On this page:

  • Properties of threads
  • A list of material densities (specific gravities)

Properties of fly tying threads

The descriptions of fly-tying threads as currently offered by the manufacturers is utterly unhelpful to us. The '00' system is meaningless since the scale is different for each brand. It has been suggested that a statement of the denier would be more helpful. This is probably true, but it occurs to me that there are many other relevant properties.

An obvious one is strength and this is often quoted as being a great point in favour of some new-tech materials, e.g. gel spun polypropylene. But why (apart from some specialist applications, deerhair spinning perhaps) is high strength assumed to be so desirable? In most tying high tension is not necessary and there is no difficulty in handling a thread with a b.s. of, say, 1lb. It might be that the advantage of a high strength material is to provide thinner threads, not high b.s. where it is not needed.

Elasticity has some importance. Polyamides (Nylon) and polyesters have an appreciable degree of elasticity and this means that such a thread will be forgiving of variation in tension while tying to the extent that it will not totally release its hold unless it is allowed to go very slack. Whereas a thread with virtually no elasticity will lose all hold on the materials the moment the tension is allowed to slacken. This is particularly bad for beginners and will probably result in a habit of applying far more tension that is necessary.

Also relevant to the thread’s “holding power” is its surface friction, or how slippery it is. A twisted thread is likely to grip more securely than continuous filament. Gel-spun threads are very slippery. This can, of course, be improved by the use of wax.

Weight of thread applied to the fly must have some bearing particularly in the case of dry flies. But this depends on two things – the amount of thread used and its density. For example, polyester with a density of about 1.4 will weigh almost half as much again as an equal volume of one of the polypropylenes, but to assess the value of this information it would be necessary to compare the actual volume of the two materials as used in a typical dressing. The weight of the fly in use, however, depends also on its propensity for soaking up water. This might depend mainly on the bulk of thread used, a high level of which is probably desirable in the case of wet flies; but not dry flies where use of the smallest possible amount of thread will avoid the problem of water saturation. This suggests that the finest possible thread should be used for dry flies. A further point to bear in mind with regard to material density is that polypropylene has a density of slightly less than 1 therefore it floats in water.

Along with strength, thickness is the most immediately obvious property. In tyings where the thread is not required to add bulk but only to retain the other materials then the less that is used the better. And since a certain minimum number of turns are probably necessary to retain the materials then the only way to restrict the bulk of thread used is to choose the thinnest, provided it can withstand the required tension. However, it is not necessarily a simple matter to assess the thickness of a thread. The directly measured thickness depends not only on the actual total cross-sectional area of the fibres in the yarn but also on the yarn’s construction. In the case of a twisted thread, the slacker the twist the more apparent thickness there will be and in the case of multiple untwisted continuous filaments, thickness is almost meaningless. It is not possible to measure thickness by measuring across a counted number of touching turns because examination with a lens shows that the thread flattens and spreads to an extent that depends on the radius of the wrapping. Measuring with a micrometer is difficult since again, the thread flattens and if the thread is consolidated by twisting tightly it becomes unrealistic.

At this point it occurs to me that what we are really interested in is not the arbitrarily measured thickness of the thread, but how quickly its windings create bulk regardless of whether it is a twisted yarn or a “flat-winding” thread. Bulkiness and strength are probably the most noticeable properties to the fly dresser. It has been said that there is a need for all threads to be labelled with their denier, which is a measurement of weight per unit length, the intention being that this should give a direct indication of the bulk of the thread. But this is not strictly true, since materials with different densities are not comparable. The most every-day materials are nylon with a density of about 1.1 and polyester about 1.4. In the case of two threads of these two materials, each stated to have the same denier, it is therefore to be expected that given equal number of windings the polyester will create less bulk than the nylon. Also, the construction of a thread can affect how it compacts as it is wound.

What is needed therefore to provide a direct reference to the rate at which a thread builds up is a simple measurement system. This will be the subject of another paper entitled
Fly-Tying Threads: measurement of Bulk Factor

Specific gravities:

  • epoxy 1.6 – 2
  • rubber 1.5
  • regenerated cellulose (Rayon)1.45
  • Kevlar 1.44
  • polyesters 1.4
  • cellulose acetate 1.3
  • acrylic 1.2
  • Nylon 66 1.14
  • Nylon 6 1.07
  • Water 1.0
  • HD polypropylene 0.95
  • gel spun polyethylene 0.97
  • polyethylene high density 0.95
  • polyethylene low density 0.92
  • polypropylene copolymer 0.9