarbon results was due to a slip in the knot at the 4.5 pound load range on the first test. I retied and ran again successfully up to the 5 pound load using the same piece of line that had already been stretched once. The same overall pattern emerged, but it is interesting to note that the second time around the line stretched slightly more initially before being identical at the 2 pound load, and then being a little less elongated after that at the higher levels. Might just be an anomaly, but is interesting none the less.
The final test, which I don't have data to show, was at just a single load of less than 2 ounces (50g) on the two 6 pound lines. The thought was that perhaps only under very light loads would the difference be apparent, but again, the fluoro and the mono stretched almost identical distances (1" over a 120" length).
So the take home message is that fluorocarbon is not low stretch, at least not under typical testing loads and conditions, and is really no better than regular monofilament in this category. As such, another claim potentially busted, though there is still the potential for shock loading differences, which isn't exactly 'stretch' but more rate of stretch. So what are it's positive attributes, its correct claims, and why does it seem to be more sensitive?
So you're probably beginning to wonder what in the hell fluorocarbon is actually good for. I wanted to get the two biggest misconceptions out of the way first, so that we can now focus on the other attributes. So let's jump right into it, taking the easy ones out next.
One of the weaknesses of nylon monofilament is it's unstability when exposed to sunlight, in particular ultraviolet radiation which is the primary cause of the degradation. Tests show that the loss can be as great as 20% of its strength in the first 100 hours of exposure. After that, mono can lose another 20% over the next 100-200 hours. This degradation is usually in the form of oxidation.
But you haven't changed your line all season and you don't really notice a difference in its strength. There are several reasons for this. One is that 100-300 hours of exposure is a long time, especially with the usage of rod lockers now days on modern bass boats. Additionally, most of the outfits are being fished with throughout the day thereby limiting further exposure. Another is that only the line on the surface of your reels and strung through your guides is directly exposed to the UV and subsequently getting the exposure, and you're constantly cutting off end sections and retying, thereby always getting rid of the old and exposing the new.
Fluorocarbon on the other hand is largely transparent to UV radiation and therefor suffers much lower UV absorption. As such, the material will not be degraded by sunlight. So score one positive for the PVDF.
DENSITY(as relates to sink rate):
Everyone has probably heard the numbers relating to density for the two materials, as well as the claims of faster sinking for fluorocarbon. While this is true, what you rarely read are the actual rates and their subsequent practicality. It is easy to think in terms of faster sinking and relate to worm weights, or jigs, or 1' per second and all that other stuff. In reality, life is much slower. If you take two pieces of line, one nylon monofilament and one fluorocarbon, sink them in a column of water and time their drop rate, what you'll find is that it takes the nylon monofilament about 40 seconds to drop one foot in the water column. This is because monofilament has a density of about 1.1 relative to water at 1.0, not much denser.
Fluorocarbon on the other hand has a density of about 1.78. So how does this translate under the same test conditions? A similar piece of fluorocarbon will make that same 12" journey in about 15 seconds, still 3 times faster than mono but not overly fast in the big scheme of things. If you think of your average cast and retrieve taking somewhere between 30-60 seconds, under its own accord the fluoro will have only had the chance to move 2'-4' down into the water column in that time period.
Now you still have the fact of the lure and any tension caused by that lure to help pull down the line quicker, but you also still end up with a sort of arc down to your bait instead of this really direct, straight line that people invision. So for things like shallow running crankbaits, jerkbaits and spinnerbaits, fluorocarbon would work well. For topwaters, it pretty much sucks as it wants to slowly pull your bait down into the water which is not what you want with a floating topwater. For all other baits like worms, jigs or other things typically fished deep or on the bottom, it probably helps a little, just not as much as you might have thought.
WATER ABSORPTION (Tensile Strength):
Another potential benefit comes in the form of resistance to water absorption. Depending on the exact makeup of a particular monofilament, it would not be unusual to have between 3-10% absorption of water (by weight) with nylon. That leads to both good properties and bad. On the good side, a nylon mono that absorbs water swells slightly, becomes easier to handle, becomes more limp, makes knot tying easier and will even cast better. On the negative side though is that water absorption weakens the line, increases stretch and decreases strength. So depending on your perspective this might be good or it might be bad.
If you look at the numbers for fluorocarbon though, it is practically impervious to water. Water absorption rates are typically <0.04% for pure fluorocarbon. What this means is that none of its physical properties change after a good soaking. Line strength stays the same, stretch stays the same, but so too does any stiffness or unruliness. Again, a mixed bag depending upon how you look at it.
