The fly fishing line weight needs to be the correct one for the rest of the system. Fly fishing systems are graded according to the weight of the line, and the weight of the system you use depends on the intended use. The lines are graded from 1 to 15, with one being the lightest and fifteen being the heaviest. Rods must be numbered to match lines i.e. a number 1 rod needs a number 1 line in order to work properly. This is because the amount of spring or flex that a rod has is balanced to the line weight. If the line is too heavy it will overload the rod and it won't be able to cast it forward properly. Too light a line won't stress the rod enough to get the best spring and again the line won't travel as far as it could.
Variations on this situation occur though when you are attempting to make either quite short or quite long casts. You average rod/line combination is set for about six metres of line being used for casting at any one time. Obviously this amount of line has a given weight. If there is more or less line being used the weight will obviously vary, potentially affecting performance. So if you are using less line, such as when making short casts of fast water, you may want to use a heavier line to get the best from the rod. Or if you are making long casts, such as on lakes or other still water where the fish can see you more easily, you may want to switch to a lighter line so as to not overload the rod.
Fly fishing line weights.
When starting out, most people choose a rod/line weight of 6. This is a good all round weight that will do for most fishing situations. It has sufficient ability to cope with the longer casts needed for lakes, but could still be used for the shorter casts needed for many rivers. However if you know you are going to be doing one form of fishing more than others it may be worth buying a rod/line combo more suited to that style of fishing from the outset. Many people also start to expand their collection of rods and lines as they get better at fly fishing.
The smaller rods and lines in the 1-3 range are particularly suited to small rivers. This is where you need a delicate presentation of small flies on fine leaders and a large clunky line and rod simply won't mange this! However as the flies and leaders get larger and heavier, and the casts longer you will need to start looking at heavier combos. Weights 4 to 6 are good for average size rivers.
Once you get beyond size sixes you are into rods designed for large rivers and lakes. Casting heavy flies and leaders into the wind takes some real grunt and these rods can deliver that. Size 8 is generally the largest you will find for trout fly fishing. Beyond this you get into the double-handed rods used in salmon fishing.
Fly fishing line types.
Fly fishing lines also come in a variety of tapers and shape, the most common being weight-forward or double taper. This refers to the actual diameter of the line, which varies along its length.
You can also get several different types of line that vary in whether they sink or float. This enables you to get at fish at different depths in the water.
Fly fishing lines also come in a variety of colours. For beginners it is usually better to choose a brighter colour as this makes it easier to see on the water to watch for strikes, line drag or other mistakes. As you progress you may want to choose duller colours as they are less likely to spook the fish.
Like most things in fly fishing, choosing your line can quickly become overwhelming if you are not careful. However for most people starting out a number 6 floating, weight forward line on a number 6 rod will be quite adequate.
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