In a previous post, we discussed rope. Rope is of significant value in many survival situations, but there are things which rope cannot do (or at least, cannot do well) due to the thickness or stiffness of the rope. It would not be really successful to sew up a rip in your pants with rope, or try fishing with rope. Plus there are things which rope can do, but so can something thinner. Thus it is usually a good idea to include some ‘string’ in your survival kit as well as some rope.
There are many types of string, made up of various materials. Usually, you will be better off with artificial fibers, since they are less subject to damage or decay due to the environment, and generally are stronger than the equivalent natural fibers. Nylon is often a good choice, or there are ‘specialty’ fibers such as Kevlar or Spectra if a higher strength to thickness ratio is important. Always be aware of the tensile strength and weaknesses of any string you consider. For instance, Kevlar is weakened by knotting it, so use of special knots is often necessary in order to get the full benefit. If you insist on natural fibers, investigate jute. I don’t know how good a ‘string’ it is, but it is an excellent tinder.
If you have paracord in your kit, then you have some string, since paracord has several strands of nylon with a 40 to 50 pound breaking strength as a core. Of course, you need to ‘destroy’ or at least downgrade your rope in order to access the string within, and from a kit design standpoint, a lot of string takes up less space and costs less than an extra length of paracord expected to provide string.
Thus the type (or types) of string to consider for inclusion in your kit would be from the list: nylon (or specialty) cord, dental floss, fishing line and thread.
Thread is mostly only used for sewing, and most appropriate fishing line will fulfill that function quite well. Thus, including thread in a survival kit is usually not worthwhile; use the space for more fishing line. An exception may be if there is a sewing kit included, which has some thread in it.
Dental floss is primarily used for your teeth, but does provide a backup string source. The neatest way to carry some is the Floss Card (www.flosscard.com), a credit card sized dispenser. A cheaper alternative is to get a standard container of the stuff and remove the small roll of floss from the container. Test whatever floss you get to find an estimate of its breaking strength (in case you need to use it for a purpose other than flossing your teeth).
Woven Nylon cord is available in a wide variety of sizes. I would not go below #12 with a breaking strength of 106 pounds, or over #24 with a 254 pound breaking strength (unless you do not have room for any rope in your kit). My normal preference would be #18 at 170 pound breaking strength, along with both rope and fish line included for when greater strength or thinner cord is required. Some people recommend ‘tarred’ nylon twine; it would seem this might be of most use where immersion in water is likely.
In most cases, a good fishing line is an important survival kit component, not only for fishing, but for sewing as well. If you look in the tackle box of a serious fisherman, you will likely find an assortment of fishing lines, each optimal for a particular fishing scenario. This is usually not practical in a survival kit, so go with the one or possibly two most versatile fish lines. There are two general types to choose from: mono-filament or woven (multi-filament). Mono-filament has some uses for which it is superior, but not for most survival fishing. It is ‘slippery’ and thus difficult to knot, and tends to have a ‘memory’ which would tend to be a bother when deploying it after storing it in a small package. Usually you will be better off with a good woven line.
The breaking strength of the fish line is a difficult decision. There tends to be a fairly close relationship between the diameter of the line and its breaking strength. The heavier the line, the less you can get into a small space. And there may be a point where the line will no longer easily fit through the smaller hooks appropriate for most survival kits. Or through a needle’s eye if the fish line will also be used for sewing. On the other hand, a light line has a higher chance of breaking, which could be a disaster if your fishing tackle is limited. I would suggest looking for lines in the 25 to 50 pound range. Of course, modify this range based on the fishing you will likely be doing. I’m partial to TufLine XP, usually 30 pound and green in color. If I could not get that, I might try some Spectra fish line by PowerPro. A cheaper, more convenient option is cute little spools with 50 feet of 25 pound nylon line from http://www.m4040.com; they also have 100 foot micro spools of 15 pound line if a lighter line is needed.