But this brings up a larger question in my mind that I've never seen anyone address. With monofilaments, line conditioners that you spray onto your reel are pretty popular. Some work by penetrating into the nylon and performing this softening I mentioned via absorption, while others work by attaching themselves to the exterior of the line and giving a slick coating. I've seen many people recommend using these same conditioners on all lines including fluorocarbon like they're some type of magic formula, but I have to wonder if the improvement isn't just a psychological one. You're not instantly changing the physical properties of fluorocarbon by spraying this stuff, so the imperviousness to moisture still exists. Nothing is getting absorbed, so are they really working?
Odds and Ends: A couple other observations I've come across in the research.
- Knot strength - Mono wins out here. Tests with just some simple doubled loop knots, lubricated before cinching and applied to dry line show mono to retain as much as 97% of its breaking strain, while fluorocarbon retains about 77%. After soaking the lines and accounting for the previously mentioned absorption by nylons, mono weakens to 83% of original dry, unknotted breaking strain while fluoro maintains its 77% rate. Your mileage may vary depending upon the knot, the fluoro and the care used when tying.
- Weight on Spool - If you weigh an empty spool from your reel, and then load that spool to capacity with similar diameter lines and then reweigh, you'll find that a spool of fluorocarbon weighs about 23% more than a spool of mono (but just 4-5 grams total weight). If you subtract the spool weight and weigh only the line, that increase becomes 75%. What this typically translates to is decreased casting distance. If you actually throw braids into the mix, they come out best...and they tend to cast further as expected. This is largely due to the density difference and ties in with the effort required to start the spool rotating as well as with keeping that heavier line "up in the air" during the cast.
- Rockwell Hardness - I found this one interesting. Rockwell Hardness is a standard method of measuring hardness of a material. Everyone seems to state that fluorocarbon is much tougher than mono, but it depends on the specific formulation used and tested. Whereas the different types of nylon can vary with a reading between 88 -114 in the test, fluorocarbon scores 100. So though fluoro seems tougher due to a slightly higher modulus in flexure, in actual hardness tests it doesn't score appreciably better. But keep in mind these are dry tests, and once nylon absorbs water, it can lose between 30-40% of its modulus and would definitely feel (if not become) "softer" at that point.
- Thermal Conductivity, Bending Strength - You frequently hear anglers mention that you need to tighten fluorocarbon knots slowly to avoid heat buildup, but it turns out that fluoro has a lower level of heat conductivity than mono. Fluoro is rated at 0.19 W/mK while monofilaments range from 0.22-0.27 W/mK. To give you a better perspective on these numbers, if you think of typical insulating materials like Styrofoam or fiberglass, they have ratings between .03-.04 W/mK. On the other hand, something like a cast iron pot has a rating of 55.
Which brings up another interesting point. Why is it good to 'spit on' or wet your knot when tying and cinching? Of course, lubricity is probably the first thing people think of. But it also turns out that water has a thermal conductivity rating 3X greater than fluoro (0.58) and about twice that of monos. This means the water actually helps wick away excess heat generated during the knot tying process.
That said, fluoro does have a slightly higher thermal expansion rating ranging between 100-140 e-6/K, while monofilaments range from 70-120 e-6/K. The real culprit with knots is in the bending strength of each material. Monofilament ranges from 110-125 MPa, while fluorocarbon comes in at a relatively lower rating of 94 MPa. This ties back into and explains the knot strength differences as mentioned at the top of this section.
So you read all the tourney reports in BASS or over at BassFan and it seems like all the pros are using fluorocarbon. There has to be a reason, right? These guys' livelihood depends on putting fish in the boat, and they'll take advantage of every little thing that might help tip the odds in their favor. So what gives? It primarily comes down to sensitivity in the end, along with energy transmission and absorption, so let's take a look at what is happening there.
There are a couple different concepts in play when considering how "bites" get registered by anglers, most all of which primarily encompass the functions of line tension, elasticity and linear density. Let's take a look at these 3 components and the role each plays.
Since we're dealing with a comparison of just two line types, each similar in diameter, each being fished on the same equipment and using the same lures, in a sense line tension is actually a wash in this scenario. One thing to keep in mind though is that for nylon lines, elasticity is a function of tension. So to get the best possible feel with monofilament line, you want to keep as much tension on the line as possible. Think of the old tin can telephone trick you learned as a kid.
Similarly, if we were to add braid to the discussion, tension would again come into play. Braid has very low elasticity, but it also has very low density. If you fish something like a Senko weightless on braid and give it slack line to fall, you won't feel a thing. What you will see is the line jump due to the very low stretch. On the other hand, if you fish baits that keep tension on the line due to the way they are worked, you'll have very good feel, better than either of the other two lines under the same conditions, again due to the very low level of stretch (very high modulus of elasticity, 30X-40X greater than mono or fluoro).
Speaking of elasticity, also referred to as modulus of elasticity, Young's modulus, or elastic modulus, it is a reference to the tendency of an object to deform along an axis when opposing forces are applied along that axis, or what might commonly be referred to as stretch or stiffness. As mentioned in a previous post, fluorocarbon and monofilaments are not too different here when tested off the shelf. This was demonstrated in the stetch test graphs I showed. As far as actual measurements go, most monofilaments core constituents fall in a range between 1.3-2.6 x 10~3 MPa, and fluorocarbon comes in at 2.1-2.9 x 10~3 MPa (FYI, braid is ~110 x 10~3 MPa).
But there is a catch. These numbers refer to dry sample tests, and if you remember from the previous post, nylons absorb water pretty heavily while fluorocarbons don't. This has a very definite effect on modulus. Water has a plasticizing effect that facilitates molecular chain movement. This decreases stiffness (modulus) but increases flexibility. Even a few tenths of a percent of water absorption by nylon can decrease significantly the modulus value of nylon compared to its dry state reading. How much? Depending on the specific formulation, as much as a 30-40% loss in axial elastic modulus and, therefore, stiffness.
The chart above demonstrates this in a quick way. I took two nearly 20' lengths of 15# test Big Game and soaked one of them for 2 hours in water and left the other one dry. I then ran a stretch test documenting % elongation relative to their original lengths at every 1/2-pound of pressure. You can see that the soaked strand stretched further as a percentage of original length at every strain tested. What isn't nearly as obvious is by how much relative to the dry readings.
With the first half pound of pressure, % elongation doubled. At one pound it was half (50%) as much. Between 1-2 pounds it averaged 30% higher readings. From 2-3 pounds it was just over 20% more. From there all the way through to 6.0 pounds it averaged about 12.5% more stretch than the dry line. As an overall average across all strains tested, there was a 27% increase in stretch when comparing the two lines. My guess is that with the standard 5 hours of soaking that I've seen stated as a test condition before, things would have looked worse. BTW, permanent deformity over the 20' of dry line was 1" after 6.5 pounds of strain, while the deformity in the wet line was 0.5" after the same strain.
So while the two lines in a dry state start out similarly and react as such, with fluoro having a slight overall advantage, nylon quickly absorbs water and it's properties change. Since fluorocarbon is nearly impervious to water, it's modulus properties and hence its stiffness or stretch don't. And this is where the difference comes into play, and where anglers correctly identify most fluorocarbons as being stiffer or having less stretch, as much as 1/2 less. They stretch a lot, it's just all relative, and they (anglers) don't necessarily make the needed connection to use and subsequent water absorption. This extra stretch monofilament gains also turns into an energy absorber, dissipating the transfer of energy from a hookset.
The final component is mass per unit length, which is just linear density. Here as I have pointed out, there is a big difference. Monofilament density is ~1.1 while most fluorocarbons come in at 1.78. Again, water is the standard of comparison typically mentioned with a density of ~1.0. For simplicities sake, if we ignore terms like “modulus of elasticity” for a moment and just accept the following to be generally true: "The density of a molecular structure has a lot to do with its ability to transmit energy", it will go a long way in explaining this aspect. Think in terms of getting hit equally hard with a wood bat versus getting hit with a Nerf bat :)
You can transmit energy through basically solid media, more or less at the molecular level, without any obvious motion. A desk-top toy is the best example of this. Newton’s Cradle has a row of steel balls hanging on equal length strings which serve as pendulums. When one ball at the end of the row is dropped, the ball at the opposite end of the row will move, without any perceptible motion to the balls in the center. It's a demonstration of one of Newton's Laws concerning the conservation of momentum and energy.
Fluorocarbon is about 2/3 more dense than nylon monofilament and about 80% more dense than braid. This means that under similar tensions, the fluorocarbon would be better at transmitting that energy (of a bite) to the angler, and the angler using fluoro would be better at transmitting the energy from a hookset back down to the fish. It's why you can fish a bait with "slight" tension and feel the "pop" when a fish sucks it in.
So you can see that there are a variety of factors that come into play when considering why fluorocarbon is largely believed to be more sensitive than traditional monofilaments. If I had to rank them, I'd probably place density as the primary cause with "wet" modulus somewhere in second. Keeping a straighter connection to your bait due to line sinkage would be a small percentage in my opinion, at least until I see swimming pool or very clear lake pictures documenting otherwise. I think the pros have switched so heavily over to fluorocarbon because it gives them an advantage in the "feel"/hookset aspect of their game, regardless to what degree, while still having the stretch as a cushion of familiarity similar to mono when it comes to playing and landing a fish.
So that concludes my look at fluorocarbon line. It bears repeating that every brand of line has its own formulation that will affect all these parameters mentioned. As such, it's hard to make very specific claims without actually comparing a pair of lines head-to-head, and that's where you as an individual angler has to step in and determine if fluorocarbon, or any other type specialty line is right for your style of bass fishing.