There are many knots that the fly fisher uses in putting together his fly fishing system. There are, however, just a few knots that are really necessary. Any worth while fly shop will load a purchased fly line and backing to the reel. Also, if the fly line purchased doesn't have a loop a good fly shop will attach one to the end of the fly line. The fly fisher should be familiar with a loop-to-loop connection to attach the leader to the fly line; blood knot or surgeon's knot to attach the tippet to the leader; and, a improved clinch knot or non-slip loop to tie the fly to the tippet or as a dropper.
The good fly fisher will be able to tie each of these know in a matter of seconds or minutes. Remember, you can't catch a fish unless your fly is in the water. Tying a knot should be come second nature to the good fly fisher.
Knots weaken the rope in which they are made. When knotted rope is strained to its breaking point, it almost always fails at the knot or close to it, unless it is defective or damaged elsewhere. The bending, crushing, and chafing forces that hold a knot in place also unevenly stress rope fibers and ultimately lead to a reduction in strength. The exact mechanisms that cause the weakening and failure are complex and are the subject of continued study.
Strength: Relative knot strength, also called knot efficiency, is the breaking strength of a knotted material in proportion to the breaking strength of the material without the knot. Determining a precise value for a particular knot is difficult because many factors can affect a knot efficiency test: the type of material, the style of material, the size of material, whether it is wet or dry, how the knot is dressed before loading, how rapidly it is loaded, whether the knot is repeatedly loaded, and so on. The efficiency of common knots ranges between 40—80% of the rope's original strength.
Nylon monofilament line is exactly what the name says: a single strand (“mono-filament”) of nylon, produced by passing a glob of molten plastic, actually a by-product of crude oil processing, through a die that creates the thin strand of nylon that we know as fishing line. Simple process, simple line.
A more complex and expensive process is used in the creation of fluorocarbon line, in which a polymer of fluorine is actually bonded to carbon at a molecular level. The process is much more scientific than I care to understand or dive into, but the results are quite amazing.
Uses: The Surgeon's Knot is an excellent knot to tie two pieces of line together. It is especially useful to connect the leader and tippet materials together. If the diameter of the of the tippet and leader is not the same it can be a good idea to tie a triple surgeon's knot. Be sure to wet the knot before tightening.
Uses:The Improved Clinch knot is one of the basic knots that is used by a lot of fly fishers. It is traditionally used to tie the tippet material to the hook. It can also be used in a two fly rig to tie the bottom fly the bend of the fly above.
Uses: The non-slip loop has several uses. The primary use is to tie a fly to the tippet so that the fly swings losely. The knot can also be used to attach the leader material to the fly line or the leader to the tippet using a loop-to-loop attachment.
Uses: Use this knot to join sections of leader or line together. It works best with lines of approximately equal diameter. This knot, depending on who you talk to is often called the "Blood Knot" or the "Barrel Knot", or the "Clinched Blood Knot